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Phosphate Rock Dissolution in Soil: Indications from Plant Growth Studies¹

ABSTRACT

Data from three plant growth studies conducted in controlled environments with seven phosphate rocks (PR) and 18 soils demonstrated how and to what degree selected PR and soil characteristics influence rock phosphate release. Rock phosphate release was calculated as the sum of the increase in plant P uptake and soil NaOH-extractable P in treated soil as compared to untreated soil. The seven PR materials tested were carbonate apatites containing 0.4 to 7.1% CO^2–₃ substituted in the apatite lattices. Relative agronomic effectiveness of the rocks was directly related to their substituted CO^2–₃ contents. In a given soil, substituted CO^2–₃ content accounted for 71 to 84% of the variation in rock P released. For the group of 18 soils, substituted CO^2–₃ accounted for nearly 49% of the variation in rock phosphate release. Six selected soil characteristics accounted for 54 to 75% of the variation in phosphate release from a given PR. Across a group of four PR materials, no single soil characteristic appeared to have a consistent and predominate influence on phosphate release. Overlapping of soil spheres serving as a sink for dissolution products of adjacent PR particles decreases dissolution rates of individual particles. Within these spheres of influence are contained the elements (i.e., buffer powers, pH, solute concentration gradients, etc.) driving dissolution.

¹ Research supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin-Madison, and by USDA-SEA Hatch formula fund 2324. Florida Agric. Exp. Stn. Journal Series no. 5572.

² Research Assistant and Professors, respectively, Dep. of Soil Science, Univ. of Wisconsin-Madison, Madison, WI 53706. Senior author currently Assistant Professor, Agronomy Dep., Univ., of Florida/IFAS, Everglades Res. Ed. Center, Belle Glade, FL 33430.

Received for publication April 30, 1984. Accepted for publication January 28, 1985.