Download use of non-conventional phosphate fertilizers in tropical agriculture1

Lectures
39
USE OF NON-CONVENTIONAL PHOSPHATE FERTILIZERS
IN TROPICAL AGRICULTURE1
LUÍS IGNÁCIO PROCHNOW
International Plant Nutrition Institute, Brazil Program Director, Av. Independência no. 350, Sala 141, Edifício Primus Center, 13.419160, Piracicaba, SP. E-mail: [email protected]; website: brasil.ipni.net.
Introduction
This paper describes use of non-conven-
tional phosphate (P) fertilizers by focusing in new
techniques to improve the efficiency of conventional
sources, as well as the use of non-conventional
P sources. By conventional P sources the author
means totally acidulated P fertilizers, like SSP, TSP,
MAP and DAP.
Improving Efficiency of Conventional
Phosphorus Fertilizers Using
Alternative Techniques
It is known that water-soluble P can be converted to water-insoluble P after reaction with soil
minerals that can result in a decrease of P availability. Several terminologies such as P sorption,
adsorption, retention, fixation, precipitation, immobilization, etc. have been used to describe this process. The forms of reaction products depend on P
sources and soil minerals. There has been some
interest in research and development on modifying
the physical characteristics of conventional watersoluble P fertilizers in order to reduce P fixation
by soil, and thereby increasing the P efficiency for
plant uptake. Some of the recent findings are discussed below.
Coated water-soluble phosphorus
fertilizers
Recently, some fertilizer companies have
developed thin-coating of water-soluble P fertilizers
(DAP, MAP, TSP) with S or polymers as a slowrelease water-soluble P source to reduce rate of
P conversion to water-insoluble P by soil fixation.
Some claim that a polymer with a high-charge density can inhibit P precipitation by acting as a platform for sequestration of P fixing cations such as
Ca and Mg in high pH soils and Fe and Al in low pH
soils. However, there is little information on the soil
chemistry of this polymer-coated P fertilizer published in the peer-reviewed scientific journals. If the
P release meets the crop need and at the same
time minimizes P fixation, the coated water-soluble
P can be an effective P fertilizer for crop production
provided the cost/benefit is feasible as compared
with the uncoated water-soluble P fertilizers.
Urea supergranule containing
phosphorus and potassium nutrients
Results of several farmer-managed field trials conducted especially in India demonstrate that
the USG-DAP management can make the fertilizer
agronomically more efficient, economically more attractive with less risk, and reduced losses of nutrients as compared with conventional use of prilled
urea and water-soluble P fertilizers. Although no information is available on plant-available K of deepplaced USG containing K nutrient for flooded rice, it
is expected that USG containing DAP + KCl should
also be agronomically as effective as or even better than deep-placed USG with incorporated DAP +
KCl. The deep-placed USG containing DAP + KCl
should perform better than split-applied prilled urea
with incorporated DAP + KCl due to N availability.
Thus urea-based NPK compound fertilizers can be
agronomically and economically feasible in supergranule form by deep placement for flooded rice
production. Future work will be needed to test this
supposition.
Fluid versus granular water-soluble
phosphorus fertilizers
A comparison of fluid with granular WSP
fertilizers in agronomic effectiveness depends
on many factors. Some of them are (1) chemical
compounds of P sources, (2) proportion of watersoluble P and water-insoluble P in P sources, (3)
soil pH, (4) soil P-fixing capacity, (5) soil biological
Adapted from “Chien, S.H.; Prochnow, L.I.; Cantarella, H. Recent Developments on Fertilizer Production and Use to Improve
Nutrient Efficiency and Minimize Environmental Impacts. Advances in Agronomy, 102:261-316, 2009.
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16th World Fertilizer Congress of CIEC
activity, (6) soil moisture or rainfall, (7) crop species
(e.g., rooting system), (8) rate of P applied, (9) P
placement method (e.g., incorporation versus band,
no-till versus till), (10) initial versus residual P effect,
and (11) cropping systems. One benefit of applying
fluid ortho- or polyphosphate sources as compared
with solid P sources is that fluid P sources can increase soil concentration of available P with depth.
This implies that application of fluid P sources to
soil surface after crop harvest will move into the soil
profile where it will be less subject to loss in runoff
or by erosion that may minimize P environmental
impact during the winter months, and yet be available to plants the following growing season. In lieu
of recent renewal interest in research on comparing
the agronomic effectiveness of fluid with granular
ing in reactivity for different crop species. Based on
this model, FAO/IAEA has posted the PRDSS on
IAEA web site (http://www-iswam.iaea.org/dapr/srv/
en/resources).
Recently eutrophication of aquatic environments (creeks, ponds, rivers, lakes, etc.) caused
by excessive P from soil surface run-off has drawn
many researchers to find strategies to mitigate the
P pollution problem. Preliminary studies done in
New Zealand and U.S.A. have suggested that use
of reactive PR not only can sustain crop productivity
but also may minimize eutrophication problem compared with the use of water-soluble P sources because of lower P availability of PR for algal growth.
Another new field of PR research is that
PR has been increasingly used for organic farm-
WSP fertilizers for alkaline and calcareous soils in
Australia, it may be worthwhile to renewal similar
research for acid or neutral soils in other countries.
ing worldwide since chemical P fertilizers are not
allowed to be used. Organic farmers should be
aware of that not all the PR sources are the same
in reactivity. The general rule is that the higher the
reactivity of PR, the better is for organic farming as
P source. In fact, most of igneous PR sources are
high in P content but very low in reactivity due to
little carbonate substitution for phosphate in apatite
structure and therefore, they are not suitable for direct application.
Use of Non-Conventional Phosphorus
Fertilizers
Phosphate rock for direct application
Direct application of phosphate rock (PR)
can be an effective agronomic and economic alternative to the use of more expensive water-soluble
phosphate (P) fertilizers for crop production under
certain conditions, especially in acid soils of tropical and subtropical developing countries. The agronomic use of PR has been extensively studied or
reported during the past 50 years. The major factors affecting the agronomic effectiveness of PR are
chemical and physical properties of the PR, which
affect solubility, soil properties, management practices, climate, and crop species. Despite hundreds
of agronomic trials that have been conducted worldwide in the past, there is a need to integrate all of
these factors in a comprehensive system to understand how these major factors affect the agronomic
effectiveness of PR. Use of a phosphate rock decision support system (PRDSS) is a means to solve
this problem. Because it is designed to be practical,
PRDSS can be used in developing countries, especially those countries with endowed indigenous
PR deposits to assist in making decision to use
water-soluble P fertilizers or PR to supply P need
by crops. Recently, IFDC has developed and published its own PRDSS model for PR sources vary-
Mixture of phosphate rock and watersoluble P
Under certain conditions such as low PR
reactivity, high soil pH, or short-term crop growth,
etc., agronomic use of PR may not be feasible as
compared to water-soluble P. Mixing PR with water-soluble P sometimes can be an agronomic and
economic effective alternative under these conditions. Partial acidulation of low-reactive PR (PAPR)
which consists of un-acidulated PR and acidulated
water-soluble P (WSP) can be one way to achieve
this goal. Another way is to mix PR with WSP by dry
granulation (compaction). The results of many studies done by researchers have provided valuable
information on the factors affecting the agronomic
effectiveness of mixtures of PR and WSP. These
include: (1) PR reactivity, (2) degree of acidulation,
(3) degree of Fe and Al impurities of PR, (4) effect
of soil properties such as pH and P-fixing capacity
(more favorable for soils with high P-fixing capacity), (5) starter effect of water-soluble P on PR ef-
Lectures
fectiveness, (6) effect of crop species, and (7) initial
and residual P effect.
Calcined Non-Apatite Phosphate Rock
for Direct Application
Most of phosphate rocks used for chemical acidulation process or direct application contain
Ca-P minerals in the form of apatite. There are limited PR deposits in the world that contain Ca-FeAl-P minerals in the form of crandallite. Because
of its high Fe and Al content, the non-apatite PR is
not suitable for conventional chemical acidulation
process. The natural crandallite PR are very low in
reactivity and therefore, not suitable for direct application. The reactivity, however, can be significantly
increased upon calcination at temperature ranging
from 450 to 700 ○C after the hydrated water molecule of the crandallite structure is driven off and
the structure becomes amorphous.
Agronomic Effectiveness of NonConventional Acidulated Phosphate
Fertilizers
Amounts of high premium quality PR to
produce conventional acidulated phosphate fertilizers (SSP, TSP, MAP, DAP) are rapidly decreasing worldwide. As the phosphate industry becomes
more dependent on lower quality PR ore, higher
levels of phosphate impurity compounds can be expected in the final acidulated phosphates. These
impurities are generally water-insoluble forms of
Ca-P or Fe-Al-P and their composition is determined by the mineralogical constitution of the ore
and also by the process of fertilizer production. Preliminary agronomic studies showed some Fe-Al-P
compounds in acidulated P fertilizers to be, when
applied per se, less-effective when compared to the
water-soluble phosphate (WSP) compounds normally found in superphosphates and ammonium
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phosphates. Legislation in some parts of the world
has established the minimum legal content of water-soluble P in acidulated P fertilizers. When impurity compounds are mixed with reagent-grade MAP
or MCP, under experimental conditions to simulate
acidulated P fertilizers, the results suggested that
these fertilizers may contain higher proportions of
impurity compounds than expected and normally
used. Research has provided already valuable information regarding the possible agronomic use of
some non-conventional acidulated phosphate fertilizers. As a result, legislation was modified as to
make the presence of water insoluble phosphate
compounds more flexible. It is necessary to recognize that a great variety of new non-conventional
products may be offered in the future due to differences in PR chemical composition and process
of production. Due to distinct chemical composition
the fertilizers will have to be tested and approved
individually. A better understanding of how the water insoluble P compounds will form, the final chemical composition of the P fertilizers, and also how
these different non-conventional fertilizers will react
in the soil is essential for P fertilizer producers, legislators and final users to obtain and manage these
heterogeneous fertilizers in a cost-effective manner.
Only agronomic research will provide the necessary
guidance. Better utilization of PR is anticipated as a
result of this type of research.
Keywords: Phosphate Fertilizers, Non-Conventional P Fertilizers, Phosphate Sources, Plant Nutrition
with Phosphorus.
References
Several agronomic examples and references are quoted in the original paper as per the
footnote in the first page of this abstract.