Loading limits for trace elements from municipal sewage sludges applied to land should be based on sludge and soil characteristics that affect plant availability of those elements. Some evidence indicates that the rate at which a plant root absorbs a trace element such as Cd, Zn, or Cu depends on the activity of the free-ion form of that metal in solution at the root surface. The activity at the root surface depends on equilibrium reactions between solution and solid phases and the rate of transport to the root. Sludges by themselves support certain trace element activities when equilibrated with the soil solution. Adsorbing sites on the soil immobilize some of the dissolved trace element ions, causing more ions to be released from the sludge. Because the chemical composition of municipal raw sewage and the types of metal compounds that may enter a wastewater treatment plant vary widely the chemical transformations that will occur in the plant are difficult to predict.