PHOSPHORUS DESORPTION QUANTITY-INTENSITY RELATIONSHIPS IN SOILS

Phosphorus desorption quantity-intensity (Q/I) relationships reveal in formation about the P release properties of a sell. This research was carried out to evaluate the P desorption characteristics of a group of soils of diverse properties and to compare several empirical models t o describe P desorption Q/I curves. Samples of five soils (Typic Calci ustoll, Udic Pellustert, Vertic AIbaqualf, and Aridic Calciustoll surf ace soils, and Psammentic Paleudult subsoil) were enriched with five P concentrations (0, 25, 50, 75, and 100 mg P kg(-1)) and incubated for 31 d. Subsamples of each treated soil sample were equilibrated with d ifferent amounts of HCO3--saturated anion-exchange resin. After 72 h t he amount of P sorbed on the resin (Q) as well as the solution P conce ntration (I) were determined. Results from each subsample gave a point on the Q/I curve. The P release capacity of the soils used in this st udy was relatively few. In untreated samples, the solution P concentra tion when no P was desorbed (I-o), an intensity parameter, ranged betw een <0.01 and 0.19 mg P L(-1), while the maximum desorbed soil P (Q(ma x)), a quantity parameter, was in the range 4.7 to 21.1 mg P kg(-1). T he corresponding P buffering power index, the slope of the Q/I curve a t I-o, ranged between 56 and 1703 L kg(-1). These Q/I parameters were not closely related to any evaluated soil property, with the possible exceptions of CaCO3 content in calcareous soils and clay content. The P buffering power of a soil apparently depended on the availability of P sorption sites and their degree of depletion, and was not always di rectly related to the ability of a soil to release P. The P release ac tivity of a P enriched soil depended on the desorbability of the appli ed P as well as the native labile soil P. Plots of P buffering power a gainst intensity for a given soil were not affected by P enrichment. T he Q/I curves could be best described by a proposed desorption Q/I equ ation (Q = a I-(0.1) + b In(I + 1) + c) and the Barrow power function, but the proposed equation followed the theoretical expected trend. Th e proposed equation is therefore recommended to describe P desorption ell curves.