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