Abstract
In this paper, the mechanism of phosphate-release from Manganous, Calcium and Magnesium phosphaes by humic acid was studied by the same method as reported previously. The results are summarized as follows; (1) Humic acid prevents the precipitation of phosphates by Mn^<++>, Ca^<++> or Mg^<++> at various pH values, but the humates formed with those bases have the tendency to adsorb the soluble phosphates reversely in the acidic range such as pH 5〜6. (2) The mechanism of phosphate-release from Mn-, Ca- and Mg-phosphates by humic acid might be similar to that from Fe- or Al-phosphates by humic acid, as the humic acid accelerates the hydrolysis of Mn-, Ca- or Mn-phosphates. (3) The effective action of humic acid in preventing phosphate precipitations from soluble phosphates or releasing soluble phosphates from insoluble phosphates may be ascribed to the formation of humates with Fe(OH)_3,Al(OH)_3 or such bases as Mn^<++>, Ca^<++> and Mg^<++>.
