Micronutrient elements General Characteristics

Micronutrients in Soils and Crops Soil Content (kg/ha) Corn Removal (kg/ha) Boron (B) 22-225 0.07 Copper (Cu) 2-450 0.06 Iron (Fe) 11,200 – 224,000 0.12 Manganese (Mn) 112-11,200 0.09 Molybdenum (Mo) 1-8 0.03 Zinc (Zn) 22-675 0.17

Forms taken up by plants Micro nutrient Forms in soil solution Iron Fe2+, Fe(OH)+, Fe(OH)2+, Fe3+ Manganese Mn2+ Zinc Zn2+ Zn(OH)+ Copper Cu2+ Cu(OH)+ Molybdenum MoO42- HMoO4- Boron H3BO3 , H2BO3- Cobalt Co2+ Chlorine Cl- nickel Ni2+, Ni2+ Specific forms present are determined by pH and soil aeration Under acidic conditions simple cation forms dominate Hydroxyforms –dominant under alkaline codns Under acid conds MoO42- reacts like phosphorus in alkaline soils B exists as anion

General Characteristics A.  Micronutrients are required by all plants but in smaller amounts compared to the macronutrients. Deficiencies of micronutrients can reduce growth just as much as deficiencies of macronutrients. Remember the “Law of the Minimum”.

General Characteristics B The amount of micronutrient that is needed by the plant can have a narrow range. If the amount of micronutrient is too high, it is possible for toxicity to occur. Available forms 1.  Cations: Fe2+, Fe3+, Mn2+, Zn2+, Cu2+ 2.  Anions: MoO42-, Cl- 3.  Neutral: H3BO3

Micronutrient function in plants 1.  Enzymes and coenzymes a)   Structural component of enzymes b)   Involved in the activation and regulation of enzymes 2.  Oxidation and reduction reactions Micronutrients are involved in electron transport

Transformations in the soil The transformations that micronutrients undergo in the soil vary depending on the nutrient, but overall, the reactions are similar to what has already been discussed for macronutrients. a)  Mineralization and immobilization b)  Adsorption and desorption to clay or organic matter surfaces c) Precipitation and dissolution of secondary minerals

Transformations in the soil -chelation process One transformation is quite different from transformations associated with macronutrients-- chelation. The word chelate means “claw”. Chelation refers to the process in which organic molecules in the soil form a complex with micronutrient ions.   The chelated micronutrients are carried by mass flow and diffusion to the root surface.

The chelation process in the soil The chelate is broken down in rhizosphere . The micronutrient is taken up by the root and the organic molecule diffuses away. The organic molecule can complex with more micronutrients and the process is repeated.

The chelation process in the soil a lot about the chelation of micronutrients is still unknown. The organic molecule that forms the chelate may be derived from: Natural chelates (1)  Organic compounds released by the plant roots (phytosiderophores) or soil microorganism, (Siderophores) (2)  Compounds released during the decomposition of soil organic matter, such as fulvic acids (3) Man-made chelates: EDTA, DTPA EDDHA etc

The chelation process in the soil Soluble chelates increase the availability of micronutrient cations: Fe, Zn, Mn, and Cu. The chelates help protect the ions from precipitation or adsorption reactions.

Factors that affect micronutrient availability   Solubility of the micronutrient (affected by factors such as pH)   Percent of exchange sites that are occupied by the micronutrient)  Soil organic matter (1)  Affects the balance between adsorption and desorption (2) Organic matter is also involved in the chelation process