The kinetics of iron minerals' dissolution is the reason for the considerab
le variability in Tamm's reagent efficiency. The selected period of treatme
nt (2 h) is very short and fits the steep segment of the kinetic curve. Dif
ferent chemicals, e.g., soil reducers and complexing agents, as well as Fe-
II ions, catalyze the dissolution of iron minerals. The solubility of mixed
iron minerals in the Tamms's reagent differs substantially from that of in
dividual minerals. The solubility of hematite alpha Fe2O3 may increase by a
factor of 30 in the presence of siderite. The hematite solubility also ris
es with the magnetite Fe3O4 present. Calcite CaCO3 raises the oxalate solub
ility of siderite and magnetite. The pH value of the Tamm's extract is rela
tively permanent; however, its redox potential (pe) varies considerably dep
ending on the oxidation-reduction status of soil. The reduction processes i
n soils lead to a sharp increase in the solubility of Fe(III) minerals.