Micronutrients in Crop Production

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

Organic matter

  • Soils very low in organic matter will be deficient in micronutrients
  • Sandy soils are more likely to show micronutrient deficiencies than clay soils
  • Micronutrient availability generally decreases as the soil pH increases, with the exception of Mo.

Management and climatic variability

  • Soil moisture and temperature are important. For example, cold, wet soils restrict root growth, reducing the area of soil explored by the roots. Some soils can be cool and wet in the spring, at which time micronutrient deficiencies may show up, but disappear when the soils warm up
  • Deficiency of one of the macronutrients (N, P, K, or S) may also restrict the ability of the plant roots to explore for other nutrients. For example, P is important for early root formation and growth
  • Micronutrient deficiencies generally appear as patches in fields because micronutrient levels can vary greatly across landscapes
  • Land application of livestock manure can increase the amount of available Cu and Zn.

What do you do when you suspect a micronutrient deficiency?

  • The best and most important measure of performance of a micronutrient treatment is to determine if the yield increase - or, for some crops (e.g., potatoes), improvement in quality - will cover the costs of the micronutrient application and return some profit, i.e., obtain “an economic yield response.”
  • Look for “multiple evidence” before recommending a micronutrient for a whole field. A combination of crop symptoms in the field, tissue tests, soil tests, test strips, cropping history, and other techniques will be used to confirm micronutrient deficiencies and economic yield responses.
  • Many factors, such as macronutrient deficiency, drought, salinity, disease, insect, herbicide injury, or other physiological problems, can cause poor or stressed plant growth. Stressed growth may show symptoms similar to those of micronutrient deficiencies.
  • Crop nutrient deficiency symptoms are specific to each crop.

Micronutrient Supplied by Dew

Sulphate (salts)

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.

Oxide

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.

Chelate

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.

Copper (Cu)

  • If you suspect a Cu deficiency in wheat, barley or canary seed (crops most sensitive to Cu deficiency) or flax, alfalfa (less sensitive than wheat) based on a soil or tissue sample, consider a foliar application on a test strip. If there is a Cu deficiency in that field, the result will be an economic yield response
  • Copper is immobile in soil
  • Solubility and plant availability of Cu is highly dependent on soil pH. Copper solubility increases approximately 100 fold for each unit decrease in soil pH
  • Copper deficiencies will most likely show up first in wheat, barley, oats or canary seed, as these crops are highly sensitive to Cu deficiency. Canola, rye, flax, and forage grasses are much less sensitive to Cu deficiency. Crop cultivars can differ widely in sensitivity to Cu deficiency. Where Cu and Zn are both deficient, they both must be applied to obtain a yield increase.
  • High levels of soil P can also depress Cu absorption by plant roots creating the Cu deficiency.
  • Avoid blending Cu sulphate fertilizer with other fertilizers. The blend readily absorbs moisture

Manganese (Mn)

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 (Zn)

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.

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