Most compaction results from use of machinery on soil which is too wet to work well, or from overworking soil and destroying natural structure. The resultant pressure from tires and tillage tools compresses more soil into a given volume. Regardless of reason, the proportion of solid soil material relative to total volume of soil increases. In the process, natural soil aggregates are broken down and large pores become smaller, generally resulting in soil more difficult for plant roots to penetrate.
Wells, Kenneth L. and Murdock, Lloyd W., "Contending with Soil Compaction" (1988). Soil Science News and Views. 135.