Soil Test Phosphorus and Solubility Relationships in Calcareous Soils1
- J. L. Havlin and
- D. G. Westfall2
Irrigated alfalfa (Medicago sativa L.) was established in 1976 on two Colorado Soils [Keith clay loam (Aridic Argiustolls) and Ravola loam (Typic Torrifluvents)] to determine the residual value of fertilizer P. Triple superphosphate was applied annually at rates of 25 and 50 kg P ha−1, with 75 kg P ha−1 applied triennially and at seeding. Phosphorus solubility (in 0.01M CaCl2) and NaHCO3 ex tractable P were measured initially and in the third and sixth years of the study. The resulting calcium hydroxide (Ca(OH)2) and monocalcium phosphate (Ca(H2PO4)2) potentials were related to known solubility products for distinct mineral phases. Phosphorus solubility in the 0- to 7.5-cm depth increased in all fertilized treatments after three years in both soils. No further increases in P solubility, NaHCO3-P, or P uptake were observed over the next 3 yr. According to solubility diagrams residues of octacalcium phosphate (OCP) were apparently accumulating in the surface 7.5-cm layer at NaHCO3-P levels >32 mg kg−1. Below this extractable P level β-tricalcium phosphate (TCP) apparently controlled P solubility.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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