How to Get an Accurate Soil Test

Make sure you get the information you need to build healthy, organic soil.

March 1, 2011

One of the best deals for gardeners is a soil test, available through local cooperative extension offices or private laboratories. In most areas, you can get a fairly comprehensive and informative test done for $5 to $10—less than you'd spend for a bag of fertilizer. And that soil test may tell you that you don't even need that fertilizer.

When you send your soil for testing, ask the lab to tailor any recommendations for a garden. Lab recommendations for remedying soil deficiencies are typically designed to serve farmers and thus are given in terms of pounds of the proposed remedy per acre of land unless you request otherwise. (If you don't mind or have to do that math yourself, just divide "pounds per acre" by 43 to convert the recommendation to "pounds per 1,000 square feet.")


Also note on the paperwork accompanying your soil sample that you would like any remedies proposed to be in the form of organic soil amendments, as opposed to agricultural chemicals.

What is the test for?
Laboratories test the soil's pH level, nutrient content and percentage of organic matter. Most garden vegetables and flowers prefer a slightly acidic (sometimes called "sour") soil, about 6.2 to 7.0 pH. This is the range where nutrients are most available for uptake by plants' roots.

If your soil is too acidic, your soil test report will probably tell you to add lime to raise the pH. But another portion of the test results should determine what kind of lime you choose. If your soil's magnesium levels are OK, the lab will probably tell you to add calcitic lime. But if your soil needs magnesium, you'll probably want to add dolomitic lime (also called magnesium lime) to correct both problems.

If your soil is too alkaline, the test results may tell you to add sulfur to lower the pH. Choose pelleted or granular sulfur, which is also known as garden sulfur. Stay away from ground sulfur—it is so finely ground you need to wear protective gear to handle it safely.

The nutrients most commonly tested for are phosphorus, potassium, calcium, sulfur and magnesium. Most labs don't include nitrogen as part of their basic test. That's because the nitrogen content of your soil can change dramatically and quickly. If you want a test of the nitrogen in your soil, you usually have to request it; you'll pay a few dollars more for this test.

Micronutrients are essential to plant health but, as their name implies, plants use them in very small amounts. Zinc, iron and copper are all micronutrients, and the best sources of these and other micronutrients are compost and other organic materials. If you haven't been amending your soil with compost and your garden has visible problems, a micronutrient deficiency could be the cause. If you suspect this problem, ask that your soil be tested for micronutrients when you send in a sample. It will probably cost more than a standard soil test, but the resulting information will tell you just what your garden needs.

6 Steps to a successful soil sample
1. Get a trowel and a bucket. Be sure neither is rusty or made of galvanized (zinc-coated) metal, which could skew your results.

2. Scrape mulch and leaf litter from the soil surface. Dig out a wedge of soil about 6 to 8 inches deep, and set this wedge aside.

3. Now dig out a half-inch piece of soil from the hole and pour it into your bucket.

4. Repeat steps 2 and 3 at least a half-dozen times in different parts of the garden so that the soil sample represents your whole garden when mixed.

5. Use your trowel to mix the soil together thoroughly.

6. Fill the soil sample bag or container with the mixed soil, complete the paperwork and mail it all off to the lab.

Soil test laboratories
Get your soil tested as close to home as possible so that the recommendations you receive make sense for your climate and soil.