Impact of fertilizer recommendations from soil testing laboratories on soil nutrient levels

Abstract There are many resources that farmers use to determine fertilizer needs for crops such as private and public laboratories, crop advisors, and fertilizer dealers. These resources provide recommendations for a specific crop that can vary greatly. An experiment established in 2021 at 12 sites in Utah and Wyoming in alfalfa ( Medicago sativa ), small grain forage, and corn ( Zea mays ) tested and compared fertilizer recommendations from five labs. The recommendations tested were from two public labs (Utah State University and the University of Idaho) and three commercial labs located in the western United States. A composite soil sample split and sent to multiple labs for analysis revealed high variability in reported soil test results and recommendations, both for types and rates of nutrients. Differences in soil test results were influenced by analytical methods used by each lab to measure nutrient levels and the accuracy of their analyses. Much of the variability in fertilizer recommendations was likely due to the recommendation approaches used by each lab, with soil test values having some influence. When the recommendations were applied in field trials, fertilizer applications increased soil concentrations of some nutrients (phosphorus P, potassium K, sulfur S, and zinc Zn) at 21%–57% of sites, depending on the recommendation source, but the rates required and application costs varied greatly among sites and treatments. Applying higher fertilizer rates can sometimes raise soil nutrient levels, but results vary by nutrient and conditions, and the approach is often not cost‐effective.