Sulfur Needs of Alfalfa and Tools for Sulfur Management.

Alfalfa is an important forage crop in New York State (NY). Over the past decades, atmospheric S deposition has decreased, raising questions about the S status of alfalfa. On-farm S response trials were conducted in eight locations in NY, comparing two S sources (CaSO4 and K2SO4^.MgSO4 at 134 kg S/ha applied after 1st cutting in 2008) versus a no-S control in four replications. Forage quality and yield were monitored for two years following the application. Soil samples (0-15 cm depth) were taken prior to applying treatments in 2008 and in early spring in 2009. Tissue samples (top 15 cm) taken at 3^rd cutting were analyzed for total S. In 2008, four sites with <2.5 mg/kg tissue S were responsive to S addition while three sites with tissue S levels >2.5 mg/kg were non-responsive. One site had a tissue S level <2.5 mg/kg but was non-responsive to additional S, most likely due to the low pH of the site. Sulfur addition increased tissue S in both years. In 2009 only one site had significantly higher total yield where S has been applied, and all sites showed tissue S levels that had dropped below 2.5 mg/kg in both the treated and untreated plots. Milk per ton estimates were not impacted by S addition. Soil test results showed that soils with CaCl2 extractable S of 8 mg/kg or less might benefit from S addition. We conclude: (1) there is a potential for a yield response upon S addition to alfalfa in NY, (2) tissue testing is effective in identifying S deficient sites (<2.5 mg/kg S) for sites that are otherwise managed optimally (correct pH), and (3) a soil test of <8 mg/kg indicates the probability of a response to S addition.