Surface Reactions of Phosphorus Extracted by the Modified Truog Method to Predict Soil Intrinsic Pools

Abstract

Core Ideas Constant Capacitance Model was applied to soil P extraction reactions by the modified Truog test. Sulfate in Truog extractant was adsorbed to form a negative electrostatic potential at soil particle surface. Sulfate replaced phosphate by ligand exchange mechanism at soil particle surface. The surface electrostatic potential reduced the reaction rates of P extraction. Weakly and tightly adsorbed phosphorus (P) pools were proposed as a novel approach to quantify the actual amount of P adsorbed at the particle surface of tropical soils, and the extraction efficiencies (percentages) of the two pools by the modified Truog method were important for the measurements of the pools. The objectives of this study were to: (i) study the surface reactions in the method and (ii) predict the extraction efficiencies of the pools from soil properties. The first surface reaction in the method was the instant adsorption of sulfate in the extractant to form a monodentate inner‐sphere complex. The second and third surface reactions in the method were the ligand exchanges between sulfate and phosphate, and the new mechanisms proposed in this study included a step of slow formation of intermediate complexes in which both sulfate and phosphate were bound to metal cations and a subsequent step of instantly releasing phosphate from the intermediate complexes. The negative potential formed in the first reaction resulted in difficulty for sulfate to access the surface layer in the ligand‐exchange reactions. The double exponential models were derived from constant capacitance model (CCM) and kinetics of surface reactions. Results showed the models fit the experimental measurements well. The extraction efficiencies of the pools were determined by surface site density of clay. The consistency of the estimated capacitance density 2.421 F m−2 and literature indicated that CCM was valid to describe the surface reactions in the method and applicable to soil P management.