On the soil: Molybdenum can be found:

• As part of the structure of certain minerals such as calcium molybdate and molybdenum oxides.
• As a changeable anion adsorbed by soil colloids.
• Linked to organic matter.
• In soil solution.

The soil’s pH is the main factor determining the assimilability of Molybdenum.

The absorption of Molybdate anion by soil colloids becomes less frequent as pH raises, drastically falling above pH6 and practically non-existent above pH8.

In the plant:

• Mo is absorbed in the form of molybdate, not excluding the possibility of complex formation with certain components in the plant. There is an antagonistic effect of the sulfur anions (sulfates) due to direct competition with the places of root absorption, as well as other synergistic of the phosphate.
• There are also certain sugars associated with Mo mobility in the plant.
• The normal concentration of Mo in certain grains (eg: pea), allows normal mobility in the plant.

Physiology of Mo: It is a component of nitrogenase and nitrate reductase, the first one needs Mo to fix N and the second one to reduce NO3 -.

Fixing N:

• Nitrogenase fixes N in NH3 form, associating two different proteins, one of which contains Fe and Mo. The base mechanism is the same as that performed by fixing bacteria in symbiosis with higher plants.
• Mo creates favorable conditions in the synthesis of nucleic acids.
• The deficiency of Mo can generate chlorosis due to poor synthesis of chlorophyll and is associated to the metabolism of Fe and phosphoric acid.

Mo in crops

Mo deficiency:

• The normal needs of plants of Mo are low.
• In alkaline soils, the concentration of soluble Mo is higher; unlike other microelements acid soils can cause availability problems. Here the addition of organic matter is advised to avoid mineral fixation of Mo.
• The retention of Mo in ferruginous soils is particularly important, presenting deficiencies with a certain normality in this case.

Interactions:

It has been found that phosphorus facilitates Mo absorption and mobility. This has been observed after the application of fertilizers rich in P for the formation of phosphomolybdate. Sulfur acts contrary to P.

The presence of iron oxides in the soil hinders the assimilation of Mo.

Also, worth mentioning is the antagonism between Cu and Mo; aggravating the deficiency of one microelement by applying the other. The application of Cu aggravated the deficiency of Mo in spinach and in cauliflower, and conversely, the application of Mo aggravated the deficiency of Cu in carrots, spinach and lettuce.

There is also certain antagonism between Mn and Mo. Excessive Mn contents in nutrient solutions led to a low absorption of Mo in beet and cauliflower.

Cauliflower, clover, lettuce and spinach are especially sensitive to Mo deficiency.

The leguminous because of the fixing bacteria’s needs are more demanding and in cereals, oats are the most sensitive to deficiencies with a blue coating of the grain.

In citrus, Mo deficiencies in leaves, “yellow spot” are characteristic.

Other examples are deficiencies visible on cauliflower leaf and melon, which typically present “spoon” type deformations and marginal discolorations.