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Soil Test Phosphorus and Solubility Relationships in Calcareous Soils¹

ABSTRACT

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 ß-tricalcium phosphate (TCP) apparently controlled P solubility.

¹ Contribution from the Dep. of Agronomy, Colorado State Univ. Exp. Stn., Ft. Collins, CO 80523, and published as Scientific Series Paper no. 2851. This research was supported in part by the Agricultural Research Branch, Tennessee Valley Authority, Muscle Shoals, AL 35660 and the Colorado Exp. Stn.

² Former Graduate Research Assistant and Professor, Dep. of Agronomy, Colorado State Univ., respectively. Senior author is currently Assistant Professor, Univ. of Nebraska, Panhandle Station, Scottsbluff, NE 69361.

Received for publication June 15, 1983. Accepted for publication October 25, 1983.

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