Zinc uptake by vegetables: Effects of soil type and sewage sludge

Studies were carried out to investigate how sludge applied to 3 soil types to improve the yield of carrots (Daucus carota) and spinach (Spinacea oleracea) would affect the amount of Zn taken up by these vegetables. A 3 year old (type 1 sludge) and a 3 month old (type 2 sludge) sludge were applied to a vertisol, an arenosol, a chromic/calcic luvisol and a ferric luvisol at (v/v, %) ratios of 0: 100, 5: 95, 10: 90, 20: 80, and 40: 60 sludge : soil. Spinach and carrots were grown on these soil-sludge mixtures for 9 and 13 weeks, respectively, after which the concentration of Zn in both was determined. Sludge application resulted in an increase in total Kjeldahl nitrogen (TKN), available phosphorus, organic matter content, (OM) and cation exchange capacity (CEC) of soils. Percentage increase varied with soil type but ranged from 900 - 3600%, 700 - 3000%, 60 - 300% and 9 - 600% for TKN, available phosphorus, OM, and CEC, respectively. Sludge application also increased the fresh weight of spinach by up to 31% and carrots by up to 10%, these increases also varied with type of soil on which vegetable was grown. Spinach accumulated more Zn than carrots. Carrots and spinach grown on the arenosol had the highest mean concentration of Zn with values of 131.58 mg/kg and 86.33 mg/kg, respectively. Soil type, sludge age and sludge application rate may not individually affect the amount of Zn accumulated by these vegetables, but they could interact to increase the uptake.