If the sodium ions increase to about 10% or more, the soil becomes
- Aless permeable and of poorer tilthCorrect
- Bmore permeable
- Cgood tilth
- Dmore permeable and of better tilth
Solution
Concept
High sodium in soil breaks up (disperses) clay particles, so the soil loses its structure and drainage.
Formula
ESP = (Exchangeable Na / Cation exchange capacity) × 100
Step-by-step solution
1. Sodium ions replace calcium and magnesium on clay particles. 2. Sodium clay disperses: particles separate and block the pores. 3. Blocked pores mean water cannot pass easily, so permeability drops. 4. Soil becomes hard when dry and sticky when wet, so tilth is poor. 5. So, at about 10% or more sodium, the soil is less permeable and of poorer tilth. Answer is (A).
Explanation
When sodium ions are about 10% or more of the total exchangeable ions, the clay particles disperse. The soil pores get blocked, so water moves very slowly. The soil also loses good crumb structure, so its tilth becomes poor. Such soil is called alkaline or sodic soil.
Why the other options are wrong
B: sodium reduces permeability, it does not increase it. C: sodium spoils soil structure, so tilth becomes poor, not good. D: both parts are wrong, because permeability falls and tilth becomes worse.
Shortcut
More sodium means the soil is dispersed, tight and bad: less permeable and poor tilth.
Common mistake
Thinking salt makes soil loose and porous. Sodium actually disperses clay and seals the pores.
Correct answer: A) less permeable and of poorer tilth