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Phosphorus Desorption Quantity-Intensity Relationships in Soils

Dep. of Soil and Crop Sciences, Texas A&M Univ., and Texas Agric. Exp. Stn., College Station, TX 77843

*Corresponding author.

ABSTRACT

Phosphorus desorption quantity-intensity (Q/I) relationships reveal information about the P release properties of a soil. 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 to describe P desorption Q/I curves. Samples of five soils (Typic Calciustoll, Udic Pellustert, Vertic Albaqualf, and Aridic Calciustoll surface 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 different amounts of HCO^–₃-saturated anion-exchange resin. After 72 h the amount of P sorbed on the resin (Q) as well as the solution P concentration (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 study was relatively low. In untreated samples, the solution P concentration when no P was desorbed (I_o), an intensity parameter, ranged between <0.01 and 0.19 mg P L^–1, while the maximum desorbed soil P (Q_max), a quantity parameter, was in the range 4.7 to 21.1 mg P kg^–1. The corresponding P buffering power index, the slope of the Q/I curve at 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 directly related to the ability of a soil to release P. The P release activity of a P-enriched soil depended on the desorbability of the applied P as well as the native labile soil P. Plots of P buffering power against intensity for a given soil were not affected by P enrichment. The Q/I curves could be best described by a proposed desorption Q/I equation (Q = a I^–0.1 + b ln(I + 1) + c) and the Barrow power function, but the proposed equation followed the theoretical expected trend. The proposed equation is therefore recommended to describe P desorption Q/I curves.

Received for publication October 20, 1992.

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