Balance sheet of phosphorus, sulphur and potassium in a long-term grazed pasture supplied with superphosphate
1992
Williams, P.H. | Haynes, R.J.
A balance sheet of P, S and K was constructed for a long-term trial which investigates the effects of three rates of superphosphate (9% P, 11% S) on pasture production on border-strip irrigated land grazed with sheep. A balance sheet of the inputs and outputs of P, S and K to the trial over a 38 year period showed that of the nutrients applied in fertiliser, only 51-5997 of the P and 15-31% of the S were retained in the soil. Small amounts were lost in animal products (4-19% of the applied nutrients) but major losses were attributed to runoff of P as particulate matter (dung and soil particles) during irrigation and leaching of sulphate-S during irrigation. Losses of K from the site were small and had no effect on total soil K content. The distribution of soil nutrients across the border-strips was also investigated. The results showed that the concentrations of total soil P and S and exchangeable K were significantly greater at the sides of the irrigation borders than in the main strip area of pasture. This was caused by deposition of a disproportionate amount of dung and urine (and therefore nutrients) on the levees where the sheep tended to camp. It was calculated that with increasing superphosphate rates greater amounts of P were transferred to the levees due to the increased amounts of P been, recycled via the animals (as a result of increased herbage P concentration, pasture production and stocking rate).
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