According to USDA's Agriculture Research Service, most soil tests don't correctly account for soil microbes, which mineralize organic nitrogen and make more of it available to the crop. The inaccuracy can lead to farmers using more fertilizer than they should, even if they test their soil for nitrate.
Researcher Richard Haney with the USDA developed a soil test that replicates some natural processes that occur in field and accounts for microbial activity.
It's called the Soil Health Tool, and it dries and rewets soil to mimic precipitation. It also uses some of the same acids plant roots use to get nutrients.
Haney made it available to commercial and university soil testing labs. He says on average farmers who use it reduce fertilizer costs by about $10 to $15 an acre and curb runoff with less fertilizer applied.