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.
Collecting samples properly is key to getting useful results, Cogger says. “You want the sample to be representative, so even in a fairly small garden space I collect a minimum of 10 subsamples.” A soil coring device, which removes a slender, vertical section of soil, is the best tool for obtaining samples, he says. To create a uniform core with a spade, dig an inch-thick slice from the ground, then remove some of the soil from the sides of the blade to leave a section about an inch wide. Soil tests are calibrated to soil samples 6 to 12 inches deep, so be sure to dig at least 6 inches down; deeper is better. (Because grass roots don’t penetrate deeply, a 4-to-6-inch sample is adequate for a lawn.) Take samples from several spots scattered randomly across the garden, mix them together in a clean bucket, and then spread the mixture on a clean surface where it can dry for several days. Spoon about one cup of the dry sample into a sealable plastic bag and label it clearly as to its source.
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