Download Reports on the impacts of Polyhalite when used as fertilizer

Reports on the impacts of Polyhalite when used as fertilizer
Prepared for Sirius Minerals
April 2014
1
2
Contents
Summary
4
Introduction
7
Demand for fertiliser and associated environmental impacts
8
A study of the existing knowledge in respect of the influence and
implications for cropping on:
a) Soil flora (Rhizobia and free-living N fixers, Trichoderma spp,
Gliocladium spp and others)
b) Soil inhabiting disease organisms Pythium spp,
Plasmodiophora brassicae and others
46
An investigation into the environmental impact of Polyhalite used
as a fertiliser
68
3
Summary
Sirius minerals commissioned a series of reports from two leading environmental
organisations in the UK; Ricardo-AEA and The Food and Environment Research Agency
(Fera), to examine the impacts of polyhalite when used as a fertilizer and also the
environmental impacts compared with those of conventional fertilizers.
Ricardo –AEA report into the demand for fertilizer and associated environmental
impacts
•
•
•
•
•
•
•
•
•
Ricardo-AEA was commissioned by Sirius Minerals to present an independent,
evidence-based review of some factors relevant to the future demand for nutrients
supplied by polyhalite.
A tobacco crop scenario was used to illustrate that polyhalite can be part of a
fertiliser application programme that will decrease the GHG emissions (per ha of
crop) from fertiliser production. Emissions for fertiliser production to supply adequate
K and S were 310 kg CO 2 e/ha without polyhalite and 73 kg CO 2 e/ha with polyhalite.
There are many interacting factors that will influence the effects of climate change on
yield and production. Overall, the balance of the available literature supports the
optimistic conclusion that yield and production will increase in line with increasing
demand from a growing population.
The significance of climate change for fertiliser demand is related to the significance
of climate change for crop production. Predictions of greater crop production lead to
predictions of greater offtake of nutrients from agricultural land, and greater need for
those nutrients to be replaced by fertiliser applications.
An indirect effect of climate change on crop production is the increased interest in
bioenergy production from land for climate change mitigation. An increase in
bioenergy production from crops will increase total crop production and the need for
fertiliser inputs.
Many of world’s soils are considered to be deficient in K, and in many cropping
systems K offtake exceeds K inputs. This negative K balance has been suggested as
a cause of sub-optimal yields, and will cause soil K status to decline further.
There is a decreasing trend in atmospheric S deposition to soil globally, with a
concomitant increase in standard S fertiliser recommendations in many developed
countries including the USA and the UK.
A balanced nutrition approach, avoiding sub-optimal availability of other nutrients
(e.g. the K and S present in polyhalite), protects the investment in N fertiliser
applications.
Plentiful supply of K at early growth stages has been shown to influence the ability of
plants to take up N. This illustrates the importance of nutrient release timing.
4
•
Fertiliser use globally has increased at a faster rate than global cereal yields: by
2009, fertiliser use was 550% of the 1961 level (2009 cereal yields were at 250% of
1961 levels).
Fera study of the existing knowledge in respect of the influence and implications for
cropping on
-
•
•
•
•
•
•
•
Soil flora (Rhizobia and free-living N fixers, Trichoderma spp, Gliocladium
spp and others)
Soil inhabiting disease organisms Pythium spp, Plasmodiophora brassicae
and others
The addition of potash or potassium salts to soil can be shown to have beneficial
effects on specific nitrogen-fixing and plant growth promoting bacteria as well as on
some organisms with antagonistic activity towards plant pathogens.
Potash, calcium and magnesium were shown to have the ability to reduce disease
severity or symptoms caused by certain plant pathogenic fungi and other pathogens
of significant agricultural importance. As part of an integrated crop protection
approach there could therefore be a reduction in pesticide requirements.
We found potash applications support the beneficial effects of Pseudomonas
fluresences, P. aeroginosa, Memnoniella echinata, Bradyrhizobium japonicum,
nitrogen fixing organisms, Azospirillum brasilense, Trichoderma harzianum and T.
viride.
Evidence indicates reduced infection and / or survivability of Rhizoctonia solani,
Mycosphaerella gramincola, Fusarium solani, Cephaleuros parasiticus, Cochliobolus
sativus, Pyrenophora triticepentis, Pestalotiopsis pala, Cercospora kikuchii, Puccinia
striiformis, Sphaerotheca fulginea, Pythium butleri, P. ultimum, P. coloratum and P.
splendens, Pytophthora colocasiae, Fusarium solani and other species,
Streptomyces scabiei and Plasmodiophora brassicae after potash or calcium
supplementation to the soil.
We found indications for the enhancement of soil biodiversity after the addition of
potassium.
Limitation in potassium can potentially lead to an increase in soil fungi including plant
pathogenic species, which will increase biotic stress on plants that might already be
stressed by potassium deficiency itself.
The potential for Polyhalite to make a contribution in areas of soil remediation is
significant due to its content of K, Mg and Ca, its pH neutrality and low chloride
content.
5
An investigation into the environmental impact of Polyhalite used as a fertiliser
•
•
•
•
A desk based appraisal of scientific literature was undertaken by Fera to consider the
environmental impact of polyhalite when used as a fertiliser. Based on this review,
conclusions were drawn on how the potential environmental impacts of polyhalite
compare to the use of conventional fertilisers.
Consideration was given individually to the leaching potential, effect on soil structure,
and effects on ecosystems and the organisms they contain by potassium, calcium,
magnesium and sulphate.
Based on this literature appraisal it can be concluded that the use of polyhalite as a
fertiliser will show no obvious environmental impacts that could specifically be
associated with polyhalite or would not be seen using other types of fertilisers
consisting of the same compounds.
All potential environmental impacts that have been identified during this study are
associated with individual compounds of polyhalite (e.g. sulphate and potassium)
rather than with polyhalite itself. All constituents of polyhalite (K, Mg, Ca and SO 4 )
are also present in other types or mixtures of fertilisers (e.g. NPK fertilisers and
manure) and therefore should not pose any additional threats to the environment.
6
Introduction
Sirius minerals commissioned a series of reports from two leading environmental
organisations in the UK; Ricardo-AEA and The Food and Environment Research Agency
(Fera), to examine the impacts of polyhalite when used as a fertilizer and also the
environmental impacts compared with those of conventional fertilizers.
These are desk based literature studies which draw upon a wide range of published data.
Each report is produced in full as a series of independent but related documents.
7
Demand for fertiliser and associated environmental impacts.
Demand for fertiliser and associated
environmental impacts.
Customer:
Contact:
Sirius Minerals
Dr Jeremy Wiltshire
Ricardo-AEA Ltd
Gemini Building, Harwell, Didcot, OX11 0QR
t: 01235 75 3593
e: [email protected]
Ricardo-AEA is certificated to ISO9001 and
ISO14001
Customer reference:
Confidentiality, copyright &
reproduction:
This report is the Copyright of RicardoAEA Ltd and has been prepared by
Ricardo-AEA Ltd under contract to York
Potash Ltd dated 29/11/2013. The
contents of this report may not be
reproduced in whole or in part, nor
passed to any organisation or person
without the specific prior written
permission of Marc Addison, RicardoAEA Ltd. Ricardo-AEA Ltd accepts no
liability whatsoever to any third party for
any loss or damage arising from any
interpretation or use of the information
contained in this report, or reliance on
any views expressed therein.
Author:
Dr Jeremy Wiltshire
Approved By:
Hugh Martineau
Date:
26 March 2014
Ricardo-AEA reference:
Ref: ED59315- Issue Number 1
Report for Sirius Minerals
Ricardo-AEA/ED59315
Issue Number 1
Date 26/03/2014
8
Demand for fertiliser and associated environmental impacts.
Summary
Evidence and independent interpretation
•
Ricardo-AEA was commissioned by Sirius Minerals to present an independent,
evidence-based review of some factors relevant to the future demand for nutrients
supplied by polyhalite.
•
The full report is organised into four tasks: environmental impacts, trends in crop
yields and soil nutrient status, implications of climate change, and the effect of crop
nutrition on nitrogen (N) use efficiency (NUE).
•
This summary provides the main findings of the report.
Carbon footprint impact of polyhalite fertiliser
•
The estimated value of the global warming potential (GWP) of polyhalite is 0.051 kg
CO 2 e per kg product. This is low compared with other fertilisers, considerably lower
than other potassium (K) source fertilisers like muriate of potash (MOP; 0.13 – 0.265
kg CO 2 e/kg) and common sulphur (S) source fertilisers like ammonium sulphate
(0.58 kg CO 2 e/kg).
•
Polyhalite can be part of a fertiliser application programme that will decrease the
GHG emissions (per ha of crop) from fertiliser production. This is illustrated by
comparison of alternative fertiliser regimes for a three-year arable rotation in
England. Emissions for fertiliser production to supply adequate K and S were 100 kg
CO 2 e/ha without polyhalite and 81-95 kg CO 2 e/ha with polyhalite.
•
A tobacco crop scenario is used to further illustrate that polyhalite can be part of a
fertiliser application programme that will decrease the GHG emissions (per ha of
crop) from fertiliser production. Emissions for fertiliser production to supply adequate
K and S were 310 kg CO 2 e/ha without polyhalite and 73 kg CO 2 e/ha with polyhalite.
•
In addition to potential savings in emissions from fertiliser production, use of
polyhalite has the potential to decrease GHG emissions from crop production through
more efficient use of N fertiliser when other nutrients provided by polyhalite are in
optimal supply.
Crop yields and production
•
Crop yields have increased by 115% over the last four decades.
•
By 2009, total global cereal yields were at 250% of 1961 levels.
•
Broadly optimistic conclusions for the future of crop yields and production reflect the
balance of the available literature.
•
As a result of the introduction of best practices with respect to nutrient inputs and
crop protection regimes, there is an achievable potential for crop yields in the
Russian Federation to double and, in Africa, to treble.
Effects of climate change on crop yields and production
•
There are many interacting factors that will influence the effects of climate change on
yield and production. Overall, the balance of the available literature supports the
optimistic conclusion that yield and production will increase in line with increasing
demand from a growing population.
9
Demand for fertiliser and associated environmental impacts.
•
Atmospheric warming since 1981 has reduced global crop production per unit area,
albeit by less than the increases due to improved varieties and agronomic
techniques.
•
Production in northern Europe is expected to increase due to longer growing seasons
and the extended geographic range of some crops.
•
Southern Europe will see decreased production due to drought stress.
•
The geographic range for successful cultivation of C4 crops (including maize,
sorghum, sugar cane and millets) is expected to increase with climate change,
contributing to increased global production of food.
•
Climate change will lead to enhanced ranges for pest and disease organisms and
weeds, and this will challenge crop production in new and unpredictable ways.
•
Higher intensity of rainfall will cause enhanced soil degradation if management
changes are not implemented to manage this risk. Intense rain will also lead to
nutrient losses, particularly losses of soluble nutrients, such as sulphate and nitrate.
•
Farm land will be lost through rising sea levels. Although a small loss of land can
have substantial local impacts, the effect on worldwide total production is likely to be
small relative to larger changes in demand for crop products, and improvements in
production through other means, such as technological improvements.
•
Over irrigation and extraction of water from inland aquifers is already leading to
salinisation of some areas. This is increasing the need for water to maintain
production, and ultimately will intensify competition for limited water supplies and
decrease production in affected areas.
Soil nutrient status
•
Many of world’s soils are considered to be deficient in K, and in many cropping
systems K offtake exceeds K inputs. This negative K balance has been suggested as
a cause of sub-optimal yields.
•
Since K offtake is reported to exceed K input in many parts of the world, we conclude
that soil K status is expected to decline further.
•
There is a decreasing trend in atmospheric S deposition to soil globally, with a
concomitant increase in standard S fertiliser recommendations in many developed
countries including the USA and the UK.
•
A lack of nutrients (including K which can be supplied by polyhalite) is thought to be
holding back rice yields in south-east Asia by 10%.
Nitrogen use efficiency and nutrient interactions
•
Nitrogen is applied to agricultural systems as a mineral fertiliser in order to increase
crop yields. However, N is lost from soil/crop systems and causes various forms of
pollution.
•
Application of N fertiliser leads to large emission of nitrous oxide (N 2 O, a greenhouse
gas) from soil, and this emission dominates the carbon footprints of many crop
production systems. Greater efficiency mitigates overall greenhouse gas emissions
from crop production.
•
Correct nutrient balance (especially K, P and S) is required to optimise N use
efficiency. A shortage of one nutrient can restrict crop growth and development
which, in turn, may limit the uptake of other nutrients, or the effectiveness with which
those nutrients are utilised by the plant.
10
Demand for fertiliser and associated environmental impacts.
•
A balanced nutrition approach, avoiding sub-optimal availability of other nutrients
(e.g. the K and S present in polyhalite), protects the investment in N fertiliser
applications.
•
Improvements in utilisation of all nutrients can be obtained by ‘Best Management
Practices’ e.g. matching inputs to crop requirements, taking account of available
nutrients in soils and in any organic manures applied, etc.
•
Deficiencies of Ca, Mg, and micronutrients can seriously reduce the utilisation
efficiency of other nutrients.
•
Plentiful supply of K at early growth stages has been shown to influence the ability of
plants to take up N. This illustrates the importance of nutrient release timing.
Fertiliser demand and use
•
Fertiliser use globally has increased at a faster rate than global cereal yields: by
2009, fertiliser use was 550% of the 1961 level (2009 cereal yields were at 250% of
1961 levels).
•
The significance of climate change for fertiliser demand is related to the significance
of climate change for crop production. Predictions of greater crop production lead to
predictions of greater offtake of nutrients from agricultural land, and greater need for
those nutrients to be replaced by fertiliser applications.
•
An indirect effect of climate change on crop production is the increased interest in
bioenergy production from land for climate change mitigation. An increase in
bioenergy production from crops will increase total crop production and the need for
fertiliser inputs.
•
Climate change mitigation activities are leading to increased offtake of crop residues
for bioenergy, with consequent increases in the needs for fertilisers, through removal
of nutrients in crop residues (one tonne of cereal straw contains around 5 kg of K),
and through the secondary effects of a decline in soil organic matter.
•
The demand for K fertiliser is expected to increase to reach around 50 Mt of K 2 O
(41 Mt of K) per year by 2050.
•
Because of a decreasing global trend in S deposition, there will be an increased need
for S fertilisation in future years.
•
Fertiliser demand from sub-Saharan African farmers has potential to increase in
future decades, with concomitant increases in production.
11
Demand for fertiliser and associated environmental impacts.
Table of contents
List of abbreviations ............................................................................................................ 0
0
Project aim, scope and methods ............................................................................... 1
1
Task 1: Environmental impact information for fertilisers ........................................ 2
1.1 Summary ............................................................................................................ 2
1.2 Review of existing environmental impact information for fertilisers ...................... 2
2
Task 2: Trends in crop yields and soil nutrient status under current conditions 12
2.1 Summary .......................................................................................................... 12
2.2 Important sources of information ....................................................................... 13
2.3 Trends in crop yields ......................................................................................... 13
2.4 Trends in soil nutrient status ............................................................................. 17
2.5 Demand for fertilisers ........................................................................................ 21
3
Task 3: Implications of climate change for crop production and nutrient demand
of crops ..................................................................................................................... 22
3.1 Summary .......................................................................................................... 22
3.2 Climate change predictions – relevance and background ................................. 23
3.3 The influence of predicted climate change on crop physiology and yield .......... 24
3.4 The significance of climate change for fertiliser demand ................................... 28
3.5 Broad implications for changes in farming practices ......................................... 34
4
Task 4: The effect of crop nutrition on nitrogen use efficiency............................. 35
4.1 Summary .......................................................................................................... 35
4.2 Background ...................................................................................................... 35
4.3 Effects of other nutrients on uptake of N ........................................................... 37
4.4 Benefits of multi-nutrient fertilisers .................................................................... 38
5
References ................................................................................................................ 40
12
Demand for fertiliser and associated environmental impacts.
List of abbreviations
CO2e
Carbon dioxide equivalent (the unit for measuring GHG emissions and
GWP)
EEF
Enhanced efficiency fertilisers
FAO
Food and Agricultural Organisation of the United Nations
FNE
Fertiliser-N efficiency
GHG
Greenhouse gas
Grain NutE
Grain N utilisation efficiency
GWP
Global warming potential
IPCC
Intergovernmental Panel on Climate Change
LCA
Life cycle assessment
LCI
Life cycle inventory
MOP
Muriate of potash (potassium chloride)
NUE
Nitrogen use efficiency
SOP
Sulphate of potash
SSP
Single superphosphate
Total NutE
Total N utilisation efficiency
TSP
Triple super phosphate
0
Demand for fertiliser and associated environmental impacts.
Project aim, scope and methods
Ricardo-AEA was commissioned by Sirius Minerals to conduct a literature review. The aim
was to present an independent view of available evidence and interpretation on demand for
fertiliser and its associated environmental impacts.
There were four tasks:
•
Task 1: Environmental impact information for fertilisers;
•
Task 2: Trends in crop yields and soil nutrient status under current conditions;
•
Task 3: Implications of climate change for crop production and nutrient demand of
crops;
•
Task 4: The effect of crop nutrition on nitrogen (N) use efficiency.
Following the methodology stated in the project proposal, a literature review was carried out
through a targeted search of online databases of evidence and case studies. These
addressed a wide range of potentially relevant subject areas using key words designed
systematically to identify relevant studies. This initial search was supplemented by the use of
the search engines GoogleScholar and ScienceDirect. If appropriate, relevant documents
were ‘snowballed’, i.e. the literature they cited was also explored. Limits were placed on the
search where necessary, for example by searching for literature only after a given year. We
only reviewed literature in the English language.
1
Demand for fertiliser and associated environmental impacts.
Task 1: Environmental impact information for
fertilisers
Summary
Scope
The scope of this task was to evaluate embedded carbon and energy of polyhalite compared
to the major alternative commodities (sulphur (S), potash, magnesium (Mg) and calcium (Ca)
sources) by reviewing existing data in the public domain. The existing evidence has been
tabulated and analysed.
Impacts and energy use
•
Values for global warming potential (GWP, also known as carbon footprint or
greenhouse gas (GHG) emissions) and embedded energy were collated from the
reviewed references for a variety of fertiliser types. Where possible, values were used
for fertiliser production only (i.e. cradle to distribution centre, excluding subsequent
transport and agricultural use). A table of values is given in section 1.2 of this report
(Table 3).
Relevance to polyhalite use
•
The estimated value of the global warming potential (GWP) of polyhalite is 0.051 kg
CO 2 e per kg product. This is low compared with other fertilisers, considerably lower
than other potassium (K) source fertilisers like muriate of potash (MOP; 0.13 – 0.265
kg CO 2 e/kg) and common S source fertilisers like ammonium sulphate (0.58 kg
CO 2 e/kg).
•
Polyhalite can be part of a fertiliser application programme that will decrease the
GHG emissions (per ha of crop) from fertiliser production. This is illustrated by
comparison of alternative fertiliser regimes for a three-year arable rotation in England.
Emissions for fertiliser production to supply adequate K and S were 100 kg CO 2 e/ha
without polyhalite and 81-95 kg CO 2 e/ha with polyhalite.
•
A tobacco crop scenario is used to further illustrate that polyhalite can be part of a
fertiliser application programme that will decrease the GHG emissions (per ha of
crop) from fertiliser production. Emissions for fertiliser production to supply adequate
K and S were 310 kg CO 2 e/ha without polyhalite and 73 kg CO 2 e/ha with polyhalite.
•
In addition to potential savings in emissions from fertiliser production, use of
polyhalite has the potential to decrease GHG emissions from crop production through
more efficient use of N fertiliser when other nutrients provided by polyhalite are in
optimal supply.
Review of existing environmental impact information for
fertilisers
The literature search for Task 1 revealed 145 papers. Because environmental impact
conversion factors change over time, the most recent of these papers are of most interest.
Therefore a date threshold of 2011 was used to focus on the most recent papers. The titles
and abstracts of papers dated earlier than 2011 were examined for relevance, and the
2
Demand for fertiliser and associated environmental impacts.
papers deemed important were included with the more recent papers. Eighty-six papers from
2011 onwards, and nine pre-2011 papers, were reviewed (Table 1).
Table 1: Summary table of the literature reviewed for Task 1.
Literature search
Number of papers
Papers found
145
Post-2011 papers reviewed
86
Pre-2011 papers reviewed
9
Total papers reviewed
95
Many of the papers examined the Global Warming Potential (GWP)/energy use/embedded
water of individual products or processes and did not break this down to provide values for
individual fertilisers. When fertilisers were mentioned, this was typically with reference to N
fertiliser, which is outside the scope of this project. The breakdown of how many papers
looked at topics of interest is shown in Table 2.
Table 2: Summary table of the number of papers which commented on relevant topics.
Topic
Number of relevant papers
Magnesium sources
1
Sulphur sources
4
Calcium sources
5
Potash sources
20
Embedded carbon/GWP/greenhouse gas emissions
67
Water footprints/embedded water/water use
15
Embedded energy/energy use
32
Table 3 shows the global warming potential and embedded energy of the target fertilisers.
Water use is excluded from the table as no values were found for this environmental impact.
Information on the environmental impacts of the following fertilisers was not found and hence
they also do not appear in the table:
•
Kieserite
•
Ammonium thiosulphate
•
NPK + S blends
Where possible, values taken from papers were converted into units of kg CO 2 e/kg product
and MJ/kg product. It should be noted that not all values are comparable because different
methodologies may have been used. This is indicated by the large range in values for some
3
Demand for fertiliser and associated environmental impacts.
fertilisers. Where possible, values that relate to fertiliser production only have been included,
and those which include emissions from transport and field use (e.g. soil emissions as a
consequence of applying fertilisers) emissions have been excluded.
Comparison with polyhalite
The GWP value of polyhalite is 0.051 kg CO 2 e/kg product (Table 3), calculated by
conversion from Sirius Minerals energy use estimates to GWP using up-to-date emission
factors provided by Defra (Defra, 2014). The conversion from energy use data to GWP
depends on assumptions such as the source of electricity, which influences the GWP value
per unit of energy. We assumed that the production energy will be from the UK grid.
Sirius minerals have recently changed the method for transport of polyhalite from the mine to
the processing plant near the port. This change (use of a conveyor belt system within a
tunnel, rather than transport by pipeline) decreased the GWP value of polyhalite by 6.4%.
Our calculation, using energy data from Sirius Minerals for the pipeline transport system, was
0.054 kg CO 2 e/kg, compared with the value of 0.051 kg CO 2 e/kg given above (values
rounded to two significant figures).
Generally, fertilisers containing N have higher production GHG emissions than fertilisers that
do not contain N. Based on the GWP data found during the research for this project (Table
3), polyhalite has a lower GWP than the values we found for the non-N-containing fertiliser
materials K 2 O, potassium chloride (MOP) and SOP. However, polyhalite has a higher GWP
than the value we found for SSP, although these provide different nutrients. We conclude
that polyhalite has a lower carbon footprint than many fertilisers, but comparisons with those
that have a GWP value relatively close to that of polyhalite is difficult because it is not clear
that similar methodologies were used for the assessments being compared.
A true like-for-like comparison between polyhalite and other fertiliser products is not possible
because the fertiliser products provide different nutrient inputs, in different proportions: e.g.
polyhalite provides Ca and Mg, not present in SOP. A better way to compare the GWP of
different fertiliser materials is to compare the GWP of the quantities required to fertilise a
crop rotation. This requires GWP values per unit (mass) of fertiliser applied, and quantities of
fertiliser required to supply the needs of the crops in a rotation. To illustrate this approach we
have selected a three-year crop rotation of wheat (year 1), wheat (year 2) and oilseed rape
(year 3), which we estimate is the most commonly-occurring rotation in arable catchments of
eastern England, representing 30% of arable rotations based on Defra statistics of crop
production areas. Below (Table 4) are the mean annual fertiliser applications (for supply of K
and S) for this scenario.
4
Demand for fertiliser and associated environmental impacts.
Table 3: Quoted values for global warming potential (GWP) and embedded energy for
different types of fertiliser retrieved from a review of 95 papers. Numbers in bold
represent the only value found or the range when multiple values were obtained.
Empty cells indicate that no good evidence was found.
Fertiliser
GWP
8.9 MJ/kg (Fore et al., 2011**)
Sulphur
Sources
Sulphur
Ammonium
sulphate
Embedded energy
0.58 kg CO 2 e/kg (IFA, 2009*) 8.3 MJ/kg (IFA, 2009*)
0.01 kg CO 2 e/kg (IFA, 2009*) 0.1 MJ/kg (IFA, 2009*)
Single
superphosphate
(SSP)
Potassium
0.17 kg CO 2 e/kg (IFA, 2009*) Manufacture energy: 8.9 MJ/kg
potassium (Fore et al., 2011**)
Content energy: 9.7 MJ/kg
potassium (Michos et al., 2012***)
9 MJ/kg potassium (Schramski et
al., 2013**)
Range: 8.9-9.7 MJ/kg
K2O
0.33 kg CO 2 e/kg (IFA, 2009*) 6.7 MJ/kg (Soltani et al., 2013***)
Potash Sources
0.69 kg CO 2 e/kg (Liu et al.,
2013***)
5.8 MJ/kg (IFA, 2009*)
Energy intensity: 9.5 MJ/kg (Liu et
Range: 0.33-0.69 kg CO 2 e/kg al., 2013***)Range: 5.8-9.5 MJ/kg
Sulphate of
potash (SOP)
0.766 kg CO 2 e/kg (Hillier et
al., 2011**)
NPK blend
1.06 kg CO 2 e/kg (EUav06,
Brentrup and Palliere, 2008**)
1.6-2.064 kg CO 2 e/kg
(Sweden, Wood and Cowie,
2004**)
0.34-1.184 kg CO 2 e/kg
(Europe average, Wood and
Cowie, 2004**)
0.06-0.4 kg CO 2 e/kg (Europe
Mod Tech, Wood and Cowie,
2004**)
0.782 kg CO 2 e/kg (Hillier et
al., 2011**)
Range: 0.06-2.064 kg
CO 2 e/kg
5
Demand for fertiliser and associated environmental impacts.
Fertiliser
GWP
Potassium
chloride/
muriate of
potash (MOP)
0.265 kg CO 2 e/kg (Hillier et
al., 2011**)
Embedded energy
0.13 kg CO 2 e/kg (Christie et
al., 2011***)
Calcium Sources
Magnesium
Sources
Multiple nutrients
(K, S, Ca, Mg)
Range: 0.13-0.265 kg
CO 2 e/kg
Polyhalite
0.051 kg CO 2 e/kg (calculated
by Ricardo-AEA using energy
data provided by Sirius
Minerals)
Magnesium
chloride
0.1055 kg CO 2 e/kg
(Linderholm et al., 2012**)
Calcium
ammonium
nitrate (CAN)
2.380 kg CO 2 e/kg (Hillier et al, 11.28 MJ/kg (EUav06, Brentrup
2011**)
and Palliere, 2008**)
1.835 MJ/kg (Linderholm et al.,
2012**)
1.68 kg CO 2 e/kg (EUav06,
8.32 MJ/kg (BAT, Brentrup and
Brentrup and Palliere, 2008**) Palliere, 2008**)
0.75 kg CO 2 e/kg (BAT,
13.2 MJ/kg product (IFA, 2009*)
Brentrup and Palliere, 2008**)
Range: 8.32-13.2 MJ/kg
1.95 kg CO 2 e/kg (IFA, 2009*)
0.8-2.639 kg CO 2 e/kg (Wood
and Cowie, 2004**)
Range: 0.75-2.639 kg
CO 2 e/kg
Calcium nitrate
0.598 kg CO 2 e/kg (Hillier et
al., 2011**)
6.9 MJ/kg (EUav06, Breuntrup and
Palliere, 2008**)
1.49 kg CO 2 e/kg (EUav06,
5.19 MJ/kg (BAT, Brentrup and
Brentrup and Palliere, 2008**) Palliere, 2008**)
Range: 5.19-6.9 MJ/kg
0.56 kg CO 2 e/kg (BAT,
Brentrup and Palliere, 2008**)
Range: 0.56-1.49 kg CO 2 e/kg
Abbreviations: BAT = best available technology; EUav06 = EU average in 2006; Europe Mod
Tech = Europe Modern Technology.
*Values provided by IFA 2009 are expressed in terms of the ‘world today’ and are stated to
exclude energy and GHG emission from power and steam generation and transport of raw
materials. It is assumed that the term ‘World today’ reflects values accurate to 2009 when the
paper was published.
**Values provided are explicitly related to fertiliser production or manufacture.
***It is not stated within the paper whether this value is just for the production of the fertiliser
6
Demand for fertiliser and associated environmental impacts.
Fertiliser
GWP
Embedded energy
or if it also includes transport and field emissions.
N.B. Values are provided in the Kongshaug (1998) publication provided by Sirius Minerals.
However, the age of this data means that the methodologies used are potentially out of date.
For this reason, along with the fact that we are not in possession of the original PDF
document, and more relevant, recent values have been obtained, these values are not
included in the data table.
Table 4: Nutrients (K 2 O and SO 3 ) applied, kg/ha, for a rotation of winter wheat, winter
wheat and oilseed rape as reported by the British Survey of Fertiliser Practice (2012)
for Great Britain.
Crop
K2O
SO 3
Winter wheat
77
54
Winter wheat
77
54
Winter OSR
68
86
222
194
Total
Noting that the British Survey of Fertiliser Practice (2012) did not report Mg applications, we
describe below two rotation nutrition scenarios to supply the crop requirements for K and S
(in line with the actual amounts reported by the British Survey of Fertiliser Practice (2012)).
The first scenario does not use polyhalite and the second does. The approach in the second
scenario is to use polyhalite to provide all of the S, and as much K as that quantity provides,
topped up by another source of K. Polyhalite is not used to provide all of the K because this
would provide an excess of S.
Scenario 1 – polyhalite not used
RB209 (Anon., 2010) cites ammonium sulphate, kieserite and gypsum as sources of S. In
this first scenario we use gypsum (59.0% SO 3 ) as it is a straight fertiliser supplying only S
among the nutrients of interest. To supply 194 kg SO 3 requires:
194/0.59 = 329 kg of gypsum.
Using muriate of potash (MOP) (60.0% K 2 O), 222 kg of K 2 O would be supplied by:
222/0.60 = 370 kg of MOP.
Thus, total S and K fertiliser inputs = 329 kg gypsum and 370 kg MOP.
Scenario 2 – polyhalite included
Polyhalite contains 47.5% SO 3 , thus to supply 194 kg SO 3 will require:
194/0.475 = 408 kg polyhalite.
Polyhalite contains 14% K 2 O. Thus the application of 408 kg polyhalite will supply:
408 * 0.14 = 57 kg K 2 O.
7
Demand for fertiliser and associated environmental impacts.
This leaves 222-57=165 kg K 2 O to be supplied by MOP:
165/0.60 = 275 kg of MOP.
Thus, total S and K fertiliser inputs = 408 kg polyhalite and 275 kg MOP.
Using GWP values for the fertiliser materials, the total GWP over three years, is shown in
Table 5. This is for supply of K and S only, and does not include N which has much larger
GWP value, but is not provided by polyhalite.
Table 5: Global warming potential (GWP) for fertiliser materials to supply K and S to
1 ha of a 3-year crop rotation of wheat, wheat, oilseed rape. Totals are rounded to
2 significant figures.
Fertiliser material
GWP (kg
CO 2 e per
kg)
Without polyhalite
With polyhalite
Fertiliser
quantity
(kg/ha)
Fertiliser
quantity
(kg/ha)
Gypsum
0.01*
329
Muriate of potash (MOP)
0.27
370
Polyhalite
0.051
0
kg CO 2 e
per ha
3.3
Total:
kg CO 2 e
per ha
0
0
100
275
74
0
408
21
100
Total:
95
*From
http://ec.europa.eu/clima/policies/ets/cap/allocation/docs/bm_study-gypsum_en.pdf
accessed 21st of February 2014.
If polyhalite is used in the second scenario (‘with polyhalite’; Table 5) to provide all of the K,
and adequate (but excess) S, the total GWP for supply of K and S with polyhalite would be a
lower value of 81 kg CO 2 e per ha.
As a further illustration, below we show hypothetical scenarios for a tobacco crop grown in
China, using typical K and S inputs for the region (Table 6).
Table 6: Typical quantities of K 2 O and SO 3 applied, kg/ha, for a tobacco crop.
Crop
K2O
SO 3
Flue cured tobacco
200
50
Data source: IFA 2009
We describe below two nutrition scenarios to supply the tobacco crop requirements for K 2 O
and SO 3 . The first scenario does not use polyhalite and the second does. Because tobacco
8
Demand for fertiliser and associated environmental impacts.
is sensitive to chloride, fertilisers containing chloride (e.g. MOP) are not used, and the
fertilisers used provide all of the required K, with adequate (and surplus) S.
9
Demand for fertiliser and associated environmental impacts.
Scenario 1 – polyhalite not used
In this first scenario we use potassium sulphate (SOP; 50% K 2 O and 45% SO 3 ; Anon, 2010)
to supply the required K, and this also supplies adequate S. To supply 200 kg/ha K 2 O
requires:
200 / 0.50 = 400 kg of SOP.
Scenario 2 – polyhalite included
Here we use polyhalite to provide the required K, which also provides adequate S.
Polyhalite contains 14% K 2 O. Thus to supply 200 kg K 2 O will require:
200 / 0.14 = 1,428 kg polyhalite.
Using GWP values for the fertiliser materials, the total GWP is shown in Table 7, for supply
of K and S in one year of tobacco production.
Table 7: Global warming potential (GWP) for fertiliser materials to supply K and S to
1 ha of a tobacco crop. Totals are rounded to two significant figures.
Fertiliser material
GWP (kg
CO 2 e per
kg)
Without polyhalite
With polyhalite
Fertiliser
quantity
(kg/ha)
Fertiliser
quantity
(kg/ha)
kg CO 2 e
per ha
kg CO 2 e
per ha
Sulphate of potash
(SOP)
0.766
400
310
0
0
Polyhalite
0.051
0
0
1,428
73
Total:
310
Total:
73
The first example above (Table 5) shows that polyhalite can be part of a fertiliser application
programme that will decrease the GHG emissions from crop production, through decreased
emissions in fertiliser production. The second example (Table 7) shows much larger savings
in a crop with a high K requirement.
Where there are needs for Mg and/or Ca applications, the use of polyhalite will have further
benefits by avoiding the need for separate applications of these nutrients. The savings in
GHG emissions will increase (by avoiding emissions from manufacture of alternative sources
of Mg and/or Ca), and there will be practical and efficiency advantages on farms by the use
of fewer fertiliser materials.
Beyond the savings in GHG emissions from fertiliser production, use of polyhalite has the
potential to further decrease GHG emissions from crop production. The GHG emissions from
crop production include emissions from soil as a consequence of N fertiliser applications.
The emissions associated with N fertiliser (fertiliser manufacture plus soil emissions after
application) are a large proportion of the ‘pre-farm-gate’ GHG emissions from crop
10
Demand for fertiliser and associated environmental impacts.
production, often between 50% and 70% of total crop production GHG emissions. If
availability to the crop of other nutrients is sub-optimal, the use of N fertiliser by the crop will
be less efficient than use by a crop that has optimal nutrition. More efficient use of N is a key
GHG mitigation opportunity.
EcoInvent
Thirty-one of the 95 papers reviewed stated that they used EcoInvent as a source of
emissions factors or conversion factors. EcoInvent describes itself as ‘the world’s leading
database with consistent and transparent, up-to-date Life Cycle Inventory (LCI) data’
(EcoInvent, 2014). At present there are no values within EcoInvent for polyhalite, but other
relevant materials are included and their values are used in many published studies.
11
Demand for fertiliser and associated environmental impacts.
Task 2: Trends in crop yields and soil nutrient status
under current conditions
Summary
Scope
The scope of this task was to review changes in crop yields under current conditions, and
world soil nutrient status changes, based on available data. Past trends and forecast
changes have been interpreted to give implications for crop fertiliser demand.
Crop yields and production
•
Crop yields have increased by 115% over the last four decades.
•
By 2009, total global cereal yields were at 250% of 1961 levels.
•
Broadly optimistic conclusions for the future of crop yields and production reflect the
balance of the available literature.
•
As a result of the introduction of best practices with respect to nutrient inputs and
crop protection regimes, there is an achievable potential for crop yields in the Russian
Federation to double and, in Africa, to treble.
Soil nutrient status
•
Many of world’s soils are considered to be deficient in K, and in many cropping
systems K offtake exceeds K inputs. This negative K balance has been suggested as
a cause of sub-optimal yields.
•
Since K offtake is reported to exceed K input in many parts of the world, we conclude
that soil K status is expected to decline further.
•
There is a decreasing trend in atmospheric S deposition to soil globally, with a
concomitant increase in standard S fertiliser recommendations in many developed
countries including the USA and the UK.
•
A lack of nutrients (including K which can be supplied by polyhalite) is thought to be
holding back rice yields in south-east Asia by 10%.
Implications for fertiliser demand
•
Fertiliser use globally has increased at a faster rate than global cereal yields: by
2009, fertiliser use was 550% of the 1961 level (2009 cereal yields were at 250% of
1961 levels).
•
Nutrient demand will broadly follow production trends, since production removes
nutrients (offtake) from the land, and these nutrients must be replaced to maintain
production in the long-term.
•
The demand for K fertiliser is expected to increase to reach around 50 Mt of K 2 O
(41 Mt of K) per year by 2050.
•
Because of a decreasing global trend in S deposition, there will be an increased need
for S fertilisation in future years.
12
Demand for fertiliser and associated environmental impacts.
•
Fertiliser demand from sub-Saharan African farmers has potential to increase in
future decades, with concomitant increases in production.
Important sources of information
A key source of information on trends in crop production is “Global Food and Farming
Futures”, published in 2011; the “Foresight project” (Foresight, 2011). The Foresight project
studied the pressures on the global food system and considered the future up to 2050.
The project web page is here:
http://webarchive.nationalarchives.gov.uk/+/http://www.bis.gov.uk/foresight/ourwork/projects/current-projects/global-food-and-farming-futures (accessed 27th of January
2014)
It is stated that: “The Project has involved around 400 leading experts and stakeholders from
about 35 countries across the world. Drawing upon over 100 peer-reviewed evidence paper
commissioned by the Project…”
Reports for the Foresight project can be downloaded from the following web address.
http://webarchive.nationalarchives.gov.uk/+/http://www.bis.gov.uk/foresight/ourwork/projects/current-projects/global-food-and-farming-futures/reports-and-publications
(accessed 27th of January 2014)
The Foresight project analysed five key challenges for the future:
A. Balancing future demand and supply sustainably – to ensure that food supplies are
affordable.
B. Ensuring that there is adequate stability in food prices – and protecting the most
vulnerable from the volatility that does occur.
C. Achieving global access to food and ending hunger – this recognises that producing
enough food in the world so that everyone can potentially be fed is not the same thing
as ensuring food security for all.
D. Managing the contribution of the food system to the mitigation of climate change.
E. Maintaining biodiversity and ecosystem services while feeding the world.
Another important source of information is the WWF report (Holmes et al., 2013): “A 2020
vision for the global food system”, published in 2013 and available here:
http://www.wwf.org.uk/wwf_articles.cfm?unewsid=6465 (accessed 27th of January 2014)
A summary (WWF, 2013) is available from the same web page. The WWF project included
work on trends in the availability of agricultural land for food production, and trends in
agricultural yields.
The reports of these two projects include reference to most of the available publications
relevant to this task. We recommend that these reports are consulted by Sirius Minerals for
more detailed information on trends in production.
Trends in crop yields
Historical context
The Foresight Project (Foresight, 2011) reported that crop yields have increased by 115%
over the last four decades. There is much complexity behind this statement: see report C1,
from page 5 (Foresight C1, 2011). There is much regional variation: for example, in Africa the
area of production has increased with little change in yield, while in Asia production of
cereals has trebled with little change in production area.
13
Demand for fertiliser and associated environmental impacts.
The Foresight report (Foresight, 2011) also reported that, as a result of the introduction of
best practice with respect to nutrient inputs and crop protection regimes, there was an
achievable potential for crop yields in the Russian Federation to double and, in Africa, to
treble.
There is also temporal variation in the increasing production trend. Since the mid-1980s,
growth in productivity has fallen below the rate of population growth, from 2.8% per annum to
1.1% (IAASTD, 2009; Evans, 2009; Godfray et al., 2010).
There have been increases in global cereal yields and production in recent decades
(Figures 1 and 2). The exception to this is barley production, which has remained fairly
stable, (Figure 2), despite the rise in barley yield (Figure 1). Figure 3 demonstrates that total
cereal yield has increased since 1961. Interestingly, fertiliser use has increased at a much
faster rate than yields. By 2009, fertiliser use was 550% of the 1961 level, whereas cereal
yields were at 250% of 1961 levels (Figure 3).
Figure 1: Global changes in yield (Hg/ha) for major cereals from 1961-2009 (graph taken from
Foresight (2011) based on FAOstat data)
14
Demand for fertiliser and associated environmental impacts.
Figure 2: Global changes in production for major cereal crops from 1961-2011 (graph complied
from data taken from FAOstat, 2014)
Figure 3: Global trends in the intensification of crop production from 1961-2009 (graph taken
from Foresight (2011) based on FAOstat data)
15
Demand for fertiliser and associated environmental impacts.
Predictions
Future trends in yields of major crops, and in total world production (in addition to yield per
unit area, production is influenced by land availability) are uncertain and there is
disagreement in respected published views and evidence. Published evidence generally
includes climate change amongst the factors affecting yields.
There are factors that are expected to increase yield and production, and factors that are
expected to cause decreases. The WWF project (Holmes et al., 2013) discussed several of
these factors, including technology (positive), climate change (positive and negative with
variation by region; overall severely negative if average temperature increases by more than
2 degC), disease (direction of trend unclear), availability of fertiliser (direction of trend
unclear), and soil erosion (negative). Overall, this study makes broadly optimistic
conclusions, reflecting the balance of the available literature. However, the optimistic
conclusions are heavily qualified by the potential for severely negative effects of climate
change. In the box below we quote the headline conclusions of the WWF project (Holmes et
al., 2013).
Conclusions from Holmes et al., (2013):
•
Agricultural output has kept pace with rapid rises in global food demand over the past
50 years – but will this be the case over the next 50 years?
•
There is a good prospect of achieving approximately 50% increase in crop production
without the need for extra land (assuming no land is taken to produce bioenergy).
•
Socio-economic factors are a key component of the food production system and
government needs to adopt a holistic policy.
•
Breeding should allow large increases in crop yields in a CO 2 -“enriched” environment
with most airborne pests and diseases remaining controllable assuming crop
protection chemicals remain available.
•
Transgenic breeding could help control soil-borne pathogens.
•
The yield gap between potential and actual yield needs to be reduced.
There are many other published studies that estimate the need for increased future
production, and/or the future yield/production trends. Here we quote some of these to
illustrate the range of studies available.
In the future, technological developments are expected to help increase yield. For example,
in the United States of America, the combination of marker-assisted breeding, biotechnology,
and advances in agronomic practices, has the potential to double corn (maize) yields by
around 2030 (Edgerton, 2009).
In a review for the Food and Agricultural Organisation of the United Nations, FAO, (2009) the
authors reported that potential yield (PY, yield with the best varieties and agronomy and no
manageable biotic or abiotic stresses) may be estimated as:
•
maize, 21 t/ha,
•
rice, 15 t/ha,
•
wheat, 19 t/ha.
These are more than double the typical current yields in advanced agricultural systems.
16
Demand for fertiliser and associated environmental impacts.
The FAO (2009) report concluded that annual increases in yields over the next 40 years
need to be greater than the average annual yield increase over the last 50 years. For rice in
south-east Asia a lack of nutrients is thought to be holding back yields by 10% (FAO, 2009).
Peltonen-Sainio et al., (2009) concluded that there does not appear to be any decline in the
ability of selective breeding to increase crop yields. Yield increases have declined in recent
years due to constraints on inputs, including reduced interest in breeding varieties for
increased yield potential per se, rather than because plant breeders are finding any law of
diminishing returns in identifying traits which can increase potential yield.
We conclude that there is potential to continue to increase crop production until 2050. There
is still potential to increase yields both by breeding of new cultivars and developing new crop
protection strategies, and by closing the gap between potential yields and yields currently
achieved. Although the capacity to implement these actions may be compromised by the
reduction in agricultural research capacity, particularly in the developed world, this capacity is
likely to be increased through political pressure to support food production.
Trends in soil nutrient status
Soil degradation
Soil degradation through erosion, with consequent loss of nutrients, is a continuing problem.
For example, the “Foresight Project on Global Food and Farming Futures Synthesis Report
C2: Changing pressures on food production systems” (Foresight C2, 2011) states that soil
degradation and soil loss (i.e. erosion) in Africa, and especially West Africa, is resulting in a
major challenge for the development of agriculture in this region. The International Centre for
Soil Fertility and Agricultural Development has estimated that over 95 million hectares of land
have been degraded to the point of greatly reduced productivity (Bationo et al., 2007).
Special measures are needed to retain and reclaim soil fertility.
Major nutrient balance
Tan et al., (2005) reported the following whole crop N:P:K ratios:
•
Wheat - 100:21:109
•
Rice - 100:18:108
•
Maize - 100:16:66
These represent uptakes (i.e. nutrients in the crop plants) but not offtakes because some of
these nutrients are returned to the soil. The greatest amounts of N are in the grain of those
crops while K is mainly found in crop residues.
The proportions of nutrients applied as N, P and K were also reported by Tan et al., (2005):
71% as N, but only 12% and 16% as P and K respectively. The proportion of fertiliser N to P
(approximately 6:1) is in reasonable agreement with the proportions of those elements in
wheat, rice and maize (approximately 5:1) plants at harvest. However, to balance crop
uptake, K fertiliser should be applied in roughly the same amounts as those of N. This is not
currently the case, even in developed countries. If it is assumed that all of the non-harvested
fractions of crops are returned to soil, then the ratio of N to K currently applied as fertilisers
(4.5:1) approximately balances the N:K ratios in the harvested fractions (i.e. representing the
N:K ratio of nutrient offtake) of wheat, maize and rice (between 4:1 and 5:1). However, the
proportions of residue returned directly to soil can be very small, hence it appears that,
overall, current K fertiliser applications are neither balancing the need for crop uptake nor
fully replacing the K removed in crops.
17
Demand for fertiliser and associated environmental impacts.
Potassium status
At present, many of the world’s soils are considered to be deficient in K, including
approximately 75% of the paddy soils of China, and approximately 65% of the wheat belt of
Southern Australia (Romheld and Kirkby, 2010). In addition, Zorb et al., (2013) estimate that
less than 50% of K removed by crops is replaced, further worsening soil K status.
Dobermann (2007) have reported that there is evidence of depletion of soil K in India. This
depletion of K reserves has been suggested as a major reason for the lack of success in
increasing crop yields in India in recent years (Romheld and Kirkby, 2010). An analysis of the
nutrient balance of China, Indonesia, Malaysia, the Philippines, Thailand and Vietnam from
1961 to 1998 indicated an overall annual K deficit of about 11 Mt, which is 250% more than
their current K fertiliser use (Zorb et al., 2013). Zorb et al., (2013) also considered that India
and some other Asian countries are expected to drastically increase their K fertiliser
consumption in the future.
Since K offtakes are reported to exceed K inputs in many parts of the world, we conclude
that soil K status is expected to decline further.
Sulphur status
Sulphur deficiency has been reported to be increasingly common (Dick et al., 2008) because
of decreases in S deposition and increases in S offtake.
Global S deposition from pollution (especially coal combustion) is changing with
consequences for the need for S fertilisation of crops. High S deposition can adequately
supply crops with S, but crops are responsive to S fertiliser when deposition is low.
Figure 4 shows the trend of global S emissions from 1900 to 2000. Emissions increased
through the industrial revolution but, since reaching peak levels around 1975-1980, have
been declining steadily (Figure 4). Global emissions of S have fallen at an average rate of
2.7% per year since 1990 (Stern, 2005) with levels in 2000 estimated to be 10 Tg lower than
the peak values seen in 1980 (Smith et al., 2004). This decline is due to controls on coalfired power plants, changes in fossil fuel use, and increases in the amount of S removals
from non-ferrous metals and oils (Smith et al., 2004). Figures 5 and 6 demonstrate the
regional differences in S emissions.
18
Demand for fertiliser and associated environmental impacts.
Figure 4: Global anthropogenic sulphur emissions from Smith et al., (2011) and previous
studies (references found in Smith et al., 2011).
Figure 5: Global anthropogenic sulphur dioxide emissions by region, where North America
incorporates the USA and Canada, and East Asia refers to Japan, China and South Korea
(Smith et al., 2011).
19
Demand for fertiliser and associated environmental impacts.
Figure 6: The regional trends in sulphur from 1980 to 2000 (Stern, 2005).
Figures 5 and 6 show that since approximately 1980 many regions have seen decreases in S
emissions (Smith et al., 2004). In Europe, between 1990 and 2004, most countries reduced
their emissions by more than 60%, with a quarter of these countries reducing their emissions
by more than 80% (Vestreng et al., 2007; cited in Vet et al., 2014). Consequently, this
decrease in emissions reduces S deposition (Vet et al., 2014). A similar trend of significant
reductions in S emissions is in seen in North America where deposition has declined in line
with emissions reduction policies (Vet et al., 2014).
The regions showing the highest levels of S emissions are East Asia (Figure 5) and Asia
(Figure 6). Changes in China give an example of how atmospheric S deposition from
pollution (especially coal combustion) is changing with consequences for the need for S
fertilisation of crops.
From 1980 to 2005 sulphur (SO 2 ) emissions in China increased rapidly in most years
(EANET, 2011; Kuribayashi et al., 2012; Vet et al., 2014). However, within this time period, a
slight decrease was seen from the mid to late nineties due to the Asian economic crisis and
the consequential reduction in fuel consumption (Lu et al., 2010; EANET, 2011; Ge et al.,
2014). Post 2000, SO 2 emissions rose again with a 53% (21.7 Tg in 2000, to 33.2 Tg in
2006) increase in SO 2 emissions, at an annual growth rate of 7.3%, seen between 2000 and
2006 (Lu et al., 2010). After 2006, SO 2 emissions steadied and then began to decline due to
the introduction of flue-gas desulphurisation in power plants (Lu et al., 2010; EANET, 2011;
Vet et al., 2014). Between 2006 and 2009, a 39% reduction in ambient SO 2 concentration
was seen within the Pearl River Delta region (Wang et al., 2013). Following this time period,
SO 2 emissions were further decreased after the introduction of continuous emission
monitoring systems by the Chinese Government in 2010 (Zhang and Schreifels, 2011). It is
stated by Pan et al., (2013) that the concentrations of SO 2 decreased every year from 200820
Demand for fertiliser and associated environmental impacts.
2010, fitting the declining trend since 2006. By the end of 2010, SO 2 emissions across China
had decreased nationwide and in the power sector (Zhang and Schreifels, 2011). However, it
should be noted that inter-annual variation in SO 2 concentrations is still present (Pan et al.,
2013).
Status of other nutrients
Calcium (Ca) and magnesium (Mg) are the most abundant cations occupying the exchange
sites of the soil colloids, both inorganic and organic. Many soils, with the possible exception
of highly weathered, leached acid soils, contain enough Ca and Mg for crop growth
(Senthurpandian et al., 2009). Magnesium deficiencies are most likely to occur on acid,
sandy soils in humid regions and on soils with very high to excessive K availability (Mayland
and Wilkinson, 1989). High Mg fertiliser rates may be required on Mg-deficient, alkaline soils
because of the large amounts of Ca, Na and K present (Mayland and Wilkinson, 1989).
Demand for fertilisers
Nutrient and fertiliser demand by crops
Nutrient demand will broadly follow production trends, since production removes nutrients
(offtake) from the land, and these nutrients must be replaced to maintain production.
Replacement nutrients can come from atmospheric deposition (e.g. reactive N produced
naturally by lightening; nutrients from deposition of pollutants), from soil processes (e.g.
mineralisation of organic matter making N available to crops), or from addition of fertiliser
materials (including organic manures and inorganic fertilisers). For the high productivity
agriculture that dominates in highly developed economies, addition of fertilisers to provide
nutrients is necessary. Farming methods that restrict use of inorganic fertilisers may provide
other benefits, but will not secure the high yield levels required globally for a growing
population (Holmes et al., 2013). Bruinsma (2009) reported that in order to increase food
production to the amounts required by 2050, additional inputs, including nutrients, would be
required.
Sheahan et al., (2013), states that ‘it is widely perceived that sub-Saharan African farmers
are under-utilizing inorganic fertiliser’. FAO graphs taken from FAOstat (2014) demonstrate
how Africa’s fertiliser use is much below that of other countries (Figures 7 and 8).
Changes in demand for potash
In view of the need for increased food production, the current deficit in K inputs in relation to
K offtake by crops, and the evidence of limited K reserves in many soils, Magen (2011)
forecasts that the demand for K fertiliser will increase. Magen (2011) estimated that, by 2050,
additions of K 2 O will need to reach around 50 Mt of K 2 O (41 Mt of K) per year.
Changes in demand for sulphur
The demand for S fertilisers is expected to increase because of increased incidence of
deficiency, and decreasing trends in S deposition. Evidence for changes in S deposition is
given above (2.4.3).
21
Demand for fertiliser and associated environmental impacts.
Task 3: Implications of climate change for crop
production and nutrient demand of crops
Summary
Scope
The scope of this task was to review the influence of predicted climate change on crop
physiology and the implications for yields of major world crops.
Climate change predictions
•
Climate change predictions (scenarios) comprise many alternative scenarios with
associated probabilities of occurring. For each alternative scenario, it is highly
uncertain whether it will occur.
•
Global surface temperature change for the end of the 21st century is likely to exceed
1.5°C relative to 1850 to 1900 for most scenarios and, more likely than not, to exceed
2°C for at least one scenario.
•
Warming will continue to exhibit variability between years, or even decades, and will
not be regionally uniform.
•
A warmer climate is expected to reduce water availability, potentially limiting
production through water stress.
•
Land availability may change through sea level rise with large local effects, but effects
on global production are expected to be small relative to other changes in
productivity.
Effects of climate change on crop yields and production
•
Under some climate change scenarios, production is not threatened by climate
change, providing the concentration of CO 2 in the air increases as forecast (CO 2
enrichment) and farmers adapt to changing growing conditions.
•
Other climate change scenarios suggest that food production is threatened in some
regions, although global production is still expected to be adequate for a growing
population.
•
Effects of temperature increase have differing and potentially confounding effects on
yield and production. This is because temperature influences plant development (e.g.
floral initiation) as well as growth (increase in biomass). High temperatures can
advance the rate of plant development, decreasing the longevity of the foliage and,
therefore, light interception during critical periods for building yield.
•
There are many interacting factors that will influence the effects of climate change on
yield and production. Overall, the balance of the available literature supports the
optimistic conclusion that yield and production will increase in line with increasing
demand from a growing population.
•
Atmospheric warming since 1981 has reduced global crop production per unit area,
albeit by less than the increases due to improved varieties and agronomic
techniques.
22
Demand for fertiliser and associated environmental impacts.
•
Production in northern Europe is expected to increase due to longer growing seasons
and the extended geographic range of some crops.
•
Southern Europe will see decreased production due to drought stress.
•
The geographic range for successful cultivation of C4 crops (including maize,
sorghum, sugar cane and millets) is expected to increase with climate change,
contributing to increased global production of food.
•
Climate change will lead to enhanced ranges for pest and disease organisms and
weeds, and this will challenge crop production in new and unpredictable ways.
•
Higher intensity of rainfall will cause enhanced soil degradation if management
changes are not implemented to manage this risk. Intense rain will also lead to
nutrient losses, particularly losses of soluble nutrients, such as sulphate and nitrate.
•
Farm land will be lost through rising sea levels. Although a small loss of land can
have substantial local impacts, the effect on worldwide total production is likely to be
small relative to larger changes in demand for crop products, and improvements in
production through other means, such as technological improvements.
•
Over irrigation and extraction of water from inland aquifers is already leading to
salinisation of some areas. This is increasing the need for water to maintain
production, and ultimately will intensify competition for limited water supplies and
decrease production in affected areas.
Implications for fertiliser demand
•
The significance of climate change for fertiliser demand is related to the significance
of climate change for crop production. Predictions of greater crop production lead to
predictions of greater offtake of nutrients from agricultural land, and greater need for
those nutrients to be replaced by fertiliser applications.
•
An indirect effect of climate change on crop production is the increased interest in
bioenergy production from land for climate change mitigation. An increase in
bioenergy production from crops will increase total crop production and the need for
fertiliser inputs.
•
Climate change mitigation activities are leading to increased offtake of crop residues
for bioenergy, with consequent increases in the needs for fertilisers, through removal
of nutrients in crop residues (one tonne of cereal straw contains around 5 kg of K),
and through the secondary effects of a decline in soil organic matter.
Climate change predictions – relevance and background
Climate change predictions
Climate change predictions, or scenarios, are alternative predictions (i.e. there are multiple
scenarios) to assist in climate change analysis, including the assessment of impacts,
adaptation, and mitigation. For each alternative prediction, it is highly uncertain whether it will
occur.
Climate change predictions are made using climate models and Earth System Models, to
simulate changes based on a set of input data scenarios that include concentrations of
greenhouse gases. Information on scenarios is published by the Intergovernmental Panel on
Climate Change (IPCC). A recent summary for policy makers (IPCC, 2013) contains useful
information and can be downloaded from the following web address:
http://www.ipcc.ch/report/ar5/wg1/
23
Demand for fertiliser and associated environmental impacts.
Scenarios include projected changes in temperature, the water cycle (distribution of rainfall),
sea level and temperature, and ice cover. This report (IPCC, 2013) indicates that global
surface temperature increase for the end of the 21st century is likely to exceed 1.5°C relative
to 1850 to 1900 for most scenarios, and more likely than not to exceed 2°C for at least one
scenario. Warming will continue to exhibit variability between years or even decades and will
not be regionally uniform (IPCC, 2013).
The confusing nature of the previous paragraph is a symptom of the uncertainty in climate
predictions. It is not possible to give firm predictions of temperature rise. Predictions by
climate scientists generally indicate greater warming over land than over the oceans, and
greater warming near the poles than near the equator. The frequency of warm extremes is
expected to increase and the frequency of cold extremes is expected to decrease. In addition
to direct effects of temperature, the consequences for crop production relate to consequential
changes in the availability and crop demand for water, and the incidence of frost. These
factors are likely to change the geographic range, and therefore, distribution, of many crops.
Effect of fertiliser use on climate change
Although N is abundant in the atmosphere, crops need ‘reactive’ forms of N, usually as
nitrate or ammonium. Production of crops is greatly increased by adding reactive N to soil in
the form of fertiliser. Both the manufacture and use of N fertiliser can lead to emissions of
GHGs. Energy is required to fix N from the atmosphere to manufacture N fertiliser, leading to
emissions of CO 2 . In addition, there can be fugitive emissions of the powerful GHG nitrous
oxide (N 2 O) during the manufacturing process. Nitrous oxide is a GHG that has a global
warming potential of about 300 times that of carbon dioxide (CO 2 ). Following application of
N fertilisers to land, around 1% of the N is released from the soil as N 2 O as a result of
microbial metabolism of the N fertiliser.
The N cycle is more important in terms of GHG emissions from food production than the
carbon cycle. Some of this reactive N ‘escapes’ from food production systems with global
environmental consequences: polluting water, air and soils; changing natural and seminatural ecosystems; and affecting the climate. For wheat grown in the UK, about two-thirds of
the GHG emissions (sometimes expressed as a ‘carbon’ footprint) are associated with the
manufacture and use of N fertilisers. More efficient use of N fertiliser, avoiding over
application, and planning applications according to crop requirements, have potential to
decrease GHG emissions from agriculture and therefore contribute to mitigation of predicted
climate change.
Crops that are not in good health with appropriate availability of other nutrients, take up less
N from the soil than crops with optimal nutrition and growth. Therefore, adequate supply of
other nutrients is helpful in minimising loss of N to the environment, including emission of the
GHG, N 2 O.
The influence of predicted climate change on crop
physiology and yield
Background on carbon fixation by crops
The metabolic pathway for carbon fixation varies between crop species. Most crop species
use the C3 pathway, and some use the C4 pathway. The names of these pathways relate to
the number of carbon atoms in the first product of carbon fixation. There are also other
differences in the pathways leading to differing responses of plants to temperature and water
stress.
Crops with the C4 pathway include maize, sorghum, sugar cane and millets. Most other
major crops (including wheat, rice, potato, apple, and many others) use the C3 pathway.
Crops with the C4 pathway can assimilate carbon at higher temperatures, and use water
24
Demand for fertiliser and associated environmental impacts.
more efficiently than crops with the C3 pathway. Crops with the C3 pathway are more
responsive (positively) to CO 2 enrichment (higher concentration of CO 2 in the atmosphere)
than crops with the C4 pathway.
In C3 plants, the rate of photosynthesis increases up to approximately 30°C and declines at
greater temperatures. However, respiration rates increase greatly to approximately 35°C
and, at temperatures >20°C, tend to counteract increases in photosynthesis, thereby limiting
net assimilation (Porter and Semenov, 2005). In C4 plants, net photosynthesis peaks at
(typically) 38°C.
Effects of climate change on yield and production
Historical context
Atmospheric warming since 1981 has been estimated to have reduced global crop
production, albeit by less than the increases due to improved varieties and agronomic
techniques (Lobell and Field, 2007).
Effects of temperature change
There are many reports of effects on yield of changed environmental conditions, especially
temperature. Generally, growth and yield of crops increases up to an optimum temperature if
water is not limiting. The optimum temperature varies and is influenced by water stress, even
when drought conditions are not prevailing, because plants often cannot maintain the
required transpiration rate at high temperatures and leaf tissues become stressed. As
examples of optimum temperatures, Schlenker and Roberts (2008) reported that for maize,
soya and cotton, yields decrease at temperatures greater than 29°C, 30°C and 32°C
respectively.
Temperature influences plant development (e.g. floral initiation) as well as growth rate
(increase in biomass). High temperatures can advance the rate of plant development (e.g.
time from stem extension to flowering in cereals), decreasing the longevity of the foliage, and
thus decreasing light interception during the period when carbohydrate is accumulated
through photosynthesis. Therefore, there are effects of temperature increase that have
differing effects on yield and production. High temperatures at critical stages of crop
development, e.g. anthesis, can cause large reductions in yield, whereas increased
temperature during the vegetative stage may (in some cases) have little impact (Gornall et
al., 2010).
The growing season length and range of crops that can be grown in northern regions are
forecast to increase, leading to increased production in those regions (Gornall et al., 2010).
However, decreases in production in mid-latitudes are forecast to be greater than increases
in high (northern) latitudes (Gornall et al., 2010). The frequency and severity of droughts has
increased since the 1960s and drought may nullify the potential for increased production in
northern latitudes (Gornall et al., 2010).
While the increase in temperature has the potential to increase yields at northern latitudes,
increases in climate variability may lead to yield reductions greater than the increases arising
from any increase in mean temperature (Porter and Semenov, 2005).
An illustration of the complexities of forecasting the impacts of a warmer climate on crop
yields is the observed decrease in rice yields due to greater night temperatures. The effect
was not entirely explained by increased respiration at night (Peng et al., 2004).
CO 2 enrichment
The greater concentration of CO 2 in the air (CO 2 enrichment) is forecast to increase yields of
C3 crops by 13% and to reduce water consumption by all crops (Jaggard et al., 2010) due to
the reduced need for stomatal opening (stomata are leaf pores that open and close to
regulate exchange of gases with the atmosphere). The effects of CO 2 enrichment are not
25
Demand for fertiliser and associated environmental impacts.
expected to be seen until around 2050 (Porter and Semenov, 2005), and are less when N
nutrition is adequate (Jaggard et al., 2010).
Water use and availability
This potential increase in water use efficiency in a warmer climate will need to be balanced
against a likely reduction in water availability due to increased evapotranspiration and, in
some regions, reduced rainfall in summer (Gornall et al., 2010). Despite more efficient use of
water by crops, increased yields may increase the quantity of water transpired by crops,
increasing competition with other uses of water, and potentially limiting production through
water stress.
If transpiration is reduced because of lower water availability, N uptake may be reduced
(DaMatta et al., 2010). Any reduction in N uptake may reduce N-related aspects of crop
quality, e.g. protein content (DaMatta et al., 2010). Romheld and Kirkby (2010) reported that
optimising K nutrition can increase the resilience of plants to drought, heat and salt build-up
in soils.
Soil degradation
There is potential for increased soil degradation due to increased heavy rainfall, and
consequent soil erosion. There is also potential for reduced C sequestration in regions where
crop yields will be reduced. These factors have not usually been taken into account when
forecasting the impacts of climate change on crop yields (Jaggard et al., 2010).
Land availability
Rising sea levels have already caused land losses (Zhang and Cai, 2011). Land may be lost
by inundation of land, or by salinisation, such that land becomes unsuitable for growing
crops.
By the end of this century, sea level is predicted to have risen by at least 80 cm, and is
expected to continue rising for at least the next two hundred years (Zecca and Chiari, 2012).
It is widely expected that some land will be lost due to sea level rise. In Bangladesh, for
example, a one metre rise in sea level would affect the coastal area and flood plain zone,
resulting in negative consequences for national food security (Sarwar and Wallman, 2005).
For Bangladesh, an important country for rice production, it is stated that a 10 cm sea level
rise (as expected by 2020) would result in less than 1% of current total production being lost,
with a 25 cm sea level rise (as expected by 2050) resulting in 2% of current total production
being lost (Sarwar and Wallman, 2005). However, Bangladesh is a particularly vulnerable
area of the world for loss of land through sea level rise, and this amount of production loss
will not be replicated everywhere. In the United States of America, sea level rise within a
range of 0.2 to 2.0 metres above present levels would result in 26,000 to 76,000 km2 of land
inundated (Haer et al., 2013). The amount of agricultural land within these potential lost
areas is unknown.
In coastal lowlands which are insufficiently defended by sediment supply or embankments,
tidal flooding by saline water will tend to penetrate further inland than at present, extending
the area of perennially or seasonally saline soils (Brinkman and Sombroek, 1996). The rise
of eustatic sea level (i.e. mean global sea level) will lead to an increase in areas under the
influence of sea water intrusion (Varallyay, 2010).
In addition to sea level rise, climate change worsens soil salinisation because warming
increases the need for irrigation (Utset and Borotto, 2001; Andronov, 2010). This can result
in the accumulation of soluble salts in the soil from upward capillary movements from a
saline groundwater source, an effect known as secondary salinisation (Andronov, 2010). If
the leaching fraction remains unchanged, irrigation water requirements will increase,
resulting in higher water tables and increased soil salinity (Utset and Borotto, 2001).
Excessive irrigation has been shown to be the main cause of soil salinisation in more than
one million hectares in the valleys of Cuba due to the raising of saline water tables in lower
26
Demand for fertiliser and associated environmental impacts.
lands (Ortega et al., 1982; cited in Utset and Borotto, 2001). Secondary salinisation can also
occur because of long-term high application rates of fertilisers and the excessively high
nutrient accumulation in soils which results from this (Ju et al., 2007).
Higher evaporation will also increase the risk of salinisation of soils in regions where total
rainfall is restricted; salinisation will be intensified due to higher rates of evaporation and
evapotranspiration, increasing capillary transport of water and solutes from the groundwater
to the root zone with no or negligible leaching occurring (Yeo, 1999; cited in Olesen and
Bindi, 2002; Varallyay, 2010).
Estimates of crop production loss due to sea level rise have not been found for the world as
a whole and it is uncertain as to how well land is protected across the globe. It is also
important to note that whilst a small loss of land would have substantial local impacts, the
effect on worldwide total production is likely to be small relative to changes in demand and
potential changes in yield. For example, Edgerton (2009) reported that there is potential to
double corn (maize) yields in the United States over the next two decades through marker
assisted breeding, biotechnology traits and continued advances in agronomic practices.
Uncertainty
Yield and production of crops are closely related to light interception by green surfaces that
actively assimilate carbon by the process of photosynthesis. High temperatures can advance
the rate of plant development (e.g. time from stem extension to flowering in cereals),
decreasing the longevity of the foliage, and thus decreasing light interception during the
period when carbohydrate is accumulated through photosynthesis. The relationship,
therefore, between temperature and yield of crops is complex and depends on many
variables.
As indicated above, there are many interacting factors that will influence the effects of
climate change on yield and production. The uncertainty can be illustrated by reference to
Jaggard et al., (2010), who reported a range of forecast yield changes. For example, 2050
wheat yield forecasts for the EU ranged from 8.4 tonnes per hectare, on a low improvement
scenario, to 13.2 tonnes per hectare on a high improvement scenario. Jaggard et al., (2010)
also reported high variability in yield forecasts by country/region.
Under some climate change scenarios production is not threatened by climate change,
providing CO 2 enrichment takes effect and farmers can adapt (Fischer, 2009). But, under
other climate change scenarios, food production is threatened in some regions, although
global production should be adequate (Fischer, 2009).
Conclusions
Given the variability and uncertainty in the forecasts of the impacts of climate change on crop
yields and crop production it is difficult to draw firm conclusions. Nevertheless we conclude
the following:
•
In the short term, (to 2030), we expect climate change to increase crop production,
due to a combination of greater yields and a longer growing season in some regions,
leading to increases in production in northern latitudes that outweigh decreases in
production in the mid latitudes.
•
Post 2050 climate change is expected to cause further decreases in production in the
mid latitudes, outweighing any further increases in the north.
•
As sea levels rise, productive land will be lost to production. While the area forecast
to be inundated by 2100 may be relatively small, much of the vulnerable land is fertile
and highly productive.
•
In addition, many large population centres are threatened by rising sea levels. The
need to relocate affected populations will increase competition between agricultural
and urban land uses.
27
Demand for fertiliser and associated environmental impacts.
•
These factors will further increase the drive to increase crop yields per ha already
required to feed a growing population.
•
A warmer climate will lead to greater evaporative water losses and increased
competition for water resources.
The significance of climate change for fertiliser demand
Climate change and crop nutrient demands
The significance of climate change for fertiliser demand is related to the significance of
climate change for crop production. Greater crop production leads to greater offtake of
nutrients from agricultural land, and a greater need for those nutrients to be replaced.
As the climate changes, world population is also predicted to increase. This will increase the
demand for food, which must be met, at least in part, by increased crop production.
Interacting factors will include changes in the extent of food waste (likely to decrease, partly
meeting the demand for more food) and changes in diets. The balance between consumption
of crop products and livestock products will have an influence on the size of the expected
increase in the demand for fertiliser. It is the optimism that crop production will increase
(Holmes et al., 2013) leads to the view that demand for fertiliser will also increase.
As climate and world population change, the change in the supply of nutrients to crops to
maintain and increase production will also be influenced by the apparent imbalance in
nutrient supply and demand in some regions of the world. This imbalance can be seen in
Figures 7 and 8, by a wide variation in nutrient inputs, for example low nutrient inputs in
Africa. Some geographic variation is to be expected, with high nutrient inputs in regions
where potential crop productivity is high and low inputs where potential crop productivity is
low. Currently, the fertiliser inputs (N and P in Figures 7 and 8) do not match the variability in
crop productivity, with potential for more fertiliser use and more productivity, especially in
Africa.
These data (Figures 7 and 8) mask the differences between nutrients, and in areas shown as
having high nutrient inputs, the balance between inputs of major nutrients may not be ideal
(see 0).
28
Demand for fertiliser and associated environmental impacts.
Figure 7: Trend of fertiliser nutrient (N and P) use on arable and permanent crop area by continent (1992-2011) taken from FAOstat (2014), showing
low values for Africa.
29
Demand for fertiliser and associated environmental impacts.
Figure 8: Fertiliser nutrient (N and P) use on arable and permanent crop area by country (average 2002-2010), taken from FAOstat (2014).
From FAOstat (2014): “The designations employed and the presentation of material in the maps do not imply the expression of any opinion
whatsoever on the part of FAO concerning the legal or constitutional status of any country, territory or sea area, or concerning the delimitation of
frontiers. South Sudan declared its independence on July 9, 2011. Due to data availability, the assessment presented in the map for Sudan and South
Sudan reflects the situation up to 2011 for the former Sudan.”
30
Demand for fertiliser and associated environmental impacts.
Other studies also show that the use of fertiliser by farmers in Sub-Saharan Africa remains
lower than in other countries; see Table 8 (Crawford et al., 2005). Between 1980 and 2000
fertiliser use in Sub-Saharan Africa (excluding South Africa) rose by only 17%, from 1.09
million tonnes in the 1980-89 period to 1.26 million tonnes in the 1996-2000 period
(Crawford et al., 2005). Fertiliser use intensity (the kilograms of fertiliser consumed per
hectare of cultivated land) rose by only 5% over the same period (Crawford et al., 2005).
Table 8: Fertiliser use in Sub-Saharan Africa compared to other regions (Crawford et al., 2005).
Region
2000/01
2002/03
(kg of fertiliser nutrients per ha of arable land)
Sub-Saharan Africa
9
9
South Asia
109
100
East and Southeast Asia
149
135
99
73
Latin America
Crawford et al. (2005) did not give any detail of the types of fertiliser that made up these
totals. FAO statistics were examined for trends between 1990 and 2002 (the latest year
available on the website for Kenya and Nigeria. Kenya was the African country that Crawford
et al. (2005) reported as having the biggest increase in fertiliser use intensity from 1990-95
to 1996-2002, while Nigeria had the biggest decrease over that period. While N use in Kenya
increased by approximately 10% between from 1990 to 2002, K consumption more than
halved. Nitrogen use in Nigeria over the same period halved but consumption of K
decreased to one third of the amount used in 1990. In consequence the N:K ratio increased
in both countries (i.e. less K was applied per unit of N applied).
More recent data states that farmers in Sub-Saharan Africa use low amounts of fertiliser
nutrients per hectare of arable land: 13 kg/ha in Sub-Saharan Africa compared to the
developing country average of 94 kg/ha (Minot and Benson, 2009). It is also thought that
many African smallholder farms use much less fertiliser than is economically optimal (Minot
and Benson, 2009). It is stated by Minot and Benson (2009) that, due to increasing
population densities and changes in land availability, previous methods of managing fertility,
such as regularly leaving fields fallow, may not be possible in future. Thus, the need for
inorganic fertiliser use in Africa is expected to increase significantly in future years to ensure
that farmers are able to boost production to meet the food needs of the continent whilst
maintaining profits (Minot and Benson, 2009).
The need to maximise production may influence the demand for minor and micronutrients, to
support the ‘investment’ in N fertiliser applications (see Task 4).
There may also be a need to increase N fertiliser applications to maintain protein contents of
crop products (DaMatta et al., 2010). Gornall et al., (2010) also reported that increases in
production may be accompanied by adverse changes in food quality: e.g., decreased protein
and mineral nutrient concentrations are likely. If this occurs, there will be a need for greater
production to make up for the loss of quality, or for measures, such as increased fertiliser
application rates, to raise produce quality.
The points we make here are generalisations; there will be regional variation in production
changes and fertiliser demand changes. For example, increased production in northern
31
Demand for fertiliser and associated environmental impacts.
Europe, from longer growing seasons and perhaps also increased yields with greater
temperatures, and decreased production in southern Europe, due to drought stress and
increased temperatures, are forecast to lead to further intensification in northern Europe but
greater extensification in southern Europe (Olesen and Bindi, 2002). Overall, increased yield
will drive an increased demand for fertiliser.
The influence of bioenergy production
An indirect effect of climate change on crop production is the increased interest in bioenergy
production from land, for climate change mitigation. Land used for bioenergy production is
not used for food production, so changes in bioenergy production from land influences total
crop production (i.e. for food, energy and fibre), and the need for fertiliser to grow those
crops.
At present, approximately 2.5% of global cropland (40 million gross hectares) is used for
biofuel production (Popp et al., 2014). This has wide regional variation, with 8% of US
cropland dedicated to biofuel production, 5-6% of cropland in the EU, and 3% in Brazil (Popp
et al., 2014). To meet the 2050 target for biofuels share in world transport fuels, a further 100
million hectares (equivalent to about 7% of world arable land) would be required (Popp et al.,
2014). This may lead to reduced land availability for food production, unless non-productive
land is converted to agriculture to compensate, and/or there is a compensatory increase in
yields in excess of the increase required to meet the food needs of a rising population.
Whilst bioenergy production is increasing in some regions, such as Asia, growth in biofuel
investment has slowed in several countries due to concerns about competition with food
production, impacts of drought on crop productivity and policy uncertainty (Popp et al.,
2014). However, if bioenergy production increases, fertiliser applications are expected to
increase to produce more food on a smaller area of land. Lotze-Campen et al., (2010)
estimated that productivity of agricultural land will need to increase by 1.4% per annum to
meet global bioenergy demands by 2055.
A further consideration is the removal of crop residues for use as a bioenergy feedstock. The
use of large volumes of crop residues for energy production, rather than being returned to
the soil either directly or after being used for livestock bedding, may also increase the
demand for fertilisers, in particular K. One tonne of cereal straw will contain around 5 kg of
K. The ash that remains following combustion of straw and other residues will contain the K
and P that was in the residue and could be applied to land. However, while this may be both
practical and likely when residues are used on farm, e.g. to supply heat for livestock
buildings or greenhouses, where the residues are co-fired in power stations the ash from
other fuels may contain potentially toxic elements and rule out recycling to land.
The loss of soil organic carbon by residue removal is also important (Lal, 2005; USDA,
c.2006; Lemke et al., 2010; Whittaker et al., 2014), causing loss of soil structure and a
greater need for fertilisation (Lemke et al., 2010). Residue removal affects soil structure and
water holding capacity of the soil, which negatively impacts crop emergence and growth
(Nafziger, c.2011).
In summary, any increase in bioenergy production from crops will increase global crop
production and the need for fertiliser inputs. This effect of an increased fertiliser demand will
be magnified by removal of crop residues for bioenergy generation, where otherwise those
residues would have been incorporated into the soil.
The significance of climate change for K fertiliser inputs
In many cropping systems K offtake exceeds K inputs (Magen, 2011). Hence, where yields
are forecast to increase, there will be a need to increase K applications to balance K offtake
and maintain or improve soil K status. Even in regions where yields are forecast to decline,
an increase in K inputs may be needed if current inputs are substantially less than offtakes,
in order to maintain output at optimum under the changed environment.
32
Demand for fertiliser and associated environmental impacts.
The change in sulphur demand
Sulphur inputs may also need to be increased to maintain CO 2 fertilisation effect - but there
is little work in this area (Tausz et al., 2013). Tausz et al., (2013) considered that crop
nutrient requirements will need to be re-evaluated under increased CO 2 conditions.
In Task 2 we give information on S deposition and how this influences the need for S
fertiliser.
Sulphur is generally taken up by plants as the sulphate ion (SO 4 2-). Like nitrate, sulphate is
mobile in soil and readily lost by leaching over winter. Hence in regions of the world where
overwinter rainfall, and hence overwinter drainage, are forecast to increase, then so will
losses of sulphate increase, leading to a depletion of soil reserves and increasing need for S
fertiliser.
The significance of climate change for root architecture
Norby et al., (2004) reported that among deciduous trees, the response of roots to
atmospheric CO 2 enrichment was mainly in the form of an increase in fine-root production.
Our review did not find evidence for changes in root architecture of crop plants in response
to climate change.
The significance of climate change for pest and disease
Higher temperature and greater precipitation in some regions are likely to result in the
spread of weeds, plant pests and diseases. Within Europe, problems from new pests and
diseases are considered to be most likely in the northern countries as the changing climate
allows pests, pathogens and their crop hosts, to extend their range; increased risks are also
expected in Mediterranean countries. These increases in pest and/or disease attacks,
together with increased competition from weeds, are expected to frequently negate the
fertilising impact of increased CO 2 concentrations in the atmosphere unless adequate
controls on infestation can be implemented (Rosenzweig et al., 2001). Since weed growth
may also be enhanced by increased CO 2 , a changed weed ecology may emerge with
potential for increased weed competition with crops.
Overall, Jaggard et al., (2010) conclude that: “most weeds and airborne pests and diseases
should remain controllable, so long as policy changes do not remove too many types of
crop-protection chemicals”.
The latter point about policy changes is important but it is not possible to forecast the effects
of policy change over the long-term (more than a decade). Currently, pesticides usage in the
UK and EU is under pressure. Through the EU pesticide review process we have seen the
intention to withdraw products such as: methiocarb for the control of slugs, and neonicotinoid
insecticides. There is additional pressure in EU member states due to implementation of the
Water Framework Directive (2000/60/EC) and the Drinking Water Directive (98/83/EC). The
limit for pesticides in drinking water, according to the Drinking Water Directive, is 0.1 μg/L for
individual products and 0.5 μg/L for total pesticides. If these limits are to be complied with
then we could see further pesticide product withdrawals. A previous example has been the
withdrawal of isoproturon (IPU).
The significance of climate change for trace elements
There is little published evidence on the effects of climate change on micronutrient content of
crops (Lobell and Burke, 2010).
Conclusions on the influence of climate change on fertiliser requirements
Increasing crop production does not axiomatically lead to increased fertiliser requirements.
For example, crop production in the UK has continued to increase since the mid-1980s yet
fertiliser applications have, on average, decreased. The main drivers for increased fertiliser
33
Demand for fertiliser and associated environmental impacts.
use are the need to increase production to feed a growing population and the need to
increase applications of nutrients in regions where offtakes currently exceed inputs, such as
K in India, China and Brazil. Nevertheless, we conclude that a changing climate is also likely
to lead to some increase in the demand for fertilisers, at least in the short term (to 2030).
•
Yields are forecast to increase in the northern latitudes and balanced fertilisation will
require additional fertiliser inputs.
•
While yields may decrease in mid-latitudes it is in these parts of the world that the
greatest shortfalls in nutrient applications occur, particularly K. Hence inputs will still
need to be increased in those regions to optimize production.
Broad implications for changes in farming practices
The implications for farming practice will vary considerably across the globe depending on
how the forecast changes in climate interact. While both warming and increased rainfall are
predicted for many regions, the consequences for farming practices will depend on the
circumstances arising from these increases.
Changes in farming systems are difficult to predict, but may be influenced by:
•
The need to conserve water (change of crop species, changes in irrigation use, and
changes in soil cultivation techniques). This potential increase in water use efficiency
in a warmer climate will need to be balanced against a likely reduction in water
availability due to increased evapotranspiration and, in some regions, reduced rainfall
in summer (Gornall et al., 2010). Despite more efficient use of water by crops,
increased yields may increase the quantity of water transpired by crops, increasing
competition with other uses of water, e.g. urban, and potentially limiting production
through water stress.
•
The suitability of climate for different crop species.
•
The dates of first autumn frosts and last spring frosts, influencing the range of crops
that can be grown. However, forecast increases in extreme weather events may
reduce the potential benefits from a longer growing season.
•
Local demand for low value, bulky crop products that are expensive to transport (e.g.
sugar beet, biomass feedstocks).
•
Political pressure to minimise environmental impacts (e.g. the spatial integration of
livestock and crop production to allow better utilisation of nutrients from animal
manures). Improving the utilisation of manure nutrients by better distribution of
manures will reduce the demand for fertilisers. However, we judge that the reductions
in demand for fertilisers arising from improved manure distribution will be less than
the need for additional fertilisers to increase production and to ensure that offtakes of
nutrients such as K are at least balanced by inputs. As indicated earlier, in many
regions there is a deficit of K and soil-K reserves need to be increased to ensure
optimum crop yield.
34
Demand for fertiliser and associated environmental impacts.
Task 4: The effect of crop nutrition on nitrogen use
efficiency
Summary
Scope
The scope of this task was to review how crop nutrition (multiple nutrients) affects N use
efficiency (NUE).
Nitrogen use efficiency
•
Nitrogen is applied to agricultural systems as a mineral fertiliser in order to increase
crop yields. However, N is lost from soil/crop systems and causes various forms of
pollution.
•
Application of N fertiliser leads to large emission of nitrous oxide (N 2 O, a greenhouse
gas) from soil, and this emission dominates the carbon footprints of many crop
production systems. Greater efficiency mitigates overall greenhouse gas emissions
from crop production.
•
Improvements in utilisation of all nutrients can be obtained by ‘Best Management
Practices’ e.g. matching inputs to crop requirements, taking account of available
nutrients in soils and in any organic manures applied, etc.
Effects of other nutrients
•
A shortage of one nutrient can restrict crop growth and development which, in turn,
may limit the uptake of other nutrients, or the effectiveness with which those nutrients
are utilised by the plant.
•
For optimal crop production and utilisation of N, other nutrients must not be suboptimal.
•
Deficiencies of Ca, Mg and micronutrients can seriously reduce the utilisation
efficiency of other nutrients.
•
Plentiful supply of K at early growth stages has been shown to influence the ability of
plants to take up N. This illustrates the importance of nutrient release timing.
Implications for fertiliser use
•
Correct nutrient balance (especially K, P and S) is required to optimise N use
efficiency.
Background
The nitrogen cycle and environmental impacts of nitrogen fertiliser use
Nitrogen is applied to agricultural systems as a mineral fertiliser in order to increase crop
yields. However, N may be lost from the soil/crop system and cause various forms of
pollution. Nitrate (NO 3 -) may enter ground and surface waters (Foster et al., 1982).
Ammonia, (NH 3 ), when deposited to terrestrial and aquatic ecosystems, increases N
eutrophication and soil acidification (Roeloffs and Houdijk, 1991). Nitrous oxide (N 2 O)
35
Demand for fertiliser and associated environmental impacts.
contributes to global warming (Bouwman, 1990) and breakdown of stratospheric ozone
(Crutzen, 1981). Only emissions of dinitrogen (N 2 ) are environmentally benign.
Farming practices may be adjusted to reduce N emissions to the wider environment and
increase the efficiency with which N fertiliser is taken up and utilised by crops. To do this
effectively requires a quantitative understanding of the N cycle since N emissions are part of
that cycle and measures to reduce one N pollutant may increase the emissions of another.
The key to reducing N emissions following N fertiliser application is to match the amounts of
N fertiliser applied to the amount needed to ensure optimum economic yield. Crop-specific
advice on the amounts of fertiliser needed by crops are provided in the Defra Fertiliser
Manual RB209 (Anon., 2010) or the software ‘PLANET’ for example. Nitrogen fertiliser
applications to individual fields need to take into account the amounts of N available during
the season from the soil and from any organic manures applied. Soil-available N is
determined by the previous crop(s) and the amounts of N applied as fertiliser or organic
manures over the previous seasons. Nitrogen available from organic manures applied in the
season in which the crop is grown will depend upon the amount and type of manure applied,
together with the time of year the manure is applied and the method of application. In
general, soil-available N will be greatest following grass, in rotations which receive frequent
additions of manure, and following crops that have a large ratio of applied N to N removed at
harvest, e.g. oilseed rape and leafy vegetable crops such as cauliflower. The greatest
amounts of manure-N are available following application of poultry manures and when
slurries are applied in late winter or spring by injection or trailing hose machines, or when
solid manures are applied in late winter or spring and incorporated within 4 hours of
application.
There are concerns that methods which increase the availability of manure-N, such as slurry
injection and rapid incorporation of manure, may merely substitute one form of N pollution
(ammonia) with another (nitrate leaching or nitrous oxide emission). These concerns are
often over-stated: providing that N-rich manures, such as slurry and poultry manure, are not
applied in late summer or autumn, the risk of leaching can be largely prevented, and much of
the N conserved by reducing ammonia emissions can be recovered by crops (Webb et al.,
2013). While there may be some increase in N 2 O emissions following injection of slurry,
these increases are small and reduced by deeper injection (Webb et al., 2010). The impact
of rapid incorporation of solid manures on N 2 O emissions appears to be related to soil type,
and rapid incorporation may be as likely to reduce N 2 O emissions as to increase them
(Webb et al., 2014).
Nitrogen use efficiency
Barraclough et al., (2010) reported that N efficiency (commonly termed ‘N use efficiency’)
has many meanings in the context of crop production and the literature contains a plethora
of definitions. Barraclough et al., (2010) reduced these definitions to two primary efficiencies:
•
fertiliser-N efficiency;
•
crop-N efficiency.
Fertiliser-N efficiency (FNE) is the fraction of freshly applied fertiliser-N that is recovered in
the current crop. Crop-N efficiency can be defined in a number of ways, including total N
utilisation efficiency (total-NutE) and grain N utilisation efficiency (grain-NutE). The first of
these, total-NutE, is the total dry matter yield (grain plus straw) divided by total N in the
above-ground parts of the crop at maturity. By contrast, grain-NutE is the grain-only dry
matter yield divided by total N in the above-ground parts of the crop at maturity. A large FNE
does not necessarily lead to large grain-NutE. For example, FNE is greater on sandy soils
than on heavier clay soils. But the utilisation of N by cereal crops grown on sandy soils tends
to be limited by moisture stress later in the growing season, leading to less grain per kg N
recovered by cereals grown on sandy soils than by cereals grown on clay soils (Webb et al.,
1998).
36
Demand for fertiliser and associated environmental impacts.
Effects of other nutrients on uptake of N
Principles
Here we consider NUE primarily as FNE, i.e. we consider the impacts of other nutrient inputs
on the uptake of N by the crop.
Nitrogen is the mineral nutrient usually required in the greatest amount by crops. The
amounts of different nutrients recovered from soil by a crop, range from less than 20 g of
molybdenum ha-1 to up to approximately 250 kg of N ha-1 (e.g. White and Brown, 2010). A
shortage of one nutrient can restrict crop growth and development which, in turn, may limit
the uptake of other nutrients, or the effectiveness with which those nutrients are utilised by
the plant. This is generally referred to as Liebig’s ‘Law of the minimum’. Hence N uptake,
FNE and total-NutE can be reduced, thereby reducing crop yield and quality, if the supply of
other nutrients is inadequate. For example, Roberts (2008) reported a review based on 241
site-years of experiments in China, India, and North America, which found that balanced
fertilisation with N, P, and K increased first-year N recovery to an average of 54% compared
with an average N recovery of only 21% where N was applied alone. Tan et al., (2005)
considered that the inadequate application of K in many parts of the world was resulting in
unbalanced fertilisation, leading to application of surplus N fertiliser.
Sulphur
Nitrogen uptake can be improved by adequate S nutrition. Adequate S nutrition increases
overall N use efficiency (NUE, i.e. FNE and total-NutE) when N inputs are not limiting. The
increase in NUE arises from greater FNE and not from increased efficiency of N within the
plant (total-NutE) (Salvagiotti et al., 2009).
Potassium
In many cropping systems, potassium (K) offtake exceeds K inputs, and this negative K
balance has been suggested as a cause of yield stagnation and decline (Magen, 2011).
However, this may be a direct consequence of inadequate K supply to the crop rather than
an effect of the impacts of K fertilisation on NUE. Data clearly indicate (e.g. Johnston and
Milford, 2012) that when the available K content is limited, crops will not reach their optimum
yield from the application of N alone. Adequate K fertilisation is essential under those
circumstances. Hence, for those regions of the world identified where soils are deficient in K,
and where K inputs are less than K offtakes, increased inputs are needed to ensure that
crops fully respond to N fertiliser applications.
A study by Koch and Mengel (1974) looking at the influence of the level of K supply on the
incorporation of labelled N (15N) in young tobacco plants, revealed that during a four hour
period, plants which were well supplied with K absorbed a greater amount of labelled N than
those which lacked K supplies. Utilisation of this N for the synthesis of organic N compounds
also occurred more rapidly in the plants which were well supplied with K (Koch and Mengel,
1974).
In tobacco plants well supplied with K, 32% of the total N taken up within five hours was
incorporated into protein, but in K deficient plants this was only 11% (Koch and Mengel,
1974). The uptake and assimilation of N into amino acids was unaffected by low
concentrations of K, however when K was restricted, the incorporation of N into protein was
decreased (Koch and Mengel, 1974; cited in Leigh and Jones, 1984).
Best management practices
Improvements in both fertiliser-N uptake and NUE can be obtained by ‘Best Management
Practices’ (Dobermann, 2007) e.g.:
•
Taking full account of available N in soils.
37
Demand for fertiliser and associated environmental impacts.
•
Taking full account of available N in any organic manures applied.
•
Matching N inputs to crop N requirements.
•
Appropriate timing to ensure N is available at times of greatest crop N uptake.
•
Minimising N losses following fertiliser and manure application, in particular losses of
NH 3 following application of livestock manures and urea.
In conclusion, an adequate supply of P, K and other nutrients can ‘protect’ the investment in
N fertiliser application by ensuring that the crop response to fertiliser-N is not restricted by
the lack of any other nutrients. This helps to optimise production because the plants can use
more N and yield more if other nutrients are not sub-optimal.
Impact of N fertiliser formulation on N uptake by rice
Flooded rice soils may suffer from reduced fertiliser-N efficiency because of both NH 3
volatilisation, due to the high pH of the water to which the N is applied, and losses arising
from denitrification.
As well as the best management practices outlined above, much effort has been spent on
improving fertiliser technology to produce ‘enhanced efficiency fertilisers’ (EEF) in order to
increase the uptake of fertiliser-nutrients (e.g. Chien et al., 2009).
Granulated fertilisers have now been formulated to include P and K. Crop P uptake from
such fertilisers by rice has been shown to be as good as from triple superphosphate (TSP)
while water pollution by P was reduced (Chien et al., 2009). Uptake of K was also increased.
N uptake can be improved by selective breeding
N uptake can be improved by selective breeding (Barraclough et al., 2010). However, while
breeding can reduce the N requirement to obtain optimum yield, this may not lead to a
reduction in N fertiliser application. Nitrogen inputs may need to be maintained to ensure
adequate grain protein concentration (Barraclough et al., 2010).
Benefits of multi-nutrient fertilisers
We did not find any peer-reviewed evidence that supply of other major nutrients (P, K, S) in
compound fertilisers increased NUE more effectively than the application of the same
amounts of P, K and S as individual nutrients. In general, advice on crop nutrition and
fertiliser application has been that there is no inherent need for compound fertilisers and
field-specific requirements can be met by use of ‘straight’ (i.e. single nutrient) N, P, and K
fertilisers. However, while there does not appear to be any fundamental advantage in crop
nutrient uptake from using a single compound to supply N, P, K and S, there can be practical
and business efficiency advantages. The relative advantages of using one compound
fertiliser or a combination of straight (single nutrient) fertilisers will depend upon the number,
amounts and timing of nutrients required. If, for example, both K and S need to be applied to
a crop, then a compound containing both may be cost-effective, because the two nutrients
can be applied together.
Given the need to increase K applications in some important agricultural regions, and
decreasing atmospheric S deposition in many parts of the world, polyhalite provides a
valuable source of both these nutrients and has the added advantage that it also supplies
Mg.
There is much interest in the control of fertiliser nutrient release rates so that the timing of
nutrient supply can be matched to crop nutrient demand. This control has the advantage of
minimising losses of nutrients to the wider environment (less leaching of soluble nutrients),
and maximising crop production (nutrients available when the crop needs them). There is
also an advantage in minimising the number of tractor passes across a field (decreases in
38
Demand for fertiliser and associated environmental impacts.
energy use and soil damage) because more nutrients can be applied at one time if there is a
lower risk of loss from the soil. It is our understanding that the release rate of the nutrients in
polyhalite can be manipulated by appropriate formulation of the fertiliser product (e.g.
appropriate particle size), and it is our opinion that this will increase the benefits of the
polyhalite fertiliser product and also minimise losses to the environment after application.
39
Demand for fertiliser and associated environmental impacts.
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45
A study of the existing knowledge in respect of
the influence and implications for cropping on
a) Soil flora (Rhizobia and free-living N fixers,
Trichoderma spp, Gliocladium spp and
others)
b) Soil inhabiting disease organisms Pythium
spp, Plasmodiophora brassicae and others
Report to Sirius Minerals Plc
Dr Melanie Sapp, Dr Richard Thwaites
19th of March 2014
Contact:
Tel No:
01904
462745
Date:
11 April
2014
Email:
[email protected]
Dr M. Sapp
Reviewed By:
Ian Cox
46
Contents
Summary
48
Introduction
49
A-General positive effects of Polyhalite or its components on beneficial
microorganisms in soil
50
B-Effects of Polyhalite or its components on plant pathogens
56
Discussion
60
References
64
47
Summary
•
•
•
•
•
•
•
The addition of potash or potassium salts to soil can be shown to have beneficial
effects on specific nitrogen-fixing and plant growth promoting bacteria as well as
on some organisms with antagonistic activity towards plant pathogens.
Potash or its components have the ability to reduce disease severity or
symptoms caused by certain plant pathogenic fungi and other pathogens of
significant agricultural importance. As part of an integrated crop protection
approach there could therefore be a reduction in pesticide requirements.
We found potash applications support the beneficial effects of Pseudomonas
fluresences, P. aeroginosa, Memnoniella echinata, Bradyrhizobium japonicum,
nitrogen fixing organisms, Azospirillum brasilense, Trichoderma harzianum and
T. viride.
Evidence indicates reduced infection and / or survivability of Rhizoctonia solani,
Mycosphaerella gramincola, Fusarium solani, Cephaleuros parasiticus,
Cochliobolus sativus, Pyrenophora triticepentis, Pestalotiopsis pala, Cercospora
kikuchii, Puccinia striiformis, Sphaerotheca fulginea, Pythium butleri, P. ultimum,
P. coloratum and P. splendens, Pytophthora colocasiae, Fusarium solani and
other species, Streptomyces scabiei and Plasmodiophora brassicae after potash
or calcium supplementation to the soil.
We found indications for the enhancement of soil biodiversity after the addition of
potassium.
Limitation in potassium can potentially lead to an increase in soil fungi including
plant pathogenic species, which will increase biotic stress on plants that might
already be stressed by potassium deficiency itself.
The potential for Polyhalite to make a contribution in areas of soil remediation is
significant due to its content of K, Mg and Ca, its pH neutrality and low chloride
content.
48
Introduction
With increasing pressure on agricultural land to deliver higher crop yields for an
increasing population, the need to find appropriate fertilisation regimes is very high. The
York Potash mine aims to extract Polyhalite, a multi-nutrient mineral which could be used
as plant growth promoting fertiliser containing four macro-nutrients required for plant
growth namely potassium, sulphur, magnesium and calcium (K, S, Mg and Ca
respectively).
This report is the summary of a literature review study on the existing knowledge of the
influence of Polyhalite on the soil flora including beneficial microorganisms such as:
• Rhizobia,
• free-living nitrogen fixers,
• Trichoderma and Gliocladium spp..
Furthermore, its effects on soil inhabiting disease organisms such as Pythium spp.,
Plasmodiophora brassicae and others are also summarised as well as current
knowledge linking Polyhalite and nutrient cycling.
The importance of certain macro-nutrients for plant growth is well known. For example,
the need of plant available potassium for the production of high quality crop is based on
its role in the plant’s primary metabolism among other functions (Zörb et al., 2013). The
relationship between Polyhalite/potash, plant growth and plant growth promoting or
pathogenic microorganisms was researched in several studies. In this review we
summarise the current literature related to this topic focussing on its major component
potassium. Harris ((Harris, 1997)) concluded that potash application suppressed plant
pathogens of cotton. It is further assumed that a balanced nutritional status of the crop
with special importance of potassium (K) plays a key role in disease suppression (Magen
and Imas, 2004). Plant nutrition plays an important role in integrated pest management
practice. K in particular is the effective ion reducing disease thus contributing to pesticide
reduction which is in line with the movement towards residue –free food by retailers and
registration authorities worldwide (EU sustainable use directive, EU Directive
2009/128/EC).
Furthermore, the combination of plant growth promoting microorganisms and nutrition
has been widely studied resulting in higher crop yield. Kishan et al. ((Kishan et al., 2002))
found higher yield and higher net profit using fertilisation with specific phosphorous,
potassium and nitrogen (PKN) ratios in combination with Rhizobium spp. application
when applied to Indian soil used for growing cowpea. This could also be shown for
Rajmash with an inorganic fertiliser combination of 40 kg ha-1 N, recommended P and 30
kg ha-1 K addition to soil with a baseline of potassium of 211 kg ha-1 (Sharma and Verma,
2011). Muriate of potash in particular appears to have low toxicity to antagonistic
microorganisms. This could be shown for levels up to 2000 ppm tested with Trichoderma
spp.. revealing very low reductions in radial growth (Ranganathswamy, 2013).
Limitation in potassium can potentially lead to an increase in soil fungi including plant
pathogenic species (Yakimenko, 2013), which will increase biotic stress on plants that
might already be stressed by potassium deficiency itself. Variations in effects of
potassium on plant pathogens were observed for different pathogen-host systems
(Huber and Arny, 1985). Overall, disease symptoms decreased in 78% of bacterial, 70%
49
of fungal, 60% of viral and 30% for nematode-host systems in relation to potassium
highlighting a general positive effect of this macronutrient for disease reduction, which is
thought to be acting in relation to specific environmental conditions, host susceptibility
and other essential nutrients in soil (Deliopoulos et al., 2010).
In the following sections we summarise a) the positive effects of Polyhalite on beneficial
microorganisms and b) its negative effects on plant pathogenic organisms.
A-General positive effects of Polyhalite or its components on beneficial
microorganisms in soil
Specific outcomes of studies carried out assessing the effects of Polyhalite, its
components or potash on beneficial plant growth promoting microorganisms are
summarised in Table 1. These studies were carried out on a variety of crops including
legumes and non-legumes and were combined with different plant pathogens in 12
studies (Table. 1).
Its beneficial influence on organisms including:
nitrogen fixing bacteria - Azospirillum brasilense, Bradyrhizobium japonicum,
Rhizobium sp. and trifolii,
chemolithotroph organisms involved in nutrient cycling - Acidithiobacillus spp.,
plant growth promoting organisms - Pseudomonas fluorescens and P.
aeroginosa,
plant pathogen suppressing species - Aspergillus niger, Memnoniella echinata,
Trichoderma harzianum and T. viride
were shown in various studies. The majority of studies were pot trials with less than half
of the studies being field experiments (Table 1).
The addition of potassium or potash resulted in increased nitrogen fixation linked to
increases in crop yield indicates a generally beneficial effect on nitrogen fixing
organisms. A limited number of studies combined the effect of potassium/potash on
nitrogen fixing and pathogenic organisms. The results for disease severity appeared
variable and dependent on both the crop and pathogen tested (Parveen et al., 2008).
Non-conclusive results were found for plant growth promoting bacteria (PGPB) and their
reaction to potash and potassium. Unfortunately, these studies were undertaken in
mixed environments including plant pathogenic organisms (see Table 1, (Parveen et al.,
2008)), which makes an assessment of fertiliser effects on PGPB performance difficult.
A strong indication of positive effects on plant pathogen suppressing organisms by
potash and potassium additions could be shown in the studies presented with the
exceptions of the effectiveness of Aspergillus niger against Fusarium oxysporum in
muskmelon (Chattopadhyay and Sen, 1996). Calcium addition resulted in a growth
50
stimulating effect observed for Rhizobium meliloti which was linked to improved growth
at lower pH (Reeve et al., 1993).
Interestingly, in some cases lower concentrations of the studied compound showed
better effects than higher concentrations (Sharma et al., 2007, Simek et al., 1993).
51
Table 1: Effect of Polyhalite components on beneficial microorganisms
Organism
Disease
Effect on plant
pathogens
Experiment
Treatment
type
Plant
pathogen
Crop
Reference
Pseudomonas
fluorescens
Root galling
Slight reduction of
nematode growth
Pots, soil, K
(muriate of
potash)
Meloidogyne
incognita
Tomato
(Siddiqui et al.,
2001)
Increase in infection
No effect on
infection
Reduction in
infection
Pots, soil,
K 2 SO 4
0.1 g kg
Fusarium solani
Rhizoctonia solani
Macrophomina
phaseolina
Tomato
(Parveen et al.,
2008)
Charcoal rot
Damping Off
Reductions in root
infections
Pots, soil, K
(potash)
0.12 g kg
Mungbean
(Siddiqui et al.,
2000)
Charcoal rot
Damping Off
Reductions in root
infections
Pots, soil, K
(potash)
0.12 g kg
Mungbean
(Siddiqui et al.,
2000)
Root galling
Reduction in
disease severity at
lower
concentrations
Pots, soil, KCl,
CaCl
100 and 200
ppm
50 and 100 ppm
Meloidogyne
incognita
Okra
(Sharma et al.,
2007)
Fusarium wilt
Reduced disease
suppression
Pots, soil,
KCl(SO)
112.04 kg ha ;
0.5% solution as
foliar spray
Fusarium
oxysporum
Muskmelon
(Chattopadhyay
and Sen, 1996)
Foot rot
Reduction in
infection
Damping Off
Increase in infection
Tomato
(Parveen et al.,
2008)
Charcoal rot
No effect
Pseudomonas
aeroginosa
Memnoniella
echinata
Fusarium root rot
Damping Off
Charcoal rot
Aspergillus niger
498 mg K in 1 g
potash per
treatment
-1
Macrophomina
phasolina,
Rhizoctonia solani
Macrophomina
phasolina,
Rhizoctonia solani
-1
-1
-1
Bradyrhizobium
japonicum
Fusarium solani
Pots, soil,
K 2 SO 4
-1
0.1g kg
Rhizoctonia solani
Macrophomina
phaseolina
When the tested compound was added in a different way than mixing in soil, it is stated under “treatment”
52
Table 1 continued:
Organism
Effect on plant
pathogens
Experiment
Treatment
type
N/A
Increase in yield
Field, soil, K
(potash)
N/A
Increase in yield
Disease
Plant
pathogen
Crop
Reference
N/A
Cowpea
(Swaroop et al.,
2001b)
N/A
Cowpea vs
Arka
N/A
Pea
N/A
Faba bean
-1
60, 120 kg ha
combined with P
and N fertiliser
-1
60, 120 kg ha
combined with P
and N fertiliser
Increase in growth
and nodulation
Increase in growth
and nodulation
Field, soil, K
(muriate of
potash)
Field, soil,
K2O
Field, soil,
K2O
N/A
Increase in growth
Field, soil, K
30 kg ha
N/A
Rajmash
bean
N/A
Increase in yield
Field, soil, K
K in combination
with N and P
(20:40:20 kg)
N/A
Pigeonpea
(Singh et al.,
2009)
N/A
Increase in growth
and nodulation
Pots, soil, K
unknown
N/A
Lablab
purpureus
(L.) Sweet
Kashrangeeg
(Younis, 2010)
Rhizobium meliloti
N/A
Survival at low pH
Culture
Increase of Ca
in medium
N/A
N/A
(Reeve et al.,
1993)
Nitrogen fixing
bacteria,
symbiotic
N/A
Increased nitrogen
fixation
Pots, soil, K
0.8 and 3.0 mM
N/A
Faba bean,
Common
bean
(Sangakkara et
al., 1996)
N/A
Slight increase in
nitrogen fixing
organisms,
decrease in
nitrogenase activity
at high dose
Field, soil, KCl
(60%)
80 and 200 kg
-1
ha per year
N/A
Spring barley
(Simek et al.,
1993)
N/A
Rhizobium spp.
Nitrogen fixing
organisms
N/A
-1
30 -60 kg ha
-1
60, 120 kg ha
-1
(Swaroop et al.,
2001a,
Swaroop, 2006)
(Kanaujia et al.,
1997)
(Hussein et al.,
1997)
(Sharma and
Verma, 2011)
When the tested compound was added in a different way than mixing in soil, it is stated under “treatment”
53
Table 1 continued:
Organism
Rhizobium trifolii,
Azospirillum
brasilense
Azospirillum
brasilense
Trichoderma
harzianum
Disease
Effect on plant
pathogens
Experiment
Treatment
type
N/A
Increased nitrogen
fixation
N/A
Increase in
Actinomycetes and
bacteria in
rhizosphere
coupled with
increase in yield
N/A
Promotion of growth
N/A
Increase in yield
Grey mould
Collar rot, root rot,
damping off and
wire stem
Promotion of
mycelial growth and
stimulation of
antagonistic effect
Promotion of
antagonist and
reduction in
pathogen growth
Damping off
Rice sheath blight
Reduction in
disease severity
Plant
pathogen
Crop
Reference
Pots, soil, K
Optimal
concentration
for potassium
(unknown)
N/A
Clover,
Barley
(Mikhailovskaia,
2000)
Pots, soil, KCl
Up to 200 mg
-1
kg
N/A
Wheat
(Shalaby and
Rizk, 2002)
60% K at 1002000 ppm
N/A
N/A
(Shylaja and
Rao, 2012)
1g L
N/A
Mustard,
Tomato
(Manjurul Haque
et al., 2012)
Petri dishes,
K2O
1-100 ppm
combined with
N; foliar fertiliser
Botrytis cinerea
Rhizoctonia solani
N/A
(Dluzniewska,
2008)
Pots, soil, KCl
100 ppm
Pythium
aphanidermatum
N/A
(Jayaraj, 1998)
Pots, soil, K
(muriate of
potash)
40 kg ha in
combination
with N and P
Rhizoctonia solani
Rice
(Khan and
Sinha, 2006)
Petri dishes, K
(muriate of
Potash)
Field, soil,
KNO 3
-1
-1
When the tested compound was added in a different way than mixing in soil, it is stated under “treatment”
54
Table 1 continued:
Organism
Experiment
Treatment
type
Plant
pathogen
Crop
Reference
Pots, soil, KCl
100 and 200
ppm
Meloidogyne
incognita
Okra
(Sharma et al.,
2007)
Pots, soil, K
(muriate of
potash)
unknown
N/A
Sesame
(Usharani et al.,
2008)
Fusarium wilt
Reduced disease
suppression
Pots, soil,
(KCl(SO))
112.04 kg ha ;
0.5% solution as
foliar spray
Fusarium
oxysporum
Muskmelon
(Chattopadhyay
and Sen, 1996)
Astragal Stem Rot
Reduction in
pathogen growth
Pots, soil, Ca
(CaCl 2 , CaO,
Ca(NO 3 ) 2 )
0.0025% each
compound
Rhizoctonia solani
N/A
(Chung and
Yun, 2000)
Disease
Root galling
Trichoderma
viride
N/A
Effect on plant
pathogens
Reduction in
disease severity at
lower concentration
Promotion of growth
very low against
control
-1
Gliocladium virens
When the tested compound was added in a different way than mixing in soil, it is stated under “treatment”
55
B-Effects of Polyhalite or its components on plant pathogens
Generally, potash or potassium additions reduced infections induced by plant pathogens.
The effects of potash or its components on plant pathogens are summarised in Table 2.
The concentrations used in field trials ranged widely between 30 to 2260 kg ha-1. Crops
studied included cucumber, rice, tomato, wheat, taro, tea, mustard, alfalfa, potato, maize,
soybean, apple trees, coconut and strawberry. The effect appeared consistent for
pathogens belonging to fungi, Chromalveolata (oomycota), algae and bacteria, with the
summarised studies focussing on fungi (Table 2). Exceptions were Phytophthora root rot
and russet scab with the latter being increased by treatment with potash. The findings for
Phytophthora root rot might be linked to very high moisture levels in the soil studied thus
leading to higher mobility for ions like potassium (Kuchenbuch et al., 1986), which could
have reduced their beneficial effect. Similarly, reduced soil moisture was linked to
increased russet scab symptoms despite high potash addition (Khanna et al., 1999)
which could be linked to reduced mobility of potassium in soils (Kuchenbuch et al.,
1986).
Also calcium additions reduce growth and thus effects of plant pathogens. Evidence for
this effect is particularly strong for club root caused by Plasmodiophora brassicae
(Humpherson-Jones et al., 1992, Myers and Campbell, 1985, Niwa et al., 2008, Webster
and Dixon, 1991). Furthermore, the effects of take-all in wheat caused by
Gaeumannomyces graminis var. tritici can be reduced by the addition of magnesium
(Reis et al., 1983).
56
Table 2: Effect of Polyhalite/ component on plant pathogenic microorganisms
Organism
Reduction in
infection
Reduction in
infection
Reduction in
pathogen level
Reduction in
pathogen level
Reduction in
disease
incidence
Experiment
type
Pots, soil, K
(muriate of
potash)
Pots, soil,
K 2 SO 4
Field, soil
Field, foliar,
K 2 SO 4
Pots, soil,
K 2 SO 4
Pots, soil,
K 2 SO 4
Pots, soil,
CaCN 2
Pots, soil,
CaCN 2
Field, foliar, K
(muriate of
potash)
Reduction in
disease severity
Reduction in
disease severity
Disease
Effect
Sheath Blight
Reduction of
survivability
Damping-off
Reduction in
infection
Mycosphaerella
graminicola
Septoria tritici
blotch
Reduction of
infection
Macrophomina
phaseolina
Charcoal rot
Rhizoctonia
solani
Foot rot
Fusarium solani
Crown rot
Fusarium
oxysporum
Vascular wilt
Cephaleuros
parasiticus
Red rust
Alternaria spp.
(A. brassicae
(Berk.) Sacc., A.
Alternaria blight
brassicicola
(Schw.) Wiltshire)
Cochliobolus
sativus;
Helminthosporium
Pyrenophora
leaf blight
triticirepentis
Treatment
Host Plant
Reference
0.05%
Rice
(Sati and Sinha,
2005)
0.1 g kg-1
Tomato
(Parveen et al.,
2008)
60 kg ha-1
3 or 5g L-1
Wheat
(Arabi et al.,
2002)
0.1 g kg-1
Tomato
0.1 g kg-1
Tomato
100 g m-2
Cucumber
80 and 200 g m-
(Parveen et al.,
2008)
(Parveen et al.,
2008)
(Bourbos et al.,
1997)
2
Cucumber
(Shi et al., 2009)
200 L ha-1 of 1%
concentration
Tea plant
(Ramya et al.,
2013)
Field, soil, K
(potash), S
40 kg ha-1
20 kg ha-1
Mustard, Varuna (Meena et al.,
of India
2011)
Field, soil, K 2 O
30 or 60 kg ha-1
Wheat
(Sharma et al.,
2005)
When the tested compound was added in a different way than mixing in soil, it is stated under “treatment”
57
Table 2 continued:
Organism
Disease
Pestalotiopsis
palmarum
Leaf blight
Powdery mildew
Powdery mildew
Cercospora
kikuchii
Leaf blight
Puccinia striiformis Stripe rust
Sphaerotheca
fuliginea
Powdery mildew
Pythium butleri
Damping Off
Pythium ultimum
Collar rot
Pythium coloratum
Cavity spot
disease
Pythium
splendens
Seedling
damping-off,
fruit rot
Phytophthora
colocasiae
Leaf blight
Phytophthora spp.
Root rot
Fusarium spp or
Maize stalk rot
Effect
Reduction in
disease severity
Reduction in
disease severity
Reduction of
disease
incidence
Reduction of
disease severity
Reduction in
disease severity
Reduction in
pathogen growth
Reduction of
disease
symptoms
Reduction in cell
numbers of
pathogen
Reduction in
sporangial
germination
Reduction in
disease severity
No significant
reduction in
disease severity
Reduction of
Experiment
type
Treatment
Host Plant
Field, soil, K 2 O
2.4 kg palm-1
year-1
Coconut
Pots, soil,
K 2 O 3 Si
unknown
Strawberry
Field, soil, KCl
150; 300; 450
and 600 kg ha-1
Soybean
(Ito et al., 1993)
Field, soil, KCl
376, 1130 or
2260 kg ha-1
Winter Wheat
(Russell, 1978)
0.1 M each
Cucumber
pots, foliar, KCl,
KNO 3 or K 2 SO 4
Pots, sand, KCl,
CaCl 2
Pots, soil, KCl,
K 2 SO 4
300 µg mL-1
40 µg mL-1
30g K around
one apple tree
stem
Tomato
Reference
(Karthikeyan et
al., 2002)
(Kanto et al.,
2006)
(Reuveni et al.,
1995)
(Tripathi and
Grover, 1975)
Apple
(Sharma and
Gupta, 1988)
Ca (Lime)
4000 kg ha-1
Carrot
(El-Tarabily et al.,
1996)
Pots, soil, Ca
0.6% Ca in soil
Cucumber (root
extract)
(Kao and Ko,
1986)
Field, soil, K 2 O
(60% K)
40, 80,
120 kg ha-1
Taro
(Das et al., 2003)
Alfalfa
(Alva et al., 1985)
Maize
(Hong et al.,
Pots, soil, K
Field, soil, K
120 and
240 μg g‐1
150 kg ha-1
58
Pythium spp.
disease
symptoms
Streptomyces
scabiei
Common scab
Reduction in
disease severity
Streptomyces spp.
Russet scab
Gaeumannomyces
Take-all
graminis var. tritici
Plasmodiophora
brassicae
Clubroot
Increase of
disease
symptoms
Reduction in
disease
symptoms
Reduction in
disease
symptoms
Reduction in
infection
Reduction in
infection, effect
pH dependent
Reduction in
disease severity
2004)
Field, soil, S
(sulfate of
potash)
180 kg K20 ha-1
Potato
(Jan et al., 1995)
Field, soil, K
(potash)
150-200 kg ha-1
Potato
(Khanna et al.,
1999)
Pots, sand, Mg
48-144 mg L-1
Wheat
(Reis et al., 1983)
Pots, soil, Ca
unknown
Chinese
cabbage
(Webster and
Dixon, 1991)
Pots, soil,
CaCO 3
25 g kg−1
Brassica rapa
var.
Perviridis
(Mustard
Spinach
Komatsuna)
(Niwa et al.,
2008)
Pots, sand,
Ca(NO 3 ) 2
0.5-25 mM
Broccoli
(Myers and
Campbell, 1985)
Field, soil,
CaOH
5 and 25 t ha−1
Cauliflower
(Tremblay et al.,
2005)
When the tested compound was added in a different way than mixing in soil, it is stated under “treatment”
59
Discussion
Since no peer-reviewed studies could be found assessing the effects of Polyhalite on soil
microorganisms, potential responses were inferred from research using slightly similar
fertilisers. Polyhalite is known to release Ca, K, S and Mg into aqueous soil
environments so that the approach is assumed to be applicable.
The addition of potash or potassium salts to soil could be shown to have beneficial
effects on specific nitrogen-fixing and plant growth promoting bacteria as well as on
some organisms with antagonistic activity towards plant pathogens. Furthermore,
potash, calcium and magnesium have been shown to reduce disease severity or
symptoms caused by certain plant pathogenic fungi and other pathogens as reported in
the studies summarised. A limited number of community approaches were used in this
area of research. Examples of such studies showed in a pot trial that an addition of
potash equivalent of 67 mg K kg−1 (c. 150 kg/ha K2O) to soil from Chinese rice fields
resulted in an increase in functional diversity based on the substrates that were utilised
by soil microorganisms as assessed by Biolog ECO micro-plates (BIOLOG, Hayward,
USA) (Zhang et al., 2012). Additionally, work published in 1979 showed a rise in maize
yield that was related to an increase in microbial rhizosphere populations after addition of
K 2 O (Rai et al., 1979). The rhizosphere microflora of tobacco plants was subjected to
different levels of potassium. Small changes were observed for organisms utilising
phenolic acid (Yang et al., 2011), whilst functional diversity remained largely unchanged.
Similarly, the use of calcium cyanamide to control Fusarium vascular wilt caused short
term reductions in microbial diversity followed by recovery of microbial populations (Shi
et al., 2009).
As many relevant soil borne plant pathogens were not studied their reduction due to
Polyhalite addition would need to be addressed. For instance it has been estimated that
there are 40 soil borne pathogens for potato alone (Fiers et al., 2012) which illustrates
the limited knowledge available.
Although the addition of KCl to soil was shown to be mainly beneficial, it has been
reported to potentially increase methane oxidation (Zheng et al., 2013) as shown for
Chinese paddy soil. The transformation of ammonium to nitrate called nitrification is vital
to make nitrogen available for uptake by plants. Inhibiting effects on nitrification were
observed by potassium sulphate (Martikainen, 1985), which was thought to be based on
the lowering of pH. A similar effect was reported for the addition of KCl (Agrawal et al.,
1985) with the strongest effect in acidic soils. Polyhalite being pH neutral and containing
only small levels of chloride, it would not have this implication (www.siriusminerals.com,
2014). However, such findings strengthen the need for information on soil characteristics
when the effects of Polyhalite or components are assessed. Since the currently available
literature largely failed to specify soil types, mode of application and background rates for
added salts leaving a gap of knowledge necessary for further extrapolations to deduce
the effects for different agricultural settings, future research should take into account
such background information will be essential to holistically assess effects of Polyhalite
on soil ecosystems.
It is thought that the addition of potash would not deleteriously affect soil health since no
changes compared to other fertilisers could be observed in soil respiration (Barathan and
60
Gobinath, 2013). Although soil respiration is an important ecosystem process, it is not
directly linked to the presence of certain microbial species (diversity) and is not
informative of changes in microbial communities due to the ability of different species to
carry out the same function in a community (redundancy) (Nannipieri et al., 2003, Naeem
and Li, 1997).
Since only a small fraction of soil microorganisms can be cultivated and studied
independently from the whole community, it is not possible to extrapolate the conclusions
of the effects of Polyhalite on beneficial organisms beyond the specific case parameters.
Positive results found on the plant pathogens studied would suggest broadening the
study range would be of interest.
Based on the available literature, products like Polyhalite which supply a range of
beneficial nutrients not disrupting soil electrical conductivity (EC) or pH, could be useful
for numerous applications in soil including remediation or increasing nutrient levels in
otherwise nutrient poor soils.
The currently available literature largely failed to specify soil types, mode of application
and background rates for added salts which leaves a knowledge gap that would need to
be filled to allow further extrapolation for different agricultural settings. Future research
taking into account such background information will be essential to holistically assess
the effects of Polyhalite on soil ecosystems.
With regards to agriculture, Polyhalite has been reported to increase the yield of crops
such as Sorghum-sudangrass, for which higher yields were achieved with finely ground
Polyhalite (<0.15 mm) at addition rates of between 600 to 100 mg K kg-1 (Barbarick,
1991). Although some studies summarised in this review measured an increase of crop
yield, most researchers measured crop growth itself or a reduction in infection or
symptom severity.
Root infections of young / establishing plants can critically influence plant survival.
Establishment is important factor in setting seed rates. Chronic root infections will
undoubtedly have deleterious effects on plant health and consequential implications for
yield. The degree to which a crop suffers will be governed by aerial and soil environment
and the species under question. Thus, reductions in pathogen infection potential
mediated by additions of potassium or calcium (Myers and Campbell, 1985, Niwa et al.,
2008, Arabi et al., 2002, Parveen et al., 2008, Shi et al., 2009) are highly likely to
increase plant survival rates.
In conclusion, the addition of potash or potassium or calcium salts to soil has been
shown to have beneficial effects on specific nitrogen-fixing and plant growth promoting
bacteria as well as on some organisms with antagonistic activity towards plant
pathogens.
The implications of choosing Polyhalite as a source for K and/or Ca are:
•
•
•
Suppressive effects on persistent soil dwelling pathogens
Support for N-fixing organisms and hence fertiliser nutrient use efficiency
Support for plant health via reduction of disease combined with reductions in
pesticide use
61
•
•
Potential for positive effects on crop yield
Potential seed rate reductions combined with possible implications for crop
rotations
62
Methodology
The databases used were Web of Science, Science Direct and Google
scholar.
The keywords used in this literature study were:
•
Polyhalite (78 hits in Web of science, no hits related to
microorganisms; 2470 hits in google scholar, no hits related to
microorganisms)
•
Potash and soil (12266 hits in Web of Science, 71800 hits were
revealed by google scholar, adding the keyword “micro*”
revealed 1531 and 20000 hits respectively)
•
Soil and potassium and micro*
•
Soil and potash and micro*
•
Potash and Rhizobia
•
Potash and Trichoderma
•
Potash and Gliocladium (no relevant hits in Web of Science)
•
Potash and Pythium
•
Potash and Plasmodiophora brassicae(no relevant hits in Web
of Science)
•
Potash and Streptomyces
•
Potash and calcium and soil
•
Potash and sulphate and micro* (no relevant hits in Web of
Science)
•
Soil and calcium and micro*
•
Calcium and Trichoderma (no relevant hits)
•
Calcium and Rhizobia (no relevant hits)
•
Calcium and Gliocladium
•
Calcium and Plasmodiophora brassicae
63
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67
An investigation into the environmental impact
of Polyhalite used as a fertiliser
By: Karen Tiede and Chris Sinclair
68
About the Authors
Dr Karen Tiede
Environmental Scientist
Centre for Chemical Safety and Stewardship
Food and Environment Research Agency
Sand Hutton
York
YO41 1LZ
UK
Dr Chris Sinclair
Senior Environmental Chemist
Centre for Chemical Safety and Stewardship
Food and Environment Research Agency
Sand Hutton
York
YO41 1LZ
UK
69
Contact:
Dr Chris Sinclair
Tel No:
01904 462037
Email:
[email protected]
Contents
Executive Statement ..........................................................................................................71
Objectives ....................................................................................................................73
2
Methodological approach ...........................................................................................74
3
Introduction .................................................................................................................75
3.1 Fertilisers ...............................................................................................................75
3.2 Polyhalite ...............................................................................................................75
4
Overview of environmental issues associated with the use of fertilisers ............... 76
4.1 Water pollution .......................................................................................................76
4.2 Contamination with impurities ................................................................................77
4.3 Atmosphere............................................................................................................79
4.4 Impacts on yield and plant health ...........................................................................79
4.5 Impacts on soil .......................................................................................................80
4.6 Impacts on ecosystems and organisms..................................................................81
5
Environmental issues associated with polyhalite and its compounds ................... 83
5.1 Polyhalite ...............................................................................................................83
5.2 Potassium ..............................................................................................................84
5.3 Calcium ..................................................................................................................86
5.4 Magnesium ............................................................................................................87
5.5 Sulphate .................................................................................................................87
6
Discussion & recommendations ................................................................................89
7
References...................................................................................................................91
8
Appendix 1 - Trace element testing results for York Potash polyhalite .................. 97
9
Appendix 2 – Product specification sheet for Poly4.................................................98
70
Executive Statement
A desk based appraisal of scientific literature was undertaken to consider the
environmental impact of polyhalite when used as a fertiliser. Based on this review,
conclusions are drawn on how the potential environmental impacts of polyhalite compare
to the use of conventional fertilisers.
Scientific literature considering the specific environmental impacts of polyhalite when
used as a fertiliser were scare, therefore literature considering the potential
environmental impact of individual polyhalite constituents or combinations of the
constituents were considered as surrogate information. Consideration was given
individually to the leaching potential, effect on soil structure, and effects on ecosystems
and the organisms they contain by potassium, calcium, magnesium and sulphate.
Based on this literature appraisal it can be concluded that the use of polyhalite as a
fertiliser will show no obvious environmental impacts that could specifically be associated
with polyhalite or would not be seen using other types of fertilisers consisting of the same
compounds.
All potential environmental impacts that have been identified during this study are
associated with individual compounds of polyhalite (e.g. sulphate and potassium) rather
than with polyhalite itself. All constituents of polyhalite (K, Mg, Ca and SO 4 ) are also
present in other types or mixtures of fertilisers (e.g. NPK fertilisers and manure) and
therefore should not pose any additional threats to the environment. Detailed below are
some of the positive environmental effects of the use of polyhalite as a fertiliser.
•
•
•
•
•
•
•
The use of potash fertilisers do not contribute directly to greenhouse gas
emissions.
As the sulphate content of polyhalite Is not expected to be released as aerosols
then there is expected to be no contribution to global climate change or acid rain.
Trace mineral elements in the soil are gradually depleted by crops and many
inorganic fertilisers may not replace these however polyhalite does make a
contribution.
Soil aggregate stability, microporosity and available water capacity were
significantly lower in soils when fertilised with N only or with a limited nutrient
range than balanced fertilisation. Polyhalite used as part of a balanced
fertilisation program could minimise this effect.
Balanced fertilisation tended to increase the abundance of soil insects compared
with no fertiliser and unbalanced fertilisation.
The potential contamination of soils with unwanted or even toxic substances
through the application of York Potash polyhalite is low as trace element testing
indicates that these elements are either undetectable or at levels lower than
0.0004%.
York Potash polyhalite has a typical salt (halite) content of 3% which is
considered non-toxic and would therefore be a low threat to salt sensitive plants
and soils.
71
•
Polyhalite makes a lower contribution to soil conductivity and therefore soil
salinity compared to other fertilisers.
• Polyhalite will have no acidification effect on soil except under reducing
conditions or soils with high aluminium affecting cation exchange.
• Leached potassium to the environment causes no major environmental concern.
• Higher amounts of potassium are retained by soil when applied with sulphate, as
in polyhalite.
• Potassium fertilisers enhance soil water holding capacity and improve structural
stability of sandy soils. Magnesium and calcium have greater effect in this
respect. Since polyhalite contains potassium, calcium and magnesium it may
have an improved effect on improving soil structural stability than fertilisers
containing only one of these nutrients.
• We found no literature evidence of fertiliser calcium causing ecotoxicity.
• There is evidence that calcium suppresses uptake of Cd, Pb, Ni and Zn by
aquatic invertebrates.
• Sulphate is the most important source of sulphur for plants.
Several papers indicate that repeated, high sulphate-sulphur fertiliser applications do not
cause sulphur to accumulate in soil. Losses occur mainly through leaching.
72
Objectives
The aim of this project was to consider the potential environmental impact of polyhalite
when used as a fertiliser when compared to conventional nutrient sources. This was
performed as a limited desk based appraisal of some of the relevant literature.
This literature appraisal is limited to potential environmental issues that may result solely
from the use of polyhalite as a fertiliser, environmental impacts from other activities such
as mining, processing, waste disposal etc were not within the scope of this work.
73
1 Methodological approach
The scientific literature and grey literature was searched for studies looking at potential
environmental impacts of polyhalite and its constituents when used as a fertiliser as well
as other mineral fertilisers.
An emphasis of the search was on:
1) the potential environmental impact of polyhalite on traditional lower tier indicator
ecotoxicological species from different trophic levels considered during chemical
risk assessment, e.g. green algae, daphnia, fish and earthworms; and
2) the potential for fertiliser constituents to contaminate environmental waters.
The search was performed mainly using ScienceDirect, Google, Google Scholar and
Web of Knowledge. Different keywords and keyword combinations were used including,
but not limited to:
“polyhalite” OR “mineral fertiliser*” OR “inorganic fertiliser*” OR “fertiliser” OR
“potassium” OR “sulphate” OR “calcium” OR “magnesium” OR “magnesium sulphate”
OR “calcium sulphate” OR “ potassium sulphate” OR “fertilisation effect” OR “NPK”
AND/OR
“environmental impact” OR “environment*” OR “impact” OR “toxic*” OR “ecotoxic*” OR
“fish” OR “daphnia” OR “algae” OR “earthworm” OR “aquatic organism*” OR “organism*”
OR “leaching” OR “impurit*” OR “contamin*” OR “solubility” OR “retention”.
Note that some synonyms, chemical formula and latin names were also used in the
search.
The results of the literature searching when considering polyhalite produced limited
information due to a lack of peer-reviewed scientific literature on the subject of the
potential environmental impact of polyhalite when used as a fertiliser. Moreover, there
was also a lack of literature on the potential environmental impacts of polyhalite
constituents and/or the potential environmental impacts of mineral fertilisers (with the
exception of nitrogen (N) and phosphate (P) containing fertilisers). Literature studies
identified were mostly focused on e.g. yield gain, application, application rates and ratios
of combinations of organic and inorganic fertilisers (e.g. manure and NPK fertilisers) as
well as their impact on different types of soils and carbon storage. Searches for
environmental effects or toxicology mainly retrieved studies looking at e.g. organic
compounds containing sulphate or potassium such as copper sulphate, sodium dodecyl
sulphate or potassium bromate. Other identified studies considered the potential positive
effects of adding fertilisers to surface waters (e.g. ponds) to promote fish growth.
Discussions of the environmental issues related to fertilisers in general were mostly only
found in the grey literature.
Abstracts of publications identified as potentially relevant were screened. Where
available, some full texts of publications identified as relevant were assessed and results
summarised and discussed below.
74
2 Introduction
2.1
Fertilisers
Fertiliser is any organic or inorganic material of natural or synthetic origin that is added to
soil to supply one or more plant nutrients essential to the growth of plants (SSSA, 2014).
Fertilisers typically provide, in varying proportions six macronutrients: nitrogen (N),
phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S).
Fertilisers are broadly divided into organic fertilisers and inorganic (or mineral or
commercial) fertilisers. Both organic and inorganic fertilisers provide the same required
chemical compounds, however, often in different forms (species) (Stewart et al., 2005).
The speciation is important for plant uptake. Plants can only absorb nutrients if they are
present in an easily dissolved form. Inorganic fertilisers are nearly always readily
dissolved and unless added have few other macro and micro plant nutrients. In general,
the nutrient release rates in organic fertilisers are lower than in inorganic fertilisers and
less readily available to plants. Organic fertilisers can originate from different sources
and feature highly variable amounts of plant nutrients in different ratios and can also
contain contaminants.
In addition to their positive effect on plant growth, plant health and food production,
fertilisers or some fertiliser constituents can have an adverse effect on the environment
and human health.
2.2
Polyhalite
Polyhalite is used as a natural mineral fertiliser providing four of the six plant
macronutrients and is low in chlorine. Polyhalite is an evaporite material occurring in
sedimentary marine evaporates. It is a hydrated sulphate of K, Ca and Mg (reported to
be without foreign anions) with the formula: K 2 Ca 2 Mg(SO 4 ) 4 2(H 2 O). It is typically
colourless, white-to-grey, although it may be brick red due to the presence of iron oxides.
The deposits found in Yorkshire, UK typically consist of K2O: 14%, SO3: 48%, MgO 6%,
CaO: 17% (Berry et al 2013) (See Appendix 1 for trace element analysis).
75
3 Overview of environmental issues associated with the use of
fertilisers
In this section environmental issues associated with the use of inorganic and organic
fertilisers are discussed with a focus on N and P, often considered as the main
contaminants among nutrient pollutants and the two macronutrients not found in
polyhalite.
Due to the absence of N and P, polyhalite will most likely be applied as fertiliser in a
complex or blended with other inorganic N, P and K sources as well as organic fertilisers
such as manure to achieve balanced valuable multi-nutrient fertiliser application.
3.1
Water pollution
The nutrients present in fertilisers can have adverse effects on the environment and
human health if they reach surface waters by run-off or leaching into groundwater. These
effects can be broad depending on the compound, ranging from organoleptic (e.g.
sulphate) to toxic effects (e.g. nitrate).
3.1.1 Groundwater pollution
Nitrogen is of high concern when leached to groundwater. The level of pollution depends
upon soil conditions, agricultural practices, agro-climatic conditions, and type of fertilisers
and methods of application. This also effects the time taken to reach the groundwater
zones, which can vary considerably. In sandy soils with a high water table and high rate
of fertiliser application, it may reach the groundwater in a few days whereas in heavy
soils with low rainfall, low rate of application and a deep water table, it may take years.
Nitrate ingestion in food and water is associated with cancer and blue baby disease.
Recommended limits of nitrate concentrations in drinking water are 10 mg NO 3 -N L-1 or
50 mg L-1 NO 3 (World Health Organization (WHO)).
3.1.2 Eutrophication
Eutrophication is caused by nutrients, e.g. phosphate, K and nitrate, reaching surface
waters mainly by run-off and erosion. Arable soils leach considerable amounts of these
substances and therefore contribute significantly to the growth of aquatic macrophytes,
algal blooms and other organisms in and contribute to low dissolved oxygen contents,
the death of fish, invertebrates and other aquatic animals (USEPA).
In freshwater systems P is often the limiting nutrient, whereas in oceans and coastal
systems it is mainly nitrogen.
After the application of P to soil it is either taken up by crops or transformed into various
insoluble forms (e.g. iron (Fe) and aluminium (Al) phosphate in acid soil and Ca
phosphate in alkaline soils) and is immobilised by clay or organic matter. However,
significant amounts of P are still lost from the soil through surface run-off and erosion
resulting in eutrophication of water bodies (Hooda et al., 2001).
76
N runoff derived from fertilisers is the primary cause of oxygen depletion in many parts of
the ocean, especially in coastal zones. This has serious adverse effects on oceanic
fauna and the water may become cloudy and discoloured.
3.2
Contamination with impurities
Many fertilisers contain varying amounts of trace elements such as Fluorine (F), Arsenic
(As), Cadmium (Cd), Cobalt (Co), Chromium (Cr), Mercury (Hg), Molybdenum (Mb),
Nickel (Ni) and Lead (Pb). The main issues concerning these potentially harmful
elements are:
1) Soil accumulation and the possibility of long-term effects on crop yields and
quality;
2) Plant uptake and subsequent exposure route for the element into animal feed
and human diets;
3) Potentially damage to the soil micro flora; and
4) Direct exposure to humans through contact and ingestion.
Mclaughlin et al. (1996) assessed the risks of contaminants accumulating in soils and
crops due to inadvertent addition of impurities in agricultural fertilisers and soil
amendments for Australian conditions. It was found that Cd and F will accumulate in
fertilised soils at a faster rate than As, Pb or Hg considering background concentrations
of these elements in Australian soils, inputs to soil in fertilisers and offtake in harvested
crops. As, Cd, and Hg are not found in York Potash polyhalite (Appendix 1).
3.2.1 Metal accumulation
Mined mineral fertilisers may vary in elemental composition and impurity levels
depending on geographical origin and production processes. This is particularly true of
phosphate fertilisers which can often contain Fluorides, Cd and Hg. The identity of
impurities and their concentrations are dependent on the P source and production
process used (Zhao and Wang, 2010). Although impurities can be removed during
processing, this greatly increases costs.
While soluble F can contribute to soil sterilisation over time, metal accumulation in soil
and can have toxic effects – either affecting the soil community and plants directly or
affecting human and animal health when taken up by plants and entering the food chain.
It has been found that application of phosphate minerals considerably increases the
contamination of soil with Cd in New Zealand (Taylor, 1997).
Wastes used as fertilisers/soil improvers including biosolids and animal manures are
also known to potentially contain high levels of heavy metals.
3.2.2 Radioactive element accumulation
Naturally occurring radionuclides e.g. radium (Ra), uranium (U), thorium (Th) and
polonium (Po), are found in fertilisers, especially in phosphate fertilisers, as the
77
radioactivity of phosphate rocks is enhanced by geological processes.
These
radionuclides include 226Ra, 228Ra, 235U, 238U, 232Th, 40K, and 210Po. Radioactive
nuclides can enter watercourses or be taken up by crops and enter the food chain. One
of the plants known to selectively uptake radionuclides is tobacco enhancing the risk of
lung cancer (Scholten and Timmermans, 1995/1996; Barisic et al., 1992).
3.2.3 Persistent organic pollutants
Dioxin congeners (as well as As), have been found in liming agents and micronutrient
fertilisers (USEPA, 1999). Persistent organic pollutants (POPs), pharmaceuticals,
personal care products can be present in organic wastes used as fertilisers such as
biosolids and animal manures.
78
3.3
Atmosphere
3.3.1 Emission of greenhouse gases
The use of fertilisers contributes significantly to the emission of greenhouse gases into
the atmosphere. These emissions are caused by:
1) Animal manures and urea, which release methane, nitrous oxide, ammonia, and
carbon dioxide in varying quantities depending on their form and management;
and
2) Fertilisers that use nitric acid or ammonium bicarbonate, the production and
application of which results in emissions of nitrogen oxides, nitrous oxide,
ammonia and carbon dioxide
Methane (CH 4 ) emissions from cropped fields are increased through the application of
ammonium-based fertilisers, e.g. readily decomposable organic matter (Marañóna et al.,
2011). For example, rice paddy fields are a major source of CH 4 that is formed by the
anaerobic decomposition of organic matter.
The impact of mineral fertilisers on CH 4 emissions is not clear, but is assumed to be
minor. In the case of silicate fertiliser applied to rice fields, CH 4 production activity could
be reduced. However, carbon dioxide production activity was increased (Ali et al., 2008).
Through the increasing use of nitrogen fertiliser, nitrous oxide (N 2 O) has become the
third most important greenhouse gas after carbon dioxide and CH 4 . It has a global
warming potential 296 times larger than an equal mass of carbon dioxide and it also
contributes to stratospheric ozone depletion (UNESCO – SCOPE, 2007). The use of
phosphate and potash fertilisers do not contribute directly to greenhouse gas emissions,
but all forms of nitrogen fertilisers may lead to N 2 O emissions.
3.3.2 Deposition
Volatilised N in the form of ammonia (NH 3 ) can be oxidised to N 2 O in the atmosphere,
acting as greenhouse gas. It also forms salts with acidic gases which can be transported
long distances. NH 3 and ammonia salts can be washed out and re-deposited on
terrestrial ecosystems. Deposition of NH 3 contributes to acidification of soils and water
surfaces, cause plant damage and reduce plant bio-diversity in natural systems. Excess
of NH 3 deposited causes eutrophication (Dalemoa et al., 1998). Nitrogenous fertilisers
contribute substantially towards emissions of ammonia.
3.4
Impacts on yield and plant health
3.4.1 Increased pest fitness
The excessive use of fertiliser and thus the increase of available soil nutrients has been
reported to boost pest problems such as aphids by increasing the birth rate, longevity
and overall fitness of certain agricultural pests (Jahn, 2004; Jahn et al., 2001; Jahn et al.,
2005)
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3.4.2 Over-fertilisation
Over-fertilisation can cause “fertiliser burn” resulting in drying out of the leaves and
damage or even death of the plant. Fertilisers vary in their tendency to burn roughly in
accordance with their salt index. Overfertilisation can also have an impact on yield and
quality of the produce (Fernández-Escobar, 2006).
3.4.3 Lack of long-term sustainability
The production of inorganic fertilisers cannot be continued indefinitely as resources used
in the process are not renewable (e.g. K and P are mainly mined). Artificial N fertilisers
are typically synthesized from also limited resources mainly fossil fuels. However, more
effective fertilisation practices may decrease the usage of inorganic fertilisers. Better
knowledge of crop production can also potentially decrease fertiliser usage without
reducing the need to improve and increase crop yields (Cordella et al., 2009; Dawson
and Hilton, 2011), which will also have positive environmental effects and reduce costs
(Fodor et al., 2011).
3.5
Impacts on soil
3.5.1 Soil structure
Fertilisers are known to have an impact on soil characteristics (e.g. soil structure,
aggregate stability, water retention), which is highly dependent on the type of fertiliser,
application rate as well as the soil type. Hati et al. (2008) for example studied the
“impact of long-term application of fertiliser, manure and lime under intensive cropping
on physical properties and organic carbon content of an Alfisol soil”. They found that the
application of a balanced fertiliser product along with manure (NPKM) or lime (NPKL)
was found to improve soil aggregation, soil water retention, microporosity and available
water capacity and reduce bulk density of the soil in upper horizons. In contrast, soil
aggregate stability, microporosity and available water capacity were significantly lower in
soils when fertilised with N only. Celik et al. (2010) studied the penetration resistance
and bulk densities of soils treated with compost and mineral fertilisers. They found that
the lowest bulk density was better when mycorrhizal inoculated composts were used
than when compared to unamended soils or those receiving mineral fertiliser
applications.
3.5.2 Trace mineral depletion
Trace mineral elements in the soil are gradually depleted by crops and many inorganic
fertilisers may not replace these. Enhanced trace mineral depletion has been linked to
acidulation and subsequent dissolution in soil water by free sulphuric acid sourced from
superphosphate fertiliser. In Western Australia deficiencies of zinc (Zn), copper (Cu),
manganese (Mn), iron (Fe) and molybdenum (Mo) were identified as limiting the growth
of broad-acre crops and pastures in the 1940s and 1950s. Soils in Western Australia are
very old, highly weathered and deficient in many of the major nutrients and trace
elements. Since this time these trace elements are routinely added to inorganic fertilisers
used in agriculture in this region. Many soils around the world are deficient in zinc,
leading to deficiency in plants and humans (Lawrence, 2004; Moore, 2001). York Potash
80
polyhalite is a source of trace elements essential for plants, such as Mo, Ni, Cu, Zn, Mn
and Bo (Appendix 1).
3.5.3 Soil acidification
Soil acidification can be caused by nitrogen-containing inorganic and organic fertilisers
and may lead to a decrease in nutrient availability, which may be offset by liming.
Regular use of acidulated fertilisers generally contributes to soil acidity which affects
aluminium availability and toxicity. This especially affects soils in tropical and semitropical regions and enhances soil degradation (Guo et al, 2010)
3.5.4 Fertiliser dependency
The use of high levels of inorganic fertilisers can lead to soil sterilisation, where the soil
microflora including ithe mycorrhizal fungi previously essential for converting soil
minerals into plant available nutrients are no longer functioning resulting in a
dependency on applied fertilisers (Young et al 2011; Hipps and Samuelson, 2006;
Carroll and Salt 2004)
3.6
Impacts on ecosystems and organisms
3.6.1 Biodiversity
There is accumulating evidence for a rapid loss in biodiversity with the major concern
that it may lead to changes in ecosystem functioning, with threats to the stability and
resilience of agricultural systems. Mozumder and Berrens (2007) investigated the
empirical relationship between the intensity of inorganic fertiliser use and biodiversity
risk. They concluded that the amount of inorganic fertiliser use per hectare of arable
land is significantly related to increasing biodiversity risk. The surge in the use of
inorganic fertiliser over the last few decades may be having differential effects for
different groups of organisms (e.g. mammals, birds, amphibians, earthworms,
arthropods, microarthropods, and microorganisms), and there may be varying
differences (inorganic versus organic) between N, P and K in terms of their impacts on
biodiversity risk.
3.6.2 Amphibians
There is some evidence that fertilisers harm amphibians and are partially responsible for
population declines. For example, Hamer et al. (2004) state that historical and
experimental evidence suggests that the elevated N and P concentrations in waterbodies
in 1974–1975 contributed to the decline of Litoria aurea (bell frog) in its former range in
south-eastern Australia.
It has also been suggested that the terrestrial life stages of amphibians can be very
prone to harm caused by fertilisers applied to field crops due to the effects on water and
particle intake through skin (Berger et al., 2012). During a two-year field study
investigating the temporal coincidence between the breeding of adult amphibians and
fertiliser applications it was identified that up to 60% of the amphibian populations
81
experienced a temporal coincidence with field management. The level of speciesspecific coincidence was dependent on the crops grown and the fertilisation type of
farms, determined by the timing and the number of fertiliser applications. Early migrating
species such as Rana arvalis or Triturus cristatus were much more effected with the
fertilisation of winter cereals than later active species or with the fertilisation of maize
crops.
3.6.3 Arthropods
The presence of abundant and diverse communities of macro-arthropods is considered
an indicator of sustainability in agro-ecosystems. Lin et al. (2013) investigated the soil
insect diversity and abundance following different fertiliser treatments. The treatment
regimes included treatments with application of mineral fertilisers (N, NK, NP, PK, NPK)
and treatments with NPK in combination with organic materials as well as different crop
rotations. It was shown that soil insect diversity was affected by fertilisation and
sampling date. Balanced fertilisation tended to increase the abundance of soil insects
compared with no fertiliser and unbalanced fertilisation such as N, NK, and PK. The
addition of crop residues and large amounts of organic manure in combination with NPK
substantially increased the abundance of individuals and families. Summer maize and
soybean intercropping in rotated sequence might have negative effects on abundances
and families. N, P and K fertilisers and incorporation of organic materials were found to
be favourable factors for abundance and diversity in arable soil. The finding that soil
macro-arthropod abundance is significantly positively correlated with soil moisture
content may mean the effects of fertiliser applications and crop rotation on soil insects
were partly masked by abiotic factors such as soil moisture and temperature.
The findings by Lin et al. (2013) are however inconsistent with the research of Kautz et
al. (2006) and Petersen (2002), who found that management practices, especially tillage,
limited the species composition of soil micro-arthropods overshadowing possible effects
of fertilisation on diversity.
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4 Environmental issues associated with polyhalite and its
compounds
Very little information concerning the potential environmental effects of polyhalite when
used as fertiliser were identified in the peer-reviewed and/or grey literature. Therefore, in
this section, the potential environmental impacts of the individual constituents of
polyhalite (K, Ca, Mg and SO 4 ) when applied to arable land in fertilisers is discussed.
4.1
Polyhalite
4.1.1 Contamination with impurities
Polyhalite is described as “hydrous sulphates without foreign anions” (e.g. EuroMin) and
impurities are not discussed in the literature identified for this specific study. Depending
on the purity grade, the potential contamination of soils with unwanted or even toxic
substances seems therefore low for polyhalite compared to other mineral fertilisers and
organic fertilisers. However this issue cannot be totally excluded and will be dependent
on the deposit, polyhalite could be associated with other minerals or show impurities
ultimately affecting its potential environmental impact. For example, polyhalite is often
associated with halite (rock salt, NaCl) and although halite is not toxic as such, chloride
would be unfavourable for salt sensitive plants and soils. York Potash polyhalite has a
low halite content of 3% (Appendix 2). Detailed analysis of the trace element
composition of York Potash polyhalite would indicate that contamination with metals are
either below the analytical limits of detection (As, Cd and Hg) or less than 4 part per
million (<0.004%) (Co, Cr, Cu, Mo, Mn, Ni and Pb) (Appendix 1). Therefore the potential
for environmental impacts through contamination of soil with impurities through the
application of York Potash polyhalite is low.
4.1.2 Leaching
A study by Barbarick (1991) investigated the effects of polyhalite application to sorghum
sudangrass and polyhalite leaching behaviour in soil columns and compared these to
soluble fertiliser treatments. The leaching study demonstrated that Ca and K were
leached more readily while Mg and SO 4 -S were leached to a lesser extent in the
polyhalite treatments than the soluble-fertiliser treatments. Overall it could be expected
that K, Ca and Mg will interact or adsorb to soil to some extent, although K is believed to
be more prone to leaching as Mg and Ca could also form salts of low solubility and
precipitate. Sulphate is believed to be very mobile and leach similar to nitrate.
4.1.3 Impact on soil
Barbarick (1991) concluded further that acid, infertile soils would typically benefit from
the addition of polyhalite. In this study it was also found that electrical conductivity of
saturated soil-pastes was higher in the soluble fertiliser treatments compared to the
polyhalite nutrient source. This may suggest a lower contribution of polyhalite to soil
conductivity and therefore soil salinity compared to other fertilisers. As a neutral salt,
polyhalite is believed to have no major effect on soil acidification. Due to the lack of
carbonate, neutralising properties are also not to be expected. Soil pH should only be
83
affected by polyhalite under reducing conditions (sulphate to S) or in soils with very high
levels of Al affecting cation exchange.
4.2
Potassium
The major form of K fertiliser applied is mined sylvinite. Potassium sulphate and
potassium nitrate, both side products from K mining, are also commercially available but
normally more expensive. Chlorine free potassium fertilisers are preferred for crops that
are sensitive to chloride, such as potatoes, banana, citrus, grapes, and peach (Zörba et
al., 2013).
4.2.1 Leaching
K is very mobile in soil compared to e.g. P. In its ionic form, K+, is absorbed by roots
from soil solution. Although K+ can be retained to some extent by negative charges on
clay surfaces, it can be easily displaced into the soil solution by Ca or Mg ions (Ca2+,
Mg2+). K as contained in polyhalite is very soluble, similar to KCl. Therefore, K is
susceptible to loss by leaching, run-off and erosion, but should carry no major
environmental concern (Silva and Uchida, 2000).
The leaching behaviour is dependent, among others, on the accompanying anion.
Sharma and Sharma (2013) found that in general, the K leaching in presence of the
accompanying anions followed the order of SO 4 ≤ H 2 PO 4 < NO 3 = Cl: Highest amounts
of extractable K was retained when K was applied along with SO 4 and H 2 PO 4 anions,
and the least was retained when accompanying anion was Cl. The influence of anions
was more pronounced in light textured soil and at high amounts of K application. Higher
levels of K application resulted in higher losses of K, especially in sandy loam soils as
observed in the leachate concentration.
Cuttle et al. (1995) describe in their study the leaching behaviour of NPK fertilisers and
lime. Concentrations of nutrients were measured in drainage water from a drained but
otherwise unimproved area of grassland and from a similar area following pasture
improvement in year 1of the 4 year study. Application of lime and fertilisers to the
improved area and reseeding with a grass/white clover mixture had little effect on
concentrations of nitrate and ammonium-N in drainage water except for one year when
nitrate concentrations were increased to a maximum of 24 mg N L-1 for a brief period
following application of a nitrogen fertiliser. Concentrations of organic-N, K, P and Ca
and the pH of water samples increased following pasture improvement and were
consistently greater than corresponding values for the surrounding area. By year 4
almost all of the K fertiliser applied to the reseeded area in years 1 and 2 had been
leached. In the same period, about 12% of the P fertiliser and 24% of the lime application
had been leached. (Cuttle, 1995).
Kohlahchi and Jalali (2007) looked into the effect of water quality on the leaching of K in
form of K+ from sandy soil. They state that the application of K fertilisers to sandy soils
84
with low clay content and small buffer capacity, in which K does not interact strongly with
the soil matrix, results in localised increases the K concentration in the soil solution.
Losses of K depend on the concentration of calcium (Ca2+) as a competing ion in the
leaching water and the amount of water that passes through the soil. In their study, they
examined the adsorption and movement of applied K+ in columns of sandy soil in native
soil or Ca2+-saturated soil. Native soil was leached with distilled water and CaCl 2
solutions of various concentrations. In the Ca2+-saturated soil, a pulse of K+ was leached
with CaCl 2 solutions of various concentrations or distilled water. They found that
increasing the CaCl 2 concentration of the leachate resulted in earlier break through, a
higher peak concentration of K+, and greater amounts of leached K+. The breakthrough
curve for K+, when leached with distilled water, showed very low concentrations and was
more delayed than the other treatments. In Ca2+-saturated soil, the amount of K+
leached increased as Ca2+ concentration increased. Thus they conclude that the
presence of Ca2+ in irrigation water and soil minerals able to release Ca2+ is important in
determining the amount of K+ leached from soils and that large amounts of K+ are
leached from soils in areas where crops are irrigated with water that contains significant
concentrations of Ca2+ and other cations.
One way to reduce K leaching is to incorporate organic soil amendments such as
compost or green manure. Organic materials usually have a large cation exchange
capacity, enabling them to retain K ions effectively. Among soil types, K deficiency
occurs more often in sandy than in clayey soils, more often in highly weathered soils
than in young soils.
4.2.2 Effect on soil structure
Recent investigations have raised awareness of the impact of K on the soil structure and
its ability to capture water. It has been reported that the application of mineral K
fertilisers enhances the water-holding capacity of soils and also improves the structural
stability of sandy soil in particular (Holthusen et al., 2010). The effect of K fertilisation on
soil stability is likely due to an increase in the electrolyte concentration in the soil
solution, causing flocculation and precipitated salt crystals (van Olphen, 1977). Similarly,
Holthusen et al. (2010) suggested that a higher concentration of K in soil solution may
cause an increase in micro shear resistance that may explain the change in water
retention. On the other hand, Mg2+ and Ca2+ are more effective cations for stabilizing soil
structure as relative flocculating power for K+, Mg2+and Ca2+ was 1.7,27.0 and 43.0,
respectively (Rengasamy and Sumner, 1998). Higher later retention plays a key role in
securing the soil productivity in water-limited areas. However, more information is
needed in order to understand the effect of K fertilisation on the soil’s physical properties
and soil water holding capacity but since York potash polyhalite contains K, Mg and Ca
then it may benefit soil structure when compared to fertilisers containing only one
nutrient.
4.2.3 Radioactivity of potassium
One of the sources of radioactivity other than those of natural origin is mainly due to
extensive use of fertilisers. Chauhan et al. (2013) analysed the concentrations of natural
85
radionuclides (226Ra, 232Th and 40K) in different chemical fertilisers used in agricultural
soil in order to assess the implications of the extended use of phosphate fertilisers in
recent years. The natural radioactivity levels in terms of radium equivalent activity
measured in the samples were below the recommended limits except in super
phosphate fertiliser and potash fertiliser. However, these samples satisfied the universal
standards (UNSCEAR, 2000) limiting the radioactivity within the safe limits of 1000,1000
and 4000 Bq kg−1 for 226Ra, 232Th and 40K, respectively.
4.3
Calcium
4.3.1 Leaching
Leaching of total and dissolved P, Ca, Mg and K from fallow, unfertilised and fertilised
barley and grass ley was studied in a four-year experiment carried out in lysimeters filled
with clay, silt, coarse sand and Carex peat. Half of the lysimeters were irrigated with
290–480 mm of water annually. Irrigation increased the nutrient leaching. Phosphorus
losses were anyway small (total P 0.02–0.26 kg ha−1 yr−1 from the fallowed soils) owing
to omission of surface runoff. Leaching of cations was more comparable with field
observations. The annual average varied in the bare soils from 50 to 120 and from 30 to
70 kg ha−1 yr−1 for Ca and Mg, respectively. The average leaching of K was small (4–24
kg ha−1 yr−1) in other than sand soil (56–94 kg ha−1 yr−1). Cropping of soil generally
decreased the leaching of dissolved and total phosphorus and cations compared with the
fallow treatment. Fertilisation did not have any consistent effect (Ylärantaa, 1996).
Leaching of calcium (Ca), potassium (K) and magnesium (Mg) from urine patches in
grazed grassland represents a significant loss of valuable nutrients. The effect on cation
loss of treating the soil with a nitrification inhibitor, dicyandiamide (DCD), which was used
to reduce nitrate loss by leaching was studies by Di and Cameron (2004). The soil was
a free-draining Lismore stoney silt loam and the pasture was a mixture of perennial
ryegrass (Lolium perenne) and white clover (Trifolium repens). The treatment of the soil
with DCD reduced Ca2+ leaching by the equivalent of 50%, from 213 to 107 kg Ca ha-1 yr1
on the field scale. K leaching was reduced by 65%, from 48 to 17 kg K ha-1 yr-1. Mg
leaching was reduced by 52%, from 17 to 8 kg Mg ha-1yr-1. It was suggested that the
reduced leaching loss of these cations was due to the decreased leaching loss of nitrate
under the urine patches, and follows from their reduced requirement as counter ions in
the drainage water. The treatment of grazed grassland with DCD thus not only
decreases nitrate leaching and nitrous oxide emissions as reported previously, but also
decreases the leaching loss of cation nutrients such as Ca2+,K+ and Mg2+.
4.3.2 Effect on soil structure
Calcium is reported to have a positive effect on soil stability. This is mainly due to the
contribution of interfacial processes in mechanical strength and the major role of calcium
bridges in the stability of aggregates (Chaplain, 2011).
4.3.3
Ecotoxicity
86
No literature on the ecotoxicty of Ca could be identified during the literature search of this
study. However, positive effects of Ca2+ were reported on the uptake of toxic metals in
the freshwater invertebrate Daphnia sp.. Komjarova and Blust (2009) suggested that Ca
had a suppressing effect on the uptake of Cd, Ni, Pb and Zn. Specifically, Cd and Ni
uptake rate constants decreased with increases in calcium concentrations. The uptake
of Zn and Pb was significantly suppressed only at 2.5mM Ca.
4.4
Magnesium
4.4.1 Leaching
See chapter 5.3.1.
4.4.2 Ecotoxicity
Similar to Ca, Mg demonstrates a positive effect on the toxicity of some metals.
Deleebeek et al. (2009) found that increasing Mg concentrations and decreasing pH
decreased Ni toxicity to the green alga Pseudokirchneriella subcapitata.
4.5
Sulphate
4.5.1 Leaching and impact on soil
Sulphate is the most important source of sulphur for plants. In the soil solution is it in
equilibrium with SO 4 adsorbed to Al and Fe oxides. At low soil pH SO 4 adsorption
increases and therefore addition of lime increases the amount of SO 4 in soil solution.
The addition of P fertilisers also increases the availability of SO 4 as sorption declines in
the presence of phosphate anions. Sulphate can further precipitate as Ca, Mg or Na
sulphate.
Förster et al. (2012) state that no significant increase of total S was reported after 150
years supply of S containing mineral fertilisers, which may be due to the fact that S is
mainly applied as inorganic SO 4 which is water soluble and therefore susceptible to
leaching. However, depending on the location and climatic conditions, fertiliser type and
application rate, different results may be obtained. It should be taken into consideration
that S applied as sulphate in form of NPKS fertilisers is taken up by plants in high
amounts, which also affects total S in soil.
Knights et al. (2011) used samples from the Rothamsted Broadbalk Experiment,
England, to evaluate long-term effects of changing S inputs from atmospheric deposition
and fertilisation on soil S pools and soil S isotope ratio since 1843. The effects of
changing land uses were also investigated. They found that large S inputs from
atmospheric deposition and from sulphate fertilisers did not result in any significant
accumulation of soil organic or inorganic S in the arable plots where organic C remained
stable. Inputs of sulphate in excess of crop uptake were lost mainly through leaching.
Organic S accumulated markedly in the arable plot receiving farmyard manure or where
arable land was allowed to revert to woodland or grassland state. The authors conclude
that land use had a large effect on the S cycling in soils, which is driven mainly by soil
87
organic C cycling. Without accumulating soil organic C, there appears to be little scope
for S retention in temperate soils with neutral pH.
4.5.2 Impacts on ecology and ecotoxicity
It has been found that sulphate can have positive effects on aquatic organisms, for
example Walleye in inorganically fertilised ponds grew faster, and were almost twice the
weight of those in the control ponds at harvest (Fox et al., 1992). However, adverse
effects have also been reported on aquatic organisms. In a study by Mihaljevic et al.
(2011), the genotoxic potency of sulphate-rich surface waters on medicinal leech (H.
medicinalis) and human leukocytes was assessed. It was found that the levels of DNA
damage in leeches chronically exposed (for 28 days) to water samples from a Gypsum
mine and lake Mali Kukar increased with the duration of exposure.
Ram et al. (1987) exposed Channa punctatus to safe (100 ppm) and sublethal (500 ppm)
concentrations of the commonly used fertiliser ammonium sulphate for 6 months, from
January to June. Hepatocytes revealed initial hypertrophy followed by exhaustion as
evidenced by degranulation, nuclear pyknosis, and focal necrosis. They further found
that thyroid follicles exhibited various degrees of hypertrophy, hyperplasia, hyperemia,
and reduction in colloid content. Their results suggest that ammonium sulphate fertiliser,
which can be washed into the water system, may cause dose-dependent dysfunction of
liver and thyroid.
4.5.3 Eutrophication and sulphur reduction
Under reducing conditions sulphate can be transformed to the toxic sulphide (H 2 S). In
freshwater wetlands, sulphate reduction normally plays a modest role, but becomes the
most important biogeochemical process, inducing severe eutrophication and sulphide
toxicity, if sulphate is introduced into these systems in larger amounts (wash-off,
deposition etc.). Lamers et al. (2002) observed differences between the responses of
two freshwater marshes to sulphate: On one location sulphate addition resulted in strong
P mobilization without sulphide accumulation, whereas high sediment sulphide
concentrations, known to be toxic to wetland macrophytes, were reached in the other
marsh without eutrophication occurring. The results are explained by differences in
groundwater iron discharge and nutrient contents of the peat sediments.
Lloyd et al. (1976) exposed Bluegill sunfish (Lepomis macrochirus) eggs, fry, juveniles,
and adults to H 2 S concentrations and determined LC 50 concentrations. 72 h LC 50 for
eggs was 0.0190 mg L-1; 96-h LC50 for 35-day-old fry was 0.0131 mg L-1, for juveniles
0.0478 mg L-1, and for adults 0.0448 mg L-1. They observed that exposure to lower
levels of H 2 S resulted in some acclimation. Chronic exposure to sublethal levels of H 2 S
for up to 826 days resulted in no egg deposition at 0.0022 mg L-1and reduced deposition
after 97 days at 0.0010 mg L-1. Growth was adversely affected at levels from 0.0031 to
0.0107 mg L-1 H 2 S depending on the life history stage at which chronic exposure was
started.
88
5 Discussion & recommendations
From this literature appraisal it can be assumed that the use of polyhalite as a fertiliser
will show no obvious environmental impacts that could specifically be associated with
polyhalite or would not be seen using other types of fertilisers consisting of the same
compounds.
All potential environmental impacts that have been identified during this study are
associated with individual compounds of polyhalite (e.g. sulphate and potassium) rather
than with polyhalite itself due to a lack of data on the fate and environmental impact of
polyhalite used as a fertiliser. All compounds of polyhalite (K, Mg, Ca and SO 4 ) are also
present in other types or mixtures of fertilisers (e.g. NPK fertilisers and manure) and
therefore should not pose any additional threats to the environment.
Polyhalite components could leach into groundwater to some extent depending on e.g.
soil characteristics, rainfall, solubility, uptake by plants and retardation/sorption. In
drinking water however nitrate apparently shows higher toxicity in humans (e.g. blue
baby syndrome, carcinogen, limit: 50 mg L-1) than sulphate (e.g. taste threshold,
diarrhoea, limit 200 mg L-1), magnesium (e.g. gastro-intestinal problems, limit 100 mg L-1)
or calcium (e.g. kidney stones, gout, limit; 200 mg L-1), although magnesium has been
found to cause cardiac arrest in humans in very high dosages in the form of MgSO 4 .
Adverse health effects due to potassium consumption from drinking water are unlikely to
occur in healthy individuals and it is believed that K is of no major environmental
concern.
Similar to N and P containing fertilisers, some compounds of polyhalite are also
expected to reach surface waters and contribute to eutrophication. For example,
sulphate will increase eutrophication, but will also increase sulphide toxicity in freshwater
wetlands (e.g. under reducing conditions in sediments). Sulphate has also been
reported to have adverse effects on aquatic organisms such as leeches. At one location
sulphate addition resulted in strong P mobilization without sulphide accumulation. While
it has been found that sulphate can have positive effects on aquatic organisms, for
example Walleye in inorganically fertilised ponds grew faster.
The natural radioactivity levels contained in polyhalite are assumed to be well below the
recommended threshold and polyhalite impurities potentially adversely affecting the
environment when applied with fertilisers such as toxic metals are believed to be low
compared to impurities contained in mined phosphorus applied as fertiliser. Further, the
macronutrients contained in polyhalite are not expected to contribute directly to
greenhouse gas emissions compared to nitrogen and its gaseous forms. The impact on
mineral fertilisers on methane production e.g. in paddy rice fields are not clear, but
assumed to be minor. As sulphate is not expected to be released into the atmosphere in
form of aerosols when applied as fertiliser, it is assumed that there will be no contribution
to global climate change and acid rain.
Positive effects of Ca, Mg, and K can be expected for soil structure and Ca and Mg have
been found to reduce the uptake of toxic ions such as Cd and Ni in aquatic species.
89
In many cases environmental issues associated with the application of fertilisers cannot
be linked to a certain compound of a fertiliser as such (e.g. decline in amphibians
associated with the application of mineral fertilisers, effects on biodiversity e.g.
macroarthropods).
The environmental fate and behaviour (e.g. leaching, run-off, sorption, uptake) of
fertilisers and their components is always dependent on the type of fertiliser and its
compounds including the mixture, chemical form, application rate and time of application,
the soil (e.g. soil characteristics, soil community), climate (e.g. rainfall), geology (e.g.
Karst), and agricultural usage (e.g. type of crop, crop rotation, intensification, fertilising
regime, etc).
90
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7 Appendix 1 - Trace element testing results for York Potash
polyhalite
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8 Appendix 2 – Product specification sheet for Poly4
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