Organic phosphorus

When superphosphate is applied to a subtropical soil, 40% may appear as organic P within 28 days of application (Dalal, 1977). It is also an accepted fact that from 30% to 85% of the total P of most soils is in organic combination, and that the phosphorus in manure is as available to plants as that in superphosphate (Sauchelli, 1965, pp. 79 and 195). The amount of P on a fresh-weight basis in cattle farmyard manure is approximately 1.54 kg t-1 (3.5 kg t-1 of P2O5) rising to 11.0 kg t-1 (25 kg t-1 P2O5) in broiler/turkey litter (Chambers etal., 2001). When applied to the soil, soluble forms of phosphate are largely rendered insoluble by fixation to minerals, or immobilized by incorporation into microorganisms. Fixation may be by precipitation to form relatively insoluble forms of iron and aluminium phosphates or fluorapatite, or by chelation on to clay sesquioxides surfaces. Only a small amount remains in the soil solution, but the ability of the soil to maintain this plant-available P at an equilibrium level is the important factor. Although the microorganisms are initially more successful than plants at competing for soluble P, they serve to prevent the leaching of P, and when they die, mineralization of microbial P releases soluble P over a period of time to the benefit of the plants. There is therefore a delicate balance between immobilization and mineralization. The mineralization of acid soils is usually enhanced following liming, but not always. Some of the variation in the effect of lime may be produced by the Ca:Mg ratio effect on the mineralization and turnover of P in the soil. In alkaline soils, the ratios of organic C:organic P and total N:organic P should be greater than in acidic soils (Dalal, 1977).

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