Geoscience Reference
In-Depth Information
In addition to environmental problems caused by stubble burning and pesticides, agri-
culture is partly responsible for a third issue of environmental concern, that of nitrates
in water. From the soil point of view, it is simply wasteful to pour valuable nutrients
down the drain, and reprehensible if they cause undesirable effects subsequently. We
have mentioned already the ease with which nitrates may be leached down the soil
profile, beyond the reach of crop roots. Where there are field drains, water percolat-
ing through the soil will mostly be intercepted and flow out into ditches. This gives
a means of measuring losses by inserting devices such as a V-notch weir and flow
meter, and sampling the run-off at intervals.
Early measurements were made of leaching on the Broadbalk field at Rothams-
ted during 1878-81. Because of the planned lay-out on Broadbalk, it was possible
to compare losses of nitrate arising from different fertilizer treatments - differences
between manures and inorganic fertilizers, between ammonium-N and nitrate-N fer-
tilizers, and between spring and autumn applications. The results showed that the time
of year was the most important factor; nearly three times as much nitrogen was lost
when fertilizers were applied in autumn because the wheat could not take up the ni-
trogen before it was leached by winter rains. Farmyard manure was also applied in
the autumn but the nitrogen was released more slowly and so relatively little was lost.
Similar studies were carried out in County Antrim, N. Ireland, a century later.
Here, the experimental site was in an area of intensively managed grassland receiving
heavy doses of nitrogen - about 300 kg/ha in artificial fertilizers in three or four doses
annually, together with 60,000 litres of pig slurry containing a further 100 kg N. The
grass was grazed at intervals by cattle or sheep, and two or three cuts were taken for
silage each year. Nitrates lost by leaching were measured daily for 170 weeks, from
January 1981 to March 1984.
This intensive monitoring programme showed that nitrogen losses were correl-
ated with the flow of water through the drain; increased flow led to an increase in
N collected, with the greatest losses after heavy rains following long dry periods. In
1982, for instance, 35.5 kg/ha was lost by leaching over the whole year, but 21per cent
of this was flushed out by two days of heavy rain when 20mm fell in late September
after nearly a month of 'Indian summer' weather. Again, after a very dry summer in
1983, heavy rains in December resulted in large losses of nitrogen, reaching nearly 9
kg/ha in a week. On the other hand, much less of the applied nitrogen was leached out
in 1981 which had a wet summer.
These results were not just a local phenomenon. Exceptionally high nitrate peaks
were recorded in rivers draining arable catchments in England after the 1976 summer
which was the driest on record in much of Britain. It seems that a hot, dry summer
favours nitrification of organic reserves in the soil, and free nitrates then accumulate
