AGRICULTURAL SOILS: PRODUCTIVITY AND MANAGEMENT - The Soil - page 174

Grassland, too, has become a major recipient of N. No longer is the contribution
from clover enough to meet the demands. Most lowland pastures, indeed, contain very
little clover today. Nearly a third of recently re-sown pastures now receives at least
250 kg/ha of N to maintain production, with corresponding high inputs of P and K.
Scientists at the Grassland Research Institute have determined the optimum, value-
for-money, yield of grass at about 11 t/ha, for an input of 386 kg/ha of N. Another
tonne could be grown but only by greatly increasing the amount of N ( Fig. 57 ) . The
real secret lies in the timing and the doses. By giving applications at 3-4 week in-
tervals between March and September, the maximum amount of N (50-60%) can be
taken up from the soil; and by giving the highest dose in midseason, a more even pat-
tern of production can be achieved without reducing total output.
F IG. 57
General form of the response of grass to fertilizer N showing the key points affecting upper limits of the
response. The optimum yield, Y (optimum), is defined as the point where the response to N falls to less
than 10 kg of dry matter per kg of N applied. The mean values given are based on 21 sites in 20 counties.
(Redrawn from J. Morrison and others 1980.)
In the 1984 Macaulay Lecture, Sir Leslie Fowden, former director of Rothams-
ted, described the current approach to the use of nitrogen fertilizers. This included the
development of a computer-based model which would simulate the movement of N
in the soil. Given a knowledge of soil temperatures and rainfall, the computer could