The study of the complexation of calcium by humic substances (SH) extracted from a natural peat (TN) and from a peat treated with nitrogen oxides (TT) was followed by conductimetry and potentiometry with pH variable and pH constant. The complexing capacity increases as the pH rises: complexed calcium is 130 cmol kg*[-1) SH (TN) and 135 cmol kg*[-1) SH (TT) with pH 5, and 330 cmol kg*[-1) SH (TN) and 410 cmol kg*[-1) SH (TT) with pH 9. These reactions involve various functional groupings of pKa1 < 4 for TN and TT, pKa2 = 5.9 for TN and pKa2 = 6.2 for TT; in all cases, the medium has exhibited an increase in acidity. The complexation of calcium by HS extracted from TT could then enhance the efficiency of the phosphatic nutrition of plants
