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Prairie soil descriptions printed with permission from Dr. John Hilty, Urbana, Illinois. For more in-depth prairie information, visit Dr. Hilty's Illinois Wildflowers site.
There are different kinds of prairie depending on the moisture gradient and soil type. In the glaciated areas of Illinois (primarily the central and northern areas), black soil prairie was dominant. This was called the "Grand Prairie" on account of its luxuriant growth. There are also gravel prairies, dolomite prairies, sand prairies, and hill prairies. Prairies can be poorly drained and moist, or they can have average drainage with mesic conditions, or they can be excessively well-drained and dry. The different kinds of prairie wildflowers are often associated with these different moisture gradients and soil types. More than 200 species of native wildflowers have been observed in prairies in Illinois. Some of these wildflowers are common, while othersare quite rare. As an ecological habitat, prairies are dominated by grasses and herbaceous wildflowers (called forbs), rather than trees and shrubs (woodlands), or areas with more or less permanent water (wetlands). Nor are there extensive patches of bare ground, as can be found in deserts, except temporarily after a fire. Below are brief descriptions of the different kinds of prairie in Illinois. High quality prairies are interesting and colorful places to visit during the growing season, with high biodiversity. This is contrary to the common stereotype that a prairie is a rather dull place with one or two species of grasses and a few weeds.
Black Soil Prairie. This was the dominant type of prairie in central and northern Illinois, until it was almost totally destroyed by agricultural development during the 19th century. Black soil prairie typically has a dark top soil up to 2' deep, which is rich in organic matter. Beneath this, there is a clay subsoil that retains moisture and is rich in minerals. The lowest layer of subsoil is the mineral-rich glacial till that was left behind by the last glacier. It is typically a mixture of finely pulverized rocks, clay or sand, and loess (fine wind-borne debris). In Illinois, this layer of glacial till may easily exceed 200' before bedrock is reached. Because the soil retains significant moisture, even during the worst summer droughts, the roots of some prairie plants may extend 10-30' beneath the surface of the soil until the water table has been reached. Black soil prairies are usually moist during the spring when the rainfall amounts are higher and temperatures are cooler, but they tend to dry out near the soil surface during late summer because of hot temperatures and occasional summer droughts. The landscape of such prairies is rather flat and poorly drained because there has not been sufficient time (from the last glaciation) to develop an extensive drainage system, unlike hilly southern Illinois.
Typical grasses of black soil prairies include Andropogon gerardii (Big Bluestem), Sorghastrum nutans (Indian Grass), Tripsacum dactyloides (Eastern Gama Grass), Schizachrium scoparium (Little Bluestem), Elymus canadensis (Canada Wild Rye), Panicum virgatum (Switch Grass), Spartina pectinata (Prairie Cord Grass), and Sporobolous heterolepis (Prairie Dropseed). There are also many sedges and understory grasses. Typical wildflowers of black soil prairies include Silphium laciniatum, (Compass Plant), Silphium terebinthinaceum (Prairie Dock), Silphium integrifolium (Rosinweed), Eryngium yuccifolium (Rattlesnake Master), Parthenium integrifolium (Wild Quinine), Ratibida pinnata (Yellow Coneflower), Vernonia missurica (Missouri Ironweed), Amorpha canescens (Lead Plant), Solidago rigida (Stiff Goldenrod), Aster novae-angliae (New England Aster), Phlox pilosa (Prairie Phlox), and many others. A high quality black soil prairie has lots of wildflowers in bloom from late spring until the middle of fall. Today, small remnants of original black soil prairie can be found in pioneer cemeteries, or along old railroads. There are a growing number of high quality prairie restorations in Illinois.
Gravel, Dolomite, and Hill Prairies. Gravel and dolomite prairies were never very common in Illinois, and can be found primarily in northern Illinois, where either the rocky glacial till has been exposed, or where the underlying bedrock lies close to the soil surface. They tend to be rather dry and well-drained, although moist areas also exist where the land surface is depressed. Gravel and dolomite prairies can be rather flat, or slightly hilly; they often occur on gentle to moderate slopes where there has been some erosion of the topsoil. More recently, such prairies can be found along the gravelly ballast of railroads, where they did not formerly exist (in this case, they are degraded, and often contain adventive flora from Western states). While gravel prairies have soil that is slightly alkaline, dolomite prairies often have soil that is strongly alkaline. Unfortunately, the original gravel and dolomite prairies have been largely destroyed by modern development, although they still exist to a limited extent in parks and around abandoned military bases.
Hill Prairies. These prairies occur primarily along the Illinois and Mississipi Rivers, and a few scattered locations elsewhere. There is typically a shallow layer of topsoil, beneath which lies loess, sand, glacial drift, or bedrock (usually sandstone or limestone). In southern Illinois, hill prairies are often called "glades" because they are surrounded by forest. Hills prairies are very dry and exposed to prevailing winds from the south or west. However, glades are moister and more protected from wind. The wildflowers of hill prairies are similar to those that can be found in the drier areas of gravel and dolomite prairies. Some species that are found in hill prairies are more typical of western areas.
Typical grasses of these prairies include Schizachrium scoparium (Little Bluestem), Bouteloua curtipendula (Side Oats Grama), Sporobolus heterolepsis (Prairie Dropseed), and Poa annua (Annual Bluegrass). The species Spenopholis obtusata (Prairie Wedge Grass) and Deschampsia caespitosa var. glauca (Tufted Hair Grass) are more typical of dolomite prairies. Typical wildflowers of these prairies include Allium stellatum (Cliff Onion), Aster oblongifolius (Aromatic Aster), Brickellia eupatorioides (False Boneset), Dalea purpurea (Purple Prairie Clover), Lithospermum incisum (Fringed Puccoon), Solidago nemoralis (Field Goldenrod), Pulsatilla patens multifida (Pasque Flower), Arabis drummondii (Drummond's Rock Cress), Onosmodium molle occidentale (Marbleseed), and many others. Some rare species that are restricted to dolomite prairies in Illinois include Lespedeza leptostachya (Prairie Bush Clover), Dalea foliosa (Leafy Prairie Clover), and Hymenoxys acaulis glabra (Lakeside Daisy). This latter species has been extirpated from the state when its site was destroyed by a mining company.
Sand Prairies. These prairies can be moist, mesic, or dry, and their landscape is either flat or slightly hilly (from old sand dunes). They usually occur near current or former bodies of water, including extinct glacial lakes (e.g., the Kankakee Sand Area), Lake Michigan, and the drainage basins of the Illinois and Mississippi Rivers. Their vegetation is more sparse than black soil prairies. The grasses of sand prairie include Panicum virgatum (Switch Grass), Koeleri cristata (June Grass), Bouteloua hirta (Hairy Grama Grass), Sporobolus cryptandrous (Sand Dropseed), and Eragrostis spectabilis (Purple Love Grass). The wildflowers of sand prairies include Arabis lyrata (Sand Cress), Artemesia campestris caudata (Beach Wormwood), Ceanothus ovatus (Redroot), Coreopsis lanceolata (Sand Coreopsis), Chrysopsis camporum (Golden Aster), Monarda punctata (Spotted Bee Balm), Oenothera rhombipetala (Sand Evening Primrose), Desmodium sessifolium (Sessile-Leaved Tick Trefoil), and Tephrosia virginiana (Goat's Rue). In sandy areas that are moist or wet, can be found Rhexia virginica (Virginia Meadow Beauty), Hypericum kalmianum (Kalm's St. John's Wort), and Xyris torta (Twisted Yellow-Eyed Grass).
Updated May 2, 2006