Soybean Response to Potassium and Sulfur Additions in Single-row and Twin-row Planting Systems.

As soybean yields continue to increase, the amount of macronutrients (N, P, and K) and sulfur that is removed in the grain continues to increase.  In many areas of the Mississippi Delta, no fertilizer has been applied for many years and little soil testing has been used to monitor nutrient status.  Potassium fertilization has been common for cotton production, particularly where fast-fruiting cultivars are being grown.  Fertilizer K and S could be quite important in optimizing soybean production on traditional cotton soils as these areas shift to soybean or corn.  A multi-year study was established in 2008 to evaluate potassium (K) and sulfur (S) applications in both single-row (SR) and twin-row (TR) planting systems.  Pioneer ‘94B73’ soybean was planted on conventional 102-cm beds and in twin rows (20 cm between rows) on beds.  Fertilizer K ( 0, 45, and 90 kg K ha^-1) was applied as muriate of potash (0-0-60) while S (0, 22, and 45 kg S ha^-1) was applied as elemental S (0-0-0-90S).  Both materials were applied to the surface immediately following planting but without incorporation.  Soybean grain yields ranged from 3.95 to 4.95 Mg ha^-1 (58.8 to 73.7 bu/acre) with the highest yields measured in the twin-row systems.   When averaged across K and S rates, grain yields were 4.38 and 4.79 Mg ha^-1 (65.2 and 71.2 bu/acre) for the SR and TR systems, respectively.  There was no significant response to either K rates or S rates in 2008.  Extremely dry conditions early in the growing season limited movement of the fertilizer K and S into the soil profile.   This study will be continued with additional fertilizer materials to be added prior to planting with incorporation.  Soil testing to determine nutrient demands is key to predicting fertilizer response, especially as prices continue to increase.