The use of sewage sludge as basal dressing for vegetable cultivation

Abstract

Digested municipal sludge was found applicable as basal dressing for growing four Chinese varieties of leafy vegetables: lettuce, Lactuca sativa cv. local, Chinese flowering cabbage, Brassica parachinensis cv. 70-days, Chinese kale, Brassica alboglabra cv. late-flower, and Chinese white cabbage, Brassica chinensis cv. Kwei Sin Pak. Moderate loading rate of 50, 100, and 200 Mg/ha dewatered sludge did not cause increased plant mortality, metal toxicity symptoms, significant decrease in yield, or enhanced susceptibility to the attack of viruses, bacteria, or insect pests in the vegetables. Significant yield increase was detected (Anova, 5% confidence level) in the Chinese white cabbage between control and sludge-amended plants. The metal uptake response in the leafy portion of the vegetables demonstrated distinct species difference, with lettuce as a sensitive metal accumulator, and Chinese kale a metal excluder (non-accumulator), whereas the Chinese flowering cabbage and Chinese white cabbage were intermediates. Cu, Ni, and Zn (5.6–10.5, 0.3–8.9, 125.8–239.4 mg/kg dry wt., respectively) in the sludge-amended vegetables did not reach phytotoxic levels; Cr (except Chinese white cabbage) and Pb (0.3–2.7, 0.5–5.9 mg/kg dry wt., respectively) were generally lower than the background levels of soil. The highest Cd concentration of 1.5 mg/kg (dry wt.) found in lettuce did not exceed the WHO/FAO safety limit to a cause a health hazard for human consumers. However, it is necessary to screen for more metal non-accumulating crops for safe sludge farming.