Mineral Fertilizer Use and the Environment International Fertilizer Industry Association United Nations Environment Programme

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9.2. Phosphate and potash
Until a few years ago, it was believed that
phosphate (and potash) “fixed” by the soil went
over into plant-unavailable, inert and hence
useless forms. There has, however, been a change
of perception in recent years. Experiments have
shown that in many soils reserves of plant
available P and K can be built up over time. Soils
enriched with these reserves frequently gave
larger yields than soils without the reserves.
Hence the plant-uptake figure of 15% for P
underestimates the long-term efficiency of
phosphate fertilizers. Phosphate (and potash)
residues accumulated in the soil are not
necessarily lost - but this is not a reason for
accumulating these residues unnecessarily. There
are critical values of phosphate and potash below
which yield decreases appreciably and which
represent a financial loss to the farmer. To
accumulate P and K in the soil above these
critical levels is an unnecessary cost for the
farmer. It may also pose an environmental risk, in
that soil lost by water or wind erosion to streams,
rivers and lakes takes its nutrient load with it
(A.E. Johnston, 1997). Further work is required to
establish these critical soil levels under different
conditions. Work to improve the plant uptake of
applied P and K is also required.
Phosphate has both direct and indirect effects.
The increased availability of phosphate has a
positive effect on the quantity and quality of
agricultural outputs. Through indirect interaction
effects, phosphate increases the response of
agricultural production to the other inputs such
as nitrogen and potassium and has positive effects
on biological nitrogen fixation, soil organic matter
maintenance, water-holding capacity, soil erosion
control and other soil physical and chemical
properties. All of these positive effects result in
increased agricultural output, sustained
productivity and land conservation
(C.A. Baanante, 1998).
9.3. Products
How can these high losses of nitrogen be
reduced? In fact, improvements in fertilizer use
efficiency can be detected in most agriculturally
advanced regions, but this can be attributed to
improvements in cultivation practices, techniques
of fertilizer application and crop varieties. Apart
from some developments in coated, controlled
release fertilizers and nitrification inhibitors, there
has been little significant change in the
fundamental nature of the main fertilizer
products for many years, or even decades. There
is little incentive to invest in the research and
development of a bulky, low-priced commodity
which offers little scope for product
differentiation.