Why is beach sand mostly white and tan while rich farmland is practically black? What does soil color tell you about your soil?
Soil occurs in many different colors. Iron-rich soil tends to be reddish orange or green, while peat can be practically purple!
Go outside and collect a handful of your soil and put it in a clear container. Shake it around a little bit. Is it wet or dry? What color is it? Brown? Maybe. But I’ll bet it’s not that simple.
What does soil color tell us?
Each layer of your garden soil has a unique color. The deeper you dig, the lighter those colors tend to get. Soil color tells us which minerals are present and the level of decaying organic material. It can also tell you how old a soil is, which processes are occurring, and about local water behavior.
We are not going to explore soil age or the chemical processes that take place in soil, but you can use soil color to make better decisions about irrigating and fertilizing your garden.
Soil moisture levels
We all know that soil looks darker when it is wet. But soil color can tell you how long the soil stays wet. Soil that does not drain well and stays wet for much of the year tends to be a dull yellow or grey. Wet soil contains less oxygen than dry soil. Oxygen causes some minerals to oxidize, or rust. Iron-rich soil that contains a lot of moisture most of the time will turn grey or greenish, while drier soils expose iron to more oxygen, turning the soil red or yellow.
Soils that stay wet often have more complex color patterns, while arid soils are more uniform. If your soil colors are uniform, you know that the water table is lower and you will probably have to irrigate more often. If your soil is reddish, you will probably never need to amend with iron. Remember, the minerals found in soil are plant food.
Minerals make a difference
Other minerals in your soil can also affect its color. Knowing what these colors mean can help you select the best soil amendments and irrigation schedules.
What color is your soil?
Take a closer look at your soil sample. What do you see? Is it yellowish-brown or dark brown? Or something else entirely? When we first moved to our San Jose, California house, the soil was a pale, tan color and as hard as concrete.
For many of us, identifying a specific color can be tricky. Brown is brown, right? But soil can be all sorts of shades of brown, along with a bunch of other colors. To help you get really specific information about the color of your soil, you may want to go to the library and check out their copy of the Munsell soil color book.
Munsell’s color book
Soil color is so important that a system of soil color classification has been developed. This classification method is called the Munsell soil color system. A Munsell book is the gardener’s equivalent of a paint chip book, containing 199 color chips. Its pages are heavy card stock and they are organized by color. Underneath each color chip is a hole in the card stock that lets you hold a soil sample underneath for comparison. On the opposite page tells you the universally accepted name for that color. This is a coding system used around the world by soil scientists, farmers, and gardeners like you!
You artists out there know a lot more about this than I do, but let me give it a shot. According to my Munsell book, colors are described using hue, value, and chroma. Hue is the wavelength we see as color. Munsell’s book gives codes for red (R), yellow (Y), green (G), blue (B), and purple (P). Those wavelengths are measured in graduations of purity, ranging from 2.5, 5. 7.5 and 10. A pure hue is rated at 5. Numbers above 5 indicate the presence of other hues. Value indicates lightness or darkness. A value of zero indicates pure black, while 10 is white. Finally, chroma refers to a color’s strength or intensity, ranging from greyed out (/0) to full intensity (/14).
A Munsell soil color rating is written with the hue letter first, followed by a space and then the number value, a forward slash (or virgule), and then the chroma number. Decimals can be used to provide greater clarity.
Looking at a photo of my soil when we bought our place in 2012, I see that the color most closely matches 5YR 7/1. According to Munsell, that soil would be called 'light grey'. As noted earlier, this indicates high calcium carbonate, gypsum, magnesium, sand, and/or salt levels. It can also indicate too much moisture. Funny thing, the previous owner loved to apply fertilizer and overwater the property. According to my 2015 soil test, soil organic matter was at 3.5% and all the nutrients, except iron, were through the roof! Iron was extremely low.
Seven and a half years later, after adding lots of mulch and compost, a little nitrogen, appropriate watering and nothing else, my soil has been transformed to 2.5YR 3/0, with 7.6% organic matter and nutrient levels (slowly) dropping to where they should be. [These changes never happen overnight. When they do, beware! Something is very wrong.]
The new color is 'very dark grey' which goes along with all the chicken bedding, wood chips, and other organic materials I've been adding. And my iron levels are still way too low, which is why the chroma numbers have stayed low.
So, take another look at your soil sample. Does it tell you more than it did? If you live nearby, feel free to bring a soil sample by so we can take a look in my Munsell book together. If not, head to the library.
Did you know that carpet manufacturers use the Munsell soil color system to match local soil colors with carpet dyes so that their carpets will look cleaner longer?
Now you know.
You can grow a surprising amount of food in your own yard. Ask me how!
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