Changes in forms of soil inorganic phosphorus in Lismore stony silt loam due to application of superphosphate to pasture

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Abstract
Changes in available phosphorus (as extracted by the Truog and Olsen methods) and in phosphorus held in the soil in aluminium-, iron-, and calcium-bound forms have been followed on soil samples taken from a rates of superphosphate trial on irrigated pasture from 1952 to 1964. Truog phosphorus levels in samples taken at three-monthly intervals showed a cyclic pattern due to the annual fertiliser applications, the fluctuations being greater at higher rates of superphosphate. In samples taken just before top-dressing each autumn, 3 cwt and 4 1/2 cwt of superphosphate applied annually caused a new increase in levels of aluminium-, iron-, and calcium-bound phosphorus, the aluminium-bound form increasing more than the other two forms. However, this form was also depleted at a greater rate than iron- or calcium-bound phosphorus during the residual phase of the trial. On these samples, the Truog and Olsen methods gave a pattern of phosphorus accumulation and depletion closely correlated with the aluminium-bound form, which is considered to be the most important source of phosphorus for plant uptake on this soil type.

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