Calibration of a Manganese Availability Index for Soybean Soil Test Data

Abstract The soybean ( Glycine max (L.) Merr.) plant is quite sensitive to Mn deficiency. Most recommendations for Mn fertilization in the Southeastern United States are currently based on Mehlich 1‐extractable Mn and soil pH. Other extractants are now being used and calibration data on each is needed. Also, a single expression to simplify the interpretation of soil test results would be helpful. Two extractants, Mehlich 1 (M1) and Mehlich 3 (M3), were evaluated for their ability to predict Mn availability for soybeans. Yield and plant data from 38 experiment‐year field studies were used to relate soil conditions to yield response and plant Mn concentration. Prediction of these factors was improved by inclusion of soil pH with extractable Mn in models, and the coefficients of determination were similar for the two extractants. The critical level of soil Mn varied with pH. At pH 6 and 7 the critical levels were 4.7 and 9.7 mg Mn L −1 for the M1 extractant and 3.9 and 8.0 mg Mn L −1 for the M3 extractant, respectively. The data indicated that coarser textured soils may have a lower critical level than finer textured ones, particularly with the M1 extractant. An availability index was developed to give a single number for interpretation. The critical level was set at an availability index of 25. As this value becomes less than 25, yield responses to Mn fertilization increase and leaf Mn concentrations decrease below 22 mg kg −1 .