How To Get An Accurate Soil Test
One of the best deals a gardener can get is a soil test, which is available to most North Americans through local cooperative extension offices or similar agencies. Check the useful links at the bottom of this page for a list of soil testing labs in your state 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 the soil test may tell you that you don’t even need that fertilizer.
Get your soil tested as close to home as possible so that the recommendations you’ll receive make sense for your climate and soil. Wherever 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 have to do that math yourself, just divide “pounds per acre” by 43 to convert the recommendation “pounds per 1,000 square foot.”)
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 soil tests 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 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.
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