Should you apply sulfur fertilizer for alfalfa?

In studies, alfalfa tonnage increased by the addition of sulfur fertilizer

 

Did you know that alfalfa can remove more sulfur from the soil in a year than a corn crop? However, past research on forage crops has been inconclusive as to whether sulfur fertilizer is required for highly productive alfalfa fields with organic matter (OM) concentrations greater than 4%. In addition, most of the research out there on this topic is based on one-year trials, which may not be long enough to establish the benefits of sulfur for growers pushing for higher tonnage in their alfalfa production systems.

That’s why we sought funding from Minnesota’s Agricultural Fertilizer Research and Education Council (AFREC) for a multi-year study on sulfur fertilizer sources and application rates. The project, at the Rosemount Research and Outreach Center, is currently in its third year. Combined with another trial in Rosemount from the 2019 growing season, Minnesota-based sulfur fertilizer research has yielded some impressive results.

The 2019 trial kicked off our exploration of sulfur for alfalfa. In that study, we compared several sources of sulfur, both sulfate and elemental sulfur sources, at rates of 0, 7.5, and 15 pounds of sulfur per acre (lbs S/ac) on plots with higher soil OM concentrations and lower OM concentrations. The AFREC study is looking at fewer sources of sulfur but more rates (0, 10, 20, and 30 lbs S/ac). Sulfur fertilizer was applied twice each year in both studies: first, just before seeding and, second, after the first cutting of alfalfa was taken off.

Study results

In both studies, alfalfa tonnage was increased by the addition of sulfur fertilizer while alfalfa quality was rarely affected. For the 2019 trials, we collected data from three cuttings between the two fertilizer applications. Forage yield was consistently increased by sulfur fertilizer application on the higher OM soils. The field with lower OM did not benefit from the initial application but yield was greater following the second application of sulfur. The yield increase due to sulfur fertilizer application was roughly 500 pounds of additional forage per acre over three harvests between 2019 and 2020. The higher OM fields did average a greater forage yield compared to the lower OM soils, which required a higher sulfur application rate, 15 versus 7.5 lbs S per acre, to maximize yield.

For the AFREC study, forage yield was increased by 1,000 pounds over two harvests in 2019 and 2020 when a sulfate form of sulfur was applied. The elemental sulfur treatment resulted in less yield than the sulfate sulfur. A second cutting taken around July 1 of this year (2021) also showed a positive impact of sulfur on forage yield.

While we are still in the early stages of this study, the data seems to confirm the current U of M alfalfa fertilizer guidelines for sulfur application, which suggests applying 10 to 25 lbs S/ac depending on overall production and soil OM concentration in the top six inches of soil. Application rates higher than 25 lbs S/ac are not currently suggested even though removal may be greater for fields where more cuttings are taken in a year.