Significance of man-made sources of phosphorus: Fertilizers and farming. The phosphorus involved in agricultural systems and possibilities of its movement into natural water

Abstract

In 1970/1971 0·22 × 106 tonnes of P were used as fertilizer in Britain; the crops grown contained about as much. Phosphate fertilizers react with soil to form very insoluble products and P is not lost by leaching from soils with > 5 per cent clay. Water from land drains averages about 0.05 mg P l−1 or less from clay soils and 0·1–0·2 mg l−1 from sands. This is dissolved from phosphates naturally present in subsoils and rocks. Land drainage usually contains less P than the streams into which the drains flow. Long-term experiments show that phosphate fertilizers move very slowly down the profile.Steeply-sloping uncultivated grassland may lose by erosion 1 mm of topsoil a year (containing 10 kg P ha−1). Much soil (and P) may be lost from cultivated sandy soils by both water and wind erosion, but little from level grassland or well-managed cultivated clays. Fertilizers contribute to these losses as they are “fixed” in the topsoil which is lost first. Some estimates are that 6–12 kg P ha−1 is lost each year in U.S.A. Dispersed clay appearing in some drainage water also contains P. Eroded soil deposited in water is often rich in total P, the phosphate in muds increases in solubility in anaerobic reducing conditions and will be used by rooted water plants, probably by micro-organisms too.The excreta of British livestock contains 0·15 × 106 tonnes of P annually; some may enter watercourses by surface run-off in wet weather, by careless handling of manure, or by percolation from excessive dressings. Half of the total P in excreta returns to grazed grassland; of the remainder (from housed stock) much is still returned as farmyard manure. Most pigs and poultry are housed all year and an increasing number of cattle are handled this way. Excreta from these stock are handled as slurries, which are bulky and costly to store and spread, and difficult to return uniformly at appropriate rates. Dense stocking increases risks that liquid manures and other farm effluents may contaminate watercourses in wet or frosty weather. Wild birds cause eutrophication of lakes.Agricultural and urban sources of phosphorus are compared. At present it seems that phosphate from agricultural systems reaches watercourses in eroded soil, the total amounts entering in drainage water are small, estimated in Netherlands as 0.25 kg P ha−1. The amounts in sewage from urban areas are very much larger. Little phosphate enters water from animal farming systems at present, but more may do so if many stock are kept on small areas and, even more, if their excreta is treated by sewage processes to produce a clear effluent. Phosphate fertilizers do not pollute water directly; they help to grow larger crops so that more stock are kept. Pollution will be avoided and less fertilizer will be needed where erosion is prevented and the phosphate in all wastes from the new farming systems is conserved. Farmers may need extra payment for produce so that they can avoid pollution in disposing of their wastes.