Soil Chemistry

• Fundamental Pedological Processes in Soil Horizon Development:
• Eluviation ($E$-horizon): Translocation of suspended clay, iron, aluminum, and humus out of upper surface horizons $\rightarrow$ (I)
• Illuviation ($B$-horizon): Deposition and accumulation of translocated soil colloids in the subsurface diagnostic illuvial horizon $\rightarrow$ (II)
• Podzolization: Intense acid-mediated chemical weathering and mobilization of sesquioxides ($ ext{Fe}_2 ext{O}_3, ext{Al}_2 ext{O}_3$) leaving behind bleached albic quartz $\rightarrow$ (III)
• Calcification: Evaporative precipitation and accumulation of secondary calcium carbonate ($ ext{CaCO}_3$) in calcic horizons $\rightarrow$ (IV)
• Systematic Match: (A)-(I), (B)-(II), (C)-(III), (D)-(IV) corresponds to Option 1.

Correct option: (1) (A) - (IV), (B) - (Ill), (C) - (I), (B) - 0)

• Statement (I) is True: The Uniformity Coefficient ($C_u$) in soil mechanics and soil physics is defined as the ratio of $d_{60}$ (particle diameter corresponding to $60\%$ finer by weight) to $d_{10}$ (effective diameter, $10\%$ finer): $C_u = \frac{d_{60}}{d_{10}}$.
• Statement (II) is False: A well-graded loamy soil contains a wide spectrum of particle sizes across gravel, sand, silt, and clay, resulting in a large $d_{60}/d_{10}$ ratio ($C_u \gt 4 - 6$); uniformly sized single-grained sand has $C_u \approx 1 - 2$. Thus, well-graded loam has a higher uniformity coefficient than uniform sand, not lower.
• Conclusion: Statement (I) is true but Statement (II) is false (Option 3).

Correct option: (3) Statement (I) is true but Statement (II) is false.

• Stokes' Law Assumptions in Soil Mechanical Analysis:
• Particles must be rigid, smooth, and spherical; soil clay platelets fall slightly slower than equivalent spherical masses.
• Particle size must be significantly larger than water molecules ($\gt 0.0002\text{ mm}$) to prevent Brownian motion.
• Settling must be unhindered in an infinite fluid medium without wall effects ($\gt 0.1\text{ mm}$ clearance) and particle concentration below 2–3% solids.
• Particles must have uniform density (conventionally assumed as $2.65\text{ g/cm}^3$) in a quiescent, non-turbulent fluid.
• Conclusion: Statements (A), (B), (C), and (D) are all valid assumptions and limitations. (Correct answer: Option 3)