Potassium Fertilization for Soybean in the Mid-Atlantic US.

Potassium (K) is the nutrient required in largest quantities from fertilizer sources for soybean.  Potassium deficiencies are common and may relate to insufficient amounts of K fertilizer being applied to high-yielding crops.  Potassium requirements for full season and double cropped soybean need reassessment and recommendations may need revision for the Mid-Atlantic, United States. The goal is to determine the optimum K fertilizer rate for soils in Virginia and to update soil fertility guidelines to match high-yielding crop requirements. Thirty-four full season and double cropped experiments were conducted throughout the Coastal Plain and Piedmont of Virginia on low, medium and high testing K soils during the summers of 2013 and 2014.  Within two weeks of planting, soil samples were collected at depths of 0-15, 15-30 and 30-60 cm from random locations within each replication.  Once soil samples were collected, six K rates (0, 28, 56, 112, 168, and 224 kg K₂O ha^-1) using granular Muriate of Potash (0-0-60) were hand-applied to soybean.  K concentrations were determined using acid digestion from 10 randomly selected whole plants at V5 and 20 uppermost, fully developed trifoliate leaflets at R2.  Soybean was harvested at full maturity and yield, moisture, and seed weight determined.  Soil test reports for 2013 indicated Mehlich-1 K concentrations ranging from 51 to 196 mg K kg^-1 soil in 0-15 cm soil samples.  Soil concentrations in 2014 ranged from 8 to 98 mg K kg^-1 soil in 0-15 cm soil samples.  K concentrations averaged 2.87 % K for V5 samples and 2.46 % K for R2 samples.  There was no significant yield response to K fertilizer application in 2013.  Preliminary data suggests that current K fertilizer recommendations are adequate for soybean in the Mid-Atlantic but further research needs to be continued to determine yield response to various levels of K fertilizer.