• Assertion (A) is True: In submerged paddy soils, the redox potential ($E_h$) drops rapidly from $+400\text{ to }+600\text{ mV}$ (aerobic) down to $-150\text{ to }-250\text{ mV}$ (strongly reduced).
• Reason (R) is True: Depletion of dissolved molecular oxygen by aerobic microorganisms forces facultative and obligate anaerobic bacteria to utilize alternative terminal electron acceptors ($ ext{NO}_3^-, ext{Mn}^{4+}, ext{Fe}^{3+}, ext{SO}_4^{2-}$).
• Relationship: Both statements are accurate soil biogeochemical facts, but Reason (R) describes sequential microbial reduction rather than the primary operational definition of redox potential.
• Conclusion: Both (A) and (R) are correct but (R) is NOT the correct explanation of (A) (Option 2).

Correct option: (2) Both (A) and (R) are true but (R) is NOT the correct explanation of (A).

• Amendments in Calcareous Soils: Calcareous soils contain free calcium carbonate ($CaCO_3$) that buffers soil pH above 7.5–8.2. Ameliorating high calcareousness requires acid-producing amendments:
• Aluminium sulphate: Hydrolyzes in soil water to produce sulphuric acid and aluminum hydroxide.
• Ammonium polysulfide: Undergoes microbial oxidation to yield sulphuric acid while supplying ammonium-N.
• Elemental sulphur: Oxidized by soil Thiobacillus bacteria into sulphuric acid: $2S + 3O_2 + 2H_2O \rightarrow 2H_2SO_4$.
• Sulphuric acid: Directly neutralizes $CaCO_3$ into soluble calcium sulphate (gypsum).
• Conclusion: All four materials (A, B, C, D) are effective amendments. (Correct answer: Option 3)

• Equivalent Acidity of Ammoniacal Nitrogen Fertilizers:
• Equivalent acidity is defined as parts by weight of calcium carbonate ($ ext{CaCO}_3$) required to neutralize the acid-forming residual effect of $100$ parts of fertilizer:
• Ammonium Nitrate: Equivalent acidity $\approx 60$ $\rightarrow$ (C)
• Ammonium Sulphate Nitrate: Equivalent acidity $\approx 93$ $\rightarrow$ (D)
• Ammonium Sulphate: Equivalent acidity $\approx 110$ $\rightarrow$ (A)
• Ammonium Chloride: Equivalent acidity $\approx 128$ $\rightarrow$ (B)
• Correct Sequence of Increasing Equivalent Acidity: Ammonium nitrate ($60$) $\lt $ Ammonium sulphate nitrate ($93$) $\lt $ Ammonium sulphate ($110$) $\lt $ Ammonium chloride ($128$).
• Conclusion: Sequence is (C), (D), (A), (B), corresponding to Option 2.

Correct option: (2) (C), (D), (A), (B).