Cadmium contamination of soils and rice plants caused by zinc mining

Soil-pot and incubation experiments were conducted to examine the effect of certain agronomic practices on Cd uptake by rice plants and the behavior of heavy metals in the soil. Results are summarized as follows: 1. Surface drainage practiced from the growth stage of young panicle-forming caused the soil condition to be more oxidative and acid, producing the highest Cd rice, exceeding the allowable limit for edible food. The enhanced Cd content seemed to be associated with the physiologically increased uptake by plants around this stage. Zn and Cu were the next elements, and Pb was the last one, affected by the drainage treatment. 2. Incubation of the soils and waste ores in a submerged condition resulted in decreases in the concentration of soluble heavy metals, largely in the same order of affect, as above. The depressed solubilities of metals could be attributed to the lower soil Eh followed by the formation of insoluble metal sulfides. Mixing the rice straw powder with those, samples accelerated the changes. Heavy metals contained in the calcareous waste ores became more soluble as they were added to the acid soil. 3. The effect of applied organic manures on the depletion of the Cd content in rice was obtained with the poultry manure but not with the rice straw compost. Low Cd rice was produced only when fused phosphate fertilizer was applied with 'the compost. 4. Because of the widespread and developed reductive condition of the submerged field soil, the effects of the organic manures appeared to be not as expected unless measures are taken with the alkaline amendment materials to raise the soil pH. The paddy fields in question are recommended to be submerged in the growing season as late as circumstances permit.