Rock Prairie Soils: by Peter Bier
Rock Prairie
When last we checked in on the Valentine Bier family, they were improving their lot at various rented farms in Rock County. The family's next move would make all the difference: they are about to begin farming on the Rock Prairie. To discuss the significance of this move from a geologic and soils perspective, I've invited my brother, Major Pete Bier, to contribute as a guest blogger. Take it away, Pete
For as long as I can remember, I have heard members of the extended Bier family go on and on about how the Rock Prairie is some of the best farmland in the entire world. Growing up, I knew there were a lot farms in the area, and they all seemed to be prosperous. But the Biers also never let the truth get in the way of a good story. So I chalked all the "best farmland in the world" comments up to the spinning of a good yarn. Some years later, while studying for my Master's in Soil Science at UW-Madison, the hyperbole quickly became rooted in fact.
So for those that don't know, the Rock Prairie is a term that locals from eastern Rock County, WI, like to use to refer to the land roughly bordered by County Road A to the North, Highway 14 to the West and South, and the county line to the East. During my graduate studies, I investigated if the Rock Prairie's soil could actually make it more desirable agricultural land than other local areas. Soils are very complex, and more often than not, they exhibit very little homogeneity spatially. One can be in a field studying a particular soil, and 10 meters away there may be a completely different soil series or even a different soil order. This is not the case for the soils of the Rock Prairie. As seen in the below picture, the soils of the Rock Prairie are unusually homogeneous. In an approximately 3.25 kilometer-wide swath running east-northeast to west-southwest from the Rock County/Walworth County border to Highway 51 south of Janesville, a strong dominance of a single soil series, Plano Silt Loam, can be seen. North and south of the Rock Prairie, a more usual heterogeneity of soils can be seen. So, Rock Prairie soil is unique, but what makes it unique?
There are five factors that go into the formation of soil: climate, organisms (flora and fauna), relief (topography), parent material (what the soil is made of), and time. Now, three of the five factors are pretty equal across southern Wisconsin. The climate is generally the same, organisms are relatively similar, and the time - glacial and other geologic processes are such that the soil of southern Wisconsin is roughly the same age. The relief and parent material of the Rock Prairie are what make it special.
In the picture below, we can see that the Rock Prairie is extremely flat. Although the areas to the north and south of the Rock Prairie do not appear to be mountainous by any stretch of the imagination, there does exist a greater disparity in elevation and relief than on the prairie itself, which shows very little elevation change at all. This tremendous flatness of the area likely prevented erosion and movement during soil formation and allowed one soil series to form, which cannot be said regarding areas in the vicinity, where relief, although minor, could have played a bigger factor in soil formation. During formation, the Rock Prairie also may have acted as an area of deposition for clay and silt particles (preferable for farming) from the surrounding areas.
The final soil forming factor, parent material, plays a tremendously significant role in what makes the Rock Prairie’s soil distinct from the areas around it. The Rock Prairie was not glaciated during the late Wisconsin glaciation (35,000-11,000 years ago). The terminal moraine of the glacier is just to the north of the Rock Prairie, which causes the rolling terrain that can be seen in the above photo. The Rock Prairie was glaciated during the Illinoian glaciation (191,000-130,000 years ago), which helped to influence its flat topography, but it was not glaciated during the Wisconsin glaciation.
(Syverson, K.M., and P.M. Colgan. 2011. The Quaternary of Wisconsin: An Updated Review of Stratigraphy, Glacial History, and Landforms. In: Jurgen et al., editors, Developments in Quaternary Science 15. Elsevier, Amsterdam, The Netherlands. p. 537 – 552.)
Although the Rock Prairie was not glaciated, the Wisconsin glaciation did influence the area. It is distinct in that outwash dominates the area and thus is rich in sorted sand and gravel. The surrounding areas are either terminal or ground moraines and comprised of an unsorted mix of materials much larger than sand and gravel. This means that the Rock Prairie will have adequate drainage for farming and will not be too rocky (I know, a bit of a misnomer).
Wisconsin Geological and Natural History Survey: University of Wisconsin-Extension and State Planning Office: Wisconsin Department of Administration. 1976. Glacial Deposits of Wisconsin: Sand and Gravel Resource Potential. Land Resource Analysis Program
After the glaciers retreated, parts of the Midwest were covered with wind-blow loess (dust-like particles that were blown annually from dry riverbeds to the West. In covered areas, loess becomes the true parent material of the soil). All areas were not covered equally though. Parts of Rock County received a deeper covering of loess than other areas. Specifically, the Rock Prairie is covered with a deeper loess layer than surrounding areas. Loess is desirable due to the fact that it has a high nutrient holding and water retention capacity. In layman's terms, it's good for farming.
[caption id="attachment_3139" align="alignnone" width="660"] Enter a captionBlack: 8-16 feet thickness, maroon: 4-8 feet, red: 2-4 feet, pink: .5-2 feet, and white: 0 – 0.5 feet. Yellow indicates eolian sand. (Schaetzl, R.J., and J.W. Attig. 2012. The loess cover of northeastern Wisconsin. Quaternary Research 79: 199-214.)[/caption]
So, the Rock Prairie is comprised of one soil series, generally flat, has glacial outwash as a substratum, and is covered by 4-8 feet of wind-blown loess.
The Rock Prairie hit the geologic lottery. This is a farmer's dream.
The loess provides soft soil that is easy to plant in that has the ability to retain sufficient moisture and nutrients for crops. The outwash provides drainage during large precipitation events so that the soil will not flood or pond. The flatness allows farmers to worry less about erosion, terracing, or strip cropping, Finally, the fact that it is one soil series means that farmers can generally use similar practices across their entire field and achieve similar results. This is not the case when fields are riddled with diverse soil series.
Now, is this the only place in the world that had all of these factors come together so nicely? More than likely it is not. However, when combined with the fact that the area receives adequate sunshine and precipitation to grow crops without irrigation, the list narrows considerably. But most importantly, the Rock Prairie sustained my family for nearly 100 years and 4 generations when the Biers were farmers and many of my friends make their livings off this wonderful soil today. So you know what, as far as I'm concerned, the Rock Prairie is the best farmland in the world, and always will be.
