Question

When structure of two soils is compared, it will be better in that soil whose cumulative mean weight diameter (CMWD) is :

• A. unity
• B. zero
• C. lower
• D. higher

Hint:

Mean Weight Diameter (MWD) measures the average size of stable soil aggregates. A higher MWD/CMWD means larger, more stable aggregates, which corresponds to a healthy, well-structured soil.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil structure refers to the arrangement of individual primary soil particles (sand, silt, clay) into secondary units called aggregates or peds.
The stability and size distribution of these aggregates determine soil structure quality, water infiltration, and aeration.
Mean Weight Diameter (MWD) is a statistical index used to quantify the size distribution and water-stability of soil aggregates.

Step 3: Detailed Explanation:
Let us analyze how the Cumulative Mean Weight Diameter (CMWD) relates to soil structure quality:
- CMWD is calculated by multiplying the mean diameter of each aggregate size fraction by the weight proportion of that fraction retained on sieves during wet or dry sieving, and summing these values.
- A higher CMWD value indicates that a larger proportion of soil aggregates remained stable as larger structural units during the sieving process.
- Soil with a higher CMWD has a well-aggregated, stable structure that resists slaking, water erosion, and crusting.
- Conversely, a lower CMWD indicates that the soil aggregates easily broke down into smaller fragments and primary particles, signifying a weak, unstable soil structure prone to erosion and compaction.
Therefore, soil structure is considered superior or better in the soil that has a higher CMWD.

Step 4: Final Answer:
Soil structure is better in the soil whose cumulative mean weight diameter is higher.