Soil chart

Soil chart of plant nutrient availability.

In Sir Arthur Conan Doyle’s “Sign of Four, A Scandal in Bohemia,” Sherlock Holmes explains his approach to crime: “I never guess. It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.”

Like Sherlock Holmes, I make better decisions when I have my facts straight. Often in the garden we suspect we have a specific deficiency or pest, and at the garden store we ask what we can use to treat our suspected problem. Many gardens are being aggressively treated for conditions they don’t have. Meanwhile, conditions they do have are going untreated. Furthermore, the chemicals used to treat the wrong conditions are often creating additional problems.

We can often improve our soil by making educated guesses about what is causing whatever problems we face. However, in having soil tested by a professional lab, educated guesses are replaced with solid data. As Sherlock might say, “Follow the data.” Soil tests are affordable ($30 to $60), require little work, and we strongly recommend soil testing for homeowners to have baseline information on the health of their soil.

I recently had our lawn soil tested by Twin Falls-based Stukenholtz Laboratory, Inc. I stopped at their office on Addison and picked up the sample bag, directions, and I signed out for a metal tool for taking the soil samples. I took 25 -30 core samples, put the soil in a clean plastic bucket, mixed the soil well and transferred it into the sample bag. I then returned the filled sample bag and tool and within two days had our soil report.

The one-page report was straightforward. For me, the most important piece of information was soil pH. Soil pH directly affects the availability of soil nutrients for plants. Soil pH is measured on a scale from one to 14 with seven being neutral. Lower numbers (from one to seven) reflect decreasingly acidic soil and higher numbers (seven to 14) increasingly alkaline soil. It is when pH is in a slightly acidic range of 6.2 to 6.8 that the largest number of essential nutrients are available for plant use. If the pH is lower than that (more acidic), nutrients like phosphorous, potassium and calcium will be less available. When the pH is higher than that (more alkaline), nutrients like iron will be less available. By “less available,” I mean that the nutrient is “tied up” or bound in the soil. Plant roots cannot access it.

Plant roots access nutrients through an exchange of ions. All ions have an electrical charge. For some it’s positive, for others negative. Hydrogen ions, like most soil nutrient ions, have a positive charge. Those positively charged ions are attracted to cells in plant roots and the plant exchanges hydrogen ions for the nutrient ions. This is how plants get much of their nourishment.

Beyond pH, a lab test will indicate the percent of organic matter in the soil (4 or 5% is optimal). The test will also show how much of each essential macro and micro-nutrient is stored in the soil. If your soil is low in any of these nutrients, you will know what to add. If it is high, you can simply avoid adding more of that nutrient until plants have used up the excess.

One seemingly esoteric piece of information provided in a soil report is CEC (cation exchange capacity). While this may sound like academic gibberish, it is important in telling you how well your soil holds plant nutrients. CEC is highly correlated to soil texture. The higher the number, the higher the nutrient holding capacity of your soil. Sandy soils typically have low CEC, silt soils moderate, and clay soils high. Our area has mostly silt loams with healthy CEC levels. Information about things like cation exchange capacity and base saturation are highly useful to farmers and large crop growers, but they have limited value for home gardeners.

Finally, the report will provide recommendations. These are helpful in guiding future practice. Soil tests are well worth the expense of time and money. Local professional labs are available for home garden soil testing. The University of Idaho Analytical Sciences Laboratory in Moscow (208-885-7900) and Western Laboratories in Parma (208-722-6564) are both excellent and thoroughly professional. Tremblay Consulting Inc. in Jerome (208-324-1148) is another strong and well-respected player. In Twin Falls we are fortunate to have Stukenholtz Laboratory, Inc. at 2924 Addison Ave. E. (208-734-3050). Stukenholtz Laboratory, Inc is highly regarded throughout the Northwest.

I also recommend the booklet, “Using Soil Test Results for Garden Fertilization: A Guide for Fertilizing Home Landscapes and Gardens in Idaho,” by Amber Moore, Mike Bauer, Ariel Agenbroad, and Susan Bell. It is publication CIS 1182, published by the University of Idaho Extension. It is available online as a pdf file or through the county extension office.

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Garden Wise is presented by the Magic Valley Master Gardener Association. We will try to answer questions of general interest submitted by the community. Email questions to gardenwise@cableone.net.


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