Environmental risk of chemical agriculture - Environmental Deterioration and Contamination: Problems and their Management - page 98

Table 4.1 Possible harmful consequences of the application of high fertilizer dose effects

Fertilizers
Nitrogen

Phosphorus Potassium

Direct Transport by surface runoff to

surface waters

Transport by surface runoff to
surface waters

Soil acidification

Leaching to subsurface waters Soil acidification

Soil acidification Soil structure destruction,

Soil structure destruction, erosion

erosion
Deterioration of crop yield
quality

Indirect Soil quality deterioration Soil quality deterioration Soil quality deterioration

Water quality deterioration Water quality deterioration Hypomagnesia

Eutrophication Eutrophication

Methemoglobinemia Cadmium contamination

consequence, mention should be made of the fact that in some cases the so-called hypo-
magnesia syndrome may occur in a very small number of animals, which have grazed
on pastures over-fertilized with potassium. This is a result of a relative magnesium
deficiency due to excessive uptake of potassium.
Mainly nitrogen fertilizers—but phosphorus fertilizers also—acidify soils. Nitro-
gen fertilizers may be leached into subsurface waters, mainly in NO 3 form, as NH 4 is
less mobile in soil. Extreme nitrogen fertilization often results in the deterioration of
crop yield or quality, and reduces plant resistance. Phosphorus fertilizers transform to
a significant extent into insoluble inorganic and organic compounds, therefore subsur-
face waters are less endangered by surface runoff or erosion. Jointly applied with nitro-
gen fertilizers, however, the eutrophication of surface waters may become more rapid.
In the case of over-fertilization, nitrogen mineral fertilizers, as well as organic
manure may contaminate the soil, subsurface waters and the atmosphere with nitrogen
oxides, ammonia (urea) in gaseous form or as airborne dust.
Above a certain limit concentration, nitrate leached into the groundwater disqual-
ifies the polluted water reserve to be used as drinking water, as—primarily in the case
of infants—it causes methemoglobinemia.
Primarily phosphorus fertilizers may be contaminated with toxic metals, and as a
consequence of feeding contaminated fodder or through pollutants, organic manure,
compost and soil ameliorants may also become contaminated. The greatest risk is
attributed to carcinogenic, mobile cadmium salts (i.e., acetate, oxide, nitrate, sulfate,
sulfite). Cadmium may further be mobilized and leached into surface waters from
acidic soils.

3.2 Environmental impacts of pesticides
Upon application, the majority of pesticide spray misses its target and settles on the soil
surface. Pesticides landing on topsoil may become strongly adsorbed to soil colloids,
while water-soluble compounds may be leached rapidly or slowly into deeper soil