502 SSSAJ: Volume 74: Number 2 • March–April 2010 Soil Sci. Soc. Am. J. 74:502–511 Published online 4 Jan. 2010 doi:10.2136/sssaj2009.0199 Received 21 May 2009. *Corresponding author (chad.penn@okstate.edu). © Soil Science Society of America, 677 S. Segoe Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Isothermal Titration Calorimetry as an Indicator of Phosphorus Sorption Behavior Soil Chemistry P hosphorus retention on soils has a profound impact on agricultural produc- tion and environmental quality. Tis is due to the fact that the extent to which P retention occurs and its mechanism afects the mobility and plant availability of the nutrient. Phosphorus is ofen the most limiting nutrient in surface water ecosystems (Correll, 1998) and excessive P concentrations can promote eutrophi- cation. A major source of P in surface water is runof and subsurface drainage from soils containing “excessive” concentrations of available P (relative to plant needs; Sharpley et al., 1999). Tere are several analyses for estimating how much P a soil can retain (i.e., the P sorption capacity) and how much P is currently sorbed relative to the P sorption capacity (i.e., P saturation). Te P saturation of a soil provides an indication of the potential for P loss. For example, the P sorption maximum (S max ), as determined by the Langmuir equation, is ofen used in combination with some measure of sorbed P; sorbed P divided by S max is P saturation (Sharpley, 1995). Tis provides information on the relative amount of remaining P sorption sites and indicates the potential de- sorbability of sorbed P (Beauchemin and Simard, 1999). Similarly, ammonium oxa- late extractable Fe and Al (Fe ox and Al ox , respectively) are considered to be a measure of the main components of P sorbing sites in soils. Tus, the amount of P extracted by ammonium oxalate relative to the sum of Fe ox and Al ox is also used as an indicator of P saturation (Schoumans, 2000). Tis method is not applicable to high-pH soils because carbonates tend to neutralize the acid ammonium oxalate solution. In ad- dition, this method does not consider P sorption and precipitation by Ca minerals, which is dominant in high-pH soils (Lindsay, 1979). Although measurements of the Chad J. Penn* Hailin Zhang Dep. of Plant and Soil Sciences 367 Agricultural Hall Oklahoma State Univ. Stillwater, OK 74078-1020 Estimation of the amount, mechanisms, and strength of P retention on soils is important to agricultural production and surface water quality. Te 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. Te 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. Te 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. Abbreviations: DI, deionized; ITC, isothermal titration calorimetry; M3, Mehlich-3; P ox , Al ox , and Fe ox , oxalate-extractable phosphorus, aluminum, and iron, respectively. Published March, 2010