Zinc in plant life
Field crops with the yield bear from 75 to 2250 g/ha of zinc. Crops with increased sensitivity to zinc deficiency include buckwheat, hops, beets, potatoes, and meadow clover. Weedy plants contain more zinc than cultivated ones. Coniferous plants have an increased zinc content; the highest content is in poisonous mushrooms. Field crops need less zinc than fruit crops.
Zinc’s effect on respiratory stabilization during rapid temperature changes increases heat and frost tolerance of plants. It affects phosphorus utilization by plants. Zinc deficiency results in high concentration of inorganic phosphorus in plants. In pea and tomato plants, zinc deficiency results in increased phosphorus intake but impaired phosphorus utilization, with several times higher inorganic phosphorus content and lower nucleotide, lipid and nucleic acid content. After zinc is added to the nutrient solution, the use of absorbed phosphorus is normalized.
Zinc alters phosphorus accumulation by roots and slows down phosphorus transport to above-ground organs. Zinc is able to chemically bind soluble phosphorus compounds. Zinc deficiency inhibits conversion of inorganic phosphates into organic forms.
Zinc is involved in biosynthesis of chlorophyll precursors and in photosynthesis. Zinc protoporphyrin was found in etiolated and green corn leaves, which is probably a precursor of iron and magnesium porphyrins.
The zinc-containing enzyme carboanhydrase may be involved in photosynthesis, trapping carbon dioxide that is released into the atmosphere during photorespiration. Carboanhydrase is necessary for carbon dioxide or hydrocarbonate ions to penetrate the chloroplast envelope.
More than 200 zinc-activated enzymes are known. Carboanhydrase contains 0.31-0.34% zinc. It is also a part of alkaline phosphatase, malate dehydrogenase, alcohol dehydrogenase, glutamate dehydrogenase, etc.
Zinc-containing carboanhydrase was found in oat, parsley, pea, and tomato chloroplasts. Zinc is a component of dehydrogenases that require the presence of NAD.
Zinc deficiency in plants leads to accumulation of reducing sugars, decreases sucrose and starch content, increases accumulation of organic acids, decreases auxin content, impairs protein synthesis and accumulation of nitrogenous non-protein soluble compounds. Cell division is suppressed 2-3 times, which causes morphological changes in leaves, cell stretching and tissue differentiation are impaired, meristematic cells are hypertrophied, columnar cells of longitudinal stretching in flax are suppressed and chloroplasts are reduced in size. A large number of mitochondria are formed if the content is sufficient.
Fruit crops, especially citrus, are sensitive to zinc deficiency. Apple, apricot, peach, quince, and cherry trees show small-leaved and rosette leaves, while citrus trees show leaf spotting. Corn shows white-white, or chlorosis, on the upper leaves; tomato shows small-leaved leaves and curling of leaf blades and petioles; stunting is typical for all plants.
Zinc deficiency may occur in acidic strongly podzolized light soils, carbonate and highly humusous soils. High doses of phosphorus fertilizers and strong plowing of subsoil to arable horizon aggravate deficiency.
Zinc fertilizers increase corn yield by 0.5-0.7 t/ha, raw cotton by 0.2-0.4 t/ha, and wheat grain by 0.15-0.2 t/ha. On the background of zinc deficiency, zinc fertilizers increase the yield of garlic, peas, beans, tomatoes; the sugar content of tomato fruits increases, the vitamin C content increases, the brown spot disease incidence decreases, and the red fruit yield increases. Zinc fertilizers promote potato resistance to phytophthora and other diseases.
Zinc content in soil
The highest gross zinc content is in tundra (53-76 mg/kg) and chernozem (24-90 mg/kg) soils, the lowest – in sod-podzol soils (20-67 mg/kg). Zinc deficiency often occurs in neutral and slightly alkaline carbonate soils. In acidic soils, it is more mobile and available to plants.
In soils, it is present in cationic form, adsorbed by cation exchange in acidic or chemisorption in alkaline media. Zn2+ zinc ion is well mobile in soil. The pH value and the content of clay minerals influence the mobility. At pH<6 the mobility increases, which may lead to its leaching. The mobility of zinc ions is lost when it enters the interstitial spaces of the crystal lattice of montmorillonite. Zinc forms stable compounds with soil organic matter, so it mainly accumulates in soil layers with high humus content and in peat.
Zinc fertilizers are most effective on sod-carbonate, humus-carbonate, chestnut soils of Transcaucasia, brown, gray-meadow, gray-meadow, chernozem and sandy soils. Acidic sod-podzolic and peat-gley soils are generally high in zinc and do not require zinc fertilizers.
Zinc fertilizers are used mainly in Central Asia for cotton, in the Caucasus – for corn. Primarily applied on soils with neutral reaction, rich in organic matter. Such soils are common in the Middle and Lower Volga region, the North Caucasus, Orenburg Region, and Krasnoyarsk Territory of Russia.
The effect of zinc fertilizers depends on the content of mobile forms of zinc in the soil. Lime and soil organic matter reduce solubility of zinc and its availability to plants. Zinc enters into exchange reactions with humic and fulvic acids, and is fixed in the soil due to formation of poorly soluble compounds. Phosphates reduce zinc mobility, since resulting zinc phosphate is poorly soluble. Zinc solubility increases in the presence of mineral salts, carbon dioxide and hydrocarbonates in the soil solution.
Some industrial wastes, sulfuric zinc and complex polymicrofertilizers are used as zinc fertilizers.
Zinc sulfate (ZnSO4⋅7H2O), containing 25% of zinc, is a white crystalline powder, well soluble in water.
Zinc polymicrofertilizer – slag waste from chemical plants, for example, during the production of zinc whitewash. They are a dark-gray powder of varying composition. On average they contain 19,6% of zinc oxide, 17,4% of zinc silicate, 21% of iron and aluminium oxides, admixtures of copper, magnesium, manganese, boron, calcium, silicon, traces of molybdenum and other microelements.
Copper smelter slags may contain up to 2-7% zinc.
Application of zinc fertilizers in agriculture
Zinc fertilizers are applied when the content of zinc in mobile form in soils of the Non-Black Soil Zone is less than 0.2-1.0 mg/kg of soil, in the Black Soil Zone – less than 0.3-2.0 mg/kg of soil.
It is used for pre-planting, pre-sowing seed treatment and foliar dressing. When applied to soil, the dose is 3-5 kg/ha of zinc. Copper smelter slags at a dose of 50-150 kg/ha (depending on zinc content) or zinc sulphate are suitable for this purpose.
To reduce binding processes into inaccessible forms, frites obtained by fusing broken glass with micronutrients and subsequent grinding, and chelates are used. In these compounds, trace elements are well soluble in water and accessible to plants, they are not fixed by soil.
Seed pre-treatment is carried out by spraying or powdering with zinc sulfate. Seed treatment requires a 0.05-0.1% solution (2-4 g zinc sulphate per 4 liters of water). For 100 kg, 6-8 liters of the solution are used. To sprinkle corn seeds, 100 g of polymicrofertilizer per 100 kg. Sprinkling of seeds is carried out with zinc sulfate powder or polymicrofertilizer. For better adhesion to the seeds, it is mixed with talc. For 100 kg of seeds 400-500 grams of fertilizer.
Root feeding is carried out with a solution of zinc sulfate. Consumption is 100 g per 100 liters of water per 1 ha of seeds; if aerial spraying is used, 150-200 g per 1 ha. For foliar feeding of row crops by land sprayers, 100 gr per 300-400 liters of water per 1 ha is used. Foliar feeding of fruit crops is carried out by foliar application on dormant buds (2-3% solution), as well as during vegetation of plants (0.05-0.1% solution). Vineyards are sprayed with 0.05% solution during vegetation. In a solution of zinc sulfate add 0.2-0.5% hydrated lime to neutralize excessive acidity of the solution, to prevent leaf scorch.
Zinc fertilizers show good results on sugar beets, corn (grain), vineyards, alfalfa, fruit crops and some vegetable crops.
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry/Under ed. B.A. Yagodin. – M.: Kolos, 2002. – 584 p.: ill.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.