Cadmium Availability to Sudangrass Grown on Soils Amended with Sewage Sludge and Fly Ash

Greenhouse experiments were conducted using sudangrass ( Sorghum vulgare L.) as an indicator crop to evaluate the feasibility of co‐recycling sewage sludge and coal fly ash on croplands. The rationale was that the contrasting acid‐base properties of the two waste materials would be complementary, i.e., the acidifying tendency of protons from organic matter decomposition and N mineralization when applying sludge would offset the alkalizing tendency of the hydrolysis of CaO and MgO in fly ash. The fly ash, derived from lignite coal, was effective in raising the soil pH. Alkalinity and salinity were the two most immediate and apparent effects of fly ash application on soil properties; the former depressed Cd concentrations and uptake by plants, while the latter reduced plant growth. Leaching and carbonation of unweathered fly ash alleviated the salinity problem permitting much larger quantities of ash to be applied to soils without adverse effects on plant growth. Weathering, induced by carbonation, not only maintained the fly ash pH buffering capacity but also reduced its harmful salinizing effect. Large increases in soil pH caused by fly ash addition, concomitant with large reductions in Cd uptake by plants, indicate the potential for fly ash as a companion soil amendment for sludge.