Soil can be compacted by both natural and man made forces. The most common cause results from tillage practices. These man induced compacted layers are often called a traffic pan, disc pan, plow pan, wheel compaction, hard pan, etc. Some compaction occurs in most soils that are tilled. If compaction is Severe enough, it may reduce plant growth and yield. Soil compaction can be corrected by subsoiling and deep plowing but they are expensive methods. Soil compaction may not be a problem if one understands compaction, its effects, and how to prevent it.
Murdock, Lloyd W., "Correction and Prevention of Soil Compaction" (1981). Agronomy Notes. 89.