Soil Fertility, Plant Nutrition & Fertilizers

• Inceptisols (young, embryonic soils with minimal horizon development widely found in the Indo-Gangetic alluvial plains of India) are predominantly characterized by non-expanding 2:1 layer silicates, specifically illite (hydrous mica).
• Montmorillonite (expanding 2:1 smectite) is dominant in Vertisols (black cotton soils).
• Kaolinite (1:1 non-expanding silicate) is dominant in highly weathered Ultisols, Alfisols, and Oxisols (red and lateritic soils).
• Therefore, illite is the dominant clay mineral in Inceptisols (Option 2).

Calculation of potassium dichromate equivalent weight:

• Molecular weight of $\text{K}_2\text{Cr}_2\text{O}_7$: $(2 \times 39) + (2 \times 52) + (7 \times 16) = 78 + 104 + 112 = 294\text{ g/mol}$.
• In acidic redox titration (e.g., Walkley-Black organic carbon method), each dichromate ion gains $6$ electrons: $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$. Valency factor $n = 6$.
• Equivalent weight: $E = \frac{294}{6} = 49.0\text{ g/equivalent}$.
• Weight required for $1000\text{ mL}$ of $0.1\text{ N}$: $W = N \times E \times V = 0.1 \times 49.0 \times 1.0 = 4.9\text{ g}$.

Phosphorus ionic species and uptake dynamics:

• Statement (I) is correct: The distribution of orthophosphate ionic species is strictly pH-dependent. At $\text{pH} \lt 7.2$ (acidic soils), monovalent dihydrogen phosphate ($\text{H}_2\text{PO}_4^-$) is the predominant ionic species, whereas above $\text{pH } 7.2$ (calcareous/alkaline soils), divalent monohydrogen phosphate ($\text{HPO}_4^{2-}$) predominates.
• Statement (II) is incorrect: Plant root cell membranes absorb the monovalent $\text{H}_2\text{PO}_4^-$ ion roughly $10\text{ times}$ more rapidly and preferentially than the divalent $\text{HPO}_4^{2-}$ ion; they are not absorbed in equal amounts.