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Organic Phosphorus in Calcareous Colorado Soils¹

ABSTRACT

Organic P in calcareous soils was determined by a modified 4N HCl-0.5N NaOH extraction procedure. Organic P averaged 23% of the total P in soils from 23 locations in Colorado, 26% for 3 irrigated rotation soils of 4 rotations each, and 27% in 4 virgin soils. Organic C, N, and P were highly correlated for all soil groups tested. Carbon-to-phosphorus ratios of 95:1, 72:1, and 128:1 were obtained for the Colorado, rotation, and virgin soils, respectively. Alfalfa (Medicago sativa) and manure in in rotations increased the level of organic C, N, and P, and also the C/P and N/P ratios when compared with continuous row cropping without organic residues. Alfalfa was more effective than manure in maintaining organic C, N, and P. Nonvirgin soil contained less organic P, much less organic C and N, and lower C/P and N/P ratios than virgin soil. Organic P mineralized at an average of 3.6 ppm P for 13 Colorado soils when incubated for 21 days at 35C.

¹ Contribution from the Northern Plains Branch, Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Colorado Agr. Exp. Sta.

² Research Soil Scientists, USDA, Akron, Colorado and Fort Collins, respectively.

Received for publication May 24, 1966. Accepted for publication August 2, 1966.