Sustainable Sites Initiative

Soils

Healthy soils allow rainwater to penetrate, preventing excess runoff, sedimentation, erosion, and flooding. Soils also help clean, store, and recharge groundwater. By storing water and slowing the delivery of water to plants, healthy soils play a significant role in vegetation health.

The undervaluing of soils is one of the singular failings of the conventional development approach. For example, a frequent consequence of standard construction practices is compaction of the soil, which seriously damages soil structure by shrinking the spaces between soil particles available for air and water. If not restored, compacted soil can start a spiral of degradation.

Sediment runoff rates from construction sites can be up to 20 times greater than agricultural sediment loss rates and 1000-2000 greater than those of forested lands.¹

Compaction, which is caused by the use of heavy machinery during construction, degrades soil structure, and reduces infiltration rates,² which increase the runoff volume and flooding potential.³

Examples of Sustainable Practices

Preserve and protect healthy soils
Before site design, map out areas where soil is healthy (e.g., soil horizons and bulk densities resemble reference soils as described in NRCS Soil Surveys and/or native vegetation communities are present) and where it has been disturbed by previous land uses. During construction, retain topsoil, prevent erosion and sedimentation, minimize grading, compaction and soil disturbance, and avoid vegetation removal and disturbance.

Use plant trimmings as compost to nourish soils. Reduce waste during maintenance by recovering yard trimmings for compost and mulch. Compost reduces the need for fertilizers by supplying nutrients in a slow-release manner. It also holds more rainwater onsite, decreases runoff, and provides increased soil moisture and filtering capacity.

Soils can contain as much as or more carbon than living vegetation. For example, 97 percent of the 335 billion tons (304 billion metric tonnes) of carbon stored in grassland ecosystems is held in the soil.⁴

In addition to carbon dioxide, disturbed soils also release substantial amounts of methane and nitrous oxide, both gases that trap heat even more effectively than carbon dioxide.⁵

Improve health of degraded soils
Restore soil function in areas of previously disturbed soils to rebuild soils’ ability to support healthy plants, biological communities, and water storage and infiltration. Address soil compaction, organic matter levels, and the balance of soil organisms in existing soils. Provide adequate soil volume for plant growth—generally, 2 cubic feet per square foot of mature tree canopy.⁶ Clearly communicate with contractors about the treatment details for soil restoration.

[1] U.S. Environmental Protection Agency. 2005. Stormwater Phase II Final Rule: Construction Site Runoff Control minimum control measures. Department of the Interior, Editor.

[2] Kelling, K.A. and A.E. Peterson. 1975. Urban lawn infiltration rates and fertillizer runoff losses under simulated rainfall. Soil Science Society of America Proceedings, 39(2): p. 348-352.

[3] Hanks, D. and A. Lewandowski. 2003. Protecting Urban Soil Quality: Examples for Landscape Codes and Specifications, USDA-NRCS, Editor, p. 20.

[4] Amthor, JS et al. 1998. Terrestrial Ecosystem Responses to Global Change: A Research Strategy. ORNL Technical Memorandum, 1998/27 (Oak Ridge, TN: Oak Ridge National Laboratory).

[5] Lindsey, P and Bassuk, N. 1991. Specifying soil volumes to meet the water needs of mature urban street trees and trees in containers. Journal of Arboriculture 17, no.6: pp.141-49.

[6]Flannery, T. 2005. The Weather Makers (New York, NY: Grove Press), p. 357; KA Smith, T Ball, F Conen, et al. 2003. Exchange of greenhouse gases between soil and atmosphere: Interactions of soil physical factors and biologicalprocesses. European Journal of Soil Science 54: pp. 779-791.