MECHANISMS OF pH DECREMENT IN THREE TEMPERATE SOILS TREATED WITH PHOSPHATE

Phosphate application leads to increases or decreases in pH of soils, depending on soil characteristics. Such changes influence micronutrient availability and help evaluate P sorption mechanisms. Evaluation of P addition effects on soil pH in Eastern Canada will be useful for fertilization practice. Six top and subsoil samples of 20.0 g were equilibrated with 200 mL NaH2PO4 solutions of 0 to 9 mM in 0.03 M KClO4 at initial pH 6.0 for 72 h. The supernatant pH was found to be a negative quadratic function of sorbed P or reduced soluble Ca. Increased H+ activity (ΔH) was a result of proton production in the process of H2PO−4 sorption or proton dissociation from sorbed phosphate and of P-Ca reactions. The values of ΔH showed significant positive linear correlations with soil Fe components, suggesting a repellence between surface positive charge and the proton of sorbed P.