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

5.3. Soil acidification
Most nitrogen fertilizers, especially ammonium
sulphate and to a lesser extent ammonium
nitrate, acidify soils, although some soils are
naturally able to cope. The use of organic
residues at normal levels of application may not
avoid acidification but may slow the process.
The acidifying effects of nitrogen fertilizer can
be corrected if lime is economically available and
is applied. In temperate regions, lime, applied in
quantities of tonnes per ha, but less frequently
than fertilizers, provides optimum conditions for
growth of many temperate crop species. Apart
from neutralizing soil acidity, liming improves the
availability of other nutrients such as phosphate,
and lowers the toxicity of aluminium and
manganese. In a long-term experiment in India,
with the continuous application of fertilizers
without lime, yields fell to zero. When the soil pH
was kept near to the optimum, the system
became sustainable.
In the humid tropics, the lime requirements
are high and the effect may not last long due to
leaching losses. However, increases in crop yields
can sometimes be achieved with minimal
applications of lime due to alleviation of
aluminium toxicity and/or calcium deficiency and
care must be taken to avoid over-liming
(R.J. Haynes and R. Naidu, 1998). In many
developing countries agricultural lime is not
available at an economic cost. A possible solution
lies in the development of cultivars which are
tolerant of the direct and indirect effects of soil
acidity.

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Mineral Fertilizer Use and the Environment
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5.4. Erosion
Soil erosion is a world-wide phenomenon, but is
more serious in some regions than in others. In
regions where a dry season alternates with a
season with torrential rains soil erosion can be
very severe. At the end of the dry season the soil
is likely to have sparse vegetative cover,