The properties of humic substances, which account for the majority of organic matter in water and soil, depend on their origin. To estimate heavy metal toxicity in soils, it is important to understand which characteristics of humic substances determine the speciation of heavy metals, and to examine whether interactions among soil particles, humic substances, and heavy metal ions affect toxicity. We extracted four sets of humic acids from wood and soils and analyzed the binding of protons and cupric ions to three types of binding sites on the humic acids. The results were fitted to the NICA (non-ideal competitive adsorption)–Donnan model and compared directly with predictions based on generic parameters. Copper- binding data, which provided quantitative estimates for functional groups, showed that the properties of soil humic acids of diverse origin are homogeneous, whereas those of wood humic acids vary widely. Consistent differences between soil and wood humic acids are revealed at the level of functional group concentrations.