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An Application of Autoradiography to Study the Spacial Distribution of ³³P-Labelled Orthophosphate Added to Soil Crumbs¹

ABSTRACT

The penetration of orthophosphate (P) into soil crumbs was studied semiquantitatively using ³¹P, the low energy isotope ³³P, autoradiography on a medium grained film, and solution analysis. Sorption of added P by the soil crumbs was rapid and was followed by penetration. Phosphate was not concentrated in discrete areas, suggesting that it was retained by a sorption rather than a precipitation mechanism. Native and added P, isotopically exchangeable in 1 hour, were confined to the crumb surface. Desorption for 7 days in Cl^- or SO₄^2- had little or no effect on the spacial distribution of sorbed added P; 0 and 2.4% of the sorbed added ³¹P was desorbed by Cl^- and SO₄^2-, respectively. Desorption in HCO₃^- or citrate increased the mobility of sorbed added P resulting in a complete redistribution within the crumbs and release into solution of 14 to 18% or 10 to 61% of sorbed added ³¹P, respectively. Anomalous estimates of the amount of P sorbed or desorbed, obtained by ³¹P and ³³P analyses, could not always be explained by isotopic exchange.

¹ Contribution from the Dep. of Soil Science, Univ. of Wisconsin, Madison. Research supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin, Madison 53706. Supported by a US/AID/Brazil On-Campus Research Grant in connection with the Univ. of Wisconsin-US/AID/Brazil Contract. Approved for publication by the Director of ARDO, US/AID, Brazil.

² Graduate Research Assistant and Associate Professor of Soil Science, respectively.

Received for publication April 7, 1971. Accepted for publication May 25, 1971.