Alignment of Soil Test and Tissue Potassium for Maximizing Soybean Yield in Mississippi.

Newer multi-trait soybean varieties have resulted in greater yields bringing into question whether current soil test K correlation, calibration, and recommendations need updating to maximize yield. In the Mississippi Backland Prairie, limited research has been conducted in recent years to reflect the late season K nutritional needs of higher yielding varieties managed under rain-fed conditions and conservation tillage practices. The objective of this study was to refine soil test correlation and calibration using a range in residual soil test levels and response in temporal leaf and seed K concentrations and yield across IV and V maturity groups. To examine the response of varying soil test K levels on soybean nutrition designated plots have received annual rates of 0, 56, 84, and 112 Kg K/ha for nearly 20 years at Mississippi State University sometime in March and allowed to equilibrate prior to planting. Following planting soil samples were taken at depths of 0 to 15-cm and 15 to 30-cm and extracted using the Mississippi Soil Test method to determine the greatest correlation with yield. Leaf samples were taken at R1, R3, and R5 to determine critical K levels and correlated with soil test K and yields. Results indicate that yield is not effected by residual soil test K if above 125 mg kg^-1. Critical tissue K varied between years and maturity group. Critical K during the R3 and R5 for group IV were 13.2, 12.9 g kg-1 respectively, 12.6, and 10.9 g kg^-1 for group V respectfully. The amount of K removal by the harvested seed is directly related to soil K status and begins to plateau when soil test K reaches 168 mg kg^-1. Increased soybean yield and quality can assist in maintaining or increasing sustainable soybean production in Mississippi, while providing growers with greater income or returns on crops.