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Phosphorus Diffusion in Soils¹

ABSTRACT

The validity of activity measurements as estimates of phosphorus diffusion from P³²-tagged phosphate sources was investigated. Significant changes in the apparent specific activity of diffusing phosphorus were found; these could be accounted for by assuming exchange between diffusing P³² and native soil P³¹. Despite such changes in apparent specific activity, the overall picture of phosphorus diffusion obtained from activity measurements was not substantially different from that obtained by total phosphorus analysis. One-dimensional diffusion of tagged phosphates from Ca(H₂PO₄)₂ and KH₂PO₄ sources was studied in 10 water-saturated soils using x-ray film and Geiger tubes to estimate the resulting distribution of diffused phosphorus. The x-ray film revealed many local areas of P³² concentrations 0.5 to 2 millimeters in diameter. Large-scale irregularities in the curves relating phosphorus concentration in the soil column to distance from the source were found in three cases. In both Miami silt loam and Muscatine silt loam with Ca(H₂PO₄)₂ as source, peaks of phosphorus concentration were found at 4-cm. intervals. With Miami silt loam and KH₂PO₄ as source, one peak of phosphorus concentration was found about 1 cm. from the source. Distribution of diffused calcium, as measured by extraction with 0.5 N acetic acid, was similar to distribution of diffused phosphorus in Muscatine and Miami silt loam columns treated with Ca(H₂PO₄)₂.

¹ Soil Phosphorus Research Project of the North Central Region. Contribution from the Iowa Agr. Exp. Sta. and the B.P.I.S.A.E., U. S. Dept. of Agriculture. Journal Paper No. J-2387 of the Iowa Agr. Exp. Station, Ames, Iowa, Project 1183. Presented before Div. II, Soil Science Society of America, Dallas, Tex., Nov. 18, 1953.

² Graduate Assistant and Professor of Soils, respectively. The authors are indebted to Dr. F. R. Duke for advice during the conduct of the investigation.

Received for publication November 27, 1953.