Residual effect evaluation of rock phosphates in Cerrado soils

Abstract

The agronomic effectiveness of phosphate sources were evaluated under field conditions during six years (1975-1981). After this period, soil samples from plots which initially received 44 mg P/kg of soil in the form of soluble phosphates and rock phosphates from Patos de Minas, Araxá and Catalão, were colected in order to evaluate comparative residual effects. To meet this objective, the following procedures were applied: total soil analysis (calcium and phosphorus), P fractionation, resin successive extractions and P isotherms. From the first two procedures, it was possible to estimate that about 20% of the phosphorus broadcasted via rock phosphates, remained as apatite six years after incorporation. Considering the sum of phosphorus absorbed by the plants and extracted by ten consecutive treatments with resins, 43 to 63% of the phosphorus applied was recovered. In relation to the soluble phosphate, the rock phosphates exibited the following indexes of effectiveness: 84 to 89% for Patos, 82% for Araxá and 69% for Catalão. The results, therefore, indicated that the residual effect, after the fifth crop, should be similar considering all sources of phosphorus.