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Adsorption and desorption of phosphorus in longterm phos phate-fertilized acid soils

This paper reports the adsorption and desorption of phosphorus by and from 28 topsoil acid soils from potato and wheatgrowing land that in the past had been fertilized with unknown quantities of phosphate. The experimental data for adsorption from phosphate solutions with concentrations of 2.5-100.0 mg L -1 were fitted by both the Freundlich and Langmuir equations, although usually somewhat better by the former. Multiple linear regression analysis showed the maximum adsorption predicted by the Langmuir equation, X max , to be itself predicted well by organic carbon, oxalate-extractable aluminium and preexisting available phosphate contents (R 2 = 0.929), increasing with organic carbon and oxalate-extractable aluminium contents and decreasing with preexisting available phosphate content. The Freundlich constant K F was predicted by oxalate-extractable and exchangeable aluminium and exchangeable calcium, and the Freundlich exponent n by pyrophosphate-extractable aluminium and exchangeable potassium. Desorption of P, expressed as a percentage of P adsorbed in adsorption experiments, increased with preexperimentation KCl-extractable P content (which was especially afected for desorption following experimental exposure to low concentrations of P), and fell with increasing values of K d , the apparent affinity of the soil for P in adsorption experiments.