Question

A typical mineral soil, on volume basis, contains:

• A. Mineral material 45%, organic matter 15%, air 10-20% and water 20-30%
• B. Mineral material 55%, organic matter 5%, air 10-20% and water 20-30%
• C. Mineral material 45%, organic matter 5%, air 20-30% and water 20-30%
• D. Mineral material 55%, organic matter 5%, air 20-30% and water 10-20%

Hint:

Remember the ideal soil volumetric balance:
- Half of the volume is solid (\( 45\% \) mineral + \( 5\% \) organic).
- The other half is pore space (\( 25\% \) air + \( 25\% \) water).
This \( 50:50 \) ratio between solids and voids represents optimal soil structure.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Soil is a dynamic, three-phase system consisting of solids (mineral and organic matter), liquids (soil water), and gases (soil air).
The volumetric composition of a typical, well-structured mineral soil under ideal conditions represents a balanced environment for plant root growth and microbial activity.

Step 2: Detailed Explanation:
Let us analyze the standard volumetric composition of an ideal silt loam surface soil:
Solid Phase (\( 50\% \) of total volume):
- Mineral Material (\( 45\% \)): This consists of sand, silt, and clay particles derived from the weathering of parent rocks. It forms the structural framework of the soil.
- Organic Matter (\( 5\% \)): This consists of decomposed plant and animal residues (humus) and living microorganisms. Though small in volume, it has a major influence on soil structure and nutrient capacity.
Pore Space (\( 50\% \) of total volume):
- Under ideal conditions, the pore space is divided equally between air and water.
- Soil Air (\( 20\% \text{ to } 30\% \)): This occupies the macro-pores (non-capillary pores) and provides oxygen for root respiration.
- Soil Water (\( 20\% \text{ to } 30\% \clean \)): This occupies the micro-pores (capillary pores) and holds dissolved nutrients for plant uptake.
The relative proportions of air and water fluctuate continuously depending on rainfall, irrigation, and crop water use, but they average out to this balanced range.
This matches the composition described in Option (C).

Step 3: Final Answer:
On a volume basis, a typical mineral soil contains 45% mineral material, 5% organic matter, 20-30% air, and 20-30% water.