Dirt is fascinating. Oh, I know, you’re thinking of the dirt you wash out of your clothes, or off your car. I don’t find that kind of dirt very exciting at all. But the dirt in a garden is a whole ’nother story!

life-in-soil-exhibit-desertmuseumtucsonaz-2009-03-23-991rsActually, dirt is just one component of what gardeners prefer to call “soil.” Rocks weather and break down into smaller rocks, pebbles, gravel, and finally sand and silt. These tiny particles mix with organic matter—decomposing plants and animals—called “humus.” Then there’s air, and water. Add in weed seeds, worms, bugs, and a huge variety of microorganisms, and you have the living stuff in which we plant our gardens.

Soil has various names, according to the particle sizes of the minerals involved. Coarse grains are sand, while the finest dust becomes clay. In between there is loamy sand, sandy loam, sandy clay, loam, loam, silt loam, silt, silty clay loam, clay, clay loam, sandy clay and silty clay. You can see how it all works in this chart by the U.S. Dept. of Agriculture:


If you are curious as to what kind of soil you have, an easy test is to mix about a third of a cup of soil into a jar of water. Shake it well, and watch while the particles settle. Sand sinks quickly to the bottom, while the finer silt and clay particles may remain suspended for several days. When the water is clear, you’ll have a cross-section of particle sizes.

Most plants prefer a loamy soil containing about 5% organic matter, and equal amounts of air and water, about 25% each. However, very few parts of Colorado have such soils. More typically, we find ourselves dealing with either sticky clay or porous sand. Organic matter is often nonexistent.

I don’t recommend trying to change the particle sizes in the soil you are blessed with. For one thing, adding sand to clay is how cement is made! Yes, you can buy topsoil. But even that may not help. There are no industry standards defining what “topsoil” is. Overall, it’s much easier to improve the soil you already have. In the next posting, I’ll explain how to do this.

Another characteristic of soils is their pH. Based on the activity of dissolved hydrogen ions present, this is rated on a scale from 1 to 14, with 7 being neutral. Numbers lower than 7 indicate an acid, while those higher than 7 are called alkaline, or basic.

Acidic soils, with a pH  around 5, are commonly found in areas of high rainfall such as the northeast Alkaline soils in are present in much of the western U.S., where precipitation is lower. Here in Colorado, most of our soils are on the alkaline side. My home garden hovers between 7.5 and 8.

The pH of acidic soils is easily raised by adding ground limestone to the garden. Similarly, in many places the pH of basic soils can be lowered by the addition of sulfur. However, in Colorado, where our soils are derived from limestone,* the amount of sulfur needed to significantly lower the pH is impractical. We just have to live with what we’ve got. Don’t let that stop you from gardening. While our soil may not be suitable for azaleas and blueberries, there are plenty of gorgeous plants we can enjoy!

Next time you wash your hands after a day in the garden, think about the amazing soil that forms the foundation of your landscape.

*One fun way to determine if your soil contains limestone is to add a few drops of vinegar to a spoonful of dirt. The limestone will react with the acid in the vinegar, causing bubbles!

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