What is the C:N ratio of normal soil?

Correct option is B

The correct answer is (B) 10:1 

Explanation:
• The Carbon to Nitrogen (C:N) ratio is a measure of the relative amounts of carbon and nitrogen in soil organic matter.
• For a "normal" or average mineral surface soil, the C:N ratio is remarkably constant, typically ranging from $10:1$ to $12:1$.
• This ratio is considered a state of equilibrium in the soil, representing stable humus.
• It indicates that for every 10 parts of Carbon, there is 1 part of Nitrogen available.

Information Booster:
• The C:N ratio determines the rate of decomposition and the release of nitrogen (mineralization).
• If a material with a high C:N ratio (e.g., wheat straw, $80:1$) is added to soil, microbes will consume all available Nitrogen to break down the Carbon, leading to temporary "Nitrogen Immobilization" (Nitrogen becomes unavailable to plants).
• Legumes have a low C:N ratio ($20:1$ to $30:1$), leading to rapid mineralization.

Additional Knowledge:
• Microorganisms themselves have a C:N ratio of about $5:1$ to $8:1$.
• Arable soils usually have a slightly higher ratio than forest soils.
• As organic matter decomposes, the C:N ratio generally narrows down until it reaches the stable $10:1$ ratio of soil humus.