Question

Which one of the following soil type - steady state infiltration rate combination is incorrect?

• A. Sands - $>$ 20 mm hr$^{-1}$
• B. Loams - 5-10 mm hr$^{-1}$
• C. Clayey soils - 1.5 mm hr$^{-1}$
• D. Sodic clayey soils - $>$ 20 mm hr$^{-1}$

Hint:

Remember that ______Sodium ($\text{Na}^+$)______ is a highly dispersing agent that ruins soil structure.
Any soil described as ______sodic______ or ______alkali______ will have an extremely low hydraulic conductivity and a near-zero infiltration rate.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The steady-state (or basic) infiltration rate of a soil is the constant rate at which water enters the soil profile after it has become fully saturated.
It depends heavily on soil texture, structure, porosity, and exchangeable sodium percentage (ESP).

Step 2: Detailed Explanation:
Let us evaluate each soil type and its typical steady-state infiltration rate:
- ______Sands (A):______ Coarse-textured soils have large, interconnected macropores. They exhibit extremely high infiltration rates, typically greater than $20\,\text{mm/hr}$. (Correct combination)
- ______Loams (B):______ Medium-textured soils have a balanced distribution of sand, silt, and clay, leading to moderate infiltration rates generally between $5$ and $15\,\text{mm/hr}$ or $5\text{--}10\,\text{mm/hr}$. (Correct combination)
- ______Clayey soils (C):______ Fine-textured soils have very small micropores and slow water movement, resulting in low infiltration rates, typically around $1$ to $5\,\text{mm/hr}$ (1.5 is a highly representative value). (Correct combination)
- ______Sodic clayey soils (D):______ Sodic soils have high exchangeable sodium levels ($\text{ESP} > 15\%$).
Sodium causes clay particles to disperse and swell when wet, which completely clogs the soil pores and destroys soil structure.
This results in an extremely low, nearly zero infiltration rate (typically $< 1\,\text{mm/hr}$).
Therefore, claiming that sodic clayey soils have a high infiltration rate of $> 20\,\text{mm/hr}$ is highly incorrect.

Step 3: Final Answer:
The incorrect soil type-infiltration rate combination is sodic clayey soils, which corresponds to option (D).