The main causes of zinc deficiency in soil are usually related to the parent material, soil type, and management practices. First, soils developed from zinc-poor parent materials, such as granite, often have low levels of available zinc. Second, calcareous soils, which are common in northern regions, include types like cotton soil, bauxite soil, fluvo-aquic soil, cinnamon soil, and brown soil. These soils typically have a high pH, which reduces the solubility and availability of zinc, making them a major cause of zinc deficiency in China. Third, some paddy soils with high organic matter content can also lead to zinc deficiency because organic matter tends to bind zinc, reducing its availability to plants. Fourth, excessive use of phosphate fertilizers can interfere with zinc uptake, and soils with poor root development due to harsh environmental conditions are also prone to zinc deficiency.
Zinc deficiency can lead to various physiological disorders in crops. For example, when trees lack zinc, their seedlings become small and weak, leading to clustered leaves, a condition known as "rosette disease." In rice, zinc deficiency causes symptoms such as "hollow panicles," stunted growth, delayed tillering, reduced root development, and a higher rate of empty grains. In corn, it leads to chlorosis of the leaves, white tassels, delayed silking, and poorly developed ears. Besides rice, corn, and fruit trees, crops like flax, wheat, peanuts, soybeans, and peas are also vulnerable to zinc deficiency.
To address zinc deficiency, several methods are commonly used. Zinc sulfate and zinc oxide are the most widely applied sources. Soil application is recommended before sowing or transplanting, and it's best done during plowing. A typical rate is 1.5–5.0 kg per mu (about 0.67–3.3 kg per acre) of zinc sulfate. Foliar sprays are effective for fruit trees and vegetables, especially when deficiency symptoms appear. The recommended concentration is generally 0.05–0.1% zinc sulfate solution, while fruit trees may benefit from a 0.5% solution. Seed treatment with a 0.1% zinc sulfate solution is also common for rice and cotton, and using a 2–4% zinc oxide suspension is an economical option. When using industrial byproducts like zinc sulfate from acid hydrolysis, care must be taken to avoid contamination with heavy metals like cadmium and lead. The residual effect of zinc in the soil lasts about 3–5 years, and regular plant analysis is recommended to determine if further zinc application is needed.
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