Soil Physics

• Statement (I) is correct: In upland arid and semi-arid calcareous soils with high pH ($pH \gt 7.5$) and high free calcium carbonate ($CaCO_3$), iron precipitates into insoluble ferric hydroxides and carbonates ($Fe(OH)_3$), severely suppressing bioavailable divalent ferrous ($Fe^{2+}$) and triggering widespread lime-induced iron chlorosis in upland crops like groundnut and chickpea.
• Statement (II) is correct: Molybdenum (Mo) is a crucial metallic cofactor present in nitrogenase (within the FeMo-cofactor catalyzing biological atmospheric $N_2$ reduction) and nitrate reductase (catalyzing the reduction of nitrate $NO_3^-$ to nitrite $NO_2^-$), making it indispensable for leguminous symbiotic nitrogen fixation and microbial nitrate assimilation.
• Conclusion: Both Statement (I) and Statement (II) are correct. (Correct answer: Option 1)

• Calcium Requirement in Legumes: Leguminous crops (Fabaceae) have a significantly higher physiological calcium ($Ca^{2+}$) requirement than graminaceous cereals and grasses.
• Physiological Roles: Calcium is indispensable for root hair curling, infection thread formation, and nodulation by Rhizobium bacteria, as well as synthesizing calcium pectate in the middle lamella of rapidly expanding root and pod tissues (e.g., preventing "pops" in groundnut).
• Cation Exchange Capacity (CEC): Legume roots possess high root CEC (~40–60 cmol/kg) with strong affinity for divalent cations like $Ca^{2+}$, whereas cereal roots have much lower root CEC (~10–25 cmol/kg). (Correct answer: Option 2)

• Quantitative Volumetric Analysis Titrations and Chemical Indicators:
• Precipitation Titration: Uses Potassium chromate ($ ext{K}_2 ext{CrO}_4$) in Mohr's method (forming red $ ext{Ag}_2 ext{CrO}_4$) or Ferric alum in Volhard's method $\rightarrow$ (IV)
• Neutralization (Acid-Base) Titration: Uses pH indicators such as Thymolphthalein and Bromophenol blue $\rightarrow$ (III)
• Adsorption Indicator Titration (Fajans Method): Uses anionic dye indicators such as Fluorescein and Eosin that adsorb at the isoelectric equivalence point $\rightarrow$ (I)
• Oxidation-Reduction (Redox) Titration: Uses redox indicators such as Ferroin (1,10-phenanthroline iron(II)) and Diphenylamine $\rightarrow$ (II)
• Systematic Match: (A)-(IV), (B)-(III), (C)-(I), (D)-(II) corresponds to Option 4.

Correct option: (4) (A) - (IV), (B) - (MN), (C)~ (), (B) - (HN)