Organic Matter-Metal Ion Complexes in Forested and Cultivated Soils

Based on the complexation behaviour of humic and fulvic acids with Fe⁺⁺, Ca⁺⁺ and Zn⁺⁺, their functional groups were divided into three categories viz., (i) those which reacted with all the metals, (ii) those which reacted with Ca and Zn only, and (iii) those which reacted with Zn only. The number of moles of humic or fulvic acid which reacted with one mole of metal varied with the nature of metal ion and ranged from 0.50 to 1.34. This value was greater at pH 5 than at pH 3.5.

The stability constants (K) of the complexes due to metal ion-organic matter fractions were directly related to the number of active sites on humic or fulvic acid, and were influenced by pH of the medium. The higher values of log K noticed at pH 5 were attributed to the increased ionization of functional groups. The order of stabilities of complexes was Fe > Ca > Zn.