Effect of Carrier, Rate, and Time of Application of S on the Yield, and S and N Content of Alfalfa

In view of the increased S deficiency in the North Central region of the United States and increased fertilizer prices, additional information was needed to develop more precise fertilizer recommendations. Accordingly, field experiments were conducted at nine locations in Wisconsin to evaluate the effect of carrier, rate, and time of application of S on yield, S and N content of alfalfa (Medicago sativa L.), and efficiency of S uptake. All fertilizer materials were broadcast on the soil surface in established stands of alfalfa. Significant forage yield, S concentration, and N concentration increases to S were obtained at five of the nine locations. In the first year after application, SO₄-S carriers (K₂SO₄ and K₂SO₄ :2 MgSO₄) were more effective than prilled elemental-S (88% S and 12% bentonite) in correcting S-deficiency of alfalfa. However, in the second year after application, the various S carriers were equally effective. The results of this study indicate that 28 and 112 kg S/ha as K₂SO₄ were adequate to correct S-deficiency for a 1-year and 2-year period after application, respectively. In the second year after application, 56 kg S/ha as elemental S was required to significantly increase the yield for the first and second harvests and for the third harvest a significant response occurred only at the 112 kg S/ha rate. Spring applied K₂SO₄ was more effective than a fall application, and K₂SO₄ applied at a rate of 28 kg S/ha each year over a 2-year period was more effective than a single application of 56 kg S/ha made at the beginning of the 2-year period. No significant difference between spring and fall application of elemental S was noted. During the second year after S application, a single treatment of elemental-S made at the beginning of the experiment was more effective than the same amount split among two annual applications. Significant yield response was observed when the concentration of S in the tissue was ≤ 0.22% S.