Micronutrients in Soil

Introduction

It is known that at least 16 plant food elements are necessary for the growth of plants. These plant nutrients are called as essential elements. In the absence of any one of these essential elements a plant fails to complete its life cycle, the disorder caused can be corrected by the addition of that element. Out of 16 essential element 7 nutrients viz. Iron, Manganese, Boron, Zinc, Cooper, Molybdenum and Chlorine are used by field crops in very small quantities and hence called as micronutrients. These are also called as trace, minor or rare elements. Micro- nutrients are as essential to plant growth as the macronutrients.

Important functions and deficiency symptoms

1. Iron:

Functions – Helps in chlorophyll formation, absorption of other nutrients. Essential for the synthesis of proteins contained in the chloroplasts.

Deficiency - Causes chlorosis between the veins of leaves, the veins remaining green.

2. Manganese:

Functions- Acts as catalyst in oxidation and reduction reactions within the plant tissues. Helps in chlorophyll formation, supports movement of iron in the plant, counteracting the bad effect of poor aeration.

Deficiency- Leads to chlorosis in the inter veinal tissue of net veined leaves and parallel vein leaves. In cereals it produce grey streak, white streak dry spot and lip spot, marash spot, streak disease and pahala blight in sugarcane, yellow diseases in spinach and beans.

3. Boron:

Functions- It is a constituent of cell membrane and essential for cell division. Acts as a regulated of potassium/calcium ratio in the plant, helps in nitrogen absorption and translocation of sugars in plant.

Deficiency- In lucerne yellows and rosetting, snakehead in walnuts, die back and corking in fruits, corking and pitting in tomatoes, hollow stem and bronzing of curd Cauliflower, brown heart disease in table beets, turnips etc.

4. Zinc:

Functions- Constitute of several enzyme system which regulate various metabolic reaction in the plant. Associated with water uptake and water relation in the plant.

Deficiency- Deficiency symptoms appear in younger leaves starting with interveinal chlorosis leading to a reduction in shoot growth and the shorting of internodes. Mottle leaf, little leaf etc.in the case of trees, the buds of severely deficient maize plants become white, interveinal chlorosis and mottled leaf occur in citrus.

5. Copper:

Functions- Act as "electron carrier" in enzymes, helps in utilization of iron in chlorophyll synthesis. It neutralizes the harmful conditions in certain peat soils when applied in large quantity.

Deficiency-Variation in deficiency symptoms occurs in case of copper e.g. multiple bud formation, staining and splitting of fruits, dieback of shoots, the marginal or spotted necrosis and cholorsis of leaves.

6. Molybdenum:

Functions- Acts in enzyme systems which bring about oxidation reduction reactions. Essential for the process of atmospheric nitrogen fixation.

Deficiency –Reduces the activity of the symbiotic and non-symbiotic nitrogen fixing micro-organisms. Produces whiptail in cauliflower, broccoli and other Brassica crops.

7. Chlorine:

Functions- The exact role which, chlorine plays in plant nutrition has not yet been clearly defined. It requires for proper plant development e.g. sugarbeets, carrots, lettuce, barley, wheat, cotton and clovers. From the point of view of soil fertility, plants requires one kg of chlorine for each four thousand kg of dry matter which they produce.

Deficiency- Plants display symptoms of wilt, chlorsis, necrosis, and an unusual bronze discoloration on tomatoes.

Soil condition causes to micronutrient deficiency

1. highly leached acidic sandy soils;
2. soils with a high-water table;
3. soils with a very high content of organic matter e.g. peat and muck soils of Kerala;
4. calcareous and saline-alkaline soils very high in pH e.g. UP, Punjab and Bihar;
5. intensively cropped soil with high doses of commercial fertilizers;
6. application of high doses of lime at one time.

Range of micronutrient concentrations required for normal plant growth

Source: Chemical fertilizers and range of application

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.

Methods of application

The common methods of micronutrient application are given below:

1. 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.
2. 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.
3. Addition through mixed 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.
4. Seed soaking: - Low concentration of micronutrient solution is used to soak the seed for about 12 hours before planting e.g. Mo.
5. 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.