In the Midwest, large areas of soils with fragipans are found in Kentucky, Ohio, Indiana, and Missouri, and smaller areas are found in Illinois, Michigan, Minnesota, Wisconsin, and Kansas. They formed in loess, glacial till, weathered clastic rocks, and weathered limestone. Most of these soils are classified as Fragiudalfs or Fragiaqualfs. The loess-derived soils are most extensive. They have fragipan horizons (Bx) below argillic horizons (Bt) and may have an eluvial horizon (E or E´) between the two B horizons. Fragipans form readily in acid silty and loamy deposits that overlie less permeable material (bedrock, paleosols, or dense till) at a depth of 0.75 to 2.5 m where the slope is < 12%. In the process of fragipan formation, silicate minerals weather in upper horizons and release their weathering products to the soil solution. In the winter and early spring, Si-rich solutions leach downward and become perched over the less permeable material. During the summer, trees remove water preferentially to the silicic acid in solution, and the remaining solution is concentrated in the small pores. As water is removed from these pores, silica precipitates and forms bonds with M-O groups, where O is oxygen and M may be Al, Fe, Mg or Si, on the surfaces of particles around these pores. This bond can bridge between mineral grains. The first bonding is molecular in scale, but with time it can grow large enough to be seen with a scanning electron microscope or even with a light microscope. This bridging is responsible for the hardness and brittleness of the fragipan. Compared with trees, grasses take up more silicic acid from the soil solution and cause little or no concentration of silica in the soil pores. This might explain why fragipans do not occur in soils developed mainly under prairie vegetation.
Fragipans are important soil features in the midwestern USA. According to Soil Taxonomy (Soil Survey Staff, 1975), a fragipan usually lies below an eluvial horizon. It consists of loamy materials (mainly silt loam, loam, or sandy loam) low in organic matter. These materials are arranged in polygonal structural units (prisms) that are > 10 cm across, are brittle, have very firm consistence, have a bulk density greater than in overlying horizons, contain few roots, and have bleached surfaces.
Soils with fragipans occur in nine midwestern states. Fragipans formed in Wisconsin loess, Wisconsin glacial till, and residual and colluvial deposits derived from weathered clastic rocks and limestone (Fig. 5-1). In areas of thin loess, fragipan formation may extend into the underlying paleosol. The largest areas of soils with fragipans are in Kentucky, Ohio, Indiana, and Missouri (Table 5-1). In addition to the diversity of parent materials, the climate also varies considerably within the Midwest. This variability of parent materials and climate resulted in the formation of fragipan soils with diverse properties. This chapter will concentrate on fragipan soils formed in loess, which have received the most study.
Area and classification of soil series with fragipans in the midwestern USA.†
||Soils with fragipans
||------thousands of ha------
||Udalfs, Aqualfs, Udults
||Udalfs, Aqualfs, Udults
||Udalfs, Udults, Aqualfs, Aquults
||Orthods, Udalfs, Aquods
||Udalfs, Ochrept, Udults, Aqualfs
||Udalfs, Aqualfs, Udults, Ochrepts, Aquults
No soils with fragipans are mapped in Iowa, Nebraska, North Dakota, or South Dakota. Kentucky data from Glenn E. Kelley, state soil scientist, USDA-SCS, Lexington; other data from Gerald J. Post, Midwest Nati Technical Ctr., USDA-SCS, Lincoln, NE.
Percentage of area mapped.
Map of midwestern USA showing the location of soils with fragipans. Based on Technical Committee on Soil Survey (1960) and various state maps.
Soils with fragipans are classified mainly in Udalf and Aqualf suborders (e.g., Typic Fragiudalfs and Typic Fragiaqualfs), but they are also in Udult, Aquult, Orthod, and Ochrept and Boralf suborders (Table 5-1). Most are in fine-silty and fine-loamy particle-size families, but some are in coarse-loamy, loamy-skeletal, coarse-silty, and even in sandy and fine families.
The concepts of how fragipans formed and how they are defined both changed with time. Moreover, these concepts are interrelated. One decides if a certain horizon is a fragipan partly based on his or her ideas about how the soil formed, and, conversely, one must have in mind what a fragipan is before he or she attempts to explain how it formed. In this chapter, we will accept the authors' concepts of fragipans. We will not attempt to decide if the horizons they studied met the definition of a fragipan prevailing at the time of the study or if they meet the current definition.