PREDICTING THE DISSOLUTION OF FOUR ROCK PHOSPHATES IN FLOODED ACID SULFATE SOILS OF THAILAND S. Yampracha, T. Attanandana, A. Sidibe ´ -Diarra, A. Srivihok, and R. S. Yost The dissolution of rock phosphate (RP) in soils, a reaction funda- mental to the effectiveness of direct application of RP, depends on both the RP and soil properties. Four RP materials of varying solubility, namely, Gafsa RP, Tilemsi RP, Taiba RP, and a local RP, Kanchanaburi RP, were incubated with six flooded acid sulfate soils to quantify factors affecting the dissolution of RP. The differences between dilute NaOH-P extracted from soils treated with and without RP materials (%NaOH-P) were used to estimate the dissolution of RP materials. A nonlinear Mitscherlich equation was used to describe the changes in %NaOH-P over incubation time. The coefficients of the fitted Mitscherlich equation were then regressed on soil and RP properties to develop an RP model. Soil acidity measured by pH and KCl-extractable aluminum (Al) were the soil properties that most affected RP dissolution and thus were included in the RP model. RP solubility as assessed using the second extraction with 2% citric acid as a percentage of rock and the particle size of RP (G 100 mesh) were the RP properties found most important in predicting dissolution and were include in the model. The calcium carbonate content of the RP affected the changes of pH in six acid sulfate soils after mixing with RP materials. (Soil Science 2006;171:200–209) Key words: Acid sulfate soils, flooded, rock phosphate, dissolution. P HOSPHORUS (P) deficiency in flooded acid sulfate soils of Thailand is one of the limiting factors causing decreases in rice growth and yield (Attanandana and Vacharotayan, 1984). In order for rock phosphate (RP) to become available for crops, there are at least two reactions that are critical: dissolution of RP and sorption of the dissolved RP. In this paper, we concentrate on the dissolution of RP. RP has been recommen- ded for rice cultivation in acid sulfate soils of Thailand (Department of Agriculture). 1 The ex- treme acidity of acid sulfate soils, in which soil pH can be less than 3 to 4, is a logical choice because acidity is key to the dissolution of RP. In addition to soil and RP characteristics, dis- solution of RP depends on many factors such as crop species, crop management, and climate conditions (Sidibe ´-Diarra et al., 2004). Soil characteristics such as acidity, clay content, or- ganic matter, aluminum (Al), iron (Fe), calcium (Ca), and P status were reported as limiting factors to the dissolution of RP (Bolan and Hedley, 1990; Chu et al., 1962; Hammond et al., 1986; Kanabo and Gilkes, 1987). High soil Ca status usually reduces RP dissolution and is frequently measured as Ca saturation, ex- changeable Ca, and Ca exchangeable capacity (Babare et al., 1997; Hammond et al., 1986; MacKay et al., 1986; Wright et al., 1992). Other soil properties that control the dissolution of RP include soil pH and P sorption capacity. Chu et al. (1962) suggested that the dissolution of RP decreased with increasing soil pH but that the Fe-phosphate formed varied both with soil pH and the low free Fe content of the soil. RP dissolved more completely in the soils with low pH and free Fe oxide content. Chien et al. (1980) found that the dissolution of North Carolina RP was related to reactive Al, which 200 0038-075X/06/17103-200–209 March 2006 Soil Science Vol. 171, No. 3 Copyright * 2006 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A. University of Hawaii, Honolulu, HI 96822. Dr. R. S. Yost is corresponding author. E-mail: rsyost)hawaii.edu 1 Department of Agriculture. 2004. Soil and fertilizer management (in Thai). Available at www.doa.go.th (verified May 11, 2004). Received July 27, 2005; accepted Oct. 10, 2005. DOI: 10.1097/01.ss.0000199699.01828.21 Copyr ight © Lippincott Williams & Wilkins. Unauthor iz ed reproduction of this article is prohibited.