Micronutrients

General Facts

• Micronutrients are essential elements, required by crops through all the stages of proper growth.
• A deficiency in any one of these means that efficient nutrient regulation, metabolic processes, chlorophyll synthesis, production of carbohydrates for energy, reproductive growth, fruit and seed development, may not be possible and will lead to reduced crop yields.
• Even though essential trace elements are needed in parts per million of soil for normal growth, it is important that they be available and in a balanced fashion.
• Significant yield losses may occur when micronutrients are marginally deficient, before symptoms are even visible. In some cases with micronutrients, once deficiency symptoms can be seen, the damage done is irreversible.
• Zinc, Manganese and Copper, on its own or as a combination in a blend, are most commonly applied in-furrow or as a foliar in production regions across Australia.
• Zinc, Copper and Iron (in lower concentrations) are essential micronutrients involved in the process of photosynthesis. Toxic heavy metals can replace Magnesium in the chlorophyll molecule in plant tissue and at high concentrations, it influences photosynthesis negatively. In this case, Cu is more important than Zn and Fe.
• Although Iron is the most abundant micronutrient in the soil, note that it may not be available to plant roots due to pH, its interaction with other micro’s and in some soils that are water-logged.
• Cations (such as Zinc, Manganese, Copper and Iron) are positively charged so they can bind to negatively charged clay and organic matter.
  • Cations are most soluble in acidic conditions.
  • Cations are deficient in most calcareous soils.
• Anions (such as Boron and Molybdenum) are negatively charged and thus subject to leaching.
• Modern intensive farming methods have highlighted the need for correct timing, placement and quality formulations of micronutrient applications.
• Tissue analyses are the preferred way to detect, prevent and treat micronutrient deficiencies efficiently.
• The balance between organic matter in soil, soil solution, soil pH, cation exchange sites and insoluble compounds of micronutrients, all determine micronutrient availability.

Function In Plants

Zinc

Function in plants:

• Protein synthesis.
• Present in many enzymes and essential for production of plant hormone such as auxin.
• Results include improved growth and vigour.

Deficiency symptoms (similar to that of Manganese and Iron):

• Visible interveinal chlorosis in pale green younger leaves.
• Stunted plants due to shorter internodes.

Common occurrence:

• Can be found in over-limed or alkaline soils (high soil pH).
• Seedlings and young plants exposed to extended cold and wet weather conditions, especially in sandy and calcareous grey clay soils.
• Extreme soil moisture levels, very dry or water-logged

*Note: Conditions may vary. Please confirm with your agronomist after soil and tissue testing has been done.

Manganese

Function in plants:

• Photosynthesis and respiration.
• Results include increased sugar and protein content and green colour with an improved tolerance for high light intensity.

Deficiency symptoms (similar to that of Zinc, Magnesium and Iron):

• Visible interveinal chlorosis in pale green younger leaves.
• Stunted plants due to shorter internodes.

Common occurrence:

• Sandy, calcareous soils that are prone to leaching with high rainfall events.
• Other soil types that have had high rates of lime applied.

*Note: Conditions may vary. Please confirm with your agronomist after soil and tissue testing has been done.

Copper

Function in plants:

• Chlorophyll formation.
• Cell wall building.
• Drives many enzymatic mechanisms.
• Involved in seed formation and reproduction.
• Sufficient levels result in higher sugar content and improved flavours of produce.

Deficiency symptoms:

• Twisting of flag leaf in cereals
• Tips of young leaves die back.
• Crops show yellowing of leaves with stunted growth.
• Poor seed set
• Yellow or pale green leaves wilt and die eventually.

Common occurrence:

• Plants take up less Cu as soil pH increases.
• May be found in soils where high rates of nitrogen have been applied.
• Found in low organic matter soils and sandy soils with low exchange capacity.
• Copper deficiencies are mainly reported on sandy soils which are low in organic matter.
• Increased phosphorus availability in the soil lessens copper uptake by plants.

*Note: Conditions may vary. Please confirm with your agronomist after soil and tissue testing has been done.

Iron

Function in plants:

• Chlorophyll formation (not chlorophyll synthesis but its action with N) and energy production.
• Present in several enzymes for nitrate and sulphate reduction.
• Cell division, supporting plant growth.
• Iron is needed in the highest quantity of all micronutrients in plants.
• Higher N-applications mean more Fe will be required by crops.

Deficiency symptoms (similar to that of Manganese and Zinc):

• Visible interveinal chlorosis in pale green younger leaves.
• Stunted plants due to shorter internodes.

Common occurrence:

• Iron is less available in alkaline soils due to high pH, becoming insoluble.
• Not common, but may happen in water-logged alkaline or calcareous soils.

*Note: Conditions may vary. Please confirm with your agronomist after soil and tissue testing has been done.

Molybdenum

Function in plants:

• Symbiotic N-fixing bacteria in legumes need Mo to fix atmospheric-N.
• Essential component in enzymes that convert nitrates into ammonium and when deficient may lead to grazing stock losses due to nitrate toxicity.
• Required in the smallest quantities of all the essential micronutrients.
• Mo is the only micronutrient that is mobile in plant tissue.
• Extreme deficiency or toxicity are not very common.

Deficiency symptoms:

• Symptoms are similar to nitrogen deficiency
• In non-legume pasture species, molybdenum deficient leaves may have a mottled pale appearance, stunted growth and burn on the margins of mature leaves.

Common occurrence:

• Molybdenum is less available in acidic soils.
• Molybdenum deficiency is also more likely to occur on highly weathered limestone soils that are low in native phosphorus, potassium and calcium.
• Deficiencies usually show first in legumes such as clovers, medic and lucerne.

*Note: Conditions may vary. Please confirm with your agronomist after soil and tissue testing has been done.

Boron

Function in plants:

• With no + or – charge it is extremely mobile in soil and can easily leach out of light soils. Note: relatively immobile in plant tissue.
• Essential component in structures of cell walls, especially “carrying” calcium into plant cells.
• Cell division depends on Boron where active growth is required, like the tips of roots and new leaves.
• Carbohydrate/sugar, the product of photosynthesis, needs to be stored elsewhere in plants where it is readily available for cell division and Boron regulates the speed of this movement.
• Essential for normal development of root nodules in legumes such as lucerne.
• Essential in regulating plant hormone levels for normal growth and reproduction functions like cell wall formation, elongation of roots, flower initiation and seed development.

Deficiency symptoms:

• Rather immobile in plants, symptoms begin at vegetative growing points – yellowing and eventually death.
• Stems become thick and cracked and eventually hollow and brittle.
• Splitting of flag leaf in cereals.
• Rosettes, side shooting, and stunted bushy growth appear.
• Buds and blossoms start dropping before fruit development commences.
• Thick and twisted roots, failing to develop properly.
• Fruits and seed may also be affected. Brown sunken areas may develop in fruit.

Common occurrence:

• More commonly found during dry spells when microbial activity is reduced and movement in the soil solution to plant roots is restricted.
• Toxic levels at depth may be found in clay soils in semi-arid regions (Mallee in Victoria).
• With high mobility and possible leaching in certain soil types, more frequent but at lower rates may be required for sensitive crops.

*Note: Conditions may vary. Please confirm with your agronomist after soil and tissue testing has been done.