Question

If the field capacity of soil is 22%, its permanent wilting point is 12%, apparent specific gravity of soil specific is 1.5 g/cc. and the depth of rootzone of a crop is 100 cm, then the storage capacity of soil is

• A. 12 cm
• B. 15 cm
• C. 10 cm
• D. 16 cm

Hint:

To remember this calculation:
$\text{Available Water Fraction} = FC - PWP = 10\%$.
Multiply this fraction ($0.10$) by the root zone depth ($100\text{ cm}$) and the soil's specific gravity ($1.5$) to get the total storage capacity: $1.5 \cdot 100 \cdot 0.10 = 15\text{ cm}$.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The storage capacity of a soil (also known as the available water depth) is the maximum depth of water that a soil can store in its pore spaces for crop uptake.
This water is held between two physical limits:
- Field Capacity (FC): The upper limit of water storage after excess gravitational water has drained.
- Permanent Wilting Point (PWP): The lower limit of water storage, below which plants can no longer extract water.
Key Formula or Approach:
The depth of available water ($d$) stored in the root zone is calculated using the formula: \[ d = \text{As} \times d_{\text{root}} \times (FC - PWP) \] where: - $\text{As}$ is the apparent specific gravity (bulk density of soil divided by density of water, which is a dimensionless ratio).
- $d_{\text{root}}$ is the depth of the crop root zone (in $\text{cm}$).
- $FC$ is the field capacity (expressed as a decimal fraction).
- $PWP$ is the permanent wilting point (expressed as a decimal fraction).

Step 2: Detailed Explanation:
Let us identify the values provided:
- Apparent specific gravity ($\text{As}$) $= 1.5$
- Root zone depth ($d_{\text{root}}$) $= 100\text{ cm}$
- Field Capacity ($FC$) $= 22\% = 0.22$
- Permanent Wilting Point ($PWP$) $= 12\% = 0.12$
Substitute these values into the storage capacity equation: \[ d = 1.5 \times 100\text{ cm} \times (0.22 - 0.12) \] \[ d = 150 \times 0.10 \] \[ d = 15\text{ cm} \] Therefore, the total depth of available water that can be stored in the root zone is $15\text{ cm}$.

Step 3: Final Answer:
The storage capacity of the soil is $15\text{ cm}$.
Hence, the correct option is (B).