Restoring Construction Landscape Soils
By Ellen Vincent, Clemson University Environmental Landscape Specialist
Red, baked, and slick. Compacted soil is the landscapers’ inheritance after construction. The heavy pressure of the machinery and materials; the workers’ foot traffic during and after rains; and the absence of buffering topsoil and mulch all contribute to the creation of the hardpan. The stripped top soil and pressure from above reduces or eliminates the beneficial macropores, or air spaces between soil particles. Roots count on these air spaces for root growth and penetration.
Adding 4” of compost to these damaged soils will help repair them. Tilling replaced topsoil and 4” of compost in deeply, up to 24”deep, is another remediation practice to create tree friendly spaces. Most common landscape tillers handle 6-8” depths, so additional machinery may be required to achieve a depth of 24”. Most importantly, the machine should move backwards and not run over the tilled spaces.
Dr. Susan Day works in the horticulture department at Virginia Tech and specializes in urban soil restoration also known as soil profile rebuilding (http://urbanforestry.frec.vt.edu/documents/SoilProfileRebuildingDayuf.pdf). She advocates creating channels or veins 24” deep in the soil for compost to reside. Her studies show that the compost holds soil channels open so tree root penetration can occur over time; soil aggregates form; and the soil profile is restored. The compost and newly planted tree roots work in tandem to help build soil health.
Key items to consider when restoring compacted soil include adding quality aged compost; avoiding work when the soil is overly wet or dry; and ensuring no additional heavy pressure it applied to the surface. Mulch is one effective way to buffer occasional foot traffic; while sidewalks, stepping stones, and fences may be used to guide pedestrians.
Soil restoration processes using deep tilling are needed for planting new trees. Existing trees should never have the soil tilled under their canopy as this will damage the roots that reside in the top 6” of the soil. The majority of fibrous roots responsible for nutrient uptake are in this zone. To restore soil health in existing tree beds, equipment such as air spades or air knives that inject air into the soil are recommended as they do not appear to damage tree roots. Adding compost to this newly disturbed soil will promote healthy soil for roots to grow in and reduce stress on the trees. Certified arborists tend to own or have access to air injection equipment. At the International Society of Arboriculture, “Find an Arborist” Website (http://urbanforestry.frec.vt.edu/documents/SoilProfileRebuildingDayuf.pdf)(http://www.isa-arbor.com/findanarborist/arboristsearch.aspx) you can locate certified arborists in your area by typing in a zip code.
Restoring soils affected by construction is hero’s work. Landscapes are one of the last jobs on new construction projects and budgets, time, and patience are usually frayed by this time. The benefits of soil enhancement and restoration affect the entire ecosystem: organic matter in the soil holds moisture and reduces stormwater runoff while encouraging the growth of soil microorganisms; while the plants themselves have capacity to purify air while their roots help stabilize soils. The visual impact additionally has the potential to encourage human well-being. Soil restoration, performed by landscape professionals, is an outstanding service that benefits all.