The effect of previous P additions on sorption indices of calcareous soils determined with commonly employed methods

The effect of previous P additions on commonly used sorption parameters was studied using four alkaline‐calcareous soils of Greece, two Vertisols and two Entisols. Solution concentration and buffering capacity indices were significantly affected while sorption curves of the quantity of P removed from the solution ( x/m ) against final P solution concentration ( c ) were shifted to the right, towards the higher c concentration values. Different buffering indices (e.g. the slope of the Freundlich equation, the slope of the semi‐logarithmic sorption curve of x/m against log c , the slope of the linear portion of the high c concentration range) showed an increasing trend with increasing previous P additions to the soils, with the exception of the slope of the tangent of the Freundlich equation at c = 1 μg P/ml. However, correction for the quantity of exchangeable P already present in the soil ( q ), resulted in an increasing trend for this parameter as well, while obviously it did not have any effect on the slopes of the linear portions of the sorption curves. Chemical changes, as for example the slow formation of insoluble Ca–P salts, could possibly explain the increased capacity of previously fertilized alkaline‐calcareous soils to remove P from the experimental equilibration solution suggested by the increasing trend in the buffering indices of the present work. At any case, this effect should be taken into account, especially in studies where sorption parameters are related to various soil‐plant characteristics with respect to P.