- Compaction is most likely in soils that are at field capacity.
- Compaction reduces pore space, changes physical structure, and increases soil strength.
- Compaction prevents plant roots from developing normally, inhibits nutrient and water uptake, and causes water management problems.
- The four common types of compaction are plow layer, surface, sidewall, and deep.
While we are sometimes anxious in the spring to get busy in the fields, performing field operations when soils are too wet can cause big problems with soil compaction – problems that are not easily corrected after the damage is done. Soil compaction takes four primary forms and each has different causes. All forms of soil compaction are negative to the crop and most can be avoided with sound management and patience.
The different types of soil compaction have one thing in common – the worst compaction does not occur with heavily saturated soils, but instead with soils that are at field capacity. Field capacity is the point at which the soil will hold water against the force of gravity, although the excess has drained out. At this point, some of the pore spaces are filled with air instead of water, but there is still enough water that it can serve as a lubricant between soil particles, allowing them to slide and collapse against each other. As this happens, natural pore spaces also collapse, eliminating oxygen. Crop roots will not grow or function normally in oxygen-deprived soil.
With compacted soils, we’re really dealing with two major issues:
1. Compaction alters the natural soil structure and increases the physical strength of the soil, preventing crop root systems from fully exploring to take up nutrients and water. Soils that are compacted in layers often will hold surface ponds of water early in the season, but can become dry and impenetrable to water later in the season.
2. Compaction eliminates air (oxygen) from the soil, so crop roots cannot take up nutrients or water, even when they’re surrounded by it.
‘Plow Layer’ Compaction is the most common and widely known form of soil compaction. This develops with equipment traffic and use of certain implements, such as the disk. A well-defined layer develops just below the depth of tillage. Look for a distinctive platy or blocky structured soil layer between six and eight inches deep (see Image 1).
Surface Compaction develops when repeated tillage destroys soil structure in the top inches of the soil profile and/or heavy rainfall causes soil particles to settle together into a dense layer. Surface compaction also readily develops with no-till cropping when field traffic occurs before soils dry adequately. Surface compaction may or may not include soil crusting, and it need not be as dense as the plow layer to cause crop problems. Look for soil that appears to be lacking in pore spaces, will not easily crumble in the hands, and has a blocky or platy structure.
Sidewall Compaction is another familiar form of compaction. In wet soil, one type of sidewall compaction develops when the opener disk of the planter smears the side walls of the seed slit or furrow which roots have difficulty penetrating. In many cases, shrinkage of the drying soil causes the seed slit to pull open. Look for the distinctive seed slit with smeared walls, and roots that grow in a fan shape as they grow the only direction that they can.
Sidewall compaction can also develop with anhydrous ammonia toolbars and side-dress applicators, again when operations are done under adverse soil conditions. Sidewall compaction may interfere with lateral root growth and may result in a soil fracture that allows nitrogen to be lost to the atmosphere as ammonia gas following application. Initial diagnosis is as simple as occasionally walking over applied acres immediately following anhydrous ammonia application to see if you can detect an ammonia odor. Use a spade to cut perpendicular sections across the application track to check whether sidewall compaction is occurring.
Deep Compaction is the general compaction that develops below the eight to ten inch depth in the soil. This compaction is usually caused by heavy equipment loads on wet soils or soils at field capacity. Effects can sometimes be detected twenty inches deep in the soil or more. Correction of deep compaction can take many years and often includes deep ripping under dry soil conditions, as well as planting and maintaining alfalfa or other deep-rooted forage or cover crops for several years.
Image 1. “Platy” Soil Structure Common With Soil Compaction (source: GROWMARK, Inc.)