Question

In highly acidic pH, the following nutrients are available in toxic amounts in soil:
(A). Fe
(B). Mg
(C). Mo
(D). Mn
Choose the correct answer from the options given below:

• (A) and (B) only.
• (B) and (C) only.
• (A) and (C) only.
• (A) and (D) only.

Hint:

Except for Molybdenum, the availability of all micronutrients (Fe, Mn, Zn, Cu) increases as soil pH decreases.
Under highly acidic conditions, Fe and Mn solubility increases so much that they frequently reach phytotoxic levels.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil pH directly influences the chemical solubility and biological availability of plant nutrients.
Extreme soil pH levels can cause nutrient deficiencies or increase nutrient solubility to toxic levels.

Step 2: Detailed Explanation:
In highly acidic soils (pH \( < 5.0 \)):
- Iron (Fe) (A) and Manganese (Mn) (D) become highly soluble as their oxide and hydroxide mineral forms dissolve in the abundance of hydrogen ions:
\[ \text{Fe(OH)}_3 + 3\text{H}^+ \rightleftharpoons \text{Fe}^{3+} + 3\text{H}_2\text{O} \]
This substantial increase in solubility can result in concentrations of \( \text{Fe}^{2+} \) and \( \text{Mn}^{2+} \) in the soil solution that are toxic to plants, causing cellular damage and physiological disorders.
Aluminum (\(\text{Al}^{3+}\)) toxicity is also highly prevalent in these soils.
In contrast:
- Magnesium (Mg) (B) is a basic cation.
Under highly acidic conditions, basic cations are leached out of the soil profile, leading to severe magnesium deficiency rather than toxicity.
- Molybdenum (Mo) (C) is unique among micronutrients because its availability decreases as pH drops.
It is adsorbed strongly by iron and aluminum oxides in acidic soils, resulting in deficiency.
Therefore, only Iron (Fe) and Manganese (Mn) are available in toxic amounts under highly acidic pH.

Step 3: Final Answer:
The nutrients available in toxic amounts are (A) and (D) only.