In the rocks of the Earth's crust, copper is mainly found in the form of sulfides, whether simple or complex sulfides, integrating other metals. The total amount of copper in the soil is related to the content of the parent rock and can vary between 3 and 100 ppm depending on the type of soil, although it is higher in soils that have received copper-based fungicide treatments. The normally treated lands have at least 20 ppm of total Cu.
Total copper, however, is nothing more than a very general and imperfect measure for plant assimilation, as it includes:
- Cu present in primary and secondary minerals;
- Cu present in organic compounds;
- Exchangeable Cu in soil colloids;
- Cu in soil solution.
Most of the Cu in the soil is in non-assimilable form. The availability of Cu largely depends on the factors that influence its absorption in the soil's clay-humic complex.
