For many years, even decades, the norm has been to skim off topsoil from new construction sites, leaving landowners with yards composed largely of clay. Current Iowa regulations, enacted in 2012, require that builders return 4 inches of topsoil — in areas where there was at least 4 inches of topsoil in the first place —on tracts of an acre or more in certain Iowa cities, with the goals of reducing stormwater runoff and flooding.
When presenting to the Advisory Board for the Leopold Center for Sustainable Agriculture back in June, Iowa Learning Farms program managers Jackie Comito and Matt Helmers were approached with a question of whether we could create two trays for our Conservation Station Rainfall Simulator, demonstating side-by-side one lawn with 4” of topsoil returned, and a second lawn with only the compacted clay layer.
Student intern Lance Henrichs was tasked with creating the two trays in mid-June, one with a topsoil base and the second with a clay/sand base. He then rolled pieces of sod on top of each one. Henrichs observed, “The first time I put them in the Simulator, the top soil tray had much more infiltration than the clay-based tray.”
Fast forward two months, and the differences between the two trays are even more stark. The quality of grass is very different between the two trays. Ask any landowner who lives in an area where the topsoil was removed, and they’ll confirm that growing grass or a garden on clay alone is pretty tough! The tray with topsoil also allows for water to infiltrate, while the compacted clay layer does not allow for any infiltration, forcing all rain water to run directly off the surface of the land.
Come see it for yourself at the Iowa State Fair! Visit us in Farm Bureau Park, just off the Grand Concourse, from 9:30 – 5:00pm every day this week (9:30am – 2:00pm on Sunday).