Soil compaction
- Also known as
- compacted soil, hardpan, plow pan
Soil compaction is the pressing together of soil particles, which reduces the pore space between them. Compacted soil holds less air, lets water in more slowly and physically resists root growth, which can reduce crop yield.
What is soil compaction?
Soil is made of mineral particles, organic matter and the pores between them, which hold air and water. When weight or pressure squeezes the particles closer together, those pores shrink, and the largest ones, which carry water and air, are lost first. The result is soil compaction: a denser soil with less room for air, water and roots.
Compaction is measured as an increase in bulk density, the mass of dry soil per unit volume, or as higher penetration resistance, the force needed to push a probe into the soil.
What causes soil compaction
- Wheel traffic, especially from heavy equipment. Heavier axle loads compact soil deeper, and deep compaction is slow to reverse.
- Working wet soil. Soil at or near field capacity compacts far more easily than dry soil.
- Tillage. Repeated tillage at the same depth can create a dense layer just below it, called a plow pan or tillage pan.
- Livestock trampling, particularly on wet pastures.
- Natural layers. Some soils have naturally dense subsoil layers, sometimes called hardpans.
- Loss of structure. Low organic matter and weak aggregation make soils more prone to compaction and surface crusting.
Surface and subsoil compaction
Surface compaction, in the tilled layer, is relatively easy to relieve with tillage and natural freeze-thaw and wet-dry cycles. Subsoil compaction, below normal tillage depth, is much harder to reverse and can persist for many years.
Effects on crops and soil
- Restricted roots. Roots grow poorly through dense soil and may turn sideways at a compacted layer, limiting access to water and nutrients deeper in the profile.
- Poor water infiltration. Water soaks in more slowly, increasing runoff, erosion and ponding.
- Poor aeration. Less air in the soil can stress roots and change microbial activity, including increasing nitrogen loss by denitrification in wet conditions.
- Uneven crops. Compacted zones, such as headlands and wheel tracks, often show stunted or uneven growth and lower yield.
How to detect compaction
Digging a pit or using a spade to look for dense layers, horizontal root growth and flattened root tips is the most direct method. A soil penetrometer gives a quick reading of resistance with depth; extension services often cite a penetration resistance of around 300 pounds per square inch as the level at which root growth is severely restricted. Readings should be taken when the soil is moist, because dry soil reads artificially high.
Reducing and preventing compaction
- Keep off wet fields whenever possible.
- Control traffic. Confine wheels to permanent lanes and reduce axle loads; lower tire pressures spread the load.
- Build structure. Increase soil organic matter and biological activity; cover crops with deep taproots can help open compacted layers.
- Targeted deep tillage. Subsoiling can break a dense layer, but its benefits are often temporary if traffic continues, and it is not always economic.
- Improve water movement. Soil conditioners and soil surfactants are used to help water enter and spread through dense or water-repellent soil.
Soil compaction at Locus
Locus’s Unpac is a biosurfactant soil conditioner that Locus describes as reducing the effects of compaction by lowering the surface tension between soil particles and water, improving infiltration and root-zone function.
Frequently asked questions
The long-term fix is to stop the cause: avoid traffic on wet soil, control where wheels run, and build soil structure with organic matter, living roots and cover crops. Deep tillage can break a compacted layer but may need repeating if the cause is not addressed.
Surface compaction affects the tilled layer. Heavy equipment, especially on wet soil, can compact soil well below tillage depth, where it can persist for years.
Often, yes, particularly in dry years when roots cannot reach deeper water, and in wet years when poor drainage and aeration stress the crop. The size of the loss depends on the severity and depth of compaction and on the season.
Sources
- Soil compaction, Soil Quality Indicators, USDA Natural Resources Conservation Service
- Soil compaction: causes, effects and control, Land-grant university extension (e.g. University of Minnesota Extension)