As anyone finds after moving into a newly built home, a yard full of "sterile" subsoil is a poor substitute for healthy topsoil when it comes to growing a lawn, shrubs, or a garden. Topsoil brims with worms, microorganisms, and organic matter. Subsoil—material from below the topsoil—usually spells subpar greenery.
Mining companies face a similar problem when reclaiming strip-mined Western rangeland. But their problem is far more serious: Federal and, often, state laws require them to replant and establish native vegetation. New research points to a way to give their efforts a better chance of success.
While rangeland vegetation like sagebrush may look tough, it's delicate. The plants need all the help they can get from the soil.
"Anything that helps plants tolerate drought is critical in the arid and semiarid West, especially in disturbed, reclaimed soils," says soil scientist Gerald E. Schuman, who is with the Agricultural Research Service. "Disturbing the soil—digging, piling, spreading, and compacting it—destroys soil pores that hold water."
Mining companies, he notes, typically salvage and store topsoil as long as several years. They put it back only after they finish mining a site.
But Schuman and colleagues at the University of Wyoming at Laramie found that native vegetation is so needy that mining companies should return topsoil no more than a few months after it is removed.
Recently, the scientists learned why: Beneficial root-dwelling fungi die off in topsoil stored too long. The fungi, called mycorrhizae, have hairlike filaments that funnel water and nutrients to roots, helping plants survive drought.
The scientists learned about the mycorrhizae's role in a greenhouse study. The soil came from the site of a coal mine in northeastern Wyoming. In fresh and sterilized batches of this soil, they planted seed of Wyoming big sagebrush—a species that must be replanted by mining companies if it was present before disturbance.
The seedlings grown in fresh, fungi-rich topsoil survived 3 to 5 days longer when the soil was allowed to dry. "This could be just the time needed to tide them over until the next rain," says Schuman, who is in the ARS Rangeland Resources Research Unit at Cheyenne, Wyoming.
He recommends that topsoil stockpiling be limited to the start of mining operations and for no more than a few months.
"After that it should, whenever possible, be salvaged and respread where needed in a single process." —By Don Comis, Agricultural Research Service Information Staff..
"Topsoil Is Alive: Keep It Fresh " was published in the February 1999 issue of Agricultural Research magazine.