Modelling of the Multistep Desorption of the Phosphate Content Adsorbed by the Soil

L. TOLNER, ANAS A. WAHDAN, and G. FÜLEKY

Department of Soil Science and Agricultural Chemistry, University of Agricultural Sciences, Gödöllő (Hungary)

Summary

Phosphate ions of the incorporated phosphorous-fertilizer will be more or less strongly bounded on the surface of soil particles. The rate and reversibility of the sorption depend on the dose of phosphorous fertilizer and the time which has passed since fertilizer application.

A loamy soil of pH(H20) 7.9 was used in the model experiment. The soil was incubated with 0, 80 and 320 mg·kg^-1 phosphorus. The soil samples, incubated with for one month 0 and 80 mg·kg^-1 phosphorus were equilibrated with different rates of phosphorus. After shaking and centrifugation the solid phase was 12 times shaken with distilled water and the desorbed quantity of phosphorous was calculated.

Using the equation applied for adsorption, a new model was constructed for description of the one and multistep desorption. The model helps to calculate the reversible and irreversible part of sorbed phosphorous.

Higher fertilizer phosphorus dose produces higher rate of desorbable phosphorus. The model used in this work, developed with the combination of modified Freundlich equation and the mass balance equation, makes it possible to describe the desorption and to calculate the potentially desorbable phosphorus amount. The reversible and irreversible parts of the sorbed phosphorus could be determined.

The recovery of sorbed phosphorus increased with increasing phosphorus fertilizer doses. This was approximately 100 % in the case of high phosphorus fertilizer doses. The reversibility and the rate of its increase with increasing phosphorus doses were higher when the phosphorus was added recently.

Key words: Soil; P adsorption; P desorption; modeling