Question

In the rainfed wheat crop fields, surface soil water potential is normally

• A. Positive
• B. Zero
• C. Negative
• D. Unit

Hint:

Think of soil potential as a "suction" force.
Matric suction = $-1 \times$ Matric potential.
Unless the soil is under a pond or below the water table, the potential is always negative.
The drier the soil, the more negative the number becomes (e.g., $-15$ bars at PWP).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks about the sign (positive, zero, or negative) of the soil water potential in the surface layer of a rainfed wheat field.
Rainfed agriculture relies solely on rainfall, meaning the soil is rarely saturated.
Detailed Explanation:

• Soil Water Potential Concept:
Total soil water potential ($\Psi_t$) is the sum of various components, primarily matric potential ($\Psi_m$), osmotic potential ($\Psi_o$), and gravitational potential ($\Psi_g$).
In unsaturated soil, the most dominant component is the matric potential.

• Matric Potential in Unsaturated Soil:
Matric potential arises from the forces of adsorption and capillarity.
Because water is held under tension in the soil pores by these attractive forces, it is at a lower energy state than free, pure water at the same elevation.
By convention, the potential of free pure water is zero. Therefore, water held by soil particles in unsaturated conditions has a negative potential.

• Rainfed Field Conditions:
In rainfed fields, especially at the surface, evaporation and crop transpiration continuously remove water.
This causes the soil to dry out, increasing the tension (suction) with which the remaining water is held.
A drier soil means a more negative matric potential.

• When is it Positive?
Potential is only positive below a water table (hydrostatic pressure) or in a fully saturated soil where there is standing water.
In rainfed wheat fields, the surface is almost never in a permanent state of saturation.

Step 2: Final Answer:
The water in the surface soil of rainfed fields is held under tension; hence, the soil water potential is normally negative.