Phosphorus nutrition of wheat in acid latosols and volcanic soils
1988
McMahon, M.A.
Volcanic and latosolic acid soils present special problems regarding phosphorus use. Volcanic soils are characterized by their capability to react rapidly with large amounts of phosphorus. Consequently, the availability to plants of soluble phosphates applied as fertilizer is quickly depressed, and only about 10 percent of the applied phosphorus is utilized by most upland crops. Phosphorus fixed on the soil colloids is not totally unavailable to plants. In fact, it is thought that it slowly becomes available and that the amount of available P builds up in these volcanic soils with repeated P application. In the case of wheat, application of phosphate fertilizers in the form of granules in the row is better than broadcasting because the fertilizer is then avaliable in the early stages of growth. Latosolic acid soils are characterized by two main problems from the point of view of crop growth: low pH with its accompanying high exchangeable aluminum saturation and inherently low soil phosphorus levels. Clay content of the soil affects phosphorus availability. As clay content increases, more P needs to be applied to maintain the levels of P available for plant uptake. A good strategy for these soils may be to apply a large quantity of phosphorus to begin with and to maintain the soil fertility level with lower annual applications. Single superphosphate (SSP) is a very attractive fertilizer for these soils because it contains 60 percent by weight of gypsum (CaSO4.2H2O) and is therefore a useful source of both Ca and S. SSP has also been very useful in moving calcium down the profile. Small amounts of calcium moving down the profile should increase effective rooting depth. Because of the high phosphorus needs for this soil, a calibration method for the response of wheat to phosphorus is essential. Brazilian workers are using the Mehlich method and getting a good correlation between soil P levels and yield
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