Geoscience Reference
In-Depth Information
F
IG. 56
Examples of 'interaction' effects of fertilizers on yields. Each figure shows the increase in yield with one
or other or both nutrients, the dotted line indicating the expected yield with both nutrients if their effects
were simply additive. The first pair of figures shows little interaction with low levels of nitrogen and po-
tassium but large interaction with doubled levels. The third figure shows no effect on yield of potatoes by
adding nitrogen on its own but a large effect when combined with P + K + Na + Mg. (Sugar-beet data
from G. W. Cooke 1982; potato data from Rothamsted Annual Report for 1982.)
After a rapid increase in use, the amounts of P and K applied to soils have stabil-
ized since 1956. However, the use of N in the UK climbed steeply and continuously
from about 61 thousand tonnes in 1939 to 1450 thousand tonnes in 1983. Rates of use
continued to increase until 1986 but then levelled off and have even shown a small
decline in 1988. These overall figures, of course, disguise big differences between
crops, and trends within them, as acreages (or hectareages) grown, market prices and
subsidies have changed. From a very small base in 1970, oil-seed rape, for instance,
has increased enormously in extent and now receives more nitrogen per hectare than
any other crop - 254 kg/ha in 1980 on 93,000 ha. As shown throughout this chapter,
present-day yields are the sum of many independent developments. Potential yields,
though, cannot be realized without large inputs of fertilizers or manures, as
Figure
55
makes clear. It is possible, therefore, to calculate how much production would be
foregone without them. In the case of cereal grain, this amounts to 9.3-10.5 million
tonnes per year during 1982-3. At annual costs of fertilizers of £180-300 million, this
gives net benefits to UK farming of £536-685 million. Some of the hidden costs are
discussed in the
next chapter
.
