Standard soil tests measure key soil health indicators, such as pH, organic matter. and plant-available nutrients. Now, additional soil tests are being developed that try to measure biological variables, too, such as microbial activity, says Liz Stahl, University of Minnesota Extension crops specialist in Worthington.
The Haney Soil Test, for example, attempts to mimic the soil environment more closely than standard soil tests, in order to determine how much nitrogen and phosphorus are actually available to plants. One component of the Haney Test, called the Solvita Respiration Test, measures the amount of carbon dioxide released from the soil. It’s an indicator of microbial activity and mineralization potential, Stahl says.
The organic carbon-to-nitrogen ratio (C:N) is another measurement that tries to estimate nitrogen and phosphorus mineralization. The ideal C:N ratio for soil microbe activity is 20:1 or less, according to Ward Labs in Kearney, Neb. Another biological indicator, the Phospholipid Fatty Acids Test (PLFA), gives you “a snapshot of the microbial community and abundance at the time of sampling,” Stahl says. A high ratio of fungi to bacteria is thought to be an indicator of soil health, she says.
However, these tests of biological activity are generally quite variable and difficult to interpret, Karlen cautions. Soil scientists are still learning how to apply these test results to soil management recommendations, Stahl adds. So when it comes to fertilizer guidelines, she says, “Stick to standard soil tests and university recommendations for your state,” which are backed by “years and years of solid research.”