City Trees

September/October 2016

City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!

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Page 29 of 39

30 City Trees Urban foresters know poor soils can lead to an endless cycle of dieback and tree replacement. Even if trees do establish, growth can be underwhelming and tree health disappointing. Increasingly, project managers have been turning to soil replacement, where existing soils are excavated and removed and replaced with "recycled" or blended soils. These soils present their own challenges, however. For example, many imported blends rely on high sand contents to improve drainage, resulting in low water-holding capacity and drought stress for unirrigat- ed plantings. Resulting sharp transitions in soil texture introduce the possibility of creating a "bath tub" effect in situations where it is impossible to replace all the soil and new soils are confined to the immediate vicinity of individual trees. There is an alternative to soil replacement that is especially appropriate where there are extended open soil (unpaved) areas such as in street medians—soil rehabilitation. Soil rehabilitation can help restore important ecosystem func- tions such as stormwater transmission and vegetation support to existing native soils. In soil management, urban foresters and designers need confidence that they will get the results they desire and that soil improvements will persist for the long term. Researchers at Virginia Tech developed specifications for Soil Profile Rebuilding (SPR), a soil rehabilitation tech- nique, and have been evaluating performance for nearly a decade. This process can improve tree establishment, Soil Profile Rebuilding: An Alternative to Soil Replacement by Susan Downing Day, Associate Professor, Depts. of Forest Resources & Environmental Conservation and Horticulture, Virginia Tech This site is a good candidate for SPR because soil is compacted and has an impermeable layer that can likely be broken up by the backhoe subsoiling process. Note limestone gravel mixed in soil indicates pH will be high, which will not be altered by the rehabilita- tion process. Surface gravel should be removed if possible and underground infrastructure clearly marked. Photo by Susan D. Day The Theory behind SPR SPR works by creating veins of compost deep in the soil profile that hold soil channels open for root pen- etration. The introduction of organic matter coupled with root activity can create conditions that will lead to formation of soil aggregates over time—leading to long-term soil quality enhancement.

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