Validation of Soil Test Potassium Recommendations and Plant Tissue Analysis to Optimize Soybean Yield in North Carolina Cropping Systems.

Soybean covers the largest acreage for field crops in North Carolina accounting for an average of 1.6 million acres between 2010- 2015. Increases in average yield result in higher nutrient removals, particularly potassium (K). Approximately 40% of the soil samples received by the North Carolina Department of Agriculture and Consumer Services (NCDA&CS) in 2015 had medium to very low Mehlich 3 soil test K levels, resulting in recommendations of K₂O applications last validated in 1984. K deficiency in soybean can result in reduced leaf K levels, visual yellowing or necrosis, and decline in yield through reduction in seeds per pod, seed weight, and pods per plants. Nine sites selected across NC with varying soil types considered to have low soil test K levels had replicated soil fertilization rates (0-200 lb. K₂O per acre) broadcast applied, plus a foliar treatment. We will present data on changes in soil test K levels, as well as trifoliate leaf K concentrations sampled at V3, V5, R2, and R5 stages. Harvested yield response to changes in Mehlich 3 soil test K levels and plant tissue K concentrations will be used to evaluate soybean response to NCDA K fertilization recommendations.