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Residual Availability in the Soil of Various Sources of Phosphate as Measured by Plant Absorption of P³² and by Soil Test¹

ABSTRACT

Ordinary superphosphate, concentrated superphosphate, calcium metaphosphate, liquid phosphoric acid, fused tricalcium phosphate, rock phosphate with colloidal clay, and rock phosphate were broadcast annually on a Kenyon silty clay loam at the rate of 40 pounds of P₂O₅ per acre per year for 6 years. Rock phosphate was also broadcast annually at the rate of 100 pounds of P₂O₅ per acre per year. Yields were secured of grain, mixed legume hay, and corn grown across these treatments. The only crop showing consistent increases in yield was mixed legume hay, and the phosphates giving responses were those in which the phosphorus was in water- or citrate-soluble form or both.

The seventh year 20 pounds of radioactive superphosphate was applied across the various phosphate treatments on hay, wheat, and corn. Available phosphorus in the soil calculated from absorption data was greater—sometimes 100% or more—in the soils that had received ordinary superphosphate, concentrated superphosphate, calcium metaphosphate, fused tricalcium phosphate, and liquid phosphoric acid, than in check plots or those that had received the rock phosphates. Available phosphorus in the check and rock phosphate plots was essentially the same, as measured by absorption data. There was close correlation between Bray's adsorbed phosphorus soil test and the available phosphorus in the soil as indicated by absorption of P³², but this was not true for Bray's adsorbed and acid-soluble phosphorus or the Thornton soil test for soil phosphorus.

¹ Paper No. 3242 of the Scientific Journal Series, Minnesota Agr. Exp. Sta. This study was supported in part by funds from the Graduate School, University of Minnesota. Radioactive phosphate was supplied through the courtesy of the Division of Fertilizer and Agricultural Lime, S.W.C.R.B., A.R.S., Beltsville, Md.

² Professor of Soils, Professor of Agricultural Engineering, and Junior Scientist in Soils, University of Minnesota, respectively.

Received for publication October 16, 1955.