INFLUENCE OF FERTILIZER AND SEWAGE-SLUDGE COMPOST ON YIELD AND HEAVY-METAL ACCUMULATION BY LETTUCE GROWN IN URBAN SOILS

Previous research has demonstrated that many urban soils are enriched in Pb, Cd and Zn. Culture of vegetable crops in these soils could allo w transfer of potentially toxic metals to foods. 'Tanya' lettuce (Lact uca sativa L.) was grown in pots of five urban garden soils and one co ntrol agricultural soil to assess the effect of urban-soil metal enric hment, and the effect of soil amendments, on heavy metal uptake by gar den vegetables. The amendments included NPK fertilizer, limestone, Ca( H2PO4)(2), and two rates of limed sewage sludge compost. Soil Cd range d from 0.08 to 9.6 mg kg(-1); soil Zn from 38 to 3490 mg kg(-1); and s oil Pb from 12 to 5210 mg kg(-1). Lettuce yield on the urban garden so ils was as great as or greater than that on the control soil. Lettuce Cd, Zn and Pb concentrations increased from 0.65, 23, and 2.2 mg kg(-1 ) dry matter in the control soil to as high as 3.53, 422 and 37.0 mg k g(-1) on the metal-rich urban garden soils. Adding limestone or limed sewage sludge compost raised soil pH and significantly reduced lettuce Cd and Zn, while phosphate fertilizer lowered soil pH and had little effect on Zn but increased Cd concentration in lettuce. Urban garden s oils caused a significant increase in lettuce leaf Pb concentration, e specially on the highest Pb soil. Adding NPK fertilizer, phosphate, or sludge compost to two high Pb soils lowered lettuce Pb concentration, but adding limestone generally did not. On normally fertilized soils, Pb uptake by lettuce was not exceptionally high until soil Pb substan tially exceeded 500 mg kg(-1). Comparing garden vegetables and soil as potential sources of Pb risk to children, it is clear that the risk i s greater through ingestion of soil or dust than through ingestion of garden vegetables grown on the soil. Urban dwellers should obtain soil metal analyses before selecting garden locations to reduce Pb risk to their children.