Micronutrient Functions in Plants

  • Most micronutrients are part of the enzyme systems of plants.
  • Micronutrients play important roles in redox reactions, photosynthesis, N fixation, and protein synthesis.
  • It is complex to identify all roles. The identified roles:
    • Cl is primarily involved in osmotic and cation neutralization.
    • Cu, Fe, and Mo are active in the oxidation-reduction reactions in plants, which are essential for photosynthesis, and other metabolic processes.
    • Fe involved in chlorophyll synthesis and can be deficient in turfgrass, especially if the soil is alkaline. Any condition that restricts root growth (compaction, for example) can also cause iron deficiency.
    • Zn and Mn are essential for certain N transformations in plants.
    • Ni is essential for the function of the urease enzyme.
    • Mo is essential for the symbiotic fixation of N.
    • B is involved in cell division, and seed formation.
    • Co is essential for the growth of symbiotic microorganisms, such as rhizobia, free-living N-fixing bacteria.

Soil Factors That Influence Micronutrients Availability

  • Soil organic matter (SOM)
    • Deficiency of micronutrients is more likely to occur in soils with low SOM.
    • Usually the greater the content of active (not lignified) soil organic matter the greater the availability. This is due to the release of the micronutrients through the decomposition of OM and chelating compounds.
  • Soil pH
    • As pH decreases, the availability of Fe, Zn, Cu, Mn, Ni, and B increases
    • As pH increases, the availability of Mo increases
  • Redox potential (soil wetness and aeration)
    • Fe, Cu, and Mn are more available under waterlogged than aerated conditions
    • In well-drained (aerated or oxygenated) soils, pH controls the availability of Fe and Mn.
  • Soil texture
    • Deficiency of micronutrients are more likely in coarse texture, sandy soils
  • Mycorrhizae
    • Mycorrhizal fungi increase the uptake of micronutrients, especially Zn and Cu

Chlorine (Cl)

  • Recent research has shown many areas in PNW have soils that are low in Cl.
  • Residue removal (90% Cl remains in the residue).

Boron (B) 

  • Probably the micronutrient that is most commonly deficient.
  • pH>7.0 can cause B availability problem.
  • Sandy soils are more likely deficient.
  • Crops that have a high demand for B: alfalfa, broccoli, cauliflower.
  • Crops are sensitive to B: tomato, corn, peppers, so have to be careful with rotations with these vegetables.

Molybdenum (Mo)

  • Soil pH is the most important factor affecting Mo availability.
  • At low pH values, Mo is adsorbed by silicate clays and the oxides of Al and Fe, and Mo is not easily available when in this state.
  • Liming the soil increases availability.
  • It is easy to over-apply Mo and creates a toxic situation.

Chelates Plays an Important Role in Micronutrients Uptake

  • Chelates are soluble organic compounds that bond with metals, especially Fe, Zn, Cu, and Mn, increasing the solubility of the metals, and hence their supply to the roots.
  • Natural organic chelates in soils are products of microbial activity and degradation of soil OM and plant residues.
  • Many natural organic chelates have not been identified; however, compounds such as citric and oxalic acids formed during decomposition and exuded by roots have chelating properties.
  • Artificial chelates are widely used in micronutrient fertilizer applications.

Mobility of Micronutrients in Plants

  • Mobile (symptoms appear in older tissue): Cl, Mo
  • Immobile (symptoms appear in younger tissue): Fe, Zn, Cu, Mn, B, Ni

Mobility of Micronutrients in Soils

  • Mobile (leachable): Cl, B
  • Immobile: Zn, Fe, Cu, Mn, Mo

Iron (Fe)

Iron Deficiency in Plants

  • Young leaves develop inter-veinal chlorosis, which progresses over the entire leaf
  • Chelate dynamics increases Fe availability

Toxicity could happen in poorly drained soils.

Zinc (Zn)

Zn Deficiency in Plants

  • Yellow or white areas between leaf veins
  • Reduced tillering
  • Shortening of stem or stalk internodes, bushy, rosetted leaves
  • Small, narrow, thickened leaves, malformed by growth of only part of leaf tissue
  • Premature foliage loss
  • Malformation of fruit

Manganese (Mn)

Mn Deficiency in Plants

  • Interveinal chlorosis, start from younger leaves
  • Reduced root and shoot growth rate
  • Increases potential for root and leaf diseases

Chloride (Cl)

Cl Deficiency in Plants

  • Chlorosis in younger leaves and overall wilting
  • Chlorotic lesions on leaves (physiologic leaf spot)

Copper (Cu)

Cu Deficiency in Plants

  • Chlorosis (leaf spot) in young leaves
  • Young leaves become yellow and stunted
  • Lodging, wilting
  • Increased disease

Cu Toxicity Symptoms

  • Reduced shoot vigor
  • Poorly developed and discolored root systems
  • Leaf chlorosis

Boron (B)

B Deficiency in Plants

  • Yield reduction in small grains, poor pollination, grain set and fill
  • Thickened, wilted, or curled leaves
  • Thickened, cracked, water-soaked petioles and stems
  • Darkened, cracking, rotting fruits or roots: brown heart, black heart, hollow-hart in peanut

Molybdenum (Mo)

  • Legumes (such as Soybeans, Alfalfa, Clover) need Mo for nodule formation