Retaining soil structure to improve soil
Soil structure (how soil particles are held together to form larger structures within the soil) is recognized as an important property of a healthy soil. Grading, tilling, soil compaction and screening soils during the soil processing and mixing process damages structure. Structure makes significant contributions to improving root, air and water movement thru the soil. Soil screening is extremely damaging to structure but is included in most soil specifications.
Why do we screen soils and what happens if we do not? Prior to the mid 1970’s soils were rarely screened and landscape plants performed quite well. Installed soil was moved with clumps or peds throughout the stockpile. In the last 15-20 years farmers who have stopped tilling their soil have found significant improvements in soil performance. Several new research projects suggest that elimination of the screening and tilling processes in favor of mixing techniques or soil fracturing that preserve clumps of residual soil structure may improve landscape soils.
Bryant Scharenbroch, a soil scientist at University of Wisconsin – Stevens Point, completed a study that demonstrated that unscreened soil performed equal to or better than the screened soil mix while using less sand and requiring less processing. The experiment used three soil approaches: silty clay loam topsoil with limited screening; the same topsoil loosely mixed at 60% with 25% sand and 15% compost; and finally a typical manufactured soil mix of 25% silty clay loam topsoil, 15% compost and 60% sand with the soil screens thru a 3/8” screen and the mix thoroughly blended. Water holding capacity and plant respiration improved in the less processed soils, particularly when watering frequency was reduced.
Susan Day, a researcher at Virginia Tech, just published a long-term study on different methods to improve compacted soil. At the research site, the topsoil had been removed and the resulting sub grade compacted to well above root limiting density while a portion of the plot was left in an undisturbed condition. Four approaches were then evaluated by growing trees for 6 years.
The approaches were: 1. Undisturbed, existing soil profile with no disturbance, 2. Typical soil treatments, 4 inches of topsoil added, 3. Enhanced topsoil where the 4 inches of topsoil was tilled into the compacted topsoil and 4. Profile rebuilding where 4 inches of compost was added to the soil and then a backhoe fractured the subsoil to a depth of 2 feet by digging and dropping the soil allowing the compost to fall into the spaces between the loosened subsoil. She found that the profile rebuilding generally performed best to increase tree growth exceeding the growth rates of even the undisturbed soil.
Other studies supporting retention of large soil clumps or peds suggest that the industry standard of screening soil should be discontinued or at a minimum re-evaluated. This blog post would like to solicit other research and experience with screened and unscreened soils in the landscape industry.
R.M. Layman, S. D. Day, D. K. Mitchell, Y. Chen, J. R. Harris and W. L. Daniels; Below ground matters: Urban soil rehabilitation increases tree canopy and speeds establishment. 2016 Urban Forestry & Urban Greening Volume 16
Scharenbroch, Bryant, University of Wisconsin – Stevens Point, Soil or Sand for Planting: Research and the Soil Debate Continued, Presentation at ASLA Annual Meeting, Chicago, 2015
Kays, B., McLaughlin, R., Heitman, J., Mohammadshirazi, F., and Brown, V. (2015) Amending Soils for Enhanced Infiltration of Stormwater. International Low Impact Development Conference 2015: pp. 123-132.
James Urban, FASLA, Jimtree123@gmail.com