Comparing costs per gallon retained

Soil amendment is one of the least expensive ways to collect and manage stormwater

“Manage water where it falls.”

This sound advice is the foundation of the Milwaukee Metropolitan Sewerage District’s Regional Green Infrastructure Plan, a program that identified soil amendment as one of the least expensive ways to manage stormwater. At 28 cents per gallon, improving soil is second only to native plantings in lowest cost per gallon retained.

Green roofs? $4.72 per gallon. Those fancy-schmancy deep storage tunnels? $2.42 per gallon. At $1.59 per gallon, even pretty little rain gardens cost more than five times that of simple soil amendment.

Milwaukee is not alone in promoting soil amendment as a first line of defense for stormwater management. For example:

  • Denver and Greenley, Colorado, require compost use for new landscaping, as does Leander, Texas.
  • Some state Departments of Transportation (DOTs) now routinely specify compost. A few years ago, the Texas DOT said it was the largest single market for compost in the U.S.

In an urban environment, opportunities for soil amendment abound. City parks, athletic fields, planters, urban lawns, highway medians and easements, foundation backfill – anywhere there’s soil, there’s opportunity for inexpensive water retention.

Every 1 percent increase in soil organic matter (SOM) content adds an additional 16,000 gallons of water-holding capacity per acre foot. A site managed to maintain soil organic matter at only 2 percent can hold all the water of a typical rain event (1 inch or less), which is 27,154 gallons per acre.

In fact, at 5 percent SOM, the soil can retain the water equivalent of nearly 3-inches of rainfall. In some regions, this equals 95 percent of all storm events.

Soil amendment may not solve all rainfall issues, especially in downtown areas. But managing water where it falls can be the most sensible, efficient, environmentally- and economically-prudent strategy for “first line of defense” stormwater management.