The solid phases that control Cd2+ solubility in soils are poorly understood. This study was made to measure Cd2+ activity in nine slightly acidic to alkaline soils using competitive chelation, which avoids errors that may be caused by unknown Cd‐organic complexes that may be present. The Cd2+ activities measured in these soils varied widely, ranging from 10−6.61 to 10−9.31 M, and were inversely related to soil pH with a correlation coefficient (r2) of 0.95. The activities of Cd2+ in the soils could be predicted from the equation (Cd2+) = 106.5(H+)2. A solubility diagram was constructed for various Cd minerals using the thermodynamic data selected from the literature. The equilibrium constant (Ko) for the reaction of CdFe2O4(c) was derived assuming that the entropy (So) value of CdFe2O4(c) is similar to that of ZnFe2O4(c). The diagram suggests that CdFe2O4(c), in equilibrium with soil Fe, is a likely solid phase controlling Cd2+ activities in these soils.

Nickel activities were measured in 11 alkaline and slightly acidic soils using the chelate method. The measured activities ranged from 10−6.82 to 10−8.78 M and were inversely related to the pH of the soils. The measured Ni2+ activities in the 11 soils could be predicted using the following expression: (Ni2+) = 106.95 (H+)2. The solubility of Ni was plotted on a log (Ni2+) vs. pH solubility diagram for various Ni solid phase compounds. It is most likely that NiFe2O4 (trevorite) is the solid phase controlling Ni solubility in these soils. Identification of the presence of this mineral needs further investigation.