What solutions to reduce trace elements uptake by vegetables in market gardens? lessons from a three-year pot experiment

Food safety has become a major concern in Belgium since excessive levels of Cd and Pb were measured in vegetables grown in private and market gardens, even in areas with slight soil contaminations. Addition of amendments has been reported in the literature as a cost-effective remediation approach to mitigate Cd uptake by vegetables. A three-year pot trial was conducted with Swiss chard on acidic soil with moderate total Cd concentrations (pH CaCl2 = 5.6 – Cd = 1.4 mg.kg-1 . Each year, soil was amended with biochar and green waste compost, applied alone or in combination with lime. Effects on soil pH, extractible metals and uptake by the plants were studied. In parallel, the experiment was repeated with Swiss chard and lettuce in a market garden with similar soil Cd concentration. The results showed that multiple compost additions significantly increased the soil pH in pots (mean pH CaCl2 = 6.5) and hence decreased the CaCl2 Cd concentration. Lime addition in respect of agricultural recommendations also contributed to pH increase but in a lesser extent than compost. Large variability of vegetable Cd content was observed. Compost results in the lowest cadmium levels in Swiss chard but these still exceed the European threshold. Results were compared to field experiment. Soil pH and cadmium content could not fully explain measured plant concentrations. Climatic conditions slowed down the degradation of soil amendments, especially lime. This highlighted the need to repeat field trials on the same plots in longer term (3 to 5 years). The importance of soil pH in controlling trace elements uptake by plants is widely recognized. However, despite neutral soil in some gardens, the vegetables still did not comply with EU legislation. Therefore, further studies are needed to identify the other factors to consider for controlling the bioavailability of Cd, especially in the field.