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Soil Phosphorus Availability as Measured by Displaced Soil Solutions, Calcium-Chloride Extracts, Dilute-Acid Extracts, and Labile Phosphorus¹

ABSTRACT

Five rates of Ca(OH)₂ and four rates of Ca(H₂PO₄)₂ · H₂O were added in all combinations to samples of a P-deficient, low pH, sandy loam soil. The treated soil was subjected to several wetting-drying-pulverizing cycles to facilitate equilibration. Soil solutions were subsequently displaced from subsamples and analyzed; other subsamples were extracted for P with 0.01M CaCl₂ and dilute acide (0.025N H₂SO₄ + 0.05N HCl); "labile P" was determined on subsamples from each treatment; a sorghum-sudangrass hybrid (Sorghum bicolor X S. sudanese) was grown in the greenhouse to measure P availability.

Maximum yield of test plants was obtained at about 22 µm of P in the displaced soil solution, 12 µm of P in the CaCl₂ extract, 80 ppm P on soil-weight basis in dilute-acid extract, and 50 ppm of "labile P" on soil-weight basis.

The P uptake was highly correlated with the logarithm of P concentration in soil solution (r² = 0.93) and CaCl₂ extract (r² = 0.93), logarithm of "labile P" (r² = 0.86) and concentration of P in dilute acid extract (r² = 0.87). All indexes were nearly equal as indicators of P availability.

¹ Contribution from Dep. of Agronomy and Soils, Auburn Univ., Auburn, Ala. 36830.

² Associate Professor of Agronomy, Colorado State Univ., Fort Collins, CO 80521, Professor of Soils, and Assistant Professor of Soils, respectively.

Received for publication May 9, 1973. Accepted for publication November 2, 1973.