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Soil compaction is governed by the soil type and how wet the soil is. The soil’s classification, which captures its grain-size distribution and plasticity, determines how particles rearrange and bond under compactive energy. Granular soils (like sands and gravels) tend to densify mainly through particle rearrangement and can achieve good densities with moisture near a certain optimum. Cohesive soils (such as clays) rely more on moisture to reach a plastic state that allows particles to slide and pack more tightly under pressure.

Moisture content is the other crucial factor because it controls how easily particles can move past one another and how well they can rearrange into a dense arrangement. If the soil is too dry, there isn’t enough lubrication to permit rearrangement; if it’s too wet, excess pore water reduces inter-particle contact and limits densification. Together, soil classification and moisture content determine the achievable density and the effectiveness of compaction at a given energy.

Color and odor don’t indicate how well soil will compact, and while temperature, season, and groundwater depth can influence moisture availability, they aren’t the direct governing factors themselves.