Geoscience Reference
In-Depth Information
F
IG. 55
The effects of different nutrient combinations on yields of wheat grain from Broadbalk, Rothamsted.
Average yields between 1852-1925 (black columns), 1935-62 (cross-hatched), 1970-78 (hatched). N2 =
nitrogen rate, P = phosphorus, K = potassium, Na = sodium, C = P + K + Na + magnesium, FYM = farm-
yard manure. (Data from
Rothamsted Annual Reports
for 1968 and 1982, plots 3, 5, 10, 11, 12, 13, 7, 22.)
In practice, it is possible to build up the reserves of P and K in the soil with fertil-
izers because they are adsorbed by soil particles and then gradually released to crops
with little loss. Inorganic N, on the other hand, is like quicksilver. If it is not captured
by plant roots soon after it is applied or mineralized from organic matter, it is liable
to slip away down the soil profile and into the ground water or out through the field
drains (see
next chapter
).
Although the basic concepts of crop nutrition have been known for a hundred
years, farmers themselves regarded artificial fertilizers with suspicion until the second
world war. The County War Agricultural Executive Committees met with consider-
able opposition in their attempts to persuade farmers to use more fertilizers. It was
only experience - and observation of their neighbours - that convinced them that fer-
