A new technique for assessment of reversibly adsorbed phosphate

A new desorption technique for soil phosphate is described, in which Fe oxide coated filter paper is used as an 'infinite' sink for P. This technique is applied to study desorption kinetics for nine sandy soils. The desorption can be explained with a simple Langmuir kinetics model. The mean value of the desorption rate constants found is k(d) = 0.2 ± 0.08 h^-1. The estimated errors that are due to re-equilibration during the experiments are less than experimental error. The amounts of P. desorbed with the new technique, P(i), are compared to the amounts that desorb if a conventional dilution technique, P(w), is used. The new technique desorbs more P than the conventional method (P(i) > P(w)). By comparing P(i) with oxalate-extractable P (P(ox)) it is evident that reversible adsorbed P. rarely exceeds one-third of P(ox). Differences between the soils can be explained in terms of the oxalate-extractable amounts of Fe and Al.