# Soils are colorful natural wonders Soils are colorful and complex components of nature. I'm often asked how I became a soil scientist, and the simplest answer is that I was lucky to have someone help me see how colorful and beautiful soils are, at just the right moment. Since then, the rainbow of soil colors has continued to inspire my efforts as a scientist and artist. ## Soils are made up of four general components Soils are colorful and complex components of nature. The huge variety of soil colors sparked my curiosity and love for soils as natural wonders. Soils exist in so many different colors - red, orange, yellow, pink, blue, green, gray, black, and so so so many shades of brown. But why? To understand why soils are so colorful, we need to know what makes up a soil in the first place. Soils are comprised of four general components: minerals, organic matter, water, and air. There's a ton of variation in the composition and relative proportion of each of these components based on where a soil is found and the history of it's formation. Ultimately, that variation leads to the diversity of soil colors around the world. ![[john-kelley-profiles-colorful.png]] *Soil profile photos from various locations in the United States by John Kelley https://www.flickr.com/photos/soilscience/albums/72157622983226139* ## Soil color comes from pigmenting agents Soil color is derived from the solid components of the soil - the minerals and organic matter. These materials have a natural color, or pigment, and are what we call "pigmenting agents" within soils. Minerals within the soil are derived from the weathering of rocks and they naturally occur in many different colors. Iron minerals, in particular, contribute a lot to the color of soils. Hematite, for example, is an iron mineral (Fe$_2$O$_3$) that gives the soil a reddish color. Other minerals free from iron also give soil color. Calcite (CaCO$_3$) is a white mineral often found in soils in arid climates. By observing the color of the soil, we can begin to infer the kinds of minerals that might be present. Organic matter is the other pigmenting agent in soils. Organic matter is material derived from decaying organisms, like plants and animals, that accumulates in soils over time. Generally, organic matter contributes a dark brown color to soils. Grasslands often have thick, dark, surface soils because a lot of organic matter has been deposited into the soil by deeply rooted grasses. ## Soil color can change based on environmental conditions Soil color can also change with environmental conditions. Soils appear darker when they are wet, compared to when they are dry. When soils are completely wet, meaning all the pore spaces are full of water (we call this saturation), chemical reactions occur that can change the color of the soil. Our friend iron is key here. When soils are dry, iron is oxidized and takes on red, orange, and yellow colors - reminiscent of rust. When soils are saturated (or very wet) oxygen becomes less available in the soil environment, and the iron undergoes a chemical reaction called reduction. This process leads to gray colors appearing within the soil. So, we can use the soil color as evidence of how wet a soil is or has been in the past. ![[redox hands.png|300]] *Redoximorphic features formed from coupled iron oxidation and reduction reactions in a saturated soil in College Station, Texas. Photo by Yamina Pressler (those are my hands, too).* ## Soil scientists use the Munsell color system to record soil color The color of the soil is an important clue to its long-term formation and current conditions. The colors of soil horizons are one of the first things we notice about a soil, and they can tell us a lot about the kinds of minerals, organic matter, and chemical processes that may be occurring within that soil. When describing a soil profile, soil scientists typically record the moist color because it's easy to moisten a bit of soil, but it's not possible to dry the soil immediately in the field. Back in the lab, we may dry out the soil and record the dry color as well. Soil scientists record soil color using the [Munsell color system](https://munsell.com/). This system isn't specific to soils and is used for all kinds of [applications](https://munsell.com/color-products/). We have little [books of color](https://munsell.com/color-products/color-communications-products/environmental-color-communication/munsell-soil-color-charts/) swatches made specifically for soils that we compare to our soil samples. Each color swatch has a notation that describes 3 things: hue, value, and chroma. The hue is the dominant spectral color - is the soil dominantly red, yellow, blue, or somewhere in between? The value represents the relative lightness or darkness of a color. As soils get wet, they darken and the value decreases. The chroma is the intensity of the color. Higher chromas are more intense colors. Here's an example of the scientific notation for soil color: 10YR 3/2. The hue is 10YR, the value is 3, and the chroma is 2. The "YR" indicates that the color is somewhere between yellow and red, also known as brown. ![[lamella-munsell.png|300]] *A sandy soil with lamellae (thin bands of clay) with a Munsell Soil Color book perched on top. Photo by Yamina Pressler.* ## Take home message The soil color is often the most charismatic property of a soil profile. The colors of the soil rainbow help us understand the science and formation of a soil. Soil color is an important piece to understanding the soil puzzle and it continues to inspire art, wonder, and curiosity about soils. You don’t need to be a soil scientist to see and describe soil colors. Exploring the color of the soils around us can help us see soils as an inherently beautiful part of nature, rather than just a resource from which we can extract utilitarian value. This shift in perspective is key to soil conservation. Thank you for making the time to care about soil. Please share what you learned today 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 July 24, 2023*