Tillage or plow pan
Repeated tilling at the same depth (especially with discs, moldboard plows and sweep-type implements) and wheel traffic can result in a hard layer of subsoil compaction a few inches thick, directly below normal tillage zone (about 6-12 inches below surface).
Impact: Reduced yields in extreme weather years. In wet years: drainage problems, waterlogging and denitrification. In dry years: roots confined by compaction and have less access to deep available soil water.
Prevention/remedy: Complicated, but consider less-intensive tillage while adding organic matter; strip tilling; breaking up the pan with deep tillage/subsoiling (but only when entire tillage depth is dry and the depth of the tillage layer is known); reduce heavy traffic.