Crops require 17 essential nutrients to grow normally. Carbon (C), hydrogen (H), and oxygen (O) derived from the air comprise greater than 90 per cent of the fresh plant tissue. Macronutrients derived from the soil and needed in large amounts are nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca), and magnesium (Mg).The soil supply of N, P, K, and S is often supplemented by fertilizer and manure. The remaining essential nutrients derived from the soil are referred to as micronutrients, because they are needed in small amounts. They are boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn). Micronutrients are also important for plant growth, as plants require a proper balance of all the essential nutrients for normal growth and optimum yield.
Copper is involved in several enzyme systems, cell wall formation, electron transport and oxidation reactions. Copper is not readily transferred from older to younger leaves. In cereals, older leaves remain green and healthy with the newer leaves yellowing and wilting, and the leaf tips pigtailing. Excessive tillering, aborted heads, delayed maturity, prolonged flowering period and poor grain filling are also symptoms. These symptoms appear in irregular patches within fields. These patches have a “drought-like” appearance. Copper deficiency is often associated with increased incidence of root rot, stem and head melanosis (purpling, appears as brown patches in the field at maturity).
Soil factors that affect the availability of micronutrients
Management and climatic variability
What do you do when you suspect a micronutrient deficiency?
Micronutrient Supplied by Dew
The sulphate form of micronutrients such as: Cu, Zn, Fe and Mn, represents a water-soluble form that is plant available. Borate is the equivalent plant available form for B. Sulphates are the most commonly used form for field crops. Sulphates can be applied to the soil or foliage. Sulphate products, applied at agronomically recommended rates, can provide long term residual value.
Micronutrient elements (Cu, Zn, Fe, and Mn) bonded with oxygen form oxides. The bonds with oxygen are very strong, meaning these products are not soluble in water and are not in plant available form. An oxide of a micronutrient needs to be converted to a plant available form in the soil before being taken up by the plant. Oxides represent the final form to which other forms are eventually converted, and may then be slowly converted back to plant available form. For crop response during the growing season, plant available forms (water-soluble forms) of micronutrients need to be used.
Micronutrients such as Cu, Fe, Mn, and Zn are held within ring-type compounds. Chelated micronutrients remain in plant-available form longer because the chelated structure slows the micronutrient reaction with soil minerals. There are a large number of chelating agents. Chelated micronutrients can be soil or foliar applied.
Manganese is a component in enzyme systems. Manganese activates several important metabolic reactions, aids in chlorophyll synthesis, accelerates germination and maturity, and increases the availability of P and Ca.
Zinc is involved in enzyme systems and metabolic reactions, and is necessary for production of chlorophyll and carbohydrates. In wheat and barley, the older leaves may have light blotches between the veins. Younger leaves will have a normal green colour and will be smaller. The best practice has been to broadcast and incorporate Zn as a pre-plant application. This should provide several years’ effectiveness. Chelates are foliar applied to correct Zn deficiency during the growing season.