Study On The Toxic Effects Of Soil Zinc On Crops And The Toxicity Thresholds Of Zn For The Soil

The sensitivity to Zn toxicity of 16 vegetable species that are commonly cultivated in Fujian Province was studied by water culture. The most sensitive vegetable to Zn toxicity was selected among the vegetables. Soil experiments with cucumber and rice were carried out to estimate the toxicity thresholds of Zn for the soil. The toxicity threshold will provide important basis for establishing the benchmark of Zn for the agricultural soils in Fujian Province. The results were as follows:1. Low concentration of Zn in the culture solution enhanced the growth of the vegetables while higher Zn concentration showed toxic effects to the vegetables, aggravating with the increase in Zn concentration of the solutions and culture time. The Zn toxicity symptoms were chlorasis and curling on new leaves, spot blotches and margin-wilting on older leaves. The old leaves dead when suffered serious Zn toxicity. When the roots were suffered from Zn toxicity, the number of fibrous roots decreased, the roots became blacker and even rotted.2. Among the vegetables tested, the most sensitive vegetable to Zn toxicity was cucumber and the most tolerant ones were carrot, mustard and fast cabbage based on the Zn toxicity symptoms and the EC20 values (effective concentration causing a 20% reduction of shoot biomass) of each species.3. As the increase in Zn concentration and the experimental time, Zn toxicity symptoms became more and more serious. The typical Zn toxicity symptoms of cucumber under soil culture conditions were similar to those for cucumber under water culture. Low Zn concentration stimulated while higher Zn concentration inhibited the growth of cucumber. The inhibition degree weakened in the order of the fresh weight of aboveground part >plant height, the fresh weight of root >the length of root.The Zn concentrations of the root and the aboveground parts of cucumber were significantly increased with the increase in Zn amount added and experimental time. The Zn concentration in roots were always much higher than those in the aboveground parts.4. Low Zn concentrations promoted whereas higher Zn concentration retarded the growth of rice. When the Zn concentration was≤300 mg Zn·kg^-1, the height and basal stem diameter of rice increased, most leaves became more green than those of CK treatment, a few new leaves became yellow, tillering was advanced and the number of ears per plant increased. When the Zn concentrations were between 400 ¹000 mg Zn·kg^-1, chlorosis, spot blotches and margin-wilting were more popular on leaves. Some old leaved withered and dropped. The plant may die under high Zn concentration.The Zn concentration of various parts of rice were different, decreasing in the order of roots>stems>leaves>grain hulls>brown rice. With the increase in Zn concentration of the soil, the Zn concentrations of stems, leaves and brown rice increased first and then decreased while those of roots and grain hulls increased.5. The EC20 values (available Zn in the soil) of cucumber and rice estimated from the results of the soil culture were 27.20 mg·kg^-1 and 418.16 mg·kg^-1, respectively. In order to protect the most plants from Zn toxicity, the threshold value of Zn toxicity should be 27.20 mg·kg^-1 (soil available Zn).