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

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 NaHCO 3 ‐extractable P and in P solubility measured in 0.01 M CaCl 2 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 β‐tricalcium phosphate (TCP). Octacalcium phosphate controlled solution P if the NaHCO 3 ‐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 NaHCO 3 ‐P levels indicated that considerable Ca 8 H 2 (PO 4 ) 6 ·5H 2 O, 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 β‐Ca 3 (PO 4 ) 2 , tricalcium phosphate.