- 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 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, Mn are more available under waterlogged than aerated conditions
- In well-drained (aerated or oxygenated) soils, pH controls availability of Fe and Mn.
- Soil texture
Deficiency of micronutrients are more likely in coarse-texture, sandy soils
Mycorrhizal fungi increase the uptake of micronutrients, especially Zn and Cu
- Recent research has shown many areas in PNW have soils that are low in Cl.
- Residue removal (90% Cl remains in the residue).
- 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.
- 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 create 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.