Residual Soil Phosphorus from Various Fertilizer Phosphates Extracted by Different Solvents

A Bridgehampton silt loam of moderate acidity which had received phosphate from several sources for 48 years followed by 10 years without additional phosphates was analyzed for “extractable” phosphorus by 8 methods. Phosphorus from superphosphate, rock phosphate, Thomas slag, and ground bone, determined by these chemical tests, was correlated with yields of hay, phosphorus uptake, yields of sudangrass, and total soil phosphorus. Methods using relatively strong acid extracting solutions (Bray's 0.1 HCl + 0.03 N NH₄F and Thornton's 0.1 N HCl + (NH₄)₂ MoO₄) and those employing weaker acids with wide soil extracting solution ratios (Peech's CH₃COOH + NaC₂H₃O₂ and Truog's 0.002 N H₂SO₄ + (NH₄)₂ SO₄), gave the most consistent positive correlations. Bray's weaker acid solution (0.025N HCl) distinguished soils receiving rock phosphate from those receiving phosphorus from superphosphate. Alkaline extractants (NaOH and NaHCO₃) and Morgan's extracting solution (NaC₂H₃O₂ + CH₃COOH), proved least adapted to the soil tested. The Peech and Truog methods, although inconvenient for quick soil tests, gave a better indication in terms of probable crop response to phosphorus than did the Bray (P₂) method which, although it reflected the phosphorus status of the soil very well, extracted excessive quantities when compared to the levels of Low, Medium and High phosphorus proposed by Bray as a guide for fertilizer recommendations. The Thornton method extracted somewhat more phosphorus than had been previously established at this station for use in making recommendations based upon rapid soil tests. Subsoils accumulated less phosphorus than topsoils but there was no correlation between the level of phosphorus in topsoil and the amount recovered from the subsoil. Total phosphorus accumulation due to treatment was greatest in the rock phosphate plots, and least where triple superphosphate had been added. Residual effects from superphosphate, Thomas slag, and ground bone were intermediate.