Extraction of humic substances from leonardite and their calcium complexation properties

The complexation of Ca by the humic substances extracted trom Leonardite by NKOH at 20 degreesC (substances humic of laboratory S.L.) and by KOH(N) un der pressure at 85 degreesC (humic substances commercial S.C.) was studied by conductimetry and potentiometry with pH variable and pH constant. The co mplexing capacity of the humic substances with respect to calcium increases with the rise of the pH. The best stability of the complexes is at pH 9: 6 19 cmol Kg(-1) of humic material for solution S.L. are complexed and 397 cm ol K-1 for solution S.C. Conditions of extraction of SC (S.C.) are more dra stic and would cause the deterioration of these humic substances. Consequen tly, the choice of the protocol of extraction of humic substances would be determinant during the manufacture of the new range of fertilizers in solut ion containing humic substances.