Improving Alfalfa Production in Wisconsin with Sulfur and Potassium Fertilizer.

The longevity of an alfalfa stand is an essential component for Wisconsin’s dairy rotations.  A study was developed to determine the effects of sulfur and potassium applications on the growth and development of alfalfa stands.  The objectives of this study were to determine the effect of i) S fertilizer rate, timing and form on alfalfa yield, ii) recommended or no K fertilizer application on alfalfa yield on soils testing optimum for K, and iii) S and K application on soil test S and K levels throughout the soil profile.  Plots were established on three silt loam soils in 2011 and were evaluated through 2013. The experimental design was a randomized complete block with four replications. Treatments included annual applications of 0, 28, and 84 kg S ha^-1 as gypsum or 84 kg S ha^-1 as elemental sulfur applied in the first year at 0 or 269 kg K₂O ha^-1.

Fertilizer treatment effects on alfalfa yield, S and K concentrations in the harvested plant, and S and K removal in the harvested biomass were measured throughout the study.  In 2012, there was no significant treatment effect on alfalfa yield at all three locations. However, in 2013, 269 kg K₂O ha^-1 significantly increased cumulative yield at all locations except Lancaster. Harvested plant S or K concentrations were significantly greater where the nutrient was applied with the exception of K concentration at Lancaster in 2012. At all sites, S application, regardless of rate, timing or form, significantly increased S removal. Application of K significantly increased K removal with the exception of Lancaster and Marshfield in 2012.  Gypsum applied annually at 84 kg S ha^-1 significantly increased soil test S deeper within the soil profile, but did not result in greater yield than where 28 kg S ha^-1 was applied annually as gypsum.