Soil compaction
For natural compaction on a geologic scale, see compaction (geology); for consolidation near the surface, see Consolidation (soil).
Soil compaction occurs when weight of livestock or heavy machinery compresses 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.
http://en.wikipedia.org/wiki/Soil_compaction
