LONG-TERM EFFECT OF SUPERPHOSPHATE ON ACCUMULATION OF SOIL-PHOSPHORUS AND EXCHANGEABLE CATIONS ON A GRAZED, IRRIGATED PASTURE SITE

The annual herbage dry matter yield, herbage P concentrations and quantities of P. K, Mg and Ca cycled by grazing sheep were calculated for a 37 year-old grazed pasture supplied with superphosphate at rates of 0. 188 and 376 kg ha-1 annually. The amount and distribution of inorganic and organic P and exchangeable K, Mg and Ca in the soil below thc three grazed treatments was also measured and compared with that below a wilderness area which had not been used for agriculture. Increasing rates of superphosphate increased herbage dry matter yield, herbage P concentrations and thus the amounts of P ingested and, in turn, excreted hy thc grazing sheep. Annual quantities of K, Mg and Ca cycled back to the pasture in the form of excreta were also increased. The increased cycling of K by animals induced by increasing superphosphate applications resulted in greater losses of K and consequently concentrations of exchangeable and non-exchangeable 'fixed' K in soil decreased in the order wilderness>control>188>376. Differences were evident to a depth of 20 cm. Some losses of Mg during cycling also occurred and concentrations of exchangeable Mg followed thc order: wilderness>control>188=376. Concentrations of exchangeable Ca increased with pasture development due to additions of Ca in lime and superphosphate. Concentrations of total soil organic P (0-4 cm) increased in the order:wilderness<control<188<376 but for inorganic P and total P the control had a lower content than wilderness indicating losses of P during cycling in thc grazed control treatment. Phosphorus fractionation suggested that with increasing superphosphate rates inorganic P primarily accumulated in a form adsorbed to Al hydrous oxides and as calcium phosphate compounds whilst organic P accumulated in both labile forms and forms associated with humic compounds. In the fertilised sites both inorganic and organic P accumulated in the soil profile to a depth of 20 cm.