Sorption of labelled phosphate by a Vertisol and an Alfisol of the semi-arid zone of India

Information on phosphate sorption properties of Vertisols is scarce, but can help to explain the different responses of crops to fertilizer P on Vertisols, as compared with Alfisols.Adsorption isotherms for total adsorbed phosphate and isotopically exchangeable phosphate were measured for typical examples of a Vertisol and an Alfisol, occurring in close proximity at the ICRISAT centre. For each soil, the relationships of exchangeable P and total adsorbed P with phosphate solution concentration were described well by the Freundlich isotherm. Neither of the soils adsorbed significant amounts of P in a non-exchangeable form. The Vertisol had a higher capacity and buffer power for phosphate sorption, implying a lower response to fertilizer P. However, all adsorbed P remained in forms labile to³²P, equilibrated for 22 h, so that for equal amounts of CaCl₂ extractable P there was more labile P in the Vertisol. In the absence of added P, the data suggested that the Vertisol maintained a greater level of dissolved and labile P. These observations are in accord with the results of field experiments, where larger applications of P may be required in Vertisols, compared with Alfisols, to achieve the same yield response, but that P is more freely available to crops grown in Vertisols than is suggested by chemical extraction methods for available P.