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Competency Area 4: Soil health and compaction

PO 28. Describe different types of soil compaction, and understand their agronomic and environmental implications.

  1. Crusts
  2. Plow Layer
  3. Subsoil


Crusts are caused by the impact of raindrops on the soil surface. If the soil is unprotected and has poor structural stability, soil fines will disperse and fill large pores. Infiltration capacity is reduced quickly. When wet, this thin layer is called a seal, and when it dries up it becomes a crust. Crusts can significantly harm germination of young seedlings which have trouble breaking through the crust. Increased runoff due to crusts and seals is a threat to surface water quality.

Plow layer

A plow layer is created by repeated use of moldboard or disk plows at the same depth. The soil just below the tillage tool is compacted and becomes denser than the rest of the soil below or above it. Penetration resistance peaks at the depth of the plow pan, then decreases below it. Soils with high clay contents at tillage depth are especially sensitive to this type of compaction. One of the consequences is inhibition of root penetration through the plow pan. Roots can grow horizontal if they are unable to grow through the plow pan. Water will stagnate on top of the plow pan. This causes problems with trafficability, aeration, and compromises the effectiveness of artificial drainage systems. Increased runoff can ensue.


Subsoil compaction is caused primarily by heavy machinery traffic at times when the soil is moist (in the plastic state). Subsoil compaction is primarily related to axle load. It affects root penetration and water percolation.