A few situations in particular warrant the effort and expense of getting a soil test, Cogger says. When you create a new garden on ground that has not been gardened before, nutrient levels can be quite low. To give your crops the best start, it is useful to know your soil’s nutrient levels and to build those that are scarce. Another case: Regular additions of compost and other amendments over a period of years may allow high levels of certain nutrients to accumulate in garden soil. A soil test will tell you if it makes sense to change your nutrient management plan by decreasing fertilizer use and relying instead on cover crops to maintain fertility, Cogger says. Finally, if your plants are performing poorly—they’re yellowish, growing slowly, and just don’t look like everybody else’s plants despite good weather—a soil test will reveal any nutrient deficiencies or excesses in your soil.
Soil-testing laboratories offer three types of soil tests to home gardeners: tests that measure levels of contaminants, biologic activity, and nutrient availability. Testing for contaminants such as arsenic and lead makes sense only if you live in an area with known historical contamination, Cogger says. Manufacturing, mining, or other polluting activities, even if they occurred decades ago, can leave a toxic legacy. Soil close to the foundations of older homes is often contaminated with lead from paint.
Soil biological tests that identify and count specific microorganisms are becoming more readily available—but not enough is known about managing microbes to make these tests useful for home gardeners, Cogger says. “We really don’t have the [microbial] community structure and functional relationships nailed down, nor do we know the key parts of the community to assess. There’s a lot of research going on in that area, but a biological test won’t give you any information that you can act on in a reasonable way.”
Nutrient tests, the mostly widely used soil tests, assess nutrient availability and are just as useful for organic gardeners as they are for conventional gardeners, Cogger says. For costs ranging from $9 to $40, labs will test levels of phosphorus, potassium, calcium, magnesium, and soil pH. In arid regions, the test usually includes soluble salts. Some labs will also test for micronutrients and organic matter, although these tests don’t usually provide essential information, he adds. Depending on your geographic location, the specifics of the analysis may vary, so it’s a good idea to get it done regionally. Contact your county’s Cooperative Extension for a list of regional testing labs.
Since different types of plantings put different demands on soil, you should sample and test various garden areas separately. A vegetable garden has the highest nutrient demands and so should be at the top of the list for a soil test, Cogger says. Blueberry beds and other fruit-growing areas should also be high on the list, but since they have different soil requirements from the vegetable garden and are managed differently, they should be sampled separately. Lawns require yet another management approach and therefore a separate soil test. Since landscape beds have low nutrient needs, Cogger suggests testing only when you establish them.
The testing lab will tailor its recommendations to your growing situation based on background you provide. “It’s worth noting on the form information like ‘this soil has a history of manure applications’ or ‘this is a brand new garden,’” Cogger says, “and it’s essential to note which crops you plan to grow in each soil sampled.”
After about 2 weeks, the lab will send you the test results, which are generally presented in two parts. A table or graph will show the different nutrients tested and whether they test low, medium, high, or excessive relative to your garden’s needs, and will indicate the soil pH. The second part is a management section, indicating how much fertilizer to add, usually using both traditional nutrients and organic sources. Your state or county Cooperative Extension office may provide more information about soil-test interpretations and nutrients for your region.
Learn More: Ammending Clay in the Soil
Originally published in Organic Gardeing October/November 2012