Question

The gaseous phase of soils contains the following molecules in order of

• A. \(\text{N}_2 > \text{O}_2 > \text{CO}_2 > \text{Ne}\)
• B. \(\text{O}_2 > \text{N}_2 > \text{CO}_2 > \text{Ne}\)
• C. \(\text{CO}_2 > \text{N}_2 > \text{O}_2 > \text{Ne}\)
• D. \(\text{O}_2 > \text{N}_2 > \text{Ne} > \text{CO}_2\)

Hint:

Soil air composition hierarchy: $\text{N}_2 (78\%) > \text{O}_2 (18-20\%) > \text{CO}_2 (0.3-1\%) > \text{Trace inert gases (Ne, Ar)}$.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Soil air occupies the non-water-filled macropore space, constantly exchanging gases with the atmospheric air through molecular diffusion.

Step 2: Detailed Explanation:
Typical volumetric composition of normal well-aerated soil air compared to atmosphere:
- Nitrogen (\(\text{N}_2\)): \(\approx 78\text{--}79\%\) (Dominant gas).
- Oxygen (\(\text{O}_2\)): \(\approx 18\text{--}20.5\%\) (Slightly lower than atmosphere due to root and microbial respiration).
- Carbon Dioxide (\(\text{CO}_2\)): \(\approx 0.25\text{--}1.0\%\) (10 to 30 times higher than atmospheric concentration of 0.04%).
- Noble gases (e.g., Neon Ne): Trace quantities ($< 0.002\%$).
Therefore, the gaseous molecules decrease in the exact order: \(\text{N}_2 > \text{O}_2 > \text{CO}_2 > \text{Ne}\).

Step 3: Final Answer:
Hence, the order of gaseous molecules is \(\text{N}_2 > \text{O}_2 > \text{CO}_2 > \text{Ne}\), matching option (A).