Zinc (Zn) is an essential micronutrient but Zinc deficiency appears to be the most widespread and frequent micronutrient deficiency problem in crops worldwide particularly in countries such as India, China, Pakistan and Turkey where soils are low in available Zn, and cereals are the major source of calorie intake. It is, therefore, highly important to develop cost-effective and quick solutions to the Zn deficiency problem.
Zinc deficiency is common on many soil types including calcareous, heavy clay, alluvial, sandy soils and peats. It is especially common on soils low in organic matter and of high pH. Availability of zinc may be reduced by water logging and where root growth is restricted. Cool wet weather, low light intensity or high soil nitrogen, phosphorus or copper may intensify the deficiency.
Zinc is an essential micronutrient needed not only by people but also by crops. Almost half of the world’s cereal crops are deficient in zinc, leading to poor crop yields. Many agricultural countries around the world are affected by zinc deficiencies. In China, zinc deficiency occurs on around half of the agricultural soils, affecting mainly rice and maize.
In India, zinc-deficient soils occupy almost 50% of the agricultural area and are a critical constraint on yield, but crops are highly responsive to zinc fertilization.
In Turkey, major yield and quality benefits in wheat have been obtained with the widespread use of zinc fertilizers, where half of the cereal growing land is zinc-deficient.
Research has shown that areas with zinc-deficient soils are often regions with widespread zinc deficiency in humans.
A basic knowledge of the dynamics of Zn in soils, understanding of the uptake and transport of Zn in plant systems and characterizing the response of plants to Zn deficiency are essential steps in achieving sustainable solutions to the problem of Zn deficiency in plants and humans (Zinc in soils and crop nutrition), International Fertilizer Industry Association (IFA)and International Zinc Association (IZA).)
Zinc deficiency varies from year to year. Wet, cool, cloudy weather during the early growth season increases the deficiency. Zinc deficiency in corn is occasionally noted in June, but the deficiency disappears after the soils dry out and warm up. Crops on poorly drained
organic soils show a deficiency probably because of restricted root growth. Field and vegetable crops often show differences in response to zinc fertilizer. Dry edible beans, corn, onions, sorghum, snapbeans, spinach and sweet corn are the most responsive crops.
High soil phosphorus levels have been known to induce zinc deficiency, especially in responsive crops.
For years, the cause of this interaction was suspected to be the formation of an insoluble zinc
phosphate, which reduced the concentration of zinc in the soil solution to deficiency levels. Zinc phosphate has since been shown to be soluble in soil and is an acceptable source of zinc when finely ground. High levels of phosphorus in plants have been shown to restrict zinc
movement within the plant, resulting in accumulation in the roots and deficiency in the tops. Therefore, large applications of phosphorus fertilizer may contribute to zinc deficiency in zinc-responsive crops.
Sources of zinc
Among various inorganic sources, zinc sulphate hepta hydrate (ZnSO4.7H2O) containing 21-22%.Zn is found the most efficient, commonly available, economically cheapest zinc source for correcting Zn deficiency in most of the crops and diverse soils as compared to sparingly soluble.Zn sources, chelates and mixtures. Mono hydrated zinc sulphate (33%) and hepta hydrated zinc sulphate containing (21% Zn) were found equally efficient for correcting zinc deficiency either through soil and foliar application.
Regular application of farm yard manure (FYM)at 10-15 tonnes /ha if applied , it helps in mitigating deficiencies of all the micronutrients in long run. However if farm yard manure is not applied annually, one should apply on alternate year to prevent emerging micronutrient deficiencies. When adequate FYM is not available, and 4-5 t/ha is added then one may reduce 50% of the recommended dose of FYM.
Application of 2.5 t/ha poultry manure or piggery manure is efficiently correct the zinc deficiency in different crops.
If biogas slurry is applied then application of 2.5 kg Zn/ha is adequate to get good yield in highly deficient soils. Green manuring is found helpful in enhancing availability of native micronutrients of soil and enhances yield.
Synthetic Zn-EDTA chelates were found better than zinc sulphate in combating Zn deficiency in crops in non calcareous loamy sand soils but at par in calcareous and Aridisols soils. However, their high cost than zinc sulphate made it chetated fertilizers most uneconomical and less effective
for common use.
The efficiency of sparingly soluble Zn sources such as ZnO, ZnCO3, zinc fritts in fine textured high Zn fixing soils was at par to highly soluble ZnSO4.7H2O, however, when sparingly soluble and soluble zinc sources were compared in coarse textured soils, it was soluble zinc sources which gave best performance.
Micronutrient blended macronutrients sources such as zincated urea, zincated super and boronated super were found initially inferior to zinc sulphate or chelates in highly Zn or B deficient soils as they mismatched with the nutritional requirements of crops but long term effect of zincated urea, boronated super became as efficient as that of soluble sources
in controlling hidden/emerging micronutrient deficiencies in several crops.
Zinc oxide was found effective for roots dipping and seed coating before transplanted crops than zinc sulphate. Zinc phosphate was found efficient source for seed coating to control hidden hunger and cheaper than ZnO. Zinc frits release Zn slowly in soil.
Rate of zinc application
Amount of zinc required for alleviating zinc deficiency varied with severity of deficiency, soil types,nature of crops and cultivars. In majority of instances 5.5 kg zinc ha was found to be ideal dose.
Zinc deficiency can be best alleviated with the use of 11 kg Zn ha to wheat, rice and maize; 5.5 kgZn ha to soybean, mustard, raya, sunflower and sugarcane and with 2.5kg Zn ha groundnut, ragi, gram, linseed, green gram, lentil etc.
Soils testing very low, low and marginal needs 50, 25 and 12.5 kg zinc sulphate /ha to get optimum yields. Initially higher doses of zinc application are useful but in subsequent crops only.12.5 kg zinc sulphate /ha to each crop is enough to meet the zinc requirement of crops.
Alkali soils are generally deficient in Zn and Ca. So higher yields of rice, wheat, berseem and other crops in such soils can't be achieved unless the toxicity of Na/ deficiency Ca and Zn are corrected simultaneously. Use of 9-10 kg Zn ha to highly alkaline soil and 4.5 kg Zn ha in reclaimed alkali soils for rice-wheat/mustard/barley was found optimum.
Zinc requirement of crops in alkali soils is reduced substantially by 25-75% depending upon the levels of amendments added (25-100%GR) or reducing level of sodicities.
Fertilizer Zn requirement of crops was found to be double in coarse textured loamy sand soil than in fine textured loam or clayey soil for wheat and rice.
Methods of application
Basal application of Zn to soil through broadcast and mixed or its band placement below the seed proved superior to top dressing, side dressing or band placement, foliar sprays or soaking or coating of seeds /seedling in Zn solution/slurry as well as transplanting Zn enriched nursery because of later led delayed cure of Zn deficiency than basal use.
Foliar feeding of crops with application of 0.5 to 2.0% ZnSO4.7H2O solution is the supplement of soil application but it is not a substitute. In field crops it proved inferior in case of zinc and boron;however, in horticultural and plantation crops foliar feeding of crops with repeated foliar sprays of or
boron generally excelled to their soil application.
Seed coating of Zn materials like concentrated zinc Zn; zinc phosphate was found good in correcting Zn deficiency in bold size seed crops in marginally deficient soils. But these are ZnO proved superior but inferior to soil application of Zn in highly Zn deficient soils. However, seed treatment to potato with ZnSO4.7H2O solution proved equally effective as that of its foliar sprays or soil application.
Dipping of rice seedlings in 2-4% ZnO slurry before transplanting proved less effective with other sources in combating Zn deficiency and could not catch up with the farmers because of certain limitations as that of zinc. Dipping of vegetable seedlings in ZnO suspension and sugarcane sets could not meet full Zn requirement of these crops.
Time of application
Time of zinc application mainly depends upon its content in seed or severity of its deficiency. Best time of zinc addition is prior to sowing or transplanting of crops because maximum zinc absorption by plants takes place upto tillering or preflowering stages.
Split application of zinc sulphate in rice is recommended as 50% at the time of sowing or transplanting and remaining 50% before or upto tillering stage.
Basal application of zinc to soil is found the best. However, if it is missed, zinc deficiency can be corrected by top dressing of zinc upto 45 days. Seed coating with ZnO, Teprosyn zinc, zinc phosphate slurry successfully corrected deficiency in marginally deficient soils.
Foliar sprays of 0.5% zinc sulphate two to three times at 7-10 days interval just after appearance of its deficiency can control zinc deficiency more efficient and effectively. If deficiency persists then continue more sprays.