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
This topic is near and dear to me – regardless of how much we put in specs, this is often ignored. At times, it is because earth moving is in the Civil scope, or the requirements are simply ignored. Construction managers and contractors are concerned with building construction, and ignore the site. My plan is to request photographic documentation of the process – photo, and send via email. Quick, easy, no need for reports so it will not drive up cost. Additionally, the need to be free of weed seed in compost is also a bit of a challenge – I look forward to hearing from others on that topic.
Great article! How can we measure soil clumps or peds and include such requirements in specification language?
Paul: Right now more work is needed to add metrics to ped or clump sizes and limitations particularly in specifications. We need to look at a number of soils across the soil texture range to determine what is possible. The biggest problem is that different soils types have different ability to hold peds together during harvesting, shipping and grading. Tthe greater the clay the stronger the peds, while loamy sands and sands have almost no peds. This is further compounded by moisture. In clay soils more moisture generally results in weaker peds but they can reform in the process as the clay is sticky. Less water results in stringer peds. In sandy loans what peds that exist will be stronger with more water. Silt loans are somewhat in between. finally greater soil organic matter will strengthen peds, but the peds tend tend smaller. In my area of the mid Atlantic coastal and piedmont the soils are pretty good at maintaining peds as long as you do not screen the soil. I suspect that in the central plains areas you would have the same results. The rest of the country can key off this basic understanding.
As far as specifications, I am still using a fairly vague ped language that is primarily there to recognize if the soil has been screened. My spec reads: “Topsoil and Planting Soil shall NOT have been screened through any screen smaller than 2” square and shall retain soil peds or clods larger than 2 inches in diameter throughout the stockpile.” I would like to use a larger minimum screen size say 3″ or even 4″ but it seems that 2″ is what most soil suppliers have and the ped idea is not mature enough to expect the industry to buy different equipment. Some times other factors make some minimum screening a valid part of the process. I try to control the overall screening requirements by controlling the source stock pile approvals and looking for soil with more clay. 15- 35% clay is a good target number for a soil that will hold up under grading and handling.
Hope this helps.