Effect of organic waste amendments on zinc adsorption by two soils

Two soils (fine and coarse textured) were amended with five organic wastes or humic acid. One adsorption experiment was carried out at 1 mmol L-l Zn and at pH levels from 4 to 8, A second experiment was at pH 6 and 0 to 4 mmol/L-1 Zn, The greatest variation in Zn adsorption among organic treatments came at pH 6, with a lesser range for the fine textured soil (pH 5-6) and a wider range for the sandy soil (pH 5-7), Adsorption followed a two-site Langmuir model, and maxima were higher for the finer textured soil compared with the sandy soil. Adsorption maxima were not changed by the organic wastes for the fine textured soil, but all were increased over the controls for the sandy soil. Zinc adsorption for poultry litter was lower than the control for the sandy soil. Industrial sewage sludge and humic acid increased Zn adsorption more than did commercial compost, spent mushroom compost, and cotton litter. It was concluded that organic materials have more influence on Zn adsorption for sandy soils than for fine textured soils and that most materials will increase Zn adsorption, whereas those with high soluble C can decrease Zn adsorption.