Question

Which of the following is the available form of Molybdenum to plants?

• A. $MoO_2^{2-}$
• B. $MoO_4^{2-}$
• C. $HMoO_4^-$
• D. $HMoO_4^{2-}$

Hint:

Plants absorb molybdenum as the molybdate anion, $MoO_4^{2-}$.
Unlike most micronutrients, its availability increases with soil pH, becoming highly soluble under alkaline conditions.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Molybdenum is an essential micronutrient required by plants in small quantities.
It is a key component of enzymes like nitrate reductase and nitrogenase, which are involved in nitrogen assimilation and biological nitrogen fixation.

Step 2: Detailed Explanation:
Plants absorb nutrients from the soil solution in specific ionic forms.
Molybdenum is taken up by plant roots primarily as the divalent molybdate anion, $MoO_4^{2-}$.
The availability of molybdate in the soil solution is highly dependent on soil pH, showing a trend opposite to that of most other micronutrients (like iron, manganese, and zinc).
In acidic soils (pH $< 5.5$), the molybdate anion is strongly adsorbed to the surface of iron and aluminum oxides, reducing its availability and sometimes causing deficiency symptoms, such as whiptail in cauliflower.
In alkaline soils (pH $> 7.0$), molybdate ions are released from these mineral surfaces into the soil solution, increasing their availability to plants.
The other chemical forms listed ($MoO_2^{2-}$, $HMoO_4^-$, and $HMoO_4^{2-}$) do not represent the primary bioavailable form absorbed by plant root transporters.

Step 3: Final Answer:
The available form of Molybdenum to plants is $MoO_4^{2-}$, which corresponds to Option (B).