Phosphorus and Potassium Fertilization of Irrigated Alfalfa on Calcareous Soils: II. Soil Phosphorus Solubility Relationships1
- P. E. Fixen,
- A. E. Ludwick and
- 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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