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Phosphate Fractions in Some Texas Grumusols and Their Relation to Soil Weathering and Available Phosphorus¹

ABSTRACT

The phosphate fractions were determined in nine profiles of Grumusols from Texas, representing four soil series. Soil weathering and available P were shown to be related to certain of these fractions.

The distribution percentages for Ca-P, Al-P, and Fe-P serve as sensitive indicators of the weathering environment. These show Beaumont clay to be the most weathered and Victoria and Houston Black clay to be the least and about equally weathered. Lake Charles clay occupies an intermediate position, through none of the four have undergone much weathering.

Free salts, particularly soluble sulfate salts, were found to affect the distribution percentages of Ca-P and Al-P in the Victoria profiles.

Available P was correlated with Al-P in all but the Victoria profiles where Ca-P was correlated with available P. Indications are that the correlations between Al-P and available P are fortuitous and more realistically reflect small amounts of mineralized organic phosphates extracted by the initial extraction for Al-P.

The C/P ratios of the organic matter in the A horizons of the Grumusols range from 27 to 295. The higher ratios are associated with the Beaumont and Lake Charles soils, whereas the majority of the values for the A horizons of Victoria and Houston Black soils are below 75. The lack of response to applied P on these soils is attributed to the large amounts of organic P and accompanying low C/P ratios. Decomposition of organic matter in these soils results in mineralization of significant amounts of organic phosphates which are then available for use by the plant.

¹ Contribution of the Texas Agr. Exp. Sta. Taken from the Ph.D. dissertation of the senior author.

² Formerly Research Instructor and now employed by E. I. Du Pont, Savannah River Plant, Aiken, South Carolina, and Professor of Soil Mineralogy, respectively.

Received for publication March 18, 1965. Accepted for publication April 2, 1965.