Encyclopedia of the Great Plains

SOILS

Soil is the product of the weathering action of climate and biological organisms on the parent material as modified by the topography of the site and the time the parent material has been in place. Parent materials are those geologic deposits that have remained stable long enough for a soil to form in them. In the Plains, parent materials are for the most part medium textured and calcareous. Plains soils formed under forest and later under grasses. Most upland soils are old enough for weathering to have formed soil horizons in the upper meter of the parent material. Humus content is normally higher in soils of the Northern Plains than in those farther south; this is associated with soil temperature gradients. Lime tends to exist at a more shallow depth in soils of the drier western Plains than in those of the eastern Plains, if all else is equal. In addition High Plains soils tend to contain as much as 60 percent volcanic ash. Topography affects the microclimate of the site and often affects age and ultimately the soil that is forming there. If the condition of all of the interacting factors is known, it is possible to predict soil conditions at any given site.

Classification systems are used to name soils. The United States currently classifies soil with six levels of abstraction: order, suborder, great group, subgroup, family, and series. In Canada the same number of levels are used but with a slightly different approach: order, great group, subgroup, family, series, and type. The U.S. system recognizes "type" but considers it to be a phase of a series rather than a separate taxonomic level. Families in the U.S. system are defined on criteria such as particle size class, mineralogy, temperature groups, and several others. Canadian soils are all "frigid" by the U.S. family designation, because their average temperature at twenty inches is less than 46ºF. Mesic and thermic categories (46-59ºF and greater than 59ºF, respectively) are characteristic of the Southern Plains. Differences in the two classification systems are not great enough to interfere seriously with field operations along the forty-ninth parallel, and both serve their purposes well.

The archetypal Plains soil is classified as an Ustoll in the United States and a Chernozemic soil (Boroll in the United States) in Canada. Chernozemic soils (Borolls) formed in association with tall to short prairie grasses in a semiarid climate and in a temperature zone where the average annual soil temperature is between 32ºF and 49ºF. For example, a soil in the Elstow series in the Prairie Provinces is in the Chernozemic order, Dark Brown great group, Eluviated subgroup, Elstow series, and Elstow loam type. No family is identified. A similar soil in the United States is the Holdrege Series, which falls in the fine silty mixed mesic family, Typic subgroup, Argiustoll great group, Ustoll suborder, and Mollisol order.

Sand fields (dunes and sand sheets), as well as alluvial-colluvial deposits, also exist in many parts of the region. Soils on these deposits are very recent and are classified in the same order (Entisols) to recognize their lack of development. These sandy soils (Psamments) tend to be very low in the organic agents that bind soil particles together into aggregates. This, and the constant bombardment of sand-size particles at the surface, causes sand particles to dislodge and blow in the wind if they are exposed. Southwest Kansas, the Oklahoma and Texas Panhandles, southeastern and northeastern Colorado, and northeastern New Mexico are at risk of this. By far the largest area threatened, however, is the Sandhills region of Nebraska. Roughly one-third of the state consists of aeolian sand that is currently mobile in certain areas and is in delicate balance with its current grass vegetation and the "blow out" areas that continually form. During the 1930s–the longest drought in the Plains in recent time–as much as one-half the topsoil was lost from more than 20 million acres of Plains land. Considering parent material, topography, and soil age, in the windy climate of the Plains, wind erosion is a significant and continuing hazard.

The soils of the Plains can be very productive in most areas if there is water to irrigate them and if accepted erosion-control measures are practiced. Minimum tillage and windbreaks are very useful in this respect. Nitrogen fertilizer is needed for the high crop yields expected, but natural levels are more than adequate for the range grasses that cover large parts of the region. Organic matter has declined, but its condition is such that the amounts remain adequate on the siltier soils, at least. Shortages of elements such as phosphorus, zinc, and sulfur are critical for cropland in some areas because of the low acidity of the soils. If they are carefully managed and treated as part of a fragile ecosystem, the soils of the Great Plains can continue to supply a large part of the earth's food supply for generations.

David Lewis University of Nebraska-Lincoln

Department of the Interior. Soil Working Group on the Ogallala Aquifer. Missouri Basin-Great Plains Caucus. Washington DC: Government Printing Office, 1980: 2-11.