The solubilization of phosphates in the presence of organic acids

Author

William Lawrence Holden

Date of Award

5-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Michael E. Essington

Committee Members

Donald D. Howard, Dave L. Coffey

Abstract

Loess based soils of West Tennessee are insensitive to the Mehlich I soil P test. A more accurate assessment of plant available P may be obtained using organic acids produced in the rhizosphere. Two such organic acids, citric acid and 2-ketogluconic acid, have been shown to effectively solubilize soil P. The objectives of this study were to: 1) develop extraction procedures that utilize citric and 2-ketogluconic acid and that maximize P solubilization; 2) compare extraction capabilities of the organic acid extraction procedures to those of the Mehlich III and Olsen bicarbonate methods; 3) correlate organic acid extractable P to P solid phase speciation; 4) correlate corn (Zea mays L.) yield and ear leaf P concentration to organic acid extractable P, Mehlich III-P, Olsen-P, and to the inorganic P fractions of the soil solids; and 5) investigate the mechanism of soil P solubilization by 2-ketogluconic acid. Two extraction procedures were found to maximize P solubility from a loess based soil: 1) 10 g soil sample extracted with 25 ml of a 10 mM citric acid, pH 3.5 solution for 3 hrs (OEl), and 2) 5 g soil sample extracted with 25 ml of a 5 mM citric acid + 5 mM 2-ketogluconic acid, pH 3.5 solution for 6 hrs (0E2). Soil samples were obtained from a study that investigated the use of swine manure as a P fertilizer source (at Ames Plantation Experiment Station) and from a study that investigated the long-term influence of tillage practice and fertilizer P rates on com production (at Milan Experiment Station). Both Experiment Stations are located in West Tennessee. In the Ames soil, OEl was significantly correlated with Mehlich III-P (r = 0.84***), Olsen-P (r = 0.50*), and Al-P (r = 0.85***), but OE2 was only correlated with Mehlich III-P (r = 0.74***) and Al-P (r = 0.74***). In the Milan soil, OEl and OE2 were significantly correlated with Mehlich III-P, Olsen-P, Al-P, and Fe-P. None of the available P indices or inorganic P fractions were significantly correlated with plant P from the Ames or Milan studies. In the Milan soil, com yield was significantly correlated with OEl-P (r = 0.68**), OE2-P (r = 0.68**), Mehlich III-P (r = 0.70**), Olsen-P (0.68*), Al-P (0.67*), and Fe-P (r = 0.64*). All of the P indices were poorly correlated with yield in the Ames soil, perhaps due to the relative abundance of P in the organic P phase. Therefore, all four extractants inadequately assessed plant available P in the Ames and Milan soils. The ability of 2-ketogluconic acid to solubilize soil P may be attributed to the formation of soluble Al³⁺ and Fe³⁺ complexes and to competition with phosphates for adsorption sites. The former mechanism was investigated by examining the equilibrium solubility of gibbsite as a function of 2-ketogluconic acid concentration, pH, and ionic strength. The solubility results suggest the formation of [Al(C₆H₉O₇)²⁺], [Al(OH)(C₆H₉O₇)⁺], [Al(OH)₂(C₆H₉O₇)⁰], and [Al(OH)₂(C₆H₉O₇)^-], with log K values of 3.0, -1.1, -5.2, and - 10.6, respectively. The formation of these soluble complexes would promote the dissolution of aluminum phosphates (and possible iron phosphates), thereby releasing the soil P into the extracting solution.

Recommended Citation

Holden, William Lawrence, "The solubilization of phosphates in the presence of organic acids. " Master's Thesis, University of Tennessee, 1996.
https://trace.tennessee.edu/utk_gradthes/6814