LIMING EFFECTS ON AVAILABILITY OF CD, CU, NI AND ZN IN A SOIL AMENDEDWITH SEWAGE-SLUDGE 16 YEARS PREVIOUSLY

A field study was conducted to determine the plant uptake of metals in soils amended with 500 Mg ha(-1) of municopal sewage sludge applied 1 6 yr previously. Results showed that metals were available for plan up take after 16 yr, but that liming greatly reduced the plant availabili ty of most metals. The application of sludge also resulted in high rat es nitrification and subsequent lowering of the soil pH before the upt ake study was started. The sludge-amended soil (a mesic Dystric Xeroch rept) was adjusted with lime one month prior to planting from an unlim ed pH of 4.6 to pH 5.8, 6.5 and 6.9. Food crops grown were: (i) bush b ean (Phaseolus vulgaris L. cv. Seafarer), (ii) cabbage (Brassica olera cea L. v. capitata L. cv. Copenhagen market), (iii) maize (Zea mays L. cv. FR37), (iv) lettuce (Lactuca sativa L. cv. Parris Island, (v) (So lanum tuberosum L. cv. (vi) tomato (Lycopersicum esculentum L. cv. Bur pee VF). With the exception of maize, yields were significantly reduce d in the unlimed sludge-amended soil. However, liming increased yields above the growth level of the unlimed untreated soil for cabbage, mai ze, lettuce, potato tuber and tomato fruit. Soluble and exchangeable o f Cd. Ni and Zn were also reduced after liming the sludge-amended soil . In both limed and unlimed soils, the majority of the soil Cu was fou nd in insoluble and unavailable soil fractions. To evaluate trace meta l uptake, the edible portion of each crop was analyzed for Cd, Cu, Ni and Zn. Liming redoced uptake of Cd, Ni and Zn in most crops, but gene rally did not change Cu, This study shows the benefit of pH adjustment in reducing relative solubility and plant uptake of metals as well as increasing crop yield in acid soils.