Phosphate and Silica Adsorption and Desorption from Soils

Phosphate adsorption and desorption in conjunction with silica desorption, were studied on soils of two field experiments. The soil samples were taken from control plots and plots that had received a total of 14 ton/ha superphosphate over a three‐year period. Initially, silica desorption was increased by P added in the past as fertilizer and that added in equilibration solutions. Subsequently, after repeated washings, the rate of silica desorption was independent of the previous phosphate treatment. The adsorbed P was completely leached from the high P fertilized samples but not from those of the control. The slow reaction of phosphate with the soil was accompanied by a release of silica to the equilibrium solution. Most of the slow reaction product was not isotopically exchangeable in 48 hr.