Effects of fluoride on soil properties and yield and quality of maize

Fluorine is good for animal and human health, however, its excessive use is harmful for both animal and human health. A pot experiment was conducted to study the effects of fluorine on soil properties (pH, water soluble Ca and microbes) and on yield and quality of maize. The fluorine was added into the soil in form of NaF of 0, 100 mg·kg^-1, 200 mg·kg^-1, 500 mg·kg^-1, 1 000 mg·kg^-1 and 1 500 mg·kg^-1. The results showed that maize yield significantly decreased with increasing fluoride dose. Compared with the control, maize yield dropped on the average from 9.9% to 85.4%. However, maize grain protein content significantly increased on the average from 91.8 g·kg^-1 to 108.8 g·kg^-1. Under fluoride doses of 100~200 mg·kg^-1, starch content decreased. It, however, increased at fluoride doses of 500 mg·kg^-1, 1 000 mg·kg^-1 and 1 500 mg·kg^-1. Fluoride content in different parts of the maize plant increased with increasing fluoride dose. Fluoride contents in maize root, grain and leaf were significantly positively correlated with fluoride dose, coefficients were 0.998^**, 0.915^** and 0.852^*, respectively. The order of fluoride content in different parts of the maize plant was: root > leaf > leaf-sheath > stem > grain. With 200 mg·kg^-1 of fluoride dose, fluoride content in grains exceeded the safe agricultural product standard. Soil pH increased (from 8.05 to 8.70) and water soluble Ca decreased (from 2.71 g·kg^-1 to 1.02 g·kg^-1) with increasing fluoride dose. Compared with the control, actinomyce quantity decreased by 0.92%~65.22% with increasing fluoride dose. Low fluorine dose promoted bacteria and fungi growth, while the reverse inhibited bacteria and fungi growth.