THE ADSORPTION OF INORGANIC PHOSPHATE BY A SANDY SOIL AS INFLUENCED BY DISSOLVED ORGANIC COMPOUNDS

Abstract

I conducted a laboratory study to determine the organic compounds present in the soil solution of a coarse-textured soil and the effect of these compounds on P adsorption. Solutions of KCl at concentrations of 10-2 and 10-4 and L-1 were added to columns that contained 800 g of Valentine loamy sand (Typic Ustipsamment) after the surface application of either 0.3 or 0.72 g diammonium phosphate. The collected leachates were analyzed using gas chromatography (GC) and thin layer chromatography (TLC) to determine which organic compounds were present in solution. A subsequent adsorption study was conducted in which solutions containig 25 ppm inorganic P and 10 ppm organic anion were added to 10-g soil samples and equilibrated for 24 h, after which I determined the inorganic P concentration in solution. Results obtained from the TLC analysis indicated that phytic acid was the predominant organic P species present in the soil solution. Compounds of the cinnamic and benzoic acid series did not influence P sorption, but phytic acid strongly inhibited P sorption. Inorganic P sorption was reduced to less than 14% of the control when the phytic acid concentration in solution exceeded 8 ppm. Identification of phytic acid in the soil solution suggests that the phytic acid inhibition of inorganic P sorption may accelerate P leaching in coarse-textured soil.