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A New Technique for Assessment of Reversibly Adsorbed Phosphate¹

ABSTRACT

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 Lang-muir 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 reversibly 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.

¹ Contribution from the Dep. of Soil Science and Plant Nutrition, Agricultural Univ., De Dreijen 3, 6703 BC Wageningen, The Netherlands.

² Soil Scientist, Dep. of Soil Science and Plant Nutrition, Soil Scientist, Dep. of Physical and Colloid Chemistry, and Chemist, Dep. of Soil Science and Plant Nutrition, respectively.

Received for publication December 2, 1985.