Question

Which of the following is NOT true?

• A. Soil aeration depends on volume fraction of air-filled pores
• B. Soil air has similar gaseous composition as of atmosphere
• C. Soil aeration is restricted in compacted soil
• D. Manuring increases $CO_2$ content in soil

Hint:

Remember: Soil air is "High $CO_2$, Low $O_2$."
$CO_2$ in soil is $10$ to $100$ times higher than in the atmosphere!
This difference is the "driving force" for gas diffusion between the earth and the sky.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the incorrect statement among four descriptions of soil air and aeration.
Detailed Explanation:

• Evaluating Statement (A): Soil aeration is the process by which gases are exchanged between the soil and the atmosphere. It depends entirely on the non-water-filled pore space. This is true.

• Evaluating Statement (B): While soil air contains the same gases as the atmosphere ($N_2, O_2, CO_2$), the proportions are very different.
Atmosphere: $O_2 \approx 21\%, CO_2 \approx 0.04\%$.
Soil Air: $O_2 < 20\%$, $CO_2 \approx 0.3\%$ to $3\%$.
Due to root respiration and microbial decomposition, $CO_2$ is much higher and $O_2$ is lower in the soil. Therefore, statement B is NOT true.

• Evaluating Statement (C): Compaction reduces the volume of macropores (the air-carrying pores), which directly restricts the diffusion of gases. This is true.

• Evaluating Statement (D): Manure provides organic carbon for microbes. As microbes decompose manure, they respire and release $CO_2$ into the soil air. This is true.

Step 2: Final Answer:
The statement claiming that soil air has a similar composition to atmospheric air is incorrect because soil air is significantly enriched with carbon dioxide.