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. To counteract the loss of structure, specifiers then add to it, utilizing a mined product which has no structure with reduced amounts of organic matter and plant response.
For example, Susan Day, a researcher at Virginia Tech, has published a long-term study on different methods to improve compacted soil, to support retention of large soil clumps or peds. Her study suggests that the industry standard of screening soil should be discontinued or at a minimum re-evaluated. (https://www.urbanforestry.frec.vt.edu/SRES/). Additionally, 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.
Selecting the correct load bearing soil module for soil structure
Space is important for soil, to allow for the natural unsegmented areas of soil. In the natural world, soil structure is maintained and water, roots, and microorganisms can move freely throughout the system. When choosing an appropriate load bearing system for an urban environment, space becomes the important factor – both in terms of flexibility for allowing utilities, as well as root growth that follows water distribution and air flow.
“Fill and Cap” versus “Cap and Sieve”
The Silva Cell’s soil placement is a “fill and cap” methodology, allowing for successful installation of multiple soil structure types and textures. You place the soil in the system and because it is foot compacted, it does not subside. This provides more soil volume to the trees and limits the potential habitat for fauna colonization, all the while keeping the soil structure intact.
Other manufacturers use a “cap and sieve” systems. The system is assembled and then the dry soil is screened through the caps. This means “cap and sieve” soil cells need to have proprietary soil specs with high degrees of soil processing, expense and transport. These soils are largely sand based and have less biota and organic matter. The sieving destroys any remaining soil structure and leads to subsidence and more opportunities for vermin to live in the voids.
The Difference is Design
Maintaining soil structure is paramount to plant health and the time-tested method of foot compaction lead to maximizing the soil volume within the load bearing module. Ensure that you specify a Soil Cell that uses a “Fill and Cap” installation method by design such as Silva Cells.