Across the great American plains, where the seasons are stark and natural grasslands still dot the prairie, the land is mostly covered in the rich dark loam known as chernozemic soil which have developed since the glaciers receded a few millennia ago.
There are five key factors that determine the type of soil that is formed; parent material, climate, time, relief (i.e. topography), & organisms.
Some 10,000 years ago, glaciers slowly crept across central North America. As temperatures rose, the ice melted and left behind an unsorted mixture of sand, gravel and other collected materials known as glacial till. As it settled, this slurry of sediment became the foundation (i.e. parent material) of the soils found there today.
The harsh climate of the american plains continued to further break down the glacial deposits. Erosion, from both wind and water, as well as the severe temperature fluctuations cause larger materials to break apart. Boulders break down into gravel, which break down into sand which eventually, breaks down into clay. The resulting mixture of particle size in the soil creates the classification ‘loam’.
Over time, the mixture of sediment began to settle into soil horizons. The process is ongoing and can take hundreds of years. Young soils may closely resemble their parent materials but as they age, environmental forces (e.g. organisms & climate) reshape soil characteristics. Water and gravity move smaller particles downwards while wind and water runoff translocate materials from high to low ground. Eventually, over thousands of years, soil particles will ‘settle’ into soil horizons.
Topography, also called ‘relief’ by soil scientists, can also affect soil development on a much more local scale. The shape and slope of the land alone can cause soil variation despite all other factors being equal. Soil horizons may be thicker or thinner depending on whether they are on the top of a hill or in a depression. In north america, south facing slopes receive more sun, making them warmer and drier than north-facing slopes. While the temperature differential between north and south slopes may not be great, only a degree or two, it is enough to cause variation over time.
Life is the final factor. Organisms both big and small call the soil their home, and they all have an effect on the soil’s physical and chemical development. Larger creatures (i.e. insects and small mammals), mix up the soil horizons, translocating material both up and down. Plants draw material from the soil, processing minerals and redepositing them back in the ground in altered forms. And micro-organisms break down decaying matter, recycle minerals and produce various compounds and excretions (e.g. fulvic and humic acid) that affect both chemical weather and the ph levels of the soil.
Ultimately, soil formation is the result of thousands of years of exposure to natural environmental processes. Soil is a non-renewable resource, a precious commodity developed over time that deserves to be respected – it’s so much more than just dirt.