There are several environmental factors that influence a successful crop yield: climate, water quality, and the soil the crop is planted into. Some of these aspects are easily controlled. Others, we have no control over at all. And some of these factors, such as soil, can vary greatly from place to place. But what does an ideal soil look like for maximum crop yields in Illinois?
Soils found across the State of Illinois differ based on several factors that influence their characteristics: parent materials, vegetation, climate, topography, and time.
Parent Materials
The “parents” of soils are the original source of the rock and mineral components and are known as parent materials. The most recently developed soils are a product of flooding by active streams and rivers, which deposit Alluvium, the parent material of floodplain soils.
Most of Illinois was covered by glaciers at least once, and in some places more than once, during the Ice Ages. The few exceptions are in the far southern region and in the ‘driftless’ area in the northwest corner. Glaciers picked up rock as the ice moved across Canada and the northern U.S. After the glaciers melted, ground up rock and minerals called ‘till’ were left behind. Large amounts of sand and gravel were carried by meltwater streams and deposited as outwash in valleys and across upland plains. The smaller silt and clay particles from that meltwater were later picked up by wind and redeposited. These windblown deposits are called loess.
Much of the state has a layer of loess on top of the underlying parent materials like till, outwash, lacustrine (material from glacial lakes), or residuum (material weathered directly from bedrock). The thickness of the loess varies based on distance from the source valley or plain—the western side of Illinois has a thicker layer of loess because it didn’t have to be carried as far by the wind from the Mississippi River Valley.
To the east and south, the wind carried the loess much further, so the layer is thinner. Major source valleys for loess included the Mississippi River, Illinois River, and other smaller floodplains that were actively carrying sediment from melting glaciers. The loess parent material is a key reason for the richness and high productivity of Illinois soils.
Vegetation
The native vegetation that once grew on the soil impacts its quality today. The black soils in Central and Northern Illinois are prairie soils. Prairie grass vegetation can lead to an accumulation of organic matter, the reasoning for the soil’s dark color. Grasses have deep, fibrous root systems that constantly add organic matter into the soils. In comparison, the soils of Southern Illinois are forest soils, which do not accumulate as much organic matter. In forests, organic matter is typically only added from falling leaves and the occasional fallen tree.
Forest ecosystems are typically more acidic than prairie systems in this climatic region. This acidity causes some minerals to leach more easily, and other minerals to be less mobile within the soil. As a result, soil horizons develop very differently even when the other soil forming factors are the same (parent material, chronological age, landform, and climate).
Climate
Climate encompasses temperature and moisture. There are differences across the state in both temperature and precipitation. Climate affects the growing season, affecting which plants will grow and how well they will grow. Warmer temperatures further south can lead to greater plant biomass, but quicker mineralization of organic matter.
Topography
The landscape, landform, slope, and aspect of the location of soil affects the other soil forming factors. Sloping soils allow more runoff and therefore are more erodible. Hydrology, the movement of water across the landscape and through the soil, is significantly affected by the lay of the land. Some areas are accumulating while other areas are losing. Topography and landforms also affect the wind and humidity of a site. This affects the climate, which in turn affects the prevalence of one vegetation type over another.
Soils on a hill can lose topsoil and organic matter to erosion, which collects at the foot of the slope. These lower lying soils tend to be poorly drained and can have ponding or flooding issues, but often have more nutrients and moisture for plant growth. Plains are more exposed to prevailing wind and the effects of wind-like desiccation and evaporation. Exposure to wind also allowed prairie fires to more frequently burn on the flatter landscapes, giving prairie vegetation the advantage over forest vegetation on plains. Trees and forest more easily established on slopes and in valleys where the physiography limited the frequency of fire.
Time
There is both a chronological (how long the soils have been forming) and developmental (how strongly they have developed) aspect that reflect the age or relative age of soil. Soils in Southern Illinois appear older than those in Central and Northern Illinois. They were last covered with glaciers during the Illinoian stage which ended approximately 125,000 years ago. Soils in Southern Illinois have the same age of loess as Northern Illinois but are typically thinner and the underlying till has a strongly developed paleosol (buried ancient soil) that caused the loess to weather faster than loess of similar age in Northern and Central Illinois.
Much of Central and Northern Illinois were covered by glaciers much more recently during the Wisconsin glaciation, which ended only approximately 12,000 years ago. These soils are considered younger because the glaciers ‘reset’ the clock of formation since they covered the soils with brand new parent material and started over.
The soils of Central and Northern Illinois also have less topography which affects the water table and degree of development. As soils age, they weather and leach clays and nutrients that help define the horizon development. When an area has poor internal and surface drainage, the weathering process is slowed. In contrast, Southern Illinois soils are exposed to higher rainfall and temperatures, increasing the weathering process.
Together, each factor influences the others, ultimately influencing the resulting soil and the ability to increase crop yields. From an agronomic perspective, the ideal natural soil profile is one with a thick topsoil layer and high organic matter. The dominant soil texture is loam or silt loam with equal parts of sand, silt, and clay.
Soils are a living ecosystem and a lot of the material components our soils are comprised of are innate to the location. Illinois varied geography lends itself to many different soil types and thus different cropping production systems.
