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SOIL CHEMISTRY

Phosphate Mobilization by Citric, Tartaric, and Oxalic Acids in a Clay Loam Ultisol

^a Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008 China
^b Currently at Nanjing Univ. of Information Sci. and Technol., China
^c Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078

^* Corresponding author (yqhe{at}issas.ac.cn).

Phosphate mobilization by organic acids has been shown to be soil type dependent and controlled by the soil's intrinsic P status. The objective of this study was to evaluate the capacity and mechanisms of three organic acids in the mobilization of phosphate from an upland clay loam Ultisol that had received different long-term fertilizer treatments. Soil samples were incubated at 25 ± 1°C and 40% moisture content for 3 wk with citric, tartaric, or oxalic acid at 1.0 mmol kg^–1 of soil. Soil pH, inorganic P fractions, and plant-available P (Olsen P) were analyzed after incubation. The results indicated that Olsen P and the loosely bound P extracted with 1.0 mol L^–1 NH₄Cl (NH₄Cl-P) were significantly increased by the treatment of the three organic acids, and Fe phosphate (Fe-P), occluded phosphate (Oc-P), and Ca phosphate (Ca-P) were mobilized and released to various degrees as well. The order of increased mobilization of P by the organic acids was citric acid > tartaric acid > oxalic acid. These three organic acids have the potential to increase the availability of P in soil but need to be evaluated at the field scale.

Abbreviations: Al-P, aluminum phosphate • Ca-P, calcium phosphate • Fe-P, iron phosphate • NH₄Cl-P, loosely bound phosphorus • Oc-P, occluded phosphate