Metal Levels in Garden Vegetables Raised on Biosolids Amended Soil

Biosolids compost was applied annually over a four year period to a Christiana silt loam soil at the rates of 73 and 146 Mg/ha. Two study areas were established at the site with slightly different experimental designs. Composite soil samples were analyzed for pH and levels of Pb, Zn, Cu, Ni and Cd. Vegetables grown on these plots included bush Blue Lake green beans (Phaseolus vulgaris, L.), crookneck summer squash (Cucurbita pepo, L.), zucchini squash (Cucurbita pepo, L.) Golden Bantam sweet corn (Zea mays, L.), black-seeded Simpson lettuce (Lactuca sativa, L.), Bloomsdale long-standing spinach (Spinacea oleracea, L.) and dwarf blue curled Yates kale (Brassica fimbriata, L.). Plant tissue samples were analyzed for uptake of Pb, Zn, Cu, Ni and Cd. The soil pH was found to increase with increasing application rate. Control plots which received lime had significantly higher pH than unlimed control plots. Statisticially significant differences in soil metal levels between treated and control plots were observed in later years of this study. Metal uptake by vegetables raised in the two treatment areas decreased as the application rate increased in some cases and was unaffected in others. This effect is thought to be due to the increased soil pH due to biosolids application, the low levels of metals in the material and the strong metal-binding capacity of the biosolids.