Geoscience Reference
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because plants cannot fully assimilate them during dry conditions. With the coming
of autumn, growth is held in check by the colder weather so heavy rain then flushes
out the nitrates before they can be used. In a wet summer, some of the nitrates are lost
by leaching but much more are taken up by the active growth of plants.
One must put these nitrogen losses in perspective. Losses of gaseous ammonia
from pig slurry applied to the soil in the Irish example could well amount to 40-60
kg/ha N per year, considerably more than by leaching. (Gaseous losses could contrib-
ute to 'acid rain' but not to water pollution). Loss of phosphorus by leaching was only
about 1-2 kg/ha per year; this was less than that applied and so would indicate an
annual net enrichment of soil phosphorus levels, as has occurred in the UK generally
since the 1940s.
In Nature, alternate drying and re-wetting of soil often takes place with the con-
sequence that part of the living biomass may be converted into dead organic matter.
The latter is consumed or broken down in stages by microorganisms until it becomes
finally mineralized to inorganic constituents which serve as plant nutrients. After each
cycle of drying and re-wetting, there is a flush of microbial activity in the soil and
concomitant mineralization of the soil's biomass. Provided that other conditions are
favourable, plant growth is naturally stimulated by this provision of nutrient materi-
al. This phenomenon has been frequently observed, especially in tropical areas where
there are characteristic wet and dry seasons. In some parts of Africa, for example,
crops may be sown just prior to or at the beginning of the rainy season in order to
exploit the flush of nutrients that are released when the soil is re-wetted. If crops are
sown in good time, the leaching and loss of nitrates can be avoided. Leaching implies
movement down the soil profile, but in some areas where soils are subject to intensive
drying, the upward movement of soil water due to capillary action can transport the
dissolved nitrates to the surface. After evaporation on the surface, deposits of nitrates
can occur.
Returning to the British scene, several thousand kg/ha of organic nitrogen in old
grassland are potentially mineralizable and then subject to leaching. In one measured
example, the ploughing of virgin grassland led to the loss of more than 200 kg/ha N
by leaching during the first winter, compared with less than 5 kg/ha lost from a sim-
ilar area remaining under grass. Such nitrate losses are more insidious and less easily
measured when they occur from naturally well drained soils overlying chalk or lime-
stone. Here, dissolved nitrates filter down gradually at a rate of a metre or so a year
until they reach the underground water reservoirs. By taking measurements at various
depths and times, it is possible to calculate when a particular band of nitrate left the
soil, and how long it will take to reach the aquifer. Conversely, by running the equa-
tion backwards, one can calculate when it left the surface. Much of the present nitrate
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