Effect of several organic acids on phosphate adsorption by variable chargesoils of central China

The effect of several organic acids on phosphate adsorption by acidic soils in subtropical zone of central China was studied. Results showed: (1) citr ate and oxalate remarkably reduced the amount of phosphate adsorption, but tartrate, benzoate and acetate had only a very slight influence on phosphat e adsorption; (2) the ability of citrate in reducing phosphate adsorption w as greater than that of oxalate, moreover, the reduction percentage was dep endent on the concentration of organic ligands in the solution; (3) the eff ect of organic acids on phosphate adsorption was related to the pH value of organic acid solution. The minimum reduction in adsorption of phosphate wa s present at a specific pH value of organic acid solution which ranged from 2 to 10; (4) a minimum reduction of phosphate adsorption occurred when pho sphate was added to the soils before organic acid, whereas a maximum occurr ed when organic acid was introduced before the addition of phosphate. Meanw hile, the treatment for the mixture of two organic acids resulted in more r eduction in phosphate adsorption than each of the organic acids and less th an the total of them. Based on these observations, we suggested that the co mpetition between phosphate and organic acids relied on their relative affi nity to soil mineral surface at different conditions. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.