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

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.