An evaluation of the immiscible displacement method for studying the reaction between soil and phosphate

Abstract

The reaction between soil and phosphate was studied using a method in which a sample of the soil solution was displaced by centrifuging moist soil with a dense, immiscible organic liquid. The soil solution floats on the organic liquid and can be removed for analysis.

Phosphate concentrations in the soil solution so displaced were lower than those estimated by the null-point method using 0.01M calcium chloride. The differences were shown to be closely associated with the lower calcium concentration of the soil solution.

Adsorption from solutions of monovalent phosphate salts was accompanied by accumulation of cations in the soil solution. It was argued that this occurred because the average charge conveyed to the adsorbing surfaces was less than unity. Calculated values were between 0.90 and 0.96. The remainder of the charge was balanced by release of hydroxide ions and small increases in pH were observed.

The immiscible displacement method appeared to be potentially valuable for many studies of the reaction between chemicals and soil. Its main disadvantage is that the volume of solution available for analysis is small and this places limits on the sensitivity.