Soil and Tissue Testing for Sulfur Management of Alfalfa in New York State

Abstract

Alfalfa (Medicago sativa L.) is an important forage crop in New York. During the past decades, atmospheric S deposition has decreased, raising questions about fertilizer S needs of alfalfa and soil and tissue testing as tools for S management. On‐farm, replicated trials were conducted in eight locations in New York comparing two S sources (CaSO4 and K2SO4×2MgSO4 at 168 kg S ha−1) applied after the first cutting in 2008 vs. a no‐S control. Four of eight sites were S responsive, averaging a 17% yield increase with S fertilization. Sulfur fertilization did not impact estimated milk production per unit yield for any of the cuttings or locations despite small S‐fertilization‐induced changes in protein levels at some locations. The greatest yield and response to S were measured for Site 5 (10.0 and 8.5 Mg ha−1 with and without S fertilization, respectively). The data suggest critical values of 2.7 g S kg−1 (tissue test) and 8 mg SO4–S kg−1 (0.01 mol L−1 CaCl2 extractable soil test) at which a 95% relative yield was obtained. Monitoring of soil S concentrations during a 5‐yr corn (Zea mays L.) and 4‐yr alfalfa rotation showed that recently manured fields are not likely to be S deficient, but soil S concentrations declined during the unmanured alfalfa years and soils could become S deficient. We concluded that both tissue and soil S tests have potential as indicators of S responsiveness for alfalfa. Additional research is needed to validate the suggested soil and plant tissue critical values.