Question

At ultimate wilting point, soil water potential is

• A. - 15 bars
• B. - 20 bars
• C. - 30 bars
• D. - 60 bars

Hint:

Remember the standard soil water potential values:
- Saturation = \(0 \text{ bars}\)
- Field Capacity = \(-0.33 \text{ bars}\)
- Wilting Point = \(-15 \text{ bars}\)
- Hygroscopic Coefficient = \(-31 \text{ bars}\)

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Permanent Wilting Point (PWP) or ultimate wilting point is the soil moisture limit below which plants can no longer extract water. At this point, the turgor pressure in plant cells drops to zero, causing the plant to wilt permanently.

Step 2: Detailed Explanation:
Let us analyze the standard energy states of soil water:

Soil water potential (\(\psi_w\)) is expressed in bars, atmospheres, or megapascals (MPa). It is typically negative because solutes and soil matrix forces reduce the free energy of water compared to pure water.

AtSaturation, the soil water potential is approximately \(0 \text{ bars}\).

AtField Capacity (the moisture retained after excess water drains due to gravity), the potential is typically around \(-0.1 \text{ to } -0.33 \text{ bars}\).

As plants extract water, the remaining water is held tightly in smaller pores by capillary and adsorption forces, lowering the water potential.

At thePermanent Wilting Point, the soil matrix holds the remaining water so tightly that plant roots cannot generate a lower water potential to draw it in. This critical limit is globally standardized at\(-15 \text{ bars}\) (or \(-1.5 \text{ MPa}\)).
Thus, the soil water potential at the ultimate wilting point is \(-15 \text{ bars}\).

Step 2: Final Answer:
The correct option is (A), representing - 15 bars.