Heavy metal concentration and ion stability constant also explained the lower Cd content observed in soil amended with OFMSW than that of control soil. Cd-log Kf of different organic acids has been reported to be lower than those of other metals that compete with Cd to form organo-metallic complexes. In addition, Cd concentration in soil was lower than the other heavy metals competing with Cd. For example, Zn has been reported to depress Cd plant uptake.