Abstract Changes in the heterogeneous binding sites (organic and inorganic) which occur in soil under varying conditions of pH and ionic strength may be used to discuss the chemistry of metal partition and speciation in organic rich soil-solution systems. In this study humic acid containing soil-solution system was studied to determine how the ratio of Cu-occupied sites (SR) in the solid phase is influenced by the humic acid under varying conditions of pH and background electrolyte. It was noted that, SR is a constant which is defined by the ionic composition of the soil-solution system. And under varying conditions of background electrolyte and pH the role of humic acid in Cu partition is the provision of alternate binding sites for metal adsorption in the solid phase and routes for the population of Cu in the solid phase.