The nation's plant food larder

Abstract

The crops and grass grown in U.K. in any one year need about one million tons each of N and K,O and about one-third as much P,O,. Sales of cash crops, milk and stock, and the losses involved in feeding crops and grass on farms may result in about one-third of these quantities of plant foods being lost to agriculture. Fertilisers and imported feeding stuffs provide sufficient plant foods to balance approximately the total losses of nitrogen and potassium and they supply much more phosphorus than is lost. These sources of plant food currently provide only half of the nitrogen and less than half of the potassium that is needed for a year's cropping. In contrast, fertilisers alone supply more phosphorus in any one year than is taken up by the crops and grass grown.About 7 m. tons of N, 17 m. tons of P,O,. and 5 m. tons of K,O, have been applied to the soils of U.K. through the use of fertilisers in the last 1 2 0 years. In the 18 years from 1940 to 1957, fertilisers supplied as much nitrogen, twice as much potassium, and half as much phosphorus, as was applied in the previous 100 years. The surface layers of the cultivated soils of U.K. contain plant foods equivalent to about zoo years' supply of nitrogen, IOO years' supply of phosphorus, and 1000 years' supply of potassium a t current rates of fertiliser use. The amounts of phosphorus applied to U.K. soils have exceeded losses of this nutrient for at least 80 years, but only in recent years have the potassium fertilisers used been sufficient t o avoid depleting soil-potassium reserves. The size and nature of the reserves of soil phosphorus which have accumulated in British soils as a result of manuring tend t o be more important for the nutrition of arable crops than the supply of ' native ' soil phosphorus. In contrast, the reserves of potassium built up by residues from purchased fertilisers and feeding stuffs are relatively small and the capacity of soil minerals to supply potassium to crops is very important.Most arable crops grown for sale are now receiving about as much fertiliser as can be justified, but more nitrogen is needed to grow full crops of cereals and practically all crops grown for animal feeding are, on average, undermanured. Any major increase in the total amount of fertiliser used is likely to be achieved by better manuring of grassland.The history of the use of fertilisers in U.K. is discussed.Future prospects for increases in the use of fertiliser are examined.

Managing the mineral cycle

“…This cycle (Table 9) is best known and welldocumented (Cooke, 1958; Centre for Agricultural Strategy, 1978). When soils are first improved for agriculture the first need is usually for phosphate (and lime on acid soils).…”

Waste of fertilizers

“…The total P supplied annually is more than our crops contain. This situation has occurred in each year since the late 1940s; furthermore, for at least a century the P supplied has been greater than is lost in sales of produce from farms (Cooke 1958). The surplus P has accumulated in our soils, particularly in the arable areas with a long history of using fertilizers.…”

Nutrient cycles involving phosphorus and potassium on livestock farms in Northern Ireland

“…Additions of P and K to soils have exceeded removals in crops not only in Northern Ireland but also in Great Britain (Cooke, 1958) and the United States (Engelstad & Terman, 1966) and substantial fertilizer residues have accumulated in soils. This process is desirable because serious nutrient deficiencies in soils are eliminated, but it cannot continue unchecked because excessive P poses a threat to the environment (Wood & Gibson, 1973) and excessive K to stock health.…”