Home Soil Science Micronutrients: Factors That Affect Their Depletion and Availability

Micronutrients: Factors That Affect Their Depletion and Availability



Any country can be held to liberate if it does not have food security. We have to take appropriate measures to see that we are self-reliant on the agriculture situation. The soil contains different types of elements which are essential for the development of a healthy plant.

Soil overuse; when one practices intensified agriculture of raising more crops per unit-time and over dependence on high analysis fertilizers depletes the micronutrient reserves in the soil. Such soil in the near future will fail to sustain agricultural produce.

In India, the depletion of soil micronutrient is a cause for concern. Most soils in India are generally poor in fertility as they have consistently been depleted of their nutrient resources due to continuous cultivation. The micronutrient deficiencies of zinc, iron, manganese and boron are widespread and are associated with specific soils or soil cropping systems. This has resulted in the soil becoming a poor food crop producer.

The states severely affected include Punjab, Haryana, Bihar, Uttar Pradesh, Madhya Pradesh, Gujarat, Tamil Nadu and Andhra Pradesh. In most of these states micronutrient deficiencies are widespread.

Farming practice; responsible for micronutrient depletion:

The national thrust has been on maximization of food production to feed its expanding population. This has resulted in the depletion of micronutrient reserves. Modern agriculture, which relies heavily on intensive cultivation and use of fertilizers, has depleted the micronutrient reserves of the soil.

Unless immediate steps are taken to replenish the micronutrient reserves, the soil will lose all its fertility. The improper or imbalanced use of fertilizers has worsened the problem at many places.

The increasing use of fertilizers generally devoid of micronutrients has brought about the problem related to micronutrient deficiencies by depleting the resources in the soil. Although fertilizers are essential for increasing production, they should be used very judiciously.

A number of extensive experiments on manuring and fertilization have resulted in the depletion of the available micronutrient status of soils, particularly zinc. Pesticides have nothing to do with aggravating the micronutrient problem.

Factors that affect the soil micronutrient depletion and availability: A few important factors which affect the soil micronutrient status are extensive cultivation of fertilizer-responsive high-yielding varieties on marginal soil.

Large-scale deforestation has also led to the reduction of organic matter and depletion of soil micronutrient. In addition, long-term application of nitrogenous and phosphatic fertilizers and intensive cropping (raising of more crops per unit time involving dependence of high analysis fertilizers) have played a major role in depleting the soil micronutrient levels.

Poor Drainage: Highly leached acidic sandy soils; resulting in leaching of micronutrients, thus deficiency of micronutrients occur.

Soil Temperature and moisture: Soil temperature and moisture are important factors. Cool, wet soils reduce the rate and amount of micronutrients that may be taken up by crops.

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Cropping Intensity and Systems: The total dry matter produced determines extent of micronutrient removal. Intensification of agriculture, characterized by raising of more crops per unit time and involving heavy dependence on high analysis fertilizers has progressively depleted the soils of their available micronutrient reserves. Researches in this field have depicted depletion in available micronutrient status of soils and particularly in zinc.

Management and cultivation Practices: Reduced available contents of Zn, Mn and Cu under extremely acidic environments of permeable acid soils result from the steady leaching of the soil solution containing these nutrients and reduced concentration of these nutrients in soil solution engineers dissolution from the solid phases. Large scale deforestation is responsible for progressive decline in organic matter and depletion of micronutrient cations in tarai soils of Uttaranchal, Uttar Pradesh and soils of Andman and Nicobar islands. Emergence of Khaira disease of rice due to zinc deficiency is an example of deforestation.

Doses and nature of fertilizers: Intensively cropped soil with high doses of commercial fertilizers and long term application of nitrogenous fertilizers plays a major role in removal and consequential depletion of the micronutrients in soil. In the long run, soil acidity induced leaching will leave the soil poorer with respect to these elements.

Parent Material and Climate: Soils developed on acid eruptive rocks, already low in trace elements, have characteristically low available trace element status. Soils developed on flood plain alluvium largely from siliceous sandstone exhibited strikingly lower concentrations of Zn, Cu, Mn and Fe than some chromusterts developed on basaltic alluvium. Climates acts as a major determinant of the available status of micronutrients. Eruption of zinc deficiencies on soil marginally supplied with available Zinc during severe winters is an example of role of climate. In high rainfall areas, availability of B decreases due to leaching.

Soil Texture: Soil texture affects how well nutrients and water are retained in the soil. Clays and organic soils hold nutrients and water much better than sandy soils. As water drains from sandy soils, it often carries nutrients along with it. This condition is called leaching. When nutrients leach into the soil, they are not available for plants to use. Soils with higher amounts of clay (fine texture) are less likely to be low in plant available micronutrients. Sandy soils (course texture) are more likely to be low in micronutrients.

Organic Matter in Soils: Soils low in organic matter (less than 2.0%) may have lower micronutrient availability. Soils that have very high levels of organic matter (greater than 30% organic matter to a depth of 30 cm) often have low micronutrient availability.

Soil pH (a measure of the acidity or alkalinity of the soil): Soil pH is one of the most important soil properties that affect the availability of nutrients. Macronutrients tend to be less available in soils with low pH and micronutrients tend to be less available in calcareous and saline-alkaline soils with very high in pH e.g. UP, Punjab and Bihar soils with the exception of molybdenum.

Methods of micronutrients application: There are several ways to supply micronutrients to crops, soil application, fertigation, foliar spray, seed treatment, or combination with crop protection products. Each option has specific advantages and disadvantages depending on the nutrient, the crop and the soil characteristics. Similarly, the different product types (e.g. chelated/non-chelated, organic/inorganic, granular/fluid) have respective relative strengths. The various options have quite different impacts on the solubility and availability of the micronutrients, on the uniformity of application, as well as on health, safety and the environment. The common methods of micronutrient application as below:

a. Soils Application: The require quantities of materials are broadcast or placed by adding dry soil or fine sand before planting the crop e.g. B,Cu,Zn.

b. Seed soaking: Low concentration of micronutrient solution is used to soak the seed for about 12 hours before planting e.g. Mo.

c. Seed coating: Micronutrient mixed with a small amount of soil made into a pest is coated around the seeds, dried and then used for sowing e.g. Mo.

d. Addition through fertilizers: Uniform of spreading of the micronutrients essential for different regions are added to the spread fertilizer or to fertilizer mixture used e.g. phosphates mixed with boron, molybdenum or zinc. The most commonly-used methods are the following:

  • Dry mixing: Dry mixing  is a simple method that works well with non-granular materials. But, there are often caking problems.
  • Bulk blending: Bulk blending is a form of dry mixing, but with granular material. The main problem is segregation of the different components, unless all the materials have similar particle sizes. Segregation generally leads to uneven application. However, it lowers the macronutrient grade.
  • Complex fertilizers: This option might lead to reactions that will affect the solubility of the nutrients. Moreover, unless a micronutrient deficiency is widespread, it is often uneconomical to produce small lots of special grades.
  • Foliar Application: Low doses of micronutrients are applied through sprays on plant foliage. Crops in younger stages require less solution, while crops more foliage or fruit trees like oranges, require more solution for spraying e.g. Fe, Mn, B. Micronutrients are generally applied in combination with crop protection sprays or with foliar application of macronutrients. They are generally available as prepared mixtures to prevent reactions that create water-insoluble compounds.

Impact of soil micronutrient on animal and human health: One should be clear that animals and humans depend largely on plants for their mineral requirements and the plants in turn depend on soil micronutrients for their growth and yield. Although, these micronutrients are required in extremely small quantities, they are nevertheless as important as macronutrients. A large number of these micronutrients are very essential to animals and humans for their growth and reproduction since these micronutrients are building blocks for all life forms. A deficiency in iron and zinc is common in humans and zinc and copper in animals.

Application of high doses of lime: Lime can be added to the soil to make it less sour (acid) and also supplies calcium and magnesium for plants to use. Lime also raises the pH to the desired range of 6.0 to 6.5. In this pH range, nutrients are more readily available to plants, and microbial populations in the soil increase. Microbes convert nitrogen and sulfur to forms that plants can use. Lime also enhances the physical properties of the soil that promote water and air movement.

Chelating compounds: To increase the availability of micronutrients and make them slowly available over a longer period, chelated compounds are formed. For this Chelating agent e.g. EDTA is commonly used. This agent combines with iron, copper, calcium or magnesium to form chelated compounds that supply secondary nutrients of micronutrients. The use of also some synthetic Chelating agents are also used e.g. HEDTA, DTPA, EDDHA, NTA. The use of chelated compounds of micronutrients has become very important for correcting micronutrient deficiencies particularly in horticultural crops.

Soil Sampling and Testing: Soil tests aid in determining whether a particular nutrient is responsible for poor production and provides the basis for deciding the type and amount of fertilizer needed to correct a nutrient shortage. A soil sample used for laboratory analysis must consist of a composite of a number of samples taken from the field. Surface soil samples (0-1 5 cm) should be taken from both the affected area and an adjacent area of good crop growth for comparison. When a soil sample tests low in a micronutrient, a potential micronutrient deficiency may occur. Some soils with low micronutrient levels at the surface (0- 1 5 cm) do not respond to fertilization because they have higher levels of the nutrient in the subsoil. The DTPA (diethylenetriamine pentaacetic acid) method is used to extract the metal micronutrients.

Tissue Sampling and Testing: Plant tissue tests can aid in determining if a particular nutrient is responsible for poor crop growth. When a deficiency is detected by tissue sampling, a reduction in yield due to restricted crop growth has already occurred. As with soil analysis, tests involving plant tissue must be calibrated with field fertilizer trials. Calibration in this case is far more complex than for soil tests. The reason is that measured nutrient concentration, which is the basis of the tests, varies considerably with the stage of plant development and the portion of the plant. Special care is required in taking plant tissue samples. Representative plant tissue samples can be taken early in the growing season to assist in the interpretation of soil tests. For small grain crops the entire above ground portion should be sampled. Normally, 25 plants should be sampled to provide a good representation of the field and ensure a sufficient quantity of the sample for complete analysis. The fresh samples should be air-dried to remove excess moisture before they are send to the laboratory.

Conclusions: The government has taken a few important steps but more needs to be done. Firstly, the government has to set up coordinating centers in all the states to study the soil micronutrient problems and bring about corrective measures.
However, there is also a need to strengthen the soil testing laboratories with experts and modernize equipment. Polyclinics should be set up for providing micronutrient advisory services to the user for improving the soil productivity along with other services. If micronutrient deficiencies that constrain high productivity are not identified, monitored, and alleviated, the fertilizer use efficiency of costly chemical fertilizers and other agricultural inputs will be markedly reduced. The projected micronutrient demand for 2025 in India is expected to increase by five to 10 times the present consumption level. Areas affected by micronutrient deficiencies, the second component of importance, are challenging to estimate. Generally, approaches to defining the area of impact consider only the topsoil levels of micronutrients. The challenge for the fertilizer manufacturers, distributors and farmers is to find low cost-effective means to continually update information on the locations and areas affected by micronutrient deficiency. This would entail the use of fertilizer nutrients in the right proportion, with efficient application methods and at the appropriate time of crop growth. Also, it is essential to augment the availability of native nutrients by modifying soil environment through management practices in different crops and cropping systems.

Contributed By:

Dr. Sushant*, Dr. V.K. Sharma**, Susheel Kumar* and Dr. C.P.Tiwari*

*: Department of Agriculture Chemistry and Soil Science, Narain College, Shikohabad, (U.P.)

**: Department of Soil Science, Indian Agricultural Research Institute, New Delhi