© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

The micronutrients are elements essential for plant growth, but used in amounts smaller than for macronutrients. Sometimes the term trace elements is used.

Plant need for the micronutrients is one or more orders of magnitude smaller than for the macronutrients.

Micronutrient management importance becomes more pronounced as the following have increased

  • plant removals
  • purity of macronutrient fertilizers
  • lab analysis sensitivity
  • understanding of micronutrient roles in human nutrition

Deficiency versus Toxicity

  • Most micronutrient plant sufficiency range soil concentrations are not as broad as for the macronutrients.
  • Toxicities to plants and to humans and animals consuming the plant material are often of concern.
  • Toxicities of micronutrients (mostly metals)
    • Related non-nutrient heavy metals may be due to natural soil conditions (e.g., extreme acidity, poor aeration, high natural metal content)
    • To human activities such as contamination of soil with industrial wastes (e.g. Cu, Zn, Ni)
    • Over application of micronutrient fertilizer (e.g., B, Mo).
  • Deficiencies of micronutrients are most common on
    • Acid, highly leached sandy soils (low total content of most micronutrients)
    • Organic soils (low total content and complexation with copper)
    • Soils of very high pH (except for Mo which is typically deficient at low pH)
    • Soils that have been depleted by intensive cropping.

Roles of Micronutrients in Plants

  • Most micronutrients function as components, or activators of various enzymes that control all aspects of plant physiology.
  • Deficiency of many of the micronutrients produces characteristic symptoms on the plant.
  • A small dose of a needed micronutrient can often produce a dramatic increase in plant growth and vigor.

Source of Micronutrients

  • The primary source of the micronutrients is the primary mineral from which the original igneous rock was formed
  • Many micronutrients are also found in secondary silicates
    • In association with iron and manganese oxides
    • As exchangeable cations.
    • Organic forms are also important
      • copper
      • nickel
      • boron
  • Borate and molybdate undergo anion adsorption reactions
  • In the soil solution, iron, zinc, copper, manganese, cobalt, and nickel are found as simple cations
  • In the soil solution, boron, molybdenum, and chloride exist as dissolved anions.

Factors Influencing Availability of the Micronutrient Cations

  • The cationic micronutrients (iron, manganese, zinc, copper, nickel, and cobalt) all are influenced by soil pH and oxidation state
    • Solubility generally increasing as pH is lowered
    • For iron and manganese solubility increases as conditions become anaerobic and the are reduced.
    • Overliming commonly leads to deficiencies of one or more of these micronutrients.
    • Cationic micronutrients react with certain organic molecules to form organo-metallic complexes called chelates
      • If these complexes are soluble, the chelation increases the availability of the micronutrient metal
      • Protects the metal from precipitation reactions.
An organo-metallic complex in which two or more active groups on a single organic ligand are coordinated with a single metallic cation. The chelating agents in soils may be natural compounds produced by decomposer microorganisms or excreted by roots, or they may be synthetic chelating compounds added to enhance micronutrient availability.
  • Mycorrhizal fungi acting symbiotically with higher plants have been shown to:
    • Increase micronutrient uptake from deficient soils
    • To decrease the uptake of potentially toxic metals like copper and zinc from unusually high levels.

Factors Influencing the Availability of Anionic Micronutrients

  • Chlorine is present in soils mainly as the chloride ion which is leaches quite freely with the drainage water.
    • May accumulate to toxic levels under arid, especially irrigated conditions
    • Rarely deficient.
  • Boron is present primarily in the form of undissociated boric acid.
    • Most commonly deficient of the micronutrients.
      • Lime-induced boron deficiency can occur above pH 7
        • The adsorption of this element to both clay and humus increases as pH increases.
      • Most available in acid soils
        • Easily leached from acid sandy soils and so is often deficient on this type of soil.
  • Molybdenum availability is mainly a function of soil pH
    • The one micronutrient which becomes most available at pH values above neutral
    • Becomes quite insoluble in acid soils
    • Liming acid soils usually alleviates Molybdenum deficiency (b/c increase in pH).
    • Molybdenum availability seems to be improved by the presence of phosphate ions
    • Molybdenum availability decreased by sulfate ion

Need for Nutrient Balance

  • Micronutrient uptake and function of many micronutrients are influenced by the availability of other nutrients.
    • both macro- and micro-.
    • Synergistic is plant growth improvement from the combination of nutrients together is greater than the sum of the growth responses to each individual nutrient
      • For instance, copper utilization is favored by adequate manganese
      • Some plants need both zinc and phosphorus for optimum use of manganese.
    • Negative interactions, or antagonisms, also occur.
      • Manganese and iron can interfere with each other's uptake and utilization
      • Zinc and copper can interfere with each other's uptake and utilization

Soil Management and Micronutrient Needs

  • Micronutrient deficiencies are most common on:
    • Leached sandy soils
    • Mucks
      • Muck (sapric) describes a layer or horizon of dominated by soil organic matter (SOM) were virtually all of the SOM has undergone sufficient decomposition to limit recognition of the plant parts.
    • Soils having very high pH
      • Calcareous and high pH soils are prone to deficiency of iron, manganese, copper, and zinc, while molybdenum may be toxic
    • Soils having very very low pH values
      • In very acid soils, toxicity of manganese and iron, as well as deficiency of molybdenum (and the macronutrient phosphorus) are to be expected.
  • Maintaining Micronutrient availability can be promoted by:
    • Keeping soil pH 6.0 to 7.0 usually avoids most, but not all, pH- associated micronutrient problems
    • Flooding and poor soil aeration (reducing conditions) increases manganese and iron availability
      • Reducing conditions can increase to the point of toxicity in some soils
    • Maintaining SOM
      • Liberal additions of plant and animal residues and manures.
      • Organic amendments improve micronutrient availability because
        • They contain significant quantities of these nutrients
        • They form chelating agents that improve the availability of the supplies already in the soil
    • Inorganic micronutrient supplements
      • Most effectively applied either as chelates to the soil
      • As foliar sprays
    • Where soil pH is conducive, larger amounts of inexpensive inorganic salts