The effect of non‐instantaneous exchange on the self‐diffusion of phosphate in soil

SUMMARY The kinetics of isotopic exchange of phosphate in a soil were studied using three techniques: (a) in shaken suspension, (b) in a system where solution circulated through a thin disc of soil, and (c) in moist soil from which solution was extracted by centrifugation with a dense immiscible liquid. The exchange kinetics of isotope diffusing in moist soil were inferred from the concentration profile of the isotope after diffusion periods of 10‐60 days. The proportion of the exchange that is effectively instantaneous in this diffusion system was much lower than in systems in which isotope is uniformly applied to the soil. It is postulated that the difference arises because exchange of diffusing isotope is limited by access to exchange sites and not, during the period considered, by slow exchange at the solid‐liquid interface.