The root uptake of the trace elements. . .
IRON (Fe), ZINC (Zn), MANGANESE (Mn), and COPPER (Cu) is governed by three basic principles.
1. Their presence in the soil. Many ag soils have very low levels of native Zn, Mn, and Cu and they need to be added as fertilizer to maximize crop production. There are also soils where one or more of these trace elements is abundant but they are tied up by the soil.
2. Their presence in the soil solution. It is the soil solution (the water in the soil) that delivers all nutrients taken up by roots. Practically all nutrients are taken into the roots in their dissolved, ionic forms.
3. The ability of the plant and the rhizosphere to increase the amount of micronutrients that are in solution and available for root uptake.
A soil solution that is acidic will dissolve more metallic micronutrients from the surrounding soil than a soil solution that is alkaline. It is a chemical principle that acids dissolve minerals. But metallic trace elements are not the only essential elements we want in the soil solution so we must not get too acidic or other nutrients will be less available. Conversely, if the soil solution is alkaline, copper, zinc and manganese will get tied up and be unavailable.