How to Test Your Soil

If you are a gardener in Colorado, chances are you have stuck your shovel into the dirt and thought to yourself… Man, we have a lot of clay in our soil. But, do you actually know the true composition of your dirt? A soil test is the best way to check the growing potential of your garden. A soil test commonly refers to the analysis of a soil sample to determine nutrient content, composition, and other characteristics such as the acidity or pH level.

In chemistry, pH (potential of hydrogen) is a numeric scale used to show the acidity or alkalinity of a solution. pH is measured on a scale of 1 to 14 where 7 is neutral, lower numbers indicate acidity, and higher numbers indicate alkalinity.

In Colorado the majority of our soils are alkaline, with pH ranging from 7 to about 8.2. The optimal range for most plants is 6.0 to 7.2, but most plants can do well in our alkaline soils if nutrient deficiencies are addressed and adequate irrigation is applied.

There are several soil tests you can preform at home or you can also purchase a home testing kit, but be aware the readings are ballpark. For a complete analysis of your soil we recommend using the CSU Soil Testing Lab. Click here to go to their website for more information. We will have the CSU soil test kits available at Gulley’s for your convenience!

However, before you can test your soil, first you must collect some soil to test!

HOW TO COLLECT A SOIL SAMPLE PROPERLY

Use a clean rust-free trowel, spade, soil tube, or soil auger to collect your samples. Each sample should be a mixture of 5 to 15 different collection points (depending on the size of the area), chosen randomly through out the area you wish to test. Collect these sub-samples to a depth of 6 inches (do not include grass or the thatch layer in in your lawn). Try to dig straight down, rather than at a angle, so that equal amounts of soil are collected at each depth. Combine all samples in a clean plastic container.

Try to collect roughly the same amount of soil from each collection point. Your finished soil sample should represent a uniform area consisting of land that is similar in slope, drainage, texture, or other characteristics that make the soil properties similar within the area. For example, vegetable gardens are managed differently from lawns.

Mix the sub-samples together thoroughly. Remove rocks, plant debris and break up clods to pea size or smaller. Remove about 2 cups of soil: spread on paper towels and air dry.

IDENTIFYING SOIL TEXTURE BY FEEL

Now you are ready to preform some home tests!

The first thing you want to test is the texture. Texture of the soil is an important measurement because it influences so many other properties of the soil. It affects things such as water infiltration and retention, soil temperature, and gas exchange.

Texture basically refers to the size of the particles that make up the soil. The terms sand, silt, and clay refer to the relative sizes of the soil particles. Want to learn more? Click here to read all about soil texture.

Feel test Rub some moist soil between fingers.

• Sand feels gritty.
• Silt feels smooth.
• Clays feel sticky.

Ball squeeze test Squeeze a moistened ball of soil in the hand.

• Coarse texture soils (sand or loamy sands) break with slight pressure.
• Medium texture soils (sandy loams and silt loams) stay together but change shape easily.
• Fine textured soils (clayey or clayey loam) resist breaking.

Ribbon test Roll the clay into a cigar shape with about ½ – ¾ inch diameter. Place the roll shaped soil between your thumb and forefinger, and start to gently press it into a flat ribbon shape. As the ribbon develops, let it extend over your forefinger until it breaks from its own weight. How long is the ribbon?

• No Ribbon: Indicates a sandy texture. (Sand or loamy sand)
• Ribbon < 1”: Gritty = sandy texture (sandy soil); Not Gritty = loamy texture (high in silt)
• Ribbon 1 – 2”: Gritty = loamy texture; Not gritty = clayey texture
• Ribbon > 2”: Clayey texture

PROPERTIES OF CLAYEY AND SANDY SOILS

HOW TO TEST THE PH OF YOUR SOIL

Another test you can preform is the Fizz Test. Drip a couple of drops of kitchen vinegar onto a sample of your soil.

If the soil fizzes, it indicates:

• The presence of free lime (calcium carbonate)
• pH tends to be > 7.5 alkaline
• Lowering the ph can be troublesome when the soil contains free lime
• Prone to iron chlorosis

Don’t try to drastically lower soil pH

• Alkaline soils are well buffered, resist a change in pH
• Lowering pH by adding acidifiers like sulfur will take repeated applications and results are temporary.
• Free lime in the soil neutralizers added acidifiers
• Select plants that tolerate high pH soil

It is important to note that your soil is what it is, and while it is possible to amend and add nutrients, it is best to select plants that are adapted to the properties of your soil. Raised bed gardens will require proper soil maintenance annually.

Good Luck and Happy Gardening!

Many thanks to Jean Reeder P.H. D. Soil Ecologist, for her help and expertise with this blog.