Mo

Molybdenum Content – Forms in soils – Critical limits in soils and plants. Factors affecting their availability

The essentiality of Mo was established by Arnon and Stout (1939). The total Mo content of most soils varies from 0.2 to 5.0 ppm with a mean of 2 ppm.

Sources of Mo

Earth crust 2 ppm : and range from 0.2 to 5 ppm.

Forms of molybdenum: Molybdenum is generally present as

i) Part of the crystal lattice of primary and secondary minerals.

ii) Adsorbed as molybdate (MoO4=) on clay or exchange complex.

iii) Part of the organic matter.

iv) Water soluble compounds.

The available Mo content of Indian soils extractable with ammonium oxalate at pH 3.3 (Tamm’s extractant) varies from traces to 1.65 ppm.

Soil type Mo content

Alluvial soils 1.0 to 5.5 ppm

Black cotton soils 0.6 to 11.80 ppm

Laterite soils 1.34 to 2.0 ppm.

Critical limits in soil:

Ammonium oxalate extractable molybdenum is 0.05 to 0.2 ppm. Soils with less than 0.15 ppm of available Mo respond to the application.

Critical limits in plants:

0.1 ppm deficiency

0.5 ppm adequacy level

Not known toxic level.

Factors affecting the availability of molybdenum

1. pH: Availability of Mo is low in acid soils, with increase in pH the availability increases up to 7.9.

2. Organic matter: The role of organic matter in increasing the availability of Mo is due to the protection of molybdenum from anion adsorption.

3. Calcium carbonate: Liming of acid soils increases Mo availability. Liming makes the native molybdenum available by increasing its availability.

4. Texture: Mo availability increases with increase in clay content.

5. Phosphate and sulphate fertilizers: Phosphate and sulfate ions may compete with molybdate ions for exchange sites in the soil and plant.

6. Manganese: Mn induces Mo deficiency because of antagonism between them due to changes in pH.

7. N and K fertilizers: N fertilizers reduce the response to Mo in case of legumesdue to reduced biological N fixation. However, a positive interaction exists between K and Mo.

8. Oxides of iron and aluminum: Availability of Mo may depend upon the molybdate adsorbed to free iron oxides.

Molybdenum is a non metal anion absorbed as molybdate (MoO4). It is weak acid and form complex poly anions such as phosphomdybdate. Plant contains <1 ppm Mo.

Functions of Mo

· 1. Essential component of enzyme NO3 reductase, which catalyses NO3- to NO2-.

· 2. Structural component of nitrogenase enzyme - involved in N fixation.

· 3. Essential for absorption and translocation of Fe in plant

Deficiency of Mo

· 1. Inhibits flower formation

· 2. Imbalances various Amino Acids in plants.

· 3. Reduce activity of symbiotic and non symbiotic N fixation.

· Cauliflower - Whip tail.

Interaction with other nutrients

· Mg and P enhances Mo absorption by plants.

· High level SO4 decrease Mo absorption by plants.

· Cu and Mn decrease Mo uptake by plants.

· NO3-N encourages Mo uptake

· NH4-N reduces Mo uptake

· The beneficial effect of NO3- nutrition is related to the release of OH ions.

Reclamation of molybdenum deficiency

Molybdenum deficiency is relatively easily corrected, either by the application of small quantities of molybdenum to the soil, or by raising the soil pH. Application of sodium molybdate or ammonium molybdate at rates of 0.2-0.3 kg Mo/ha should be sufficient to correct the disorder in most situations, and may be effective for several years. Sodium molybdate may also be applied as a foliar spray. A solution of 50 g sodium molybdate/100 L water has proven successful with other crops such as sunflower. Some commercially-available fertilizer mixtures also contain molybdenum.

Add lime to soil before planting or sowing seeds.

Liming to raise the soil pH above 5.5 is usually effective in alleviating molybdenum deficiency, and may improve conditions for crop growth in other ways also, such as through improved availability of phosphorus and alleviation of manganese or aluminium toxicity.