Available water for plant indicates water held between:
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Remember the classic soil water tension limits:
Field Capacity = 0.33 bar (or 1/3 atm).
Permanent Wilting Point = 15.0 bar (or 15 atm).
Available Water Capacity (AWC) = Water at 0.33 bar minus Water at 15.0 bar.
Step 1: Understanding the Concept:
Soil water is held within soil pores with varying degrees of tension, which is expressed in units of pressure (bars or atmospheres).
Not all water present in the soil is accessible to plant roots, as the energy required to extract it varies with the soil moisture content. Step 2: Detailed Explanation:
The total soil water reservoir is divided into three main classes based on availability:
1.Gravitational Water: This is water held at tensions below 0.33 bar (or below 0.1 bar in coarse sandy soils).
It drains rapidly out of the root zone under the force of gravity within 24 to 48 hours after heavy rain or irrigation and is generally unavailable for long-term plant use.
Thus, option (D) is incorrect.
2.Field Capacity: This is the moisture content remaining in the soil after gravitational water has drained away, corresponding to a tension of 0.33 bar in medium-textured soils.
3.Permanent Wilting Point: This is the lower limit of soil moisture at which plants can no longer extract water fast enough to meet their transpiration needs, causing them to wilt permanently.
This point corresponds to a soil moisture tension of 15.0 bar.
Thus, option (C) represents water that is held too tightly and is unavailable.
4.Available Water: This is the soil moisture held between field capacity (0.33 bar) and the permanent wilting point (15.0 bar).
Plants can easily extract this water to support transpiration and cellular functions. Step 2: Final Answer:
The available soil water for plant uptake is the moisture held between the tensions of 0.33 bar and 15.0 bar.
