Soil testing can bring clarity to exactly what farmers need to do to restore soil health, which in turn leads to better crops, Nelson said. “If we are not soil sampling, we cannot pick out those areas that we do not need fertiliser on. Your highest yielding areas in the field are always going to be your lower testing soils because the plant is using up all the nutrients,” he elaborated. Farmers should test their soil every two to four years to determine nutrient deficiencies, according to Iowa State University. So, I would say somewhere around that 70% of the fertiliser being applied throughout the Midwest is being applied without a proper soil sample.