Micronutrients

The micronutrients that are managed by growers and we will discuss include:

• Iron
• Boron
• Manganese
• Zinc
• Molybdenum
• Cobalt

There are three additional micronutrients that have been classified as essential, but are generally not managed by growers. These additional three nutrients, listed below, are rather managed under experimental conditions:

• Nickel
• Chlorine
• Cobalt

Forms and Functions of Micronutrients

Iron

• Form: Iron is taken up by plants as either Fe²⁺ (ferrous cation) or Fe³⁺ (ferric cation).
• Function: Iron is involved in photosynthesis, respiration, chlorophyll formation, and many enzymatic reactions.

Boron

• Form: Boron is taken up by plants primarily as H₃BO₃ (boric acid) and H₂BO₃^- (borate).
• Function: Boron plays an important role in the movement and metabolism of sugars in the plant and synthesis of plant hormones and nucleic acids. It also functions in lignin formation of cell walls.

Manganese

• Form: The primary form of manganese uptake is Mn²⁺ (manganous ion).
• Function: Manganese is a component of enzymes and is also involved in photosynthesis and root growth. Additionally, it is involved in nitrogen fixation.

Zinc

• Form: The Zn²⁺ cation is the predominate form taken up by plants.
• Function: Zinc is a component of many organic complexes and DNA protein. It is also an important enzyme for protein synthesis. Also, zinc is involved in growth hormone production and seed development.

Molybdenum

• Form: Molybdenum is primarily taken up as MoO₄^2- (molybdate ion).
• Function: It is involved in nitrogen fixation (conversion of N₂ to NH₄⁺) and nitrification (conversion of NH₄⁺ to NO₃^-).

Copper

• Form: Copper is taken up as Cu²⁺ (cupric ion).
• Function: Copper is also a component of enzymes, some of which are important to lignin formation in cell walls. It is also involved in photosynthesis, respiration, and processes within the plant involving nitrogen.

Cycling:

Iron

The iron cycle includes both mineral and organic forms.

Iron may exist:

• in the soil solution
  • includes soluble iron and organic matter complexes in the form of chelates
• as primary minerals and/or precipitated minerals
• cation exchange site on soil particles

Fe containing minerals may dissolve to replenish the soil solution as iron is removed by plants. Little iron is retained by the cation exchange sites of soil particles as compared to base and acid cations.

Organic cycling is an important process that ensures iron availability through the processes of mineralization and immobilization.

Iron Chelation

Iron can also form strong complexes with organic matter known as chelates (a Greek word meaning “claw”).  Chelation occurs between soluble organic compounds and certain metals in the soil through processes involving microorganisms. Chelates are very important in micronutrient management because chelation increases the solubility and plant uptake of many metal micronutrients. We will encounter chelation again when discussing zinc, copper and manganese.

Manganese

The manganese cycle is very similar to the iron cycle. The manganese cycle, too, has four fractions:

• manganese cations in soil solution
  • includes soluble manganese and organic matter complexes known as chelates
• exchangeable manganese on soil particles (cation exchange sites)
• primary and secondary manganese-containing minerals
• soil organic matter

Like iron, little manganese is retained by the cation exchange sites of soil particles. Manganese may undergo precipitation/dissolution, sorption/desorption on the CEC, mineralization/immobilization, and chelation.

Zinc

Zinc cycling includes:

• zinc cations in soil solution zinc
  • includes soluble zinc and organic matter complexes known as chelates
• zinc retained by soil particles on the cation exchange sites
• primary and secondary zinc-containing minerals
• soil organic matter

Zinc bearing minerals can dissolve and supply zinc to the soil solution. Once in the soil solution, zinc can be immobilized, taken up by plants, retained by soil particles, or chelated with soluble organic matter. Organic matter containing zinc must undergo mineralization before it becomes available for plant uptake.

Copper

Like Zinc, the copper cycle includes:

• Solution copper
  • Includes soluble copper and organic matter complexes known as chelates
• Exchangeable copper on the cation exchange sites of soil particles
• Primary and secondary copper minerals
  • Copper may be occluded, or buried, within the structures of various minerals, such as iron and aluminum oxides
• Organic copper
  • Copper is more tightly bound to organic matter than the other micronutrients
  • Copper deficiencies can occur in organic soils

Copper-containing minerals can dissolve and supply Zn to the soil solution. Like zinc, copper can be immobilized by microorganisms, taken up by plants, or exchanged on soil particle surfaces. Copper may also form chelates with soluble organic matter. Organic copper must be mineralized before it is available for plant uptake.

Molybdenum

Unlike the previous metal micronutrients, molybdenum exists as an anion in the soil solution. Nonetheless, the molybdenum cycle is similar to the others. The molybdenum cycle includes:

• Soil solution
• Exchangeable molybdenum on the anion exchange sites
• Primary and secondary molybdenum minerals
• Organic matter

Instead of being held onto the cation exchange capacity, molybdenum is held to soil particles with an anion exchange capacity (including amorphous materials, iron oxides, acidic kaolin clays). Organic molybdenum undergoes mineralization and immobilization.

Boron

Boron exists in the soil as:

• soil solution boron
• exchangeable boron on the anion exchange capacity sites
• primary and secondary boron minerals
• Boron and organic matter complexes

Boron is the only nonmetal micronutrient described in this section. H3BO3 is most common form of boron in soils that have a pH between 5 and 9. The exchangeable boron buffers changes in the boron levels of the soil solution. Organic matter supplies plant available boron. Boron should be carefully managed when applied to the soil since the range between boron sufficiency and toxicity levels is very narrow.

Factors affecting micronutrient availability

Iron

• Soil pH: The availability of iron may be limited in soils with high pH, especially in arid, calcareous soils.
  • Excessive liming can induce iron deficiencies.
• Soil Moisture and Aeration: Poorly aerated soils with excessive moisture in calcareous soil can promote iron deficiencies.
  • However, flooding of non-calcareous soils can improve iron availability.
• Organic Matter: Organic matter improves iron availability due to chelation, which increases iron solubility. Additions of manure can increase chelation.
• Interactions with other nutrients: Excessive amounts of other micronutrients, particularly copper, manganese, zinc and molybdenum, can decrease iron availability

Manganese

• Soil pH: Soils with high pH have limited manganese availability since manganese precipitates at high pH.
  • Overliming soils can cause Mn deficiencies.
• Soil Moisture and Aeration: High soil moisture and poor aeration increases the availability of manganese due to an increase in solubility.
• Organic Matter: Manganese availability increases with the addition of natural organic matter (i.e. compost) due to favorable chelation which increases the level of exchangeable and solution manganese.
• Climate: Wet conditions and warm temperatures increase manganese availability.
• Interactions with other nutrients: High amounts of copper, iron, and zinc may induce manganese deficiency.

Zinc

• Soil pH: Zinc availability decreases as pH increases.
  • Overliming decreases Zn solubility.
• Zn adsorption: Though the relative amount of zinc on the cation exchange capacity is low, zinc is attracted and held tightly to magnesite, dolomite and CaCO3 minerals. As a result, soils containing these minerals can develop zinc deficiencies.
• Organic Matter: Soluble zinc chelates increase zinc availability.
• Climate: Cool, wet weather generally has a negative effect on zinc availability.
  • Increasing soil temperatures increases zinc availability.
• Flooding: Flooding generally decreases zinc availability.
  • However, lowering the pH of flooded soils may increase zinc availability.
• Interactions with other nutrients: Copper, iron, manganese, and phosphorus can interfere with zinc uptake. 

Copper

• Soil texture: Copper availability is lower in highly leached, coarse textured soils.
• Soil pH: Copper availability decreases as pH increases, primarily due to decreased solubility of copper minerals.
• Organic matter: Copper forms very tight bonds with organic matter (more so than any other micronutrient), which may reduce its availability in organic (peat and muck) soils.
• Buried Cu: Copper may be occluded, or “buried,” within the structure of clay minerals and oxides. Occluded Cu is not available to plants.
• Interactions with other nutrients: Copper availability to plants may be reduced when zinc, iron, and/or phosphorus contents are high in the soil solution.

Molybdenum

• Soil pH: Unlike the other micronutrients, the availability of molybdenum increases with increasing pH.
  • As a result, liming acidic soils increases molybdenum availability.
• Fe/Al oxides: Molybdenum is strongly held onto the surfaces of aluminum and iron oxides, which reduces its availability.
• Interactions with other nutrients: Copper and manganese can reduce the uptake of molybdenum by plants. Phosphate enhances molybdenum uptake.
• Soil moisture: Low levels of soil moisture reduce molybdenum availability.

Boron

• Soil pH: Boron availability decreases as pH increases.
  • Liming can temporarily induce boron deficiencies, or lessen boron toxicities.
• Soil organic matter: Organic matter increases boron availability.
• Soil texture: Highly leached, coarse textured soils tend to have low boron availability.
• Plant factors: The range between boron sufficiency and boron toxicity is very narrow. Crop sensitivity to boron varies, and it is important to become familiar with the boron sensitivity of your crop.
• Interactions with other nutrients: Crops are less sensitive to boron when there is ample amount of calcium. This is because calcium acts to reduce boron availability. Boron may become deficient when the Ca:B range is greater than 1,200:1.
• Soil Moisture: Dry environments reduce the availability of boron.