THE EFFECT OF SOME ORGANIC ANIONS ON PHOSPHATE REMOVAL FROM ACID AND CALCAREOUS SOILS

Abstract

We studied the dependence of the organic anion form on pH and its effect on phosphate removal from phosphorus-enriched acid and calcareous tropical soils. The results indicate that the release of phosphate by organic anions is affected by pH (3–12) in a rather specific form. For both kinds of soil at low pH (3–6), phosphate desorption might be attributed to: (1) a direct exchange between the dissociated organic anion and the phosphate; (2) dissolution and chelation of “active” forms of iron and aluminum; (3) partial hydrolysis of organic compounds (very low pH). At neutral pH, desorption reaches a minimum for acid soils. At pH between 7–12, removal of phosphate from the acid soils is due to chelation and exchange of hydroxyl ions and phosphate. As expected, the phosphate dissolved from the calcareous soil depends on pH; the higher the hydrogen ion concentration the more phosphate that is dissolved from relatively insoluble calcium phosphates. Howover, the citrate solution dissolved more phosphate at pH 6–9 than did malate or oxae.