# Soils form in profiles and horizons ## Soils are continuous Soils are continuous across space a time. The boundaries between soils and other components of nature are often blurred. Soils exist along gradients of properties, and it's often difficult to discern a hard transition between soil and rock, soil and plants, and soil and water. Soils integrate across all three. I like to think of soils like a quilted blanket across a landscape. They are woven together with more or less the same materials (rocks, minerals, organic matter, water, and air), but with a ton of variation in shape, form, and color depending on where, when, and from what they form. ## Soil scientists study soil profiles Soil scientists often study soil profiles. A soil profile is the straight on view of a soil - from the surface down to the bedrock below. It is the vertical cross section of a soil that reveals the layers forming beneath the surface. We consider the soil profile the basic unit of soil investigation. It's a good place to start when studying a new soil, and it's also an important perspective to come back to as we learn more over time. When we categorize and name soils, we do so at the profile scale. ## Soil profiles reveal soil horizons The key components of the soil profile are the horizons. Horizons are layers of soil material that vary in their physical, chemical, and biological properties. Horizons vary in their color, structure, texture, mineralogy, nutrient availability, pH, organic matter content, biodiversity... the list goes on and on. We define horizons based on differences in these properties. You can think of it like a layered cake - some layers are vanilla, others are chocolate, and others are filled with sprinkles. All are made of some form of sugar, flour, and eggs, but each have distinctly different outcomes based on how the layer was made. Some may even have additional or different ingredients to add to the mix. When we cut into a cake, we are often looking for perfectly clean layers. The horizons within soils are more often blending into one another, so it's more of a gradient, rather than a clean stack. This blending effect comes from interactions between the properties of the soil. For example, as water moves down through the soil profile it picks things up (like clays and organic matter) and brings it along for the ride. So, we end up with horizons that overlap in their properties over time. Here are two photos of soil profiles with the horizons designated by white lines. On the left is a soil developing in the floodplain of the Mississipi River in Wisconsin. On the right is a soil forming on a hillside in a coastal California grassland. The horizons vary in thickness, color, and shape, along with many other properties we can't readily see in a photograph. ![[WI CA soil profiles with horizon lines.png]] Soil profiles and horizons differ in their properties due to the influence of the soil forming factors (climate, organisms, topography, parent material, time, and humans). The soil forming factors create unique soils by modifying soil forming processes (additions, losses, transformations, and translocations). There are seemingly endless combinations of soil forming factors and processes. As a result of all these interactions, our world is filled with a huge variety of soils. ## Take home message Soils are not just one homogenous thing. The soil you see at the surface, perhaps right beneath your feet, is often quite different than what's hidden below. Soils vary in their properties as you dig deeper, and there's always something new to discover. Variation in soil horizons is the key to understanding how soils form and function. Soil scientists study soil profiles to look for clues about how a soil got there in the first place, so that we can interpret how it might change in the future. All that helps us make decisions about how to conserve and manage soils. Learning more about soil is a critical part of advocating for it's conservation. Thank you for making the time to care about soil. Please keep sharing what you learn with others! #### Postscript >This essay is part of [[One thing about soil - an educational series]] created by Dr. Yamina Pressler. The essay was originally shared on substack. *updated on April 6, 2023*