The following are the major characteristics of micronutrients;
They are present in small amounts in the earth’s crust and surface soils. The range is about 20 to 1000ppm except for iron which occurs in about 5% of the earth’s crust.
They all originate from weathering of primary minerals where they occur as impurities mixed with micronutrient cations.
Clay soil is a good source of micronutrients and they may be ten times as high as sandy soils. Calcareous soils generally are low in micronutrients except for boron as CaBo3.
They have the ability to undergo chelation reaction by:
Forming either covalent bond with other elements whereby there is sharing of one or two of their outermost electrons with another element as in H2.
Forming coordinate-covalent bonds between molecules which have incomplete shell orbitals or covalent orbitals such as in Boron tetrafluoride, HBF4: where a covalent orbital is provided by BF3 to accommodate two electrons supplied by HF.
A popular example of chelation or coordination bond formation is Ca – EDTA i.e. calcium-ethylenediaminetetraacetate. Ca is the element that is chelated to EDTA.
The formation of a coordination complex completely changes the solubility of the elements being chelated. For example, Fe is soluble at pH2.5, but Fe-chelate is only soluble at pH 9-10. Therefore, chelates are used widely to supply micronutrients in fertilizers.
The metal chelate is soluble when the organic chelating compound (ligand) is soluble; the reverse is true, the metal chelate is insoluble when the ligand is insoluble. Both soluble and insoluble chelates occur in soils.
As organic matter in soil decomposes, there is the formation of chelating agents, organic compounds which are capable of complexing with certain metallic cations by means of multiple chemical bonds.
The metallic cations are protected from leaching losses in the soil and at the same time made absorbable by plants. Chelate complexes are formed by Fe, Zn, Cu, and Mn. One common iron chelate is Ferric ethylene-diamine tetraacetate (Fe EDTA).
Micronutrients are only added when it is certain that there is a deficiency of the nutrient. The soil contents of micronutrients are generally regarded as sufficient. When deficiency occurs, micronutrients are applied as sprays to the crop.
Deficiency or Toxicity of Micronutrients are Affected by Soil Factors Such as Follows:
The micronutrients may be deficient in soils formed from parent materials in which they are absent.
The micronutrients rarely act independently; the presence of one affects the uptake of the other. For example, excess Mn suppresses the uptake of Fe. In other words, they have negative relationships.
pH: Metallic micronutrients (Fe, Cu, Zn, and Mn) are soluble under acid conditions or solutions; but Mo is more soluble at high pH. The word “soluble” here means that they are so soluble that they may be washed off the soil. Their toxicity can be cured by just liming to bring the pH up and therefore make them less soluble.
Redox state: The soil oxidation-reduction state affects the availability of micronutrient elements. They are less soluble at oxidized Ferrous Fe2+.
They have a common peculiarity in that their deficiency appears during rapid plant growth, then the deficiency disappears again. Ultimately, there may be no noticeable decrease in yield at the end of the growing season.
Intensive cultivation tends to increase the demand for micronutrients. If the soil cannot supply the demand for crops, deficiency may occur.
Microorganisms have requirements for micronutrients e.g. Cobalt. In the rhizosphere, microorganisms and plant roots tend to make micronutrients more available.
Read Also: Neutralizing Value of Liming Materials
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