Comparison of Bray-1 and Mehlich-3 extraction of P and K in Wisconsin silt loam soils

Phosphorous (P) and potassium (K) fertilizer rate recommendations are based on soil nutrient concentrations, which can be measured using a variety of lab methods. In Wisconsin, Bray-1 extractant is used to measure soil test P (STP) and soil test K (STK), while most neighboring states measure STP and STK using Mehlich-3 soil test extractant. This research aims to quantify the relationship between Bray-1 and Mehlich-3 extraction methods for both STP and STK for silt-loam soils in southern Wisconsin. Soil samples were collected from three fields in November 2021. STK and soil test phosphorus STP were measured using both Bray-1 and Mehlich-3 extraction methods. The relationship between Bray-1 and Mehlich-3 extractants was quantified using linear regression. Soil test phosphorus as measured using Bray-1 extractant was linearly related to STP as measured using Mehlich-3 extractant (p < 0.001; Adj. R² = 0.97), and the relationship between STK results from Bray-1 extraction and STK results from Mehlich-3 extraction was also linear (p < 0.001; Adj. R² = 0.91). For the same soil samples, Mehlich-3 extractant removed slightly more P and K as compared to Bray-1 extractant. Due to the strong linear relationship between Mehlich-3 and Bray-1 extractants, simple regressions can be used to estimate Bray-1 STP or STK from a soil test report that was generated using Mehlich-3 lab methods, and vice versa, on WI silt-loam soils.