Soil Test Basics: Why and How to Take a Soil Test

Nothing is more critical to the success of a garden than its soil. Learn why soil tests are so beneficial and get tips on how to properly conduct a soil test.

by Jessica Walliser
PHOTO: Jessica Walliser

There’s nothing more critical to the success or failure of a farm or garden than its soil. We growers take our dirt very seriously and we go to great lengths to work on its friability and fertility, adding organic amendments and fertilizers in hopes of maximizing plant health and yields. We can see what good, crumbly, rich soil looks like with our eyes, but our eyes aren’t capable of seeing what’s happening in our soil at a microscopic level which, as it turns out, is farm more important than our soil’s outward appearance. For that, we rely on soil testing. Taking a soil test isn’t hard, but there are a few important items to consider when it’s time to test your soil.

What Does a Soil Test Analyze?

There is so much to know about your soil that can help you grow a better, more productive garden. Traditional soil tests are offered by most Cooperative Extension Services at land grant universities across the U.S. Soil tests are also available from independent soil testing laboratories easily discovered via a quick internet search. These standard soil tests tell you about your soil’s fertility by measuring phosphorus, calcium, potassium and magnesium levels, and sometimes trace nutrients, too. They’ll also inform you of your soil’s pH, which influences the availability of many plant nutrients contained in the soil, along with supplying recommendations on how to adjust the pH according to which crop(s) you’re growing. These soil tests even tell you what nutrients you have in excess that might inhibit the uptake of other nutrients.

Soil tests sometimes also have the option of testing for organic matter content. This is valuable to farmers and gardeners as it can reflect the availability of nitrogen, a plant nutrient that not included in soil tests because it comes and goes so quickly from the soil it’s nearly impossible to measure. Soils high in organic matter tend to have more nitrogen available for plant use over time.

Soil tests are also available to test for contaminants, like lead and other heavy metals.

How to Take a Soil Test

Now that you know why taking a soil test is valuable, here are some tips on how to do it properly.

For all soil tests, it’s important to obtain a representative soil sample. To do this, start with a clean, non-reactive metal or plastic bucket and a clean trowel. Choose 10 to 20 different sampling sites within the testing area. For each of the 10 to 20 sampling sites, remove all the surface debris and extract a core of soil about 6 inches deep and place it into the bucket. Once you’ve collected all your cores in the bucket, mix it gently but thoroughly.

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Spread the soil onto a newspaper for 24 to 48 hours. Stir the soil once or twice during that time to allow it to dry out a bit. Once the excess moisture has evaporated, measure out the amount of soil specified in the test instructions and place it into the provided sampling bag. Fill out the appropriate forms and send it on its way.

In a few weeks, you’ll get the results back in the mail. These results will not only include the data discovered during the test, it will also include actions you can take to correct any deficiencies or nutrient excesses.

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