SUMMARYAn important factor in plant availability of copper is its anomalous binding to soil humic substances, for which aggregation phenomena have been implicated. Two alternative models of copper‐induced humate aggregation are theoretically explored to characterize binding behaviour. In the first model the copper modifies the humic acid so that it self‐associates indefinitely. Copper binding is found to be dramatically enhanced, except at the very low concentrations corresponding to copper‐deficiency. In the second model bivalent copper forms bridges between different humate molecules to yield an array of large cross‐linked complexes. In the latter cross‐linking model, binding of copper is markedly enhanced at low copper concentrations, a behaviour still found when only relatively few sites sterically permit cross‐linking. The cross‐linking model accounts, at least in part, for the low availability and immobility of copper in some problem soils and is consistent with several items of experime