Montmorillonite: Slippery When Wet!

Have you ever tried to walk across some wet bare ground, perhaps in the badlands or Red River Valley, only to find yourself face down in the mud, faster than you can say “super slippery.” Or maybe you stayed upright but gained about five pounds of that sticky stuff plastered to your shoes? Have you driven a prairie trail in the western part of the state after a good rain and found your vehicle sliding and careening uncontrollably down a slippery slope heading towards the great beyond? If you can answer yes to one of these questions you can probably thank, or curse, montmorillonite!

The mineral components of soil are sand, silt, and clay. Montmorillonite is a type of clay that typifies North Dakota soils. It is the main player, along with organic matter, when it comes to holding moisture in the soil. Unlike some other types of clays, potter’s clay for example, montmorillonite has a structure than attracts and holds large amounts of water and has a high shrink-swell. When fully wetted, these clay particles may swell to up to ten times their dry volume. As a result, the soil becomes extremely slippery with the consistency of very wet putty or pudding. Drying, of course, reverses the process, shrinking the clay into a substance nearly as hard as brick, which is kind of is. Driving a steel fence post through a layer of this dry clay will challenge the strongest man.

“Fargo clay” is perhaps the most famous and infamous clayey soil in the state. It may produce bountiful crops, but as many Red River Valley home or business owners know, the endless cycle of wetting and drying of this clay is tough on basement walls and foundations.

It has been a wet spring, and the clayey soils may remain wet for some time. So as we deal with the wet soil conditions in our gardens, fields, roads, and elsewhere, give some consideration to the clay and its water holding capacity. And do not forget that a lot of montmorillonite makes soil slippery when wet. By the way, if you get an opportunity this summer to drive a prairie trail over some clayey soils in the badlands after a good soaker…pass it up!

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.