Isothermal Titration Calorimetry as an Indicator of Phosphorus Sorption Behavior

Estimation of the amount, mechanisms, and strength of P retention on soils is important to agricultural production and surface water quality. The purpose of this study was to determine if isothermal titration calorimetry (ITC) could provide any information on the P retention behavior of soils. Twenty‐eight benchmark Oklahoma soils were titrated with 0.01 mol L −1 NaH 2 PO 4 , analyzed using ITC, and the results compared with P sorption–desorption isotherms and other soil characterization data. The ITC provided an indicator of the P retention potential, and characteristics of the heat patterns and thermograms provided limited insight into a P retention mechanism. Strongly endothermic patterns were indicative of Al and Fe phosphate precipitation, while heat patterns dominated by exotherms indicated Ca phosphate precipitation, carbonate dissolution, acid–base neutralization, and ligand exchange of P onto Al and Fe minerals. Carbonate dissolution among high‐pH and Ca‐rich soils could potentially mask endotherms or overlap with exotherms produced from other P retention reactions. The use of ITC heat patterns and thermograms from P additions to soils should be restricted to qualitative interpretation with regard to discerning a P retention mechanism.