Soils from around the world differ greatly. One reason for that is color, as evidenced in the header of this website. Colors were first standardized by Professor Albert H. Munsell on a system with three components: hue, value, and chroma. It was standardized primarily for industry. As an example, the orange thread used in the stitching of your Levi jeans has an exact hue, value, and chroma. A factory can call a supplier and order that exact same thread by stating what hue, value and chroma of the thread they want. The United States Department of Agriculture later adopted the Munsell system as their official classification of soil colors, and later helped develop the Munsell Soil Color book.
Let’s talk about the three characteristics of soil color. Looking at the figure above, think of it as a wagon wheel. You can think of hue as a spoke, and that spoke would be called yellow, red, blue, green, etc. It includes all of the colors of the color wheel we all learned about in elementary school art class.
Value is the position on the axle of the wagon wheel. It is a measure of how much light is reflected. Since pure light is white, if a color has a high value it would reflect almost everything. If it has a low value, it would adsorb the light and look dark. Think of a hot summer day when you wore a black shirt. The shirt got really hot right? That’s because the black shirt was absorbing more light from the sun which heated you up. Same concept here.
Chroma is the concentration of color, or how far from the axle you are on the wagon wheel. Low chroma appears grey and is close to the center of the wheel next to the axle. High chroma gives you bright colors that are far away from the center. Think of taking red grape juice (pardon me while I keep it appropriate for all audiences) in a glass. Pure wine would have a high chroma and would be a very bright red. if you dilute it with water, there would be less color, or a lower chroma. If you add a lot more water it would dilute it so much that you could barely tell it is red. That would be a low chroma color. That’s the basics of soil color. You now might be asking “how does this relate to soil?”
Soil gets its color primarily from two components – iron and organic matter. Oxidized iron gives soil red or yellow colors. If there is a lot of oxidized iron, those colors will be very bright. If there is very little iron, or none at all, a soil could appear grey. Thus the more oxidized iron present, the higher the chroma and the brighter the color. The minerology is what provides the hue (red, yellow, or a mix of red and yellow). In the three soil profiles below (pictures from SoilScience.info) we see a soil with a red hue (left), a yellow hue (center) and low chroma (right). Iron oxides create the high chroma red and yellow, while the reduction of iron combined with movement of that reduced iron creates what we call a depleted matrix, or a matrix that has low iron, creating grey colors. We’ll talk about that process in a future blog post.
Organic matter is the other coloring agent in soils. For this, think of potting soil or peat. This highly organic material is very dark in color, right? Organic material (decayed leaves, roots, animals, etc.) leads to a lower value making for a darker color. That is why the surface horizons of soil are usually darker, because this is where the roots are, making for more organic matter, and soils lower value in terms of color.
The Munsell soil color book consists of multiple pages with a series of different color chips. It is set up so each page is a specific hue. The value increases as you move up the page. The chroma increases as you move from left to right.
The color of each chip is reported as Hue_value/chroma. You can remember the order of hue, value, and chroma by remembering the acronym HVAC for heating, ventilation, and cooling that plumbers work on in your house. For us, its Hue, Value, And Chroma. For the picture above, the chip in the 5th row from the top, and the 4th column from the left would be 10R 4/4. We can look at the opposite page and see the corresponding dot and “common color” name which is “weak red”. A hue of 5 indicates hue purity. For example, a hue of 5R means that the only hue present is red, a hue of 10YR means a 50/50 mixture of yellow and red, and a hue of 5Y means pure yellow. Most soils are on the 10YR page so that’s usually the starting point when soil scientists are trying to match a soil to a color chip.
A soil color book is best used under well lit conditions using natural light (i.e. near high noon with few clouds or shade). Sunglasses should not be used. Also, daylight near dawn or dusk will give off a more yellow light which may effect the color chip choice. Here’s an example of some soil scientists using a color book correctly:
Soil color is really what makes soils beautiful and unique. In a future post I will discuss how soil scientists use soil color to make inferences about how soils were formed, and how soils could be used.
Good soil color resources: