The down and dirty about soil.
In my short time on this Earth as I have continued my education through high school, college and beyond I has notice that very few people know much about the substance upon which they walk, build and live upon. This substance I speak of is soil, without soil nothing we have accomplished as a species could have happened. Without soils unique characteristics and attributes we could not feed the world, grow plants for oxygen, build our cities and pretty much do everything we have done. I seek to remedy this by providing resources to people through a series of articles that bring soil to the surface and hopefully make it importance obvious to all.
I will start off with the most basic question of all.
Where does soil come from?
The most simply answer to this question is that it come from anywhere the five soil formation factors are in action. To form a soil is an incredibly slow process, it can take up to 500 years to form one inch depending on conditions. The five soil forming factors and their interactions are what cause certain types of soils to form. These factors are,
- Parent Material
- Biotic Factors
The combination of these factors is critical in determining the characteristics of soils such as it depth, color, structure, chemical composition and pH.
Time is the base on which all the factors build off of. To form a productive soil can take anywhere from 2,000-20,000 years of the climate, parent material, topography, and living organism interacting. Over time the climate will cause mechanical and chemical weathering of the parent material, breaking it down into smaller and smaller particles until we get the building blocks of mineral soil; sand, silt and clay. At the same time as the weathering, living organisms likes plants, animals, bacteria and fungi are growing living and dying on this mineral surface both contributing to the weathering of the parent material and adding organic matter to the mineral components.
Climate after time is the most important factor in my opinion. To explain this I want to use my experience with the soils of northern North Dakota and Minnesota. This area like many others in the northern hemisphere was covered by glaciers for thousands of years up until this current warm period which began around 10,000 years ago. As the glaciers advanced and retreated they removed any top soil that had formed in the region prior to the glaciations. After the glaciers retreated for the last time they left behind a swath of material called glacial till. On this glacial till Mollisol soils were built up through the combination of climate and the living organisms that inhabit this part of the world. The areas climate is characterized by short hot summers and long cold winters. The seasonal swing from cold to hot and back to cold greatly reduced the amount of decomposition that can occur over one year. After 10,000 years of this the soil developed a thick, black, nutrient rich topsoil from all the organic matter that was not decomposing. By comparison Oxisols (tropical soils) are typically thin, grey, red or yellow in color and nutrient poor. These features are from the high rate of nutrient leaching and decomposition that occurs in those soils. All of the leaching and the high rate of decomposition can be directly related to how the climate affects the soil.
Parent materials are as varied as the soils they create. Each parent material will give the soils its unique structure and chemical properties. They can be bedrock like the Canadian Shield, eolian(Loess Hills, Iowa) or fluvial (Mississippi River Delta) deposits or even glacial till(Northern Great Plains). For example a limestone parent material will give its soils a high calcium content which is an important plant micronutrient.
Topography is often one of the most overlooked of the soil forming factors. Yet the simple action of how water hits and infiltrations into the soil it critical in how a soil forms and builds. Water has more difficulty penetrating into the soil on steeper slopes causing runoff which will erode soil that has formed into the flatter lower slopes. This causes the soil to be thicker and usually more nutrient rich in the lower parts of slopes and flat lands.
Biotic Factors are also very crucial in determining how a soil develops. Using the Northern Great Plains Mollisols again we will look how the roots of prairie plants helped steer their development. The root systems of many prairie plants are very dense and fibrous and they extend many feet down into the soil pushing organic matter and carbon deep into the soil and pulling nutrients up. When the plants die the roots stay in the soil and decompose contributing carbon, nutrients and organic matter to the soil. This same process is applied everywhere soil building is occurring to various degrees.
With all of these factors in place a soil will begin to form. Once the soil is formed it can provide a growing medium for food, trap atmospheric carbon, provide materials for building and be the foundation for everything that we need from this world
I hope you will continue to join me as I continue to explore topics in soil and soil science.
Smith , Thomas M., and Robert Leo Smith . Elements of Ecology. 6. San Francisco: Pearson Education Inc., 2006. Print.