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Self-Diffusion of Phosphorus in Clays and Soils: I. The Effect of Phosphorus Rate¹

ABSTRACT

Self-diffusion coefficients of ³²P were measured in kaolinite clay, montmorillonite clay, illite clay, Dundee silt loam soil, and Sharkey clay soil. Phosphorus in the form of concentrated superphosphate was added to each clay or soil at the rates of 10, 20, 40, 80, 160, and 320 ppm. The clays and soils were maintained near a neutral pH and at a water content in equilibrium with 173 mbar tension. The water-soluble, Ca, Fe, Al, occluded Fe, and occluded Al fractions of phosphorus were measured for each clay and soil at each rate of added phosphorus.

The self-diffusion coefficients and phosphorus rates of each clay and soil were linearly and positively correlated. The diffusion coefficients in kaolinite and montmorillonite clays were approximately 40 times larger than those in illite clay, Dundee silt loam, and Sharkey clay. No general relationship was established between the self-diffusion coefficients and the various fractions of soil phosphorus when all soils and clays were considered as a group. When each clay or soil was considered individually, there was a highly significant relationship between the diffusion coefficients and water-soluble phosphorus, aluminum phosphate, or the combination of the two. The amounts of phosphorus found in the various fractions differed greatly between soils. The kaolinite and montmorillonite clays initially contained relatively small quantities of phosphorus while illite clay, Dundee silt loam, and Sharkey clay initially contained relatively large quantities of phosphorus.

¹ Contribution from the Department of Agronomy, Arkansas Agr. Exp. Sta., Fayetteville. Presented before Div. S-2, Soil Science Society of America, Aug. 24, 1966 at Oklahoma State University.

² Associate Professor, Assistant Professor, and Professor of Agronomy, respectively.

Received for publication March 20, 1967. Accepted for publication September 21, 1967.