Micronutrients

The elements that help plant growth are called plant nutrients. There are seventeen essential nutrients, some of which are required in large quantities (macro nutrients) and some in as small a quantity as about 50 ppm, which are called micronutrient or minor nutrients. These are chlorine, iron, boron, manganese, zinc, copper, molybdenum and nickel.
The solubility and availability of micronutrient ions are affected by the presence of complexing and chelating agents secreted from roots and produced during microbial degradation of residues.
Each micronutrient plays a specific role in the growth and development of the plant. Most micronutrients, especially the transition metals (Zn, Fe, Mn, Cu, Mo and Ni) are constituents of many metalloenzymes. They function in key metabolic events such as chlorophyll synthesis, photosynthesis, respiration, protein synthesis, nitrogen fixation, assimilation of nitrates and sulphates, etc. Micronutrients are, therefore, responsible for key processes in plants including those aimed at efficient utilization of even major nutrients.
Each micronutrient has specific functions in a plant system. Micronutrients are, to an extent, present in soils. Several soil factors influence their availability to plant absorption, thus determining their deficiency. However, both the deficiency and the requirement are dynamic factors. They vary with the type and properties of the soil, characteristics of the plant, the cultivation and management practices, the level of crop productivity, the supply of fertilizers, etc
Continuous removal of nutrients from soil reserves, the ever increasing intensity of crop cultivation with the use of high yielding varieties, and adoption of intensive fertilizer applications (major nutrients) cause deficiency of one or more micronutrients in soils and/or plants.
Deficiency of micronutrients in plants is associated initially with hidden deficiency and subsequently visible deficiency. The deficiency that has not expressed itself in the form of any signs or symptoms is called hidden deficiency. In the initial stages of such a deficiency, the plant adjusts itself to the situation. Diagnosing this deficiency is extremely difficult even by leaf analysis, unless the situation is critical. However, any positive crop response to the application of micronutrients is indicative of the existence of hidden deficiency. In fact, hidden deficiency in various crops is far more widespread than it is recognized.
Visible deficiency occurs when the plant system is unable to cope up with the hidden deficiency internally. The plant then loses its nutrient balance and expresses a deficiency situation by characteristic visible symptoms. By this time, the problem of micronutrient deficiency is already aggravated. The application of micronutrients at this stage would remedy the situation, but plants may not give yields to their potential. Hence, to obtain high yields, micronutrients have to be applied prior to the occurrence of hidden deficiency and not after that of visible deficiency symptoms.
Quantification of micronutrient deficiency: A micronutrient deficiency is often quantified based on the analysis of the soil/plant samples. The critical limits established for the soil and plants under experimental conditions are used to express the extent of deficiency in the field conditions. Despite limitations, this method is universally followed.
Most research on micronutrients has been mainly focused on a single nutrient basis (e.g , zinc deficiency or iron deficiency). Fertilizers made of a single micronutrient are used to correct specific deficiencies. Deficiency of more than one micronutrient in the same soil/crop can also exist. The intensity of deficiency varies from mild to severe. The problem of multimicronutrient deficiency could be more than what is currently recognized and is bound to increase further as more areas and crops come under investigation Multimicronutrient fertilizers are being largely used on crops grown under intensive cultivation systems, plantation crops, cash crops, vegetables, fruits, ornamentals, etc
Micronutrients are applied in many ways, including foliar application, seed treatment, root dipping of transplanted seedlings and application with pesticides. However, the most popular method is to apply micronutrients with primary nutrients that the farmer regularly uses. This helps him save the time he would have otherwise spent for micronutrient application. Also, by this method, the micronutrients can be uniformly applied when mixed with a much larger volume of fertilizers.