Gradation in soil refers to what attribute?

Gradation in soil primarily refers to the distribution of particle sizes within the soil. This means how different-sized soil particles, such as sand, silt, and clay, are proportioned within a sample. Understanding the gradation is crucial for several reasons, as it influences soil properties such as permeability, compaction, and strength. For instance, well-graded soils have a good mix of particle sizes that can fill voids between larger and smaller grains, enhancing stability and drainage. In contrast, poorly graded soils may exhibit problems related to water retention and stability due to a lack of variety in particle sizes.

The other attributes listed, such as variation in moisture, soil color, and soil temperature, while important characteristics of soil, do not pertain to the specific concept of gradation. These factors can affect soil behavior and its interaction with the environment, but they do not relate directly to how the various particle sizes are distributed in the soil structure.