The effect of sheep on the recycling of phosphorus in hayed-off pastures

Abstract

The mineralization of plant organic phosphorus during digestion by sheep was examined by comparing the intake of inorganic and organic phosphorus from a range of feeds with that excreted in the dung. Up to 80% of the organic phosphorus in spring feed was mineralized but net mineralization did not occur in hayed-off summer feeds. Under field and laboratory conditions inorganic phosphorus leached slowly from intact dung pellets. Laboratory studies showed that in general a larger proportion was leached from summer samples than from either spring or autumn samples. Light crushing increased the rate of leaching in the initial stages. The unleached inorganic phosphorus was completely soluble in dilute acid and slightly soluble in water. The recovery of inorganic phosphorus indicated that mineralization of organic phosphorus had not occurred to any extent within the dung during 6 months' decomposition and leaching. Most of the organic phosphorus resisted leaching and net mineralization but a little was extracted in the first leachate. This stable organic phosphorus was soluble in sodium hydroxide but not in dilute acid. The effects of grazing on the availability of inorganic phosphorus and the accumulation of organic phosphorus in pastures are discussed in the light of these results.