Urban Soils - Part 1: Understanding Compaction

James Urban, FASLA: Excerpt from Up By Roots - published in the International Society of Arboriculture Magazine, April 2008

Abstract: The natural soil-forming process tends to create soil that is at suitable levels of compaction (or, to use the scientific term, bulk density) for ground stability and the growth of some kind of plants. Few natural places exist where the soil bulk density is so high that plants will not grow.

Human activity tends to change the compaction level in the soil, most often making it more compact, but sometimes reducing its compaction. Identifying compaction levels in the soil and when they need to be modified is a critical step in creating good growing conditions.

As soil becomes mechanically compacted, the organic bonds that were holding the soil structure are broken, and soil particles are pushed together, filling the pore space. Think of what would happen if you stepped on a bag of pop com. Your foot would crush the fluffy kernels together,and the spaces between them would be difficult to re-create.

The same goes for soil. As pore space is eliminated, space for air and water is lost, and roots must push harder to get through the soil. The plant has access to less water and grows fewer roots in the more difficult conditions.

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Effect of Urban Soil Compaction on Infiltration Rate

Dr. J.H. Gregory, M.D. Dukes, P.H. Jones, and G.L. Miller: Journal of Soil and Water Conservation, 2006

Abstract: Inadvertent soil compaction at the urban lot scale is a process that reduces infiltration rates, which can lead to increased stormwater runoff. This is particularly important in low impact development strategies where stormwater is intended to infiltrate rather than flow through a traditional stormwater network to a detention basin. The effect of compaction on infiltration rates on sandy soils in North Central Florida was measured with a double ring infiltrometer on urban construction sites and across various levels of compaction.

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Pennsylvania Stormwater Best Management Practices Manual; Chapter 6: Soil Amendment & Restoration, December 2006

Key Design Elements:

  • Existing soil conditions should be evaluated before forming a restoration strategy. 
  • Physical loosening of the soil, often called subsoiling, or tilling, an treat compaction.
  • The combination of subsoiling and soil amendment is often the ore effective strategy.
  • Compost amendments increase water retention.

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Soil Compaction & Trees: Causes, Symptoms & Effects

Dr. Kim Coder: University of Georgia, July 2000

Abstract: The health and structure of trees are reflections of soil health. The ecological processes which
govern tree survival and growth are concentrated around the soil / root interface. As soils, and associated
resources change, tree systems must change to effectively utilize and tolerate changing resources
quantities and qualities, as well as the physical space available. Soil compaction is a major tree-limiting
feature of community forest managers and arborists.

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Infiltration Through Disturbed Urban Soils and Compost - Amended Soil Effects on Runoff Quality and Quantity

Robert Pitt, Janice Lantrip, Robert Harrison, Charles Henry, Dongsen Xue: Environmental Protection Agency, March 1999

Abstract: This project examined a common, but poorly understood, problem associated with land development, namely the modifications made to soil structure and the associated reduced rainfall infiltration. This project examined this problem by conducting more than 150 infiltration tests in disturbed urban soils and by comparing these data with site conditions. The tests were organized in a complete factorial experiment to fully examine the effects, and interactions, of soil texture, soil moisture, and compaction. In addition, age since development was also briefly examined. It was found that compaction has dramatic effects on infiltration rates through sandy soils, while compaction was generally just as important as soil moisture at sites with predominately clay soils. Moisture levels had little effect on infiltration rates at sandy sites. Because of the large amounts of variability in the infiltration rates found, it is important that engineers obtain local data to measure the infiltration rates associated with local development practices.

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