Soil Compaction Fundamentals

Soil compaction is often associated with its physical properties. It is when soil particles are pressed together and pore space is decreased. Pore space can account for fifty percent depending on soil type. This can be physically altered through natural and mechanical influences.

In the pore spaces of soil, water and air are in a constant back and forth balance. As soil solution increases due to precipitation weather events or capillary action, there is less air present in pore space. Contradicting this, the soil dries from lack of precipitation and more air is present. Water infiltration and capillary action are affected by soil type and soil compaction.

There are soil types that naturally are more resistant to compaction. The higher the sand content, usually, the less compaction occurs. Soils with more clay tend to compact more and further in depth. They have a higher water holding capacity, smaller pore space and tighter particle bonds.

Compaction can occur at various levels in the soil profile. Tillage practices can influence many compaction points, but on the soil surface it experiences multiple situations. How can some no-till fields have such a hard top layer? Heavy rain events cause lots of surface compaction. What can make this worse is a seedbed preparation tillage pass before such event. This will cause crusting of the soil surface with little pore spacing for germinated seedlings to emerge.

Each pass in the field, whether it be from machine or foot, compresses the soil limiting pore space and compacting as well. A tool to help measure these actions is a penetrometer. It is a solid probe with an indicator dial on top that is pressed into the soil. As it travels through the profile, the needle on the dial will show what the PSI is at the probe tip ranging from 0-300+. Using a ¾ inch tip, 0-200 is considered optimal, 200-300 roots are restricted and anything over 300 is very compacted.

Plow layers, or subsurface compaction, is caused by smearing of the soil and done on a routine basis. These can usually be found around 7-9” deep depending on the region and tools used. These are also mistaken for soil horizon changes. Such as Topsoil A Horizon, to Subsoil B Horizon as soil changes from higher organic matter to structureless massive soils with an anerobic environment.

To manage compaction, it starts with limiting soil surface exposure. Leaving residue or practicing minimal tillage. Not applying too much down pressure with the planter gauge wheels, proper tire inflation or the use of tracks, and not disturbing the soil when field conditions are marginal to saturated.