Phosphate sorption and extractable iron in soils during irrigated rice-upland crop rotations

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Abstract
Two field experiments were conducted, in which phosphate sorptivity, acetate-(pH 4.8) and oxalate-(pH 3.0) extractable iron levels were monitored during irrigated rice-wheat-wheat and rice-fallow maize rotations in two clay soil types. Flooding for the rice crop markedly increased levels of phosphate sorptivity and extractable iron. On drainage of the rice crops, levels decreased rapidly but did not immediately return to levels occurring in the soils before flooding. Thereafter, levels of phosphate sorptivity and acetate-extractable iron slowly decreased during the following 21 months. Levels of oxalate-extractable iron in drained rice soils showed no consistent trends. The changes in phosphate sorptivity are described in terms of possible changes in the form of the iron hydrous oxides in the soil. The results indicated that the effectiveness of phosphate fertilizer to crops after rice should increase with time after drainage of the rice crop. More efficient use of phosphate fertilizer may be achieved by delaying the sowing of the following crop for as long as possible. There appeared to be little prospect for increasing the rate of decrease in phosphate sorptivity by management practices.