Question

Point out the mobile nutrients in soil

• A. $Cu^{2+}, NO_3^-, Fe^{2+}, H_3BO_3^-$
• B. $NO_3^-, Cl^-, H_2PO_4^-, MoO_4^{2-}$
• C. $NO_3^-, Cl^-, H_3BO_3$
• D. $NO_3^-, Cl^-, H_3BO_3^-, NH_4^+$

Hint:

Anions (like $NO_3^-$ and $Cl^-$) and neutral molecules (like $H_3BO_3$) do not bind to the negatively charged soil clay particles, making them highly mobile and susceptible to leaching.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Nutrient mobility in soil is governed by the chemical charge of the nutrient ions and their interaction with the solid soil phase (clay minerals and organic matter).
Because soil clay particles and humus have a net negative charge, they exhibit cation exchange capacity (CEC), attracting and retaining positively charged cations.
Anions, carrying negative charges, are repelled by the soil matrix (anion exclusion) and remain dissolved in the soil solution, moving freely with water.

Step 3: Detailed Explanation:
Let us analyze the mobility of the nutrient groups listed in the options:
- Group (C): $NO_3^-$ (Nitrate), $Cl^-$ (Chloride), and $H_3BO_3$ (Boric acid):
These are highly mobile in the soil.
Nitrate ($NO_3^-$) and Chloride ($Cl^-$) are highly soluble anions that do not bind to the cation exchange complex, making them highly susceptible to leaching.
Boric acid ($H_3BO_3$) is a neutral, uncharged molecule at standard soil pH levels, meaning it does not interact electrically with clay surfaces and moves freely with soil water.
- Let us evaluate why other options are incorrect:
- Option (A): Contains $Cu^{2+}$ and $Fe^{2+}$, which are cations that bind strongly to the soil CEC and organic matter, making them highly immobile.
- Option (B): Contains dihydrogen phosphate ($H_2PO_4^-$), which binds strongly to iron, aluminum, and calcium oxides via chemisorption, making it highly immobile.
- Option (D): Contains ammonium ($NH_4^+$), which is held by the soil cation exchange complex and is therefore relatively immobile compared to anions.
Thus, Group (C) contains only highly mobile soil nutrients.

Step 4: Final Answer:
The highly mobile nutrients in soil are $NO_3^-$, $Cl^-$, and $H_3BO_3$.