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Organic Anions' Effect on Phosphorus Release from Spodic Horizons

Dep. of Chemistry, Univ. of Florida, Gainesville, FL 32611

N. B. Comerford^*

Soil and Water Science Dep., McCarty Hall 2169, Univ. of Florida, Gainesville, FL 32611

T. R. Fox

Rayonier, P.O. Box 819, Yulee, FL 32097

*Corresponding author (nbc{at}gnv.ifas.ufl.edu).

ABSTRACT

The purpose of this research was to investigate the release of P from the spodic horizons of common forested soils in the Lower Coastal Plain of Florida and Georgia. This was considered important because (i) these soils have a history of variable responses to P fertilization, (ii) roots are known to grow in this material, and (iii) previous research has suggested that the P in this material might be available to plants. Desorption experiments using 0.001 M concentrations of a variety of organic ligands (pH 4.3 for initial solutions) with a range of Al complexation constants were conducted by reacting the ligand solutions with spodic horizon soil. Inorganic P, organic P, and Al were measured in the resulting extracts. Results showed that the amount of extractable inorganic P in these soils was variable. Some soils had detectable levels of extractable inorganic P, while others are essentially devoid of it. In those soils that had extractable P, the inorganic P release was correlated to the release of Al as the release of both moieties was initiated by organic anions with logK_Al values, the Al stability constant with that anion, of about 3.2 to 4.2 or greater. This suggests that root exudates with logK_AlS less than these values would not be effective in making inorganic P in spodic horizons more available to plants. Organic P is a major component of the soil solution and is released by a mechanism unrelated to the logK_Al of the organic ligand.

A contribution of the Florida Agric. Exp. Stn. Journal Series no. R-04325.

Received for publication May 23, 1994.

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