Validation of Soil Test Based Phosphorus and Potassium Fertilizer Recommendations for Soybean in West Tennessee

Soil tests provide accurate information about nutrient availability. Several soil test extractants, commonly Mehlich 1 (M1) and Mehlich 3 (M3), are used in the Southeastern United States. Differences in soil test reports and fertilizer recommendations arise when laboratories use different extractants, leading to frustration for producers who rely on M3 results from commercial labs but prefer or must use M1 recommendations from Land Grant Universities such as the University of Tennessee. This study's first objective is to evaluate the relationship between M1 and M3 extractants for phosphorus (P) and potassium (K) and to develop conversion equations between the extractants for these nutrients. A total of 520 samples were collected over 11 site years at five locations in West Tennessee. Samples were analyzed for M1P, M3P, M1K, M3K, and pH. Linear regression analysis revealed strong correlations between the extractants for both P and K. Two equations for converting M1P to M3P were developed based on soil pH, which affected the amount of extractable P: for pH < 6, M3P = 2M1P; for pH ≥ 6, M3P = 1.7M1P. The equation developed for converting M1K to M3K is M3K = 1.33*M1K + 33.62. Additionally, the study aimed to validate current soil test-based fertility recommendations for soybean in West Tennessee. Changes in soybean varieties, management practices, and demonstrated producer interest have created a need for validation of current recommendations, which have not been updated since 2009. Soybean response trials were conducted at three locations over four site years, with three sites receiving six P and K application rates and one site receiving five rates. The GLIMMIX procedure in SAS version 9.4 was used to analyze yield responses. No significant crop response to P fertilization was observed; however, K fertilization showed a response at two of four site years, with maximum yields at the 140 lb K2O/A rate. These results suggest that the University of Tennessee’s P recommendations are adequate, while K recommendations may need to be increased.