Soil Science- ICAR JRF/SRF 2024

• Schofield's pF Scale: Defined as the common logarithm of the soil moisture suction head expressed in centimeters of water column ($cm\ H_2O$):

• Calculation for 1 bar Suction:
• $1\text{ bar} = 10^5\text{ Pa} = 100\text{ kPa} \approx 1,020\text{ cm } H_2O \approx 1,000\text{ cm } H_2O$

• Reference Values: At field capacity ($0.33\text{ bar}$), $pF \approx 2.53$; at permanent wilting point ($15\text{ bars}$), $pF \approx 4.18$. (Correct answer: Option 2)

• Tensiometer Mechanism: A tensiometer consists of a sealed water-filled tube with a porous ceramic cup at the lower tip and a vacuum gauge at the top.
• Matric Suction Measurement: Water molecules move freely across the porous cup between the instrument and the soil until hydraulic equilibrium is established. Because dissolved salts diffuse without osmotic resistance through the ceramic pores, the tensiometer measures only matric suction (matric potential), not solute (osmotic) or pneumatic potential.
• Moisture Content: It does not directly measure moisture content; a moisture retention characteristic curve is required for conversion. (Correct answer: Option 2)

• Darcy's Law for Saturated Flow: Governs the steady flow of water through saturated porous soil media:

• Hydraulic Diffusivity: Used in unsaturated water flow equations (Buckingham-Darcy equation: $q = -D(\theta) \nabla\theta$), not in the basic saturated Darcy relationship. (Correct answer: Option 4)

• Physical Transport Laws Governing Soil Transport Processes:
• Fick's Law of Diffusion ($J = -D \frac{dc}{dx}$): Governs molecular gaseous diffusion (e.g., soil aeration) and solute diffusion $\rightarrow$ (II)
• Darcy's Law ($q = -K \frac{dH}{dx}$): Governs viscous saturated water flow through porous soil media $\rightarrow$ (III)
• Fourier's Law ($q_h = -k \frac{dT}{dx}$): Governs thermal conduction through the solid-liquid-gas soil matrix $\rightarrow$ (IV)
• Second Law of Thermodynamics ($\Delta S \ge 0$): Dictates spontaneity of natural processes and entropy generation $\rightarrow$ (I)
• Systematic Match: (A)-(II), (B)-(III), (C)-(IV), (D)-(I) corresponds to Option 2.

Correct option: (2) (A) - (Il), (B) - (ill), (C) - (IV), (B) - (1)

• Assertion (A) is True: Medium-textured soils (silt loam, loam, and very fine sandy loam) exhibit the highest susceptibility to water erosion (highest soil erodibility factor $K$ in USLE).
• Reason (R) is True: Silt and very fine sand particles lack strong physicochemical cohesion (unlike clay particles) and are too small to resist detached transport (unlike coarse sand), which significantly increases detachment and wash-off.
• Relationship: Because high silt and very fine sand fractions reduce aggregate stability while remaining readily transportable by surface runoff, Reason (R) directly explains Assertion (A).
• Conclusion: Both (A) and (R) are true and (R) is the correct explanation of (A) (Option 1).

Correct option: (1) Both (A) and (R) are true and (R) is the correct explanation of (A)