Humic substances are organic compounds that form complexes with heavy metals. Humic substances are classified into three types, humin, fulvic acids, and humic acids, of which the latter most easily forms complexes and humic acid is most easily extracted. Humic substances are easily degraded by microorganisms. Humic acids were extracted from humus collected at regular intervals from a site with fallen leaves and their properties were investigated, including exchangeable proton capacity and the adsorbed amounts of Cu2+, Cu2+ in the presence of Ca2+, and 2,4-dichlorophenoxyacetic acid dimethylamine, a typical pesticide applied to agricultural land. The exchangeable proton capacity of humic acidssw was found to increase with humification time, but no relationship was found between the adsorbed amount of Cu2+ and humification time. The presence of Ca2+ significantly changed the amount of Cu2+ adsorbed by some humic acids. The adsorption isotherms of 2,4-dichlorophenoxyacetic acid dimethylamine could be fitted to the Freundlich model, and the adsorption coefficient was found to decrease as humification time increased.