It’s sometimes easy to take the soil beneath our feet for granted. Yet soil has always been with us—it is the foundation of our houses and roads, and from the soil comes our food, fiber, and paper. Soil provides a grassy surface for our parks and fodder for our gardens. Scientists look at soil as a record of the integration of the climate and life of an area. The scientist, the engineer, and the gardener may all look at the soil below them in different ways, but perhaps no one has a more integral relationship with soil than a farmer. The economic success of producing crops is intimately tied to the quality of the soil upon which those crops grow, and the most successful farmers are very well versed in the science of their soil.

Known for some of the richest soil in the US, the Midwest is home to some of the most productive agricultural soil in North America. According to the USDA, the US contains only about 5% of the world’s population, but it provides more than 25% of the world’s food supply. The Midwest is home to the Corn Belt, the largest corn producing area in the US, supporting over a hundred-billion-dollar- a-year industry that helps feed the world but also produces plastics, biofuel, livestock feed, and more. How did the soil in this area come to be so fruitful?

What is “Soil”?

Generally, soil refers to the top layer of earth—the loose surface of the Earth as distinguished from rock—where vegetation grows. The word is derived (through Old French) from the Latin solum, which means “floor” or “ground.” It is the most basic resource upon which all terrestrial life depends, and soil is one of the most important resources we have. The Midwest has a wide variety of soils, and each type of soil has a story to tell of its origin.

Soils are composed of a mixture of two key ingredients. The first is plant litter, such as dead grasses, leaves, and fallen debris. Worms, bacteria, and fungi do the job of breaking these down into the nutritious organic matter that helps soil to be so nourishing to future plant growth. The second important component of soil is the sediment derived from the weathering of rock that is then transported by wind, water, or gravity. Both of these components can influence the texture and consistency of the soil, as well as the minerals available for consumption by plants.

All soils may seem alike, but there can be vast differences in soil properties even within small areas. A single acre may have several different soil types, each with its own assets and drawbacks. Some types of soil are clayey or prone to flooding, while others are stable enough to be used as a foundation for buildings. The most identifiable physical properties of soils are texture, structure, and color, which provide the basis for distinguishing soil horizons. Texture refers to the percentage of sand, silt and clay that makes up the soil. The textures have specific names, as indicated in Figure 8.1.

Generally, the best agricultural soils are those with about equal amounts of clay, silt, and sand. A soil of that type would be called a loam. Soils that are mostly sand do not hold water very well and dry quickly. Soils with too much clay may never dry out.

Soil structure refers to the way the soil forms clumps. These clumps are known as peds. The peds are identified by the shape of the soil clods, which take the form of balls, blocks, columns, and plates. These structures are easiest to see in recently plowed fields, where the soil is often granular and loose or lumpy. Soil color is its most obvious physical property. The color is influenced by mineral content, the amount of organic material, and the amount of water it routinely holds. The colors are identified by a standard soil color chart called the Munsell chart.

Ultimately there are five variables that affect the characteristics of soil worldwide.

1. Parent material is the original geologic material from which the soil formed. This can be bedrock, preexisting soils, or other materials such as till or loess.
2. Climate strongly determines the temperature regime, amount of moisture, and type of biota that interact with the parent material. This will affect the extent of chemical and physical weathering on the soil- forming material.
3. Topography, or landscape, of the area is related to the relative position of the soil on the landscape; this includes the presence or absence of hills and the slopes between high and low areas. Topography influences natural drainage. Gravity moves water down slopes to depressions or streams and pulls free water downward through the soil. Soils on hills tend to be dry, and soils in depressions and valleys are often wet or saturated.
4. Biota or living organisms that live on or in the material affect soil development through their influence on the amount and distribution of organic matter in the soil. For example, plants contribute significantly to the formation of humus, and animals alter a soil’s characteristics by leaving behind decayed remains and wastes. Decomposers like bacteria and fungi help to free up the nutrients locked away in these remains and wastes, and these freed nutrients are then recycled and used by new life forms within the same soil. Additionally, animal burrows create spaces in the soil horizons that allow for deeper penetration of air and water, which, in turn, aid plant development. For its part, organic matter impacts the water-holding capacity of the soil, the soil’s fertility, and root penetration.
5. Time is required for soils to develop while the four elements mentioned above interact. The effects of time can be seen when comparing soils on a glaciated region to either soils formed on recent flood plain deposits or soils in a non-glaciated area at the same latitude.

Soil Orders

Just as rocks are classified into different types based on how they formed (igneous, metamorphic, or sedimentary), their mineral composition, and other characteristics, soils also have their own classification scheme. Soils are divided into 12 dominant orders based on their composition, structures, and types and number of horizons. A typical soil profile is given below (Figure 8.2). It shows the transition from the parent material (horizons R and C) to the highly developed or changed horizons (O through B), although not every soil profile will have all the horizons present.

Soils can also be categorized by their location (northern vs. southern soils), the type of vegetation growing on them (forest soils vs. prairie soils), their topographic position (hilltop soils vs. valley soils), or other distinguishing features. The system used to classify soils based on their properties is called soil taxonomy. It was developed by the US Department of Agriculture, with the help of soil scientists throughout the country.

In soil taxonomy, all soils are arranged into one of 12 major units, or soil orders. These 12 orders are defined by diagnostic horizons, composition, soil structures, and other characteristics. Soil orders depend mainly on climate and the organisms within the soil. These orders are further broken down into 64 suborders based on properties that influence soil development and plant growth, with the most important property being how wet the soil is throughout the year. The suborders are, in turn, separated into great groups (300+) and subgroups (2400+). Similar soils within a subgroup are grouped into even smaller families (7500+), and the similar soils within families are grouped together into the smallest category of all: a series. There are more than 19,000 soil series described in the United States, with more being defined every year (Figure 8.3).

Name	Description	Controlling factors	Percentage of global ice-free land surface	Percentage of US ice-free land surface
Alfisols	Highly fertile and productive agricultural soils in which clays often accumulate below the surface. Found in humid and subhumid climates.	climate and organisms	~10%	~14%
Andisols	Often formed in volcanic materials, these highly productive soils possess very high water- and nutrient- holding capabilities. Commonly found in cool areas with moderate to high levels of precipitation.	parent material	~1%	~2%
Aridisols	Soils formed in very dry (arid) climates. The lack of moisture restricts weathering and leaching, resulting in both the accumulation of salts and limited subsurface development. Commonly found in deserts.	climate	~12%	~8%
Entisols	Soils of relatively recent origin with little or no horizon development. Commonly found in areas where erosion or deposition rates outpace rates of soil development, such as floodplains, mountains, and badland areas.	time and topography	~16%	~12%
Gelisols	Weakly weathered soils formed in areas that contain permafrost within the soil profile.	climate	~9%	~9%
Histosols	Organic-rich soils found along lake coastal areas where poor drainage creates conditions of slow decomposition and peat (or muck) accumulates.	topography	~1%	~2%
Inceptisols	Soils that exhibit only moderate weathering and development. Often found on steep (relatively young) topography and overlying erosion- resistant bedrock.	time and climate	~17%	~10%
Mollisols	Agricultural soils made highly productive due to a very fertile, organic- rich surface layer.	climate and organisms	~7%	~22%
Oxisols	Very old, extremely leached and weathered soils with a subsurface accumulation of iron and aluminum oxides. Commonly found in humid, tropical environments.	climate and time	~8%	~.02%
Spodosols	Acidic soils in which aluminum and iron oxides accumulate below the surface. They typically form under pine vegetation and sandy parent material.	parent material, climate, and organisms	~4%	~4%
Ultisols	Soils with subsurface clay accumulations that possess low native fertility and are often red hued (due to the presence of iron oxides). Found in humid tropical and subtropical climates.	climate, time, and organisms	~8%	~9%
Vertisols	Clayey soils with high shrink/swell capacity. During dry periods, these soils shrink and develop wide cracks; during wet periods, they swell with moisture.	parent material	~2%	~2%

Dominant soils of the Midwestern US

The soil orders found in the Midwest are:

• Alfisols (al-fuh-sawls): Alfisols are widely distributed throughout the Midwest but are less prominent in the western portion.
• Entisols (en-ti-sawls): Entisols are most concentrated in the Central Sands of Wisconsin and the Loess Hills of western Iowa. They can also be found sprinkled about parts of Wisconsin, the western half of Michigan’s Lower Peninsula, and northern Minnesota.
• Histosols (his-tuh-sawls): As one would imagine, the Histosols are primarily clustered around the Great Lakes and can therefore be found in Michigan’s Upper Peninsula, northern Wisconsin, and northern Michigan.
• Inceptisols (in-sep-tuh-sawls): Inceptisols are not widespread throughout the Midwest, but they are highly concentrated in the northeast corner of Minnesota.
• Mollisols (mol-uh-sawls): Mollisols dominate the western portion of the Midwest along with large parts of northern and central Illinois.
• Spodosols (spod-uh-sawl): In the Midwest, Spodosols are exclusively concentrated in northern Wisconsin, Michigan’s Upper Peninsula, and the northern portion of Michigan’s Lower Peninsula.

National and Midwest Soils

Below are maps showing the locations of the predominant soils in the nation (Figure 8.4), and in the Midwest (Figure 8.5). The Midwest soil types tend to relate to topography and parent material, with some relation to climate.

Geology of the Midwest: Parent Material

A quick look at the maps below (Figures 8.6 - 8.8) shows that the dominant parent material for Midwest soils is glacial deposits from the Wisconsinan, Illinoian and Pre-Illinoian advances. These covered the Midwest from about two million years ago to the final retreat of the Wisconsinan glaciation about 10,000 years ago. The material from the glaciers is primarily till, glacial fluvial deposits (as outwash plains), and loess deposits.

Till is the unsorted material—from boulders to fine clay silt—deposited by glaciers as they advance and recede. When a glacier retreats, a line of sediment from the flowing river remains behind and can be seen as a ridge of sand and/or gravel, such as that found in kames and eskers. Beyond the edges of the glacier, as melting of the glaciers continues, sediment-laden waters create large, sandy flats, known as outwash plains. Fluvial (outwash) material is a very common parent material in the Central Lowlands region. Another aspect of glaciation in this region was the accumulation of loess (windblown silt) distributed across the landscape that was part of the outwash plains. The Mississippi and Missouri Review Rivers, as well as other rivers in the area, aided the distribution and deposition of loess to the Midwest, creating the rich agricultural area we have today.

A simplified map of the soils of the Midwest (Figure 8.4) shows that, when compared to Figures 8.7 and 8.8, the soil types are strongly correlated with the age of the glacial deposits. It is only the southern margins of the Midwest in Illinois, Indiana, and Ohio that escaped the advancing and retreating ice.