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Soil compaction occurs when weight of livestock or heavy machinery compresses the soil, causing it to lose pore space. Affected soils become less able to absorb rainfall, thus increasing runoff and erosion. Plants have difficulty in compacted soil because the mineral grains are pressed together so as to leave little space for air and water, which are essential for root growth. Burrowing animals also find a hostile environment, because the denser soil is more difficult to penetrate.

The ability of a soil to recover from compaction depends on climate, mineralogy and fauna. Soils with high shrink-swell capacity, such as Vertisols, recover quickly from compaction where moisture conditions are variable (dry spells shrink the soil, causing it to crack). But clays which do not crack as they dry cannot recover from compaction on their own unless they host ground-dwelling animals such as earthworms -- the Cecil soil series is an example.

Increasing the density of soil, along with its side effects of increased strength and decreased permeability, is usually desirable in earthwork construction and below building foundations. Compaction is accomplished by use of heavy equipment. In sands and gravels, the equipment usually vibrates, to cause re-orientation of the soil particles into a denser configuration. In silts and clays, a sheeps-foot roller is frequently used, to create small zones of intense shearing, which drives air out of the soil.

The result of soil compaction is measured by determining the bulk density of the compacted soil and comparing it to a maximum density obtained from a compaction test, to determine the relative compaction.

• Bulk density
• Compaction
• Consolidation (geology)
• Earthwork
• Soil structure

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