Electronic Field Guide » Pre-Development Issues » Soil Compaction and Structure

Prepared by Patrick Drohan (Ecosystem Science and Management)

Bulk Density and Land Use Management
Individual soil particles bind together to form peds, which help develop soil structure. Sandy soils tend to have smaller, weaker, and less developed structures than those of loamy or clay textures. Soil texture, mineralogy, organic matter, and roots play a role in structure formation.  Strong grades of soil structure create more pore space for water movement.
 
Bulk density is the mass of a unit volume of dry soil, including both solid and pore space. Bulk density values are typically used to describe various degrees of soil compaction. Soils with more solids than open pore space have higher bulk density, meaning that they are compacted. 
 
Various factors affect inherent soil bulk density, including soil texture and organic matter content. Certain soil textures, such as loamy soils, are more prone to compaction. Organic soils have low bulk densities compared to mineral soils. Increases in bulk density inversely decrease porosity, infiltration, and percolation, all of which are detrimental to plant growth and erosion and sedimentation control.
 
Land use and management that changes a soil’s physical structure or organic matter content changes its bulk density. Undisturbed forest soils, for example, naturally have high porosity and low bulk density, but can easily be compacted by machinery such as logging equipment. Organic matter amendments increase the resistance and resilience to mechanical stresses of severely degraded soils and increase total porosity.

Soil Compaction
The volume of soil disturbed during construction of Marcellus infrastructure, construction on poorly drained soils, and wet weather conditions complicates the issue of soil compaction. Topsoil compaction tends to reduce plant productivity for 1 to 5 years, but subsoil compaction may reduce site productivity for decades or longer. Topsoil compaction depends on tire pressure of machinery. Subsoil compaction is caused by very high axle loads on trucks and other equipment common on Marcellus development sites.
 
Processes that naturally alleviate topsoil compaction over time include the action of soil organisms, root growth, and freeze-thaw and wetting-drying cycles. However, subsoil compaction is not affected by these processes and causes reduced percolation. This means that water sits longer on the land surface and less of it eventually infiltrates the ground. The lack of air movement through the soil reduces root growth and other biological activity. These areas may become functionally impervious, meaning that little or no water infiltrates the soil.
 
Landowners may be able to negotiate into their gas leases stipulations that they have input into well and other infrastructure siting and that construction activities will be limited during times of high soil moisture (usually late winter and early spring). For more information about repairing compaction, see the Penn State fact sheet Avoiding and Mitigating Soil Compaction Associated with Natural Gas Development.
 
Landowners may also be able to negotiate the repair of any damaged agricultural tile drainage, installation of new tile systems, restoration of the land surface to its original or an improved contour, and mitigation of subsoil compaction.