Assessing Critical Level of Lead in Soils for Leafy Vegetables

Soil Pb extractability and critical Pb levels in soils were examined for three leafy vegetables grown in artificially Pb-contaminated soils at different Pb concentrations in a greenhouse experiment. Soil Pb extractability was quantified using eight extractants, and the Pb concentration was determined in two different parts of the leafy vegetables (edible parts and roots). The results showed that the extracted Pb concentrations in soils using aqua regia, DTPA, EDTA, 0.5 M HCl, Mehlich-3, and 0.43 M HNO had significant (p < 0.01) relationships with Pb concentrations in the edible parts and roots of the leafy vegetables. The results indicated that these extractants were highly suitable for estimating the critical Pb level in soils and describing the maximum allowable level of Pb in leafy vegetables (0.3 mg kg). Based on the quality of the linear models (R = 0.535–0.649) and the extracted Pb in the soils, we proposed the Mehlich-3 soil test as the standard for soil quality for leafy vegetable production, with the calculated critical level of Pb in soils for growing leafy vegetables, based on this test being 61.39 mg kg. Furthermore, the results indicated that Pb is more easily transferred from soil to root than from root to shoot, preventing Pb accumulation in the edible part of leafy vegetables.