Effect of Zinc Humate on Growth of Soybean and Wheat in Zinc‐Deficient Calcareous Soil

Humic acids have many benefits for plant growth and development, and these effects may be maximized if these materials are combined with micronutrient applications. In the present study, pot experiments were conducted to evaluate the effects of zinc (Zn) humate and ZnSO4 on growth of wheat and soybean in a severely Zn-deficient calcareous soil (DTPA-Zn: 0.10 mg kg21 soil). Plants were grown for 24 (wheat) and 28 days (soybean) with 0 or 5 mg kg21 of Zn as either ZnSO4 or Zn humate. Zinc humate used in the experiments was obtained from Humintech GmbH, Germany, and contained 5% of Zn. When Zn was not supplied, plants rapidly developed visible symptoms of Zn deficiency (e.g., chlorosis and brown patches on young leaves in soybean and necrotic patches on middle-aged leaves in wheat). Adding Zn humate eliminated Zn-deficiency symptoms and enhanced dry matter production by 50% in soybean and 120% in wheat. Zinchumate and ZnSO4 were similarly effective in increasing dry matter production in wheat; but Zn humate increased soybean dry matter more than ZnSO4. When Zn was not supplied, Zn concentrations were 6 mg kg21 for wheat and 8 mg kg21 for soybean. Application of Zn humate and ZnSO4 increased shoot Zn concentration of plants to 36 and 34 mg kg21 in wheat and to 13 and 18 mg kg21 in soybean, respectively. The results indicate that soybean and wheat plants can efficiently utilize Zn chelated to humic acid in calcareous soils, and this utilization is comparable to the utilization of Zn from ZnSO4. Under Zn-deficient soil conditions, plant growth and yield can be maximized by the combined positive effects of Zn and humic acids.