When it comes to managing stormwater runoff, post-construction compacted soils and concrete have a lot of similarity. It doesn’t have to be that way.
Anyone who’s had the fortune to meet Dr. Barrett Kays, horticulturist, soil scientist and landscape architect, knows that he is a great promoter of soils as the largest stormwater and water quality management system available. While undisturbed soils are best, it’s possible to enhance infiltration on compacted, urban and post-construction soils. He would know. Barrett is the go-to expert on a number of national projects to renovate large public lawns and heavily trafficked places. Think of New York’s Central Park, the National Capitol Mall or Longwood Gardens.
Turf or planted spaces on post-construction soils that have been treated to infiltrate are a very effective way to reduce stormwater runoff. During the American Society of Civil Engineer’s LID Conference in Portland, Maine Dr. Richard McLaughlin, NC State University spoke about portions of the research he and others at the university have conducted with Barrett in a session titled “Turning your soil green.” The results he presented stimulated a lot of discussion.
Developers typically scrape off the top soil and then compact what’s left at the site. New construction can be particularly bad. That’s a problem for growing plants, which have to try to penetrate densely packed soils. Highly compacted soils also cause stormwater runoff.
But, you can use an old fashioned technique, tillage, to control compaction, Richard said. Where ever you have roots, you have infiltration, he said. The problem is that plants cannot penetrate soils at 2.5 MPa (soil resistance measurement in mega pascals).
Deeply tilling compacted soils and deeply tilling with added compost reduces runoff by 80% or more based on replicated tests conducted over three years in three locations by NC State University.
Tilled soils were treated to different depths: Shallow tillage (ripped to 6” and tilled to 6”); Deeply tilled (ripped to 12” and tilled to 12”) and Deeper tillage (ripped to 18” and tilled twice to 12”). In treatments with compost, 2” of compost was spread on top of the soil and tilled to a depth of 12” (many soil conditioning BMPs specify 2” or sometimes 3” of compost). All soils in the tests were limed and fertilized based on soil tests.
Old lawns infiltrate, Richard said. Lawns, even those on new construction, will infiltrate given enough time. The problem is that could take 30 years he explained. Tilling allows plant roots to grow throughout the tilled profile.
Mower traffic can be a problem, but the NCSU tests showed that compost may help alleviate re-compaction, Richard said.
Maintenance of the finished landscape is key in long term infiltration performance, Richard said. Well growing plants are the way to maintain high infiltration that’s created by deeply tilled soils. When roots penetrate the tilled zones, they open up the soils. Non-tilled control plots had ¼ to 1/3 less grass cover.
turResearch on tilling and infiltration is now moving into other areas like studying grasses, pollinator plantings and woody species in tests being conducted with the NC Department of Transportation.
Amending Soils for Enhanced Infiltration of Stormwater by Barrett L. Kays; Richard McLaughlin; Joshua Heitman; Fatemeh Mohammadshirazi; and Virginia Brown in International Low Impact Development Conference 2015: LID: It Works in All Climates and Soils. http://dx.doi.org/10.1061/9780784479025.012
NewTerrain October 3, 2016.