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Phosphorus and Potassium Fertilization of Irrigated Alfalfa on Calcareous Soils: II. Soil Phosphorus Solubility Relationships¹

ABSTRACT

Alfalfa (Medicago sativa L.) was established at two field locations in Colorado in 1976 and triple superphosphate applied at rates of 25 and 50 kg P/ha annually or 75 kg/ha in a single application at seeding. The initial P applications were incorporated while subsequent applications were topdressed. Soil profile changes in NaHCO₃-extractable P and in P solubility measured in 0.01M CaCl₂ were determined over a three-year period. The resulting calcium hydroxide and monocalcium phosphate potentials were related to known solubility products for distinct P mineral phases including octacalcium phosphate (OCP) and ß-tricalcium phosphate (TCP). Octacalcium phosphate controlled solution P if the NaHCO₃-extractable P rose above 35 mg·kg^–1 while TCP or a similar mineral phase dominated P intensity in the range of 10 to 25 mg·kg^–1. Relating these potentials to the changing NaHCO₃-P levels indicated that considerable Ca₈H₂(PO₄)₆·5H₂O, octacalcium phosphate (OCP), accumulated in one soil. Solubility data from both soils also indicated significant buffering of soil P on or near the solubility isotherm of ß-Ca₃(PO₄)₂, tricalcium phosphate.

¹ Published as Scientific Series Paper no. 2692 of the Colorado State Univ. Exp. Stn. This research was supported in part by the Soils and Fertilizer Research Branch, Tennessee Valley Authority, Muscle Shoals, AL 35660.

² Former Graduate Research Assistant, former Professor, Dep. of Agronomy, Colorado State Univ., Ft. Collins, CO 80526, and Soil Scientist, USDA-SEA, Ft. Collins. The first two Authors are presently Assistant Professor, Dep. of Plant Science, South Dakota State Univ., Brookings, SD 57007, and Western Director, Potash & Phosphate Institute, Davis, CA 95616, respectively.

Received for publication December 10, 1981. Accepted for publication September 16, 1982.

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