Micronutrient

Chemistry and Management

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Introduction

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Role of Micronutrients

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Cycling of Micronutrients

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Sources of Micronutrients

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Deficiency versus Toxicity

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Management of Micronutrients

Introduction

 Of the 18 elements known to be essential for plant growth, nine are required in such small quantities that they are called micronutrients  Iron (Fe), manganese (Mn), zinc (Zn), nickel (Ni), copper (Cu), boron (B), molybdenum (Mo), cobalt (Co), and chlorine (Cl).

 Animals including humans also require most of these elements in their diets.

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Introduction

The term micronutrient or trace elements do not imply that these elements are somehow less important than the macronutrients  In fact, the effects of micronutrients deficiency can be very severe in terms of stunted growth, low yields, etc.

 When micronutrients are needed, small quantities produce dramatic results  Most micronutrients come from soil parent materials. All micronutrients are found in varying quantities in igneous rocks.

Interest in Micronutrient Chemistry

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Intensive plant production practices have increased crop yields, resulting in more removal of micronutrients from soil.

Use of high analysis fertilizers has reduced use of impure salts and organic manures which formally supplied some micronutrients.

Better analytical methods of plant nutrition are helping to diagnose micronutrient deficiencies that may have been unnoticed.

In some cases, food grown on soils with low levels of trace elements provide insufficient human dietary levels of certain elements that the crops show no signs of.

Problems of toxicity due to oversupply of elements.

Role of Micronutrients

 Micronutrients play very complex roles in plant nutrition – E.g., Participate in the functioning of several enzyme systems – But their specific functions in plant & microbial growth processes varies. (See Table)  Cu, Fe, Mo, act as electron carriers in enzyme systems  Zn, Mn act as bridges between enzymes and their substrates  Mo, Mn, are needed for N transformations  Ni is a component of urease  Zn is needed in protein synthesis  See Table for details

Element

Fe

Function

Component or co-factor for many enzymes. Essential for chlorophyll formation.

Mn Activator of many enzymes; essential for photosynthesis, N metabolism, and N assimilation.

Zn Present in several enzymes. Promotes growth hormones. Promotes seed maturation and production.

Ni Essential for enzymes. Needed for grain filling, seed viability. Needed for Fe absorption, and urea metabolism Cu Present in several enzymes. Important in photosynthesis, protein and carbohydrate metabolism, and probably N fixation; Stimulates lignification of all plant cell walls.

B Activates certain enzymes. Facilitates sugar translocation. Essential for cell division and development.

Mo Co Cl Essential for N transformations (fixation and N assimilation). Activator of enzyme in nitrate reduction.

Essential for nitrogen fixation; found in vitamin B 12 Influences photosynthesis and root growth

Deficiency Symptoms

Interveinal chlorosis of younger leaves.

Interveinal chlorosis of younger leaves.

Small chlorotic leaves. Scallop leaf edges.

Non-viable seeds, toxic levels of urea in legume leaves Pale yellow-bleached appearance of leaves, lack of firmness in leaves & stems. Die back of tips of stems.

Chlorosis of younger leaves. Internal browning of stems and fruit. Many flowers abort.

Uniform chlorosis of whole plant. Extreme curling of leaves giving the whip-tail effect.

Not known Not known

Deficiency & Toxicity of Micronutrients

    At low levels of a nutrient, deficiency and reduced plant growth may occur (deficiency range). As the level of nutrient is increased, plants respond by taking more of the nutrients and increasing their growth. If the level of nutrients is sufficient to meet plant needs (sufficiency range), raising the level further will have little effect in plant growth. At some level of availability, the plant will take up too much of the nutrient for its own good (toxicity range) causing adverse physiological reactions to occur (see graph)

Deficiency Versus Toxicity

Micronutrients Cycling in Soils

Plant and animal residues/wastes Minerals and precipita tes (oxides, silicates, sulfides, etc) Ions in soil solution Chelates in soil solution Adsorbed on clay and humus colloids Drainage water Soil organisms Soil organic matter

Source of Micronutrients

 Parent materials influence the micronutrient content of soils  Deficiencies and toxicities of micronutrients may be related to: – The total contents of the elements in the parent rocks – The chemical forms of the elements in the rock – Solubility and availability of the elements  Forms of Micronutrients – Inorganic forms – silicates, oxides, sulfides, etc – Organic forms – complexes of humus and the elements

Conditions under which micronutrients my be limiting:

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Highly leached acid sandy soils Organic soils Soils that have been intensively cropped and fertilized with macronutrients Soils very high in pH (except for Mo) Eroded soils Parent materials

Conditions under which micronutrients may be toxic:

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soils low in pH (except for Mo) soils with freshly exposed 1  minerals containing these or other heavy metals mine tailings (because of a & b) Other inorganic reactions

Soil Management and Micronutrient Needs

Soil Management and Micronutrient Needs

 Always keep in mind the conditions under which micronutrient deficiencies and toxicities are likely to occur – – Changes in soil acidity Soil moisture – Fertilizer applications – Plant selection and breeding