A general approach is developed by which the equilibria between chelating agents and metal ions in soils can be predicted from formation constants. Mole‐fraction diagrams are derived for EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid) in soils when the competing cations are either Fe3+, Ca2+, and H+, or Zn2+, Fe3+, Ca2+, and H+. These diagrams give the specific metal‐ligand complexes that form in soils and show how the concentration of each complex changes with pH. Evidence is presented to support the hypothesis that Zn reacts with amorphous SiO2 in soils to form ZnSiO3. The concentration of Zn2+ in equilibrium with solid phase ZnSiO3 and amorphous SiO2 is approximately 10‐6M at pH 6, and decreases 100‐fold for each unit increase in pH. This concentration of Zn2+ is approximately 100,000 times lower than that in equilibrium with ZnO or ZnCO3.