Stockpiling native topsoil: Leaving topsoil undisturbed is the best way to insure the mycorrhizal community remains intact and, presumably, healthy. Unfortunately, this strategy is not always logistically feasible or financially viable on large-scale construction projects. In most projects, topsoil and usually subsoil are removed and “stockpiled” elsewhere on site. Although subsoil contains fewer nutrients and less fungal diversity, this soil horizon is integral to the soil structure. Replacing the subsoil layer post-construction along with the topsoil layer will approximate pre-construction drainage characteristics and soil structure. This practice is more likely to result in a healthy soil community and planting success following completion of the project.

Stockpiling soil and minimizing on-site erosion of soils are good landscape construction practices. However, the method of storing the soil can have positive or negative consequences for native mycorrhizal communities.

Thompson and Sorvig (2000) advocate storing topsoil in several small batches covered by a breathable material. Smaller piles are less likely to become compacted. Thompson and Sorvig recommend piles depths of 6' for sandy soils and 4' feet for clay soils to maintain mycorrhizal health. Microbial communities need pore space in soil to remain healthy. Deep stockpiles, heavy traffic, repeated handling, and excavation when soils are extremely wet or dry compromises soil porosity and should be avoided. (Norman et. al. 1997)

Cover material for stockpiles can be geosynthetic, such as a filtration fabric, or fast-growing plantings, or both. Plantings are preferred because they can help maintain the entire microbial community, including mycorrhizae, and can be tilled under to provide additional nutrients when the project is complete. If plantings are intended to maintain mycorrhizal activity, species selection should include plants that have a mycorrhizal symbiont (see Figure 1 in “Applicablity” section). Invasive species should be avoided and sterile plants should be considered if the cover crop is intended to be temporary. If space and funds allow, the task of establishing plantings can be avoided by keeping the plant and soil structure intact using a front-end loader to remove topsoil in sections. The sections can then be stored for the duration of the project.

Ultimately, the cover material should prevent water and wind erosion and desiccation, but still allow nutrient flow. The distance between piles and shape of the mounds can help reduce wind erosion and desiccation (see Cedar River Watershed Education Center in “Case Studies” section).

Topsoil depth varies from site to site. Topsoil and subsoil depth should be examined prior to excavation. The Natural Resource Conservation Service (NRCS) has classified and mapped soils throughout the state. Information from these soil surveys can be obtained from Washington NRCS:

http://www.wa.nrcs.usda.gov/technical/soils/soil_survey.html