Download Nutrient depletion in Africa The role of soil elements in plant growth

The nitrogen cycle
Nitrogen is a critical component of plants. It is a structural
component of chlorophyll, nucleic acids (DNA, RNA) and proteins.
While abundant in air, nitrogen in the atmosphere cannot be
used directly by either plants or animals and must be converted
into a usable state. Rainfall often contains substantial quantities
of nitrogen in the form of ammonium (NH4+) and nitrate (NO3-).
On contact with the soil, both ammonium and nitrate enter the
soil solution easily and are absorbed by plant roots. The microbial
decomposition of organic matter results in the mineralisation of
organic nitrogen to release the ion NH4+ to the soil. Depending
on temperature, moisture, the level of soil aeration and presence
of some plant species, NH4+ is oxidised to NO3- , both of which
are readily available to plants. Nitrogen may also occur in the soil
as the result of mineral weathering, animal urine and through the
application of mineral fertilisers (see page 11).
Certain types of bacteria (e.g. Rhizobium) can convert atmospheric
nitrogen (N2) to ammonia (NH3) through a symbiotic relationship
with the root nodules of leguminous plants such as clover (Trifolium)
or soybean (Glycine max). This process is known as nitrogen fixing.
Plants convert the ammonia to nitrogen oxides and amino acids that
form proteins and other molecules. In return, the plant provides
sugars to the bacteria. In order to fix nitrogen, plants such as legumes
maintain an oxygen-free (anaerobic) environment near their roots
so that the bacteria can exist. Soil pH, organic matter levels and
the availability of trace elements such as copper can influence the
distribution and activity of these specialised bacteria.
In natural ecosystems, plant growth is relatively slow and the annual
uptake of nitrogen is comparatively low (e.g. 30 kg N ha-1). Cultivated
crops are much more demanding with nitrogen uptakes that are
several orders of magnitude greater (e.g. 500 kg N ha-1). In these
cases, the natural nitrogen cycle is unable to maintain optimum
growth and artificial inputs must be added to the soil. Harvesting
is a critical process as organic matter that would have normally
decomposed on the soil surface is often physically removed from the
field for processing. This means that there is an export of nitrogen
and other elements from the soil to the marketplace. Corrections
of N shortages through the addition of mineral fertilisers result in
an increase in vegetative growth, much higher protein levels and
greater yields of grain and fruit. However, excess amounts of N,
above what can be used by plants, are often flushed out of the
soil to accumulate in water bodies. Under the right conditions,
nitrogen-driven bacterial growth can deplete the oxygen in nearby
water bodies to the point that fish and other aquatic organisms die.
This process is known as eutrophication.
Volatilisation loss
Urea
CO(NH2)2
Ammonia
NH3
Ammonium
NH4
Immobilisation
Denitrification
Nitrate
NO3
Leaching
loss
The principle pathways and transformations of the nitrogen cycle. Nitrogen is
important to all life. Nitrogen in the atmosphere or in the soil can go through
many complex chemical and biological changes, be combined into living and
non-living material, and return back to the soil or air in a continuing cycle.
Animals and plants deposit organic nitrogen into the soil. Bacteria convert
organic nitrogen to plant-usable ammonium and ammonium to plant-usable
nitrates. Denitrification describes the bacterial decomposition of nitrate in
the soil to atmospheric nitrogen while volatilization turns urea fertilisers and
manures on the soil surface into gases that go directly to the atmosphere (see
also page 11). (LJ)
Nutrient depletion in Africa
Several studies have highlighted significant nutrient losses from
African soils [28-31]. Models estimate that on average, 660 kg N ha-1
have been lost during the past 30 years from about 200 million ha of
cultivated land in 37 African countries (excluding South Africa). The
FAO estimates that Africa is losing 4.4 million t N every year from
cultivated land - these rates are several times higher than Africa's
annual fertiliser consumption of 0.8 million t N [28, 32]. N loss is
driven by cultivation on nutrient-poor soils, a breakdown of traditional
soil-fertility practices and poverty in rural Africa which does not permit
effective fertiliser management practices.
The role of soil elements
in plant growth
Macronutrients
Macronutrients are elements that are essential to plant growth and
are needed in significant amounts. For more details, see [7a].
Potassium (K) is crucial to most plant functions including stomatal
control, the maintenance of turgor pressure and charge balance
during selective ion uptake across root membranes. It is also an
enzyme in many biochemical reactions. Potassium is highly mobile
and is easily leached from leaves to be taken up in high quantities
by soil microorganisms and roots. In soil, potassium may be found in
minerals such as micas and feldspars, secondary aluminium silicates
(e.g. illite) and some salts. Potassium is available when attached to clay
and humus colloids and easily available when in solution. Potassium
dissolved in soil solution as an ion is highly leachable, although loses
of potassium from runoff and erosion is not a significant problem.
Calcium (Ca) is used to build cell walls in plants. It helps keep P
available in the root zone by binding it with other ions. Because it is
bound within cell walls, it does not leach from leaves nor circulate
within the plant. Calcium deficiency leads to stunted plant growth,
the curling of young leaves and death of terminal buds. Calcium can
be easily leached from the soil and is largely absent in the soils of
central Africa.
Magnesium (Mg) is the central atom of the chlorophyll molecule
and is an important enzyme. It is very mobile in plants. Magnesium
deficiency in plants causes yellowing between leaf veins. Low soil pH
decreases the availability of magnesium to plants.
Phosphorus (P) is crucial to many plant functions, a key component
of most fertilisers, and often lacking in non-fertilised soils. Phosphorus
forms the backbone of DNA and RNA molecules and regulates cell
division, root development and protein formation (see adjacent text).
Phosphorus is responsible for crop yield increases.
The benefits of nitrogen (N) are described in the adjacent text.
Micronutrients
The phosphorus cycle
Micronutrients are elements that are essential to plant growth but are
required in very low concentrations (< 100 µg/g). They are generally
metabolically active in plants as important enzymes.
Phosphorus is another vital plant nutrient that forms the backbone
of DNA and RNA molecules, forms cell membranes and regulates
cell division, root development and protein formation.
The future of phosphorus?
Phosphorus deficiency can occur in areas of high rainfall, on acid,
clayey or poor calcareous soils. Symptoms include poor growth and
leaves that turn blue/green but not yellow. Due to the movement
of phosphorus in plants, the oldest leaves are affected first. Fruits
are small and taste acidic.
Due to the essential nature of phosphorus to living organisms, the low
solubility of natural phosphorus-containing compounds and the slow
natural cycle of phosphorus, the agricultural industry is heavily reliant
on fertilisers containing concentrated phosphoric acids (H3PO4).
About 50% of the global phosphorus reserves are in North Africa and
the Middle East. Large deposits of phosphate-bearing apatite exist in
China, Russia, Morocco and the USA.
Iron (Fe) primarily originates from chemical weathering of minerals
and is not absorbed by plants in appreciable quantities; the amount
found in plants is several orders of magnitude lower than the amount
in the surrounding mineral soil. Iron serves as an electron carrier
in enzymes. It also plays a role in nitrogen fixation and chlorophyll
formation. Its movements in soil are due to chemical processes rather
than an association with organic matter or uptake by plants. The
presence of iron oxide gives a reddish tint to soil horizons.
Recent reports have suggested that production of phosphorus may
have peaked, leading to the possibility of global shortages by 2040.
However, some scientists now believe that a "phosphorus peak" will
occur in 30 years and that reserves will be depleted in the next 50
to 100 years. [33]
Manganese (Mn) is critical to many plant functions, including
photosynthesis, respiration, and nitrogen metabolism. Manganese is
generally plentiful in acid soil and may reach toxic levels if pH is below 6.5.
It generally leaches out of acidic soils.
Due to its high reactivity, inorganic phosphorus is never found as a
free element, and geologically occurs as phosphate rocks (PO43-). In
natural ecosystems, phosphorus is released by the decomposition
of organic matter as compounds known as orthophosphates (e.g.
H2PO4-, HPO4-). These are rapidly adsorbed by soil particles or
immobilised by phosphorus-consuming bacteria (e.g. Aspergillus).
The pool of phosphorus that is readily available to plants is found in
solution or loosely bound on to soil particles.
Because of low concentrations of P in soil solutions and the
competition from soil microorganisms, many plants have developed
a symbiotic relationships with mycorrhiza – a type of fungus – which
in essence extend the root network and provide an improved
pathway for the rapid transfer of phosphorus.
Zinc (Zn) is a key component of growth control hormones in plants
and is used in protein synthesis. Almost half of the world’s cereal
crops are deficient in zinc, leading to poor yields while zinc deficiency
is the 5th leading risk factor for disease in developing countries. Zinc
in soils is tightly bound to magnesium.
Copper (Cu) is especially plentiful in acidic, sandy soils and is an
important enzyme activator found mostly in the chloroplasts of leaves.
The highly weathered, iron-rich tropical soils of Africa tend to
be deficient in plant-available phosphorus. The low pH together
with high levels of iron and aluminium oxides, tend to imobilse
phosphorus onto soil particles thus denying its availability to plants.
In such situations, large quantities of phosphorous fertiliser must be
added to the soil to make a difference in crop yields. Lime-rich soils
also imobilise phosphorus.
Toxic elements
Pollutants are contaminants that have been introduced into the natural
environment and cause instability, disorder, harm or discomfort to the
ecosystem. Artificially high levels of all elements can have harmful or
toxic effects. Aluminium (Al) is not used in significant amounts by
plants. In soils, it immobilises phosphorus and generally increases the
acidity and concentration of cations. As for most elements, aluminium
becomes toxic to some plants above 1 ppm and to most plants
above 15 ppm.
As in the case of nitrogen, it is estimated that around 0.5 million
tonnes of phosphorus is lost every year from cultivated soil in Africa
- double Africa's annual P consumption [22].
Photograph taken through
mycorrhiza (the faint, thin
around the roots of a plant
Such symbiotic relations can
phosphorus from soils. (PDI)
a microscope of
filaments) growing
(brighter features).
help plants extract
Lead (Pb) binds with organic matter in the soil and accumulates in
certain organic tissues of plants. In high enough concentrations, it
can cause brain damage in humans.
Other elements that are toxic to plants include arsenic (As), cadmium
(Cd), sodium (Na) and even iron (if concentrations are high enough).
Introduction | Soil Atlas of Africa
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