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Documentary evidence is presented in support of the view that organic matter plays a major role in the adsorption of herbicides in soil, and that organic matter content is usually the soil factor most directly related to herbicidal behavior. Major attention is given to the nature and origin of humic and fulvic acids, with special emphasis being given to the types of reactive sites responsible for the binding of pesticide chemicals. The various aspects of humus chemistry are discussed as they relate to investigations on herbicide‐soil organic matter relationships, including the inadequacies of methylation procedures for blocking specific functional groups in connection with bonding mechanism studies. Adsorption mechanisms are suggested and possible chemical transformations brought about by organic substances are outlined.

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Application was made of the potentiometric titration method for determining apparent stability constants for the complexes between Cu2+, Pb2+, and Cd2+ and three humic acids from diverse sources. Complexes of Cu2+ and Pb2+ were considerably more stable than those for Cd2+. Log K2 values, obtained from the relationship kj = bj/Ki (Ki = ionization constant) increased rather dramatically with decreasing salt concentration and were of the order of those reported in the literature for metal complexes with known biochemical compounds. Average log K2 values for the three humic acids (absence of neutral salts) were 8.9 for Cu2+, 8.7 for Pb2+, and 6.9 for Cd2+. Differences between humic acids in their ability to bind metal ions were slight. At least two major sites were involved in the binding of metal ions.

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