Biology of Soil Compaction

Soil Structure and Soil Compaction

An often underappreciated fact is that good soil structure comes from the dynamic interplay between the non-living parts of soil, such as sand, silt and clay particles, and the living, such as fungi and roots. Furthermore, even recently, living material, such as cell debris and decomposing material, also influences soil structure. As shown in Figure 1 and Figure 2, the basic units of soil structure, the micro and macroaggregates, result from the combination of the abiotic and biotic components of a healthy soil. Macroaggregates are larger than 250 µm and are composed of multiple microaggregates (< 250 µm), clay microstructures, biotic debris and fungal hyphal strands.

Good soil structure increases the water infiltration rate and oxygen availability and decreases the force required for root penetration. Micro and macroaggregates are susceptible to physical disturbances such as compaction and tillage, which can negatively impact soil properties and lower crop yields.

Soil compaction occurs when soil particles are compressed together, usually by farm machinery and vehicles. As a result, soil pore space is greatly reduced at both the surface and subsoil level. The change in physical environment can reduce crop yields by restricting root growth, reducing oxygen availability and limiting water penetration and drainage. These problems are greatly exacerbated if the soil is worked while wet. Soils low in organic matter will also be susceptible to compaction, as organic residues can help maintain soil structure in the presence of heavy traffic. Compaction also changes the belowground biotic community, which serves as the ‘glue’ for the aggregate structures.

Effects of Compaction on Belowground Communities

As soil particles are compressed together, fungi and roots are negatively affected due to changes in water, oxygen and habitat availability. Furthermore, excessive tillage disrupts fungi and root networks, which can lead to increased soil compaction down the road due to loss of biotic activity. In both situations, fungi and roots are no longer producing two important products that help microaggregates form larger macroaggregates: polysaccharides and glomalin. The presence of these two materials helps build macroaggregates, which lead to favorable conditions for plant growth.

Managing Soil Compaction

Soil compaction and the presence of a subsoil hard pan can be diagnosed in the field via a soil pit and through the use of a tool called a soil penetrometer. Once compaction is confirmed, a plan can be drawn up to a) reduce vehicle traffic on the field, b) increase soil organic matter levels and surface residue cover and c) temporarily break up compacted areas with deep tillage.  The end goal is to restore the relationship between the living and non-living entities in your soil that promote good soil structure.

Suggested Reading

Hoorman, J, J., J.C. de Moraes Sẚ, and R, Reeder. 2009.  The Biology of Soil Compaction. The Ohio State University Extension Service Fact Sheet #SAG-10-09.

- Dr. Karl A. Wyant, Desert Division Agronomist