Soil tests and their recommendations, although a critical component of fertility management, are not 100% accurate. Soil tests are an important tool, but they need to be used by farmers and farm advisors along with other information to make the best decision regarding amounts of fertilizers or amendments to apply.
Soil tests are an estimate of a limited number of plant nutrients based on a small sample, which is supposed to represent many acres in a field. With soil testing, the answers aren’t as certain as we might like them to be. A low-potassium soil test indicates that you will probably increase yield by adding the nutrient. However, adding fertilizer may not increase crop yields in a field with a low soil test level. The higher yields may be prevented because the soil test is not calibrated for that particular soil (and because the soil had sufficient potassium for the crop despite the low test level) or because of harm caused by poor drainage or compaction. Occasionally, using extra nutrients on a high-testing soil increases crop yields. Weather conditions may have made the nutrient less available than indicated by the soil test. So it’s important to use common sense when interpreting soil test results.