Current status of US soil test phosphorus and potassium recommendations and analytical methods

Soil testing is the foundation of fertilizer recommendations in the United States. Fertilizer recommendations have primarily been developed by land-grant universities with limited coordination among programs. The individual state approach to develop fertilizer recommendations has resulted in discrepancies in recommended soil sampling protocols, soil analysis methods, and fertilizer recommendations at similar soil nutrient levels. A national survey was developed to summarize the status of soil testing and fertility work in the United States to inform future collaborative efforts among states and regions and identify opportunities to harmonize recommendation guidelines. Topics included relevant funding, multi-state collaborations, state soil-test recommendations and related data, fertilization philosophies, and analytical and soil sampling methods. Responses from 48 states and Puerto Rico showed inconsistencies across state boundaries in every category. The number of faculty full-time equivalents working in soil fertility now averages 1.3 per state, a 21.5% decrease every 10 years since the 1950s. Land-grant university soil-test-based phosphorus (P) and potassium (K) recommendation philosophies were categorized as Sufficiency (37%), Build and Maintain (19%), hybrid (20%), or multiple philosophies for which recommendations are provided (20%). Respondents in two states did not know the recommendation philosophy (4%). Fertilizer-P and K recommendations for corn (Zea mays L.) were based on eight different extractants with differences across and within regions. While there have been some successful regional efforts in the past, additional multi-state collaborative efforts are needed to identify research gaps and develop comprehensive strategies to update soil-test correlation and calibration data to address modern agronomic, economic, and environmental concerns.