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This article in SSSAJ

  1. Vol. 47 No. 1, p. 112-117
    Received: Dec 10, 1981
    Accepted: Sept 16, 1982

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

  1. P. E. Fixen,
  2. A. E. Ludwick and
  3. S. R. Olsen2



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 β-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 β-Ca3(PO4)2, tricalcium phosphate.

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