The Effect of Form of Nitrogen Supplied and pH Level of the Nutrient Solution on Zinc Toxicities in Vegetable Crops

Abstract

Six species of vegetable crops were grown in solution culture in order to investigate the effect of form of N supplied and pH level of the nutrient solution on Zn toxicities in vegetable crops. Zinc was supplied at levels of 0.05, 3 and 10ppm. At each Zn level, NO₃, NO₃+NH₄(1:1) and NH₄ were supplied, with the total N concentration being held constant at 12me/l. The pH of the nutrient solution was adjusted to two levels, 4 and 6. Plants were grown under different treatments for about three weeks.
1. At the normal Zn level (0.05ppm), plants supplied with NH₄ developed various NH₄ injury symptoms. In some vegetable crops excess Zn associated with pH 6 and NO₃ developed Zn-induced chlorosis, which was not observed in NO₃+NH₄ and NH₄ treatments. Under pH 4 this symptom was scarcely observed.
2. At the normal Zn level, the growth of plants supplied with NO₃ and NO₃+NH₄ was almost equally good, but NH₄ plants produced much more inhibited growth. Excess Zn in the nutrient solution inhibited the growth of plants supplied with NO₃and NO₃+NH₄, but in most cases NO₃+NH₄ produced better growth than NO₃.The growth of plants supplied with NH₄ was almost constantly poor regardless of the Zn level in the nutrient solution. Under Zn excess condition, pH 4 often produced better growth than pH 6 in NO₃ and NO₃+NH₄ treatments.
3. Increasing proportion of NH₄ supply and low pH had effects to inhibit the accumulation of Zn in leaves. In NO₃ and NO₃+NH₄ treatments, the more the accumulation of Zn in leaves increased, the more the growth was inhibited regardless of N treatments. In NH₄ treatment, however, the growth of plants was almost constantly poor regardless of leaf Zn concentration.
4. It may be concluded that NH₄⁺ and H⁺, unless their own levels are injurious, exert marked effects in reducing Zn uptake and Zn toxicities in vegetable crops.