Availability and retention of zinc, especially in relation to the soils of Bangladesh
The availability and retention of zinc by soils and minerals of relevance to the crop, especially rice, growing areas of Bangladesh was investigated. The 32 days of continuous soil submergence caused a significant decrease in available zinc, sulphate and copper. Available iron increased significantly while manganese showed a sharp increase followed by a rapid decrease. As pH increased, adsorption and/or precipitation and co-precipitation of zinc with aged or fresh iron and aluminium hydrated oxides was partially responsible for the frequently reported fixation and unavailability of zinc added to the soil. The particle size of the products increased with increasing pH and initial concentration of the relevant elements. For synthetic oxides, the adsorption capacity of manganese appeared to be higher than that of iron or aluminium varieties. The amount of zinc adsorbed by fresh or aged iron or aluminium oxides was similar, but was much different between the fresh and aged forms. The presence of nitrate and sulphate anions made no difference to the shape of the isotherm for zinc adsorption on soil. Variations in adsorption between soils were attributed to the content and nature of the clay fraction and magnitude of CEC. Soils with the highest percentages of clay and, especially, smectites had the highest adsorptive capacities for zinc. Zinc adsorption by soils, clays and synthetic hydrous oxide minerals conformed to the Langmuir isotherm model within certain limits. It was concluded that a higher content of iron, aluminium and manganese oxide hydroxides, a higher CEC and a higher proportion of smectites in the clay fraction were the principal reasons for increased retention by soils and resultant decreased availability of zinc to plants, particularly at higher pH.