TESTING A MECHANISTIC MODEL .11. THE EFFECTS OF TIME AND OF LEVEL OF APPLICATION ON ISOTOPICALLY EXCHANGEABLE PHOSPHATE

A model for the reaction of ions with soil was improved to permit time trends to be followed at a given level of phosphate addition. Difference equations were also developed to describe the rate of reaction of ions with both vacant sites and occupied sites, while diffusive penetration of the surface was occurring. The model was applied to data for the effects of time and of level of application on exchangeable phosphate. Many of the observed values for isotopically exchangeable phosphate could be well-described if it was assumed that equilibration of P-32 with surface sites was very rapid and this was followed by a diffusive penetration into the adsorbing particles. However, for short periods of contact between soil and P-32, it was necessary to also take into account the rate of the reaction between P-32 and surface sites. This reaction was largely with vacant sites. Reaction with occupied sites-that is, true exchange-was unimportant. It is suggested that the electric potential of the surface may determine whether reaction is with occupied or vacant sites. In contrast to reaction of P-32 with occupied sites, reaction with vacant sites involves a net transfer of charge. Reaction with vacant sites would be slow if the potential was large and negative. It is shown that when reaction with vacant sites is slow, the proportion of previously added P-31 recorded as exchangeable increases with level of addition of P-31. This may explain published observations of slow and non-linear exchange in some soils.