Effect of activated phosphate rocks on growth and quality of lettuce in heavy metal contaminated soils

Two kinds of low-grade phosphate rock collected from Nanzhang and Baokang were mixed with 0.5 mol/L oxalic acid and cultured under certain conditions. A pot experiment was conducted to study the effect of oxalic acid activated phosphate rocks on the quality of lettuce grown in heavy metal contaminated soils. The results show that compared with the control, the plant heights, chlorophyll SPAD values and vitamin C contents of lettuce are increased after applying the phosphate rocks and the activated phosphate rocks and the effect order is: the activated phosphate rocks the phosphate rocks the control, while the nitrate contents are reduced. At the same time, the contents of soil available phosphorus, exchangeable calcium and magnesium are increased, and the exchangeable heavy metal contents are decreased under the applications of the phosphate rocks and the activated phosphate rocks. The absorptions of heavy metals in various parts of lettuce are reduced, for example, compared with the control, applying activated phosphate rocks can reduce the Cd and Cu contents mostly by 55.1% and 55.24% in lettuce roots, and 59.3% and 53.4% in the shoots of lettuce, respectively. The order of the reduced effects of heavy metals is: the activated phosphate rocks the phosphate rocks the control. These results indicate that the oxalic acid activated phosphate rocks have better effect for decreasing heavy metals uptake compared with the raw phosphate rocks.