An Application of Autoradiography to Study the Spacial Distribution of ³³ P‐Labelled Orthophosphate Added to Soil Crumbs

The penetration of orthophosphate (P) into soil crumbs was studied semiquantitatively using 31 P, the low energy isotope 33 P, 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 SO 4 2‐ had little or no effect on the spacial distribution of sorbed added P; 0 and 2.4% of the sorbed added 31 P was desorbed by Cl ‐ and SO 4 2‐ , respectively. Desorption in HCO 3 ‐ 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 31 P, respectively. Anomalous estimates of the amount of P sorbed or desorbed, obtained by 31 P and 33 P analyses, could not always be explained by isotopic exchange.