Zinc fertilizers

Zinc content in soil

The highest total zinc content is in tundra (53–76 mg/kg) and chernozem (24–90 mg/kg) soils, the lowest in soddy-podzolic soils (20–67 mg/kg). Zinc deficiency often occurs on neutral and slightly alkaline calcareous soils. In acidic soils, it is more mobile and available to plants.

In soil, it is present in cationic form, adsorbed by a cation-exchange mechanism in acidic conditions or as a result of chemisorption in an alkaline environment. The zinc ion Zn²⁺ is highly mobile in soil. Mobility is affected by pH and the content of clay minerals. At pH < 6, mobility increases, which can lead to its leaching. The mobility of zinc ions is lost when they enter the interlayer spaces of the crystal lattice of montmorillonite. Zinc forms stable compounds with soil organic matter, therefore it mainly accumulates in soil layers with high humus content and in peat.

Zinc fertilizers are most effective on soddy-calcareous, humus-calcareous, chestnut soils of Transcaucasia, brown soils, serozems, serozem-meadow soils, chernozems, and sandy soils. Acidic soddy-podzolic and peat-gley soils are, as a rule, characterized by a high zinc content and do not need zinc fertilizers.

Zinc fertilizers are used mainly in Central Asia for cotton, in Ciscaucasia for maize. They are applied primarily on soils with a neutral reaction, rich in organic matter. Such soils are common in the Middle and Lower Volga region, the North Caucasus, Orenburg Oblast, and Krasnoyarsk Krai of Russia.

The effect of zinc fertilizers depends on the content of mobile forms of zinc in the soil. Liming and soil organic matter reduce the solubility of zinc and its availability to plants. Entering into exchange reactions with humic and fulvic acids, it is fixed in the soil through the formation of poorly soluble compounds. Phosphates reduce the mobility of zinc, since the resulting zinc phosphate is poorly soluble. Zinc solubility increases in the presence of mineral salts, carbon dioxide, and bicarbonates in the soil solution.

Zinc fertilizers

As zinc fertilizers, certain industrial wastes, zinc sulfate, and complex polymicronutrient fertilizers (PMU-7) are used.

Zinc sulfate (ZnSO₄⋅7H₂O) contains 25% zinc and is a white crystalline powder, highly soluble in water.

Zinc polymicronutrient fertilizers (PMU) are slag wastes from chemical plants, for example, from the production of zinc white. They are a dark gray powder with a variable composition. On average, they contain 19.6% zinc oxide, 17.4% zinc silicate, 21% iron and aluminum oxides, impurities of copper, magnesium, manganese, boron, calcium, silicon, traces of molybdenum and other micronutrients.

Slags from copper smelters may contain 2–7% zinc.

Use of zinc fertilizers in agriculture

Zinc fertilizers are applied when the content of mobile zinc in soils of the Non-Chernozem zone is less than 0.2–1.0 mg/kg of soil, and in the Chernozem zone less than 0.3–2.0 mg/kg of soil.

They are used for pre-sowing application to the soil, treatment of seeds before sowing, and foliar feeding. When applied to the soil, the dose is 3–5 kg/ha of zinc. For this, slags from copper smelters at a dose of 0.5–1.5 centners/ha (depending on zinc content) or zinc sulfate are suitable.

To reduce the processes of binding into unavailable forms, frits are used, obtained by fusing broken glass with micronutrients and subsequent grinding, and chelates. In these compounds, micronutrients dissolve well in water and are available to plants and are not fixed by the soil.

Pre-sowing seed treatment is carried out by spraying or dusting with zinc sulfate. For treatment, a solution with a concentration of 0.05–0.1% is used (2–4 g of zinc sulfate per 4 L of water). Per 100 kg, 6–8 L of solution is used. For dusting maize seeds, 100 g of polymicronutrient fertilizer (PMU-7) is used per 100 kg. Seed dusting is carried out with zinc sulfate powder or polymicronutrient fertilizers. For better adhesion to the seeds, it is mixed with talc. Per 100 kg of seeds, 400–500 g of fertilizer is used.

Foliar feeding is carried out with a zinc sulfate solution. Consumption: 100 g per 100 L of water per 1 ha of crops; for aerial spraying, 150–200 g per 1 ha. For foliar feeding of row crops with ground sprayers, 100 g per 300–400 L of water per 1 ha is used. Foliar feeding of fruit crops is carried out on dormant buds (2–3% solution), as well as during the growing season (0.05–0.1% solution). Vineyards are sprayed with a 0.05% solution during the growing season. To the zinc sulfate solution, 0.2–0.5% slaked lime is added to neutralize excess acidity of the solution in order to prevent leaf burn.

Zinc fertilizers show good results on sugar beet, maize (for grain), vineyards, alfalfa, fruit crops, and some vegetable crops.

List of sources

1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.
2. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: Publishing House of VNIIA named after D. N. Pryanishnikov.