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 $0.1\text{ N}$: $W = N \times E \times V = 0.1 \times 49.0 \times 1.0 = 4.9\text{ g}$.

Chemical classification of nitrification vs urease inhibitors:

• Chloro-6-(trichloromethyl)pyridine (Nitrapyrin / N-Serve): Classic synthetic nitrification inhibitor suppressing Nitrosomonas bacteria.
• Amino-4-chloro-6-methylpyrimidine (AM): Proven chemical nitrification inhibitor.
• Dicyandiamide (DCD): Widely commercialized organic nitrification inhibitor.
• Phenyl phosphorodiamidate (PPD): An inhibitor of the urease enzyme, NOT a nitrification inhibitor. Hence, (A), (B) and (D) only form the true set.

Composition of Bray's extractant for available soil phosphorus:

• Bray and Kurtz No. 1 extractant is universally employed for estimating available phosphorus in acidic soils ($\text{pH} \lt 6.5$).
• It is composed of $0.03\text{ N NH}_4\text{F} + 0.025\text{ N HCl}$.
• The fluoride ion complex-binds with aluminum and iron, solubilizing aluminum and iron phosphates.