We want to apply sulfur ahead of soybeans to boost yields. When should we apply? What product? What rate?
The Indiana certified crop advisers panel answering this question includes Andy Like, independent crops consultant, Vincennes; Jeff Nagel, agronomist with Ceres Solutions, Lafayette; and Marty Park, agronomist with Gutwein Seed Services, Rensselaer.
Like: Typically, sulfur is applied to soybeans before planting to minimize tire damage. However, sulfur in a liquid form can be applied at planting or with a sprayer prior to emergence. The most common forms of sulfur in our area are:
- ammonium sulfate, dry, 21-0-0-24S
- K-Mag, dry, 0-0-21-21S
- ammonium thiosulfate, liquid, 12-0-0-26S
- potassium thiosulfate, liquid, 25-0-0-17S
Choice of product will depend on availability of products and application equipment. Target 15 to 25 pounds of sulfur per acre for a test. Sulfur is applied most often as ammonium sulfate with a dry spreader prior to crop emergence in combination with phosphorus and potassium fertilizer.
Nagel: Shaun Casteel, Purdue Extension soybean specialist, conducted multiyear research on sulfur response in soybeans. This is ongoing research, and they learn new things each year. The probability of response to sulfur in soybeans increases with earlier planting dates on timber soils and is more consistent on sandier soils.
Soluble sources that provide plant-available sulfate-sulfur are needed. This could be ammonium sulfate (AMS), ammonium thiosulfate (ATS), pelletized gypsum and some others. Typically, AMS or ATS are more widely available. What source you use needs to fit into your farming system.
Here are some guidelines based on our experiences with sulfur in soybeans:
- AMS. Apply broadcast from two to three weeks before planting and in-crop up to around V4. Use 15 to 20 pounds of sulfur per acre, which is 60 to 85 pounds of AMS per acre. Earlier applications in March are being researched. 2022 data showed similar response for earlier application, but stay with the higher rate. AMS fits well on sandier soils where spring applications of potassium are more common.
- ATS. AMS has been a little more consistent compared to ATS, but results are similar. ATS is a liquid. ATS fits better into a system where a spring burndown or near-planting, soil-applied herbicide is used, and where dry fertilizer is applied in the fall or over winter.
- ATS and weed control. Purdue weed science staff evaluated ATS in a greenhouse experiment with glyphosate and 2,4-D on wheat, velvetleaf and lambsquarters. ATS can antagonize glyphosate, particularly with wheat and velvetleaf. We found acceptable weed control on winter annual weeds with spring burndown applications when weeds are small. We tank-mixed 3.5 to 5 gallons of ATS, or 10 to 15 pounds of sulfur, per acre with burndown or residual herbicide programs successfully using a minimum of 15 gallons per acre spray volume. Always do a compatibility test and apply on small weeds.
Park: The most common practice in the area that I service is applying dry ammonium sulfate at 50 to 100 pounds per acre in the spring. Ideally, it is applied no more than 30 days prior to planting. Quite often, it is applied with other dry fertilizers. There are other newer product combinations, like elemental sulfur and sulfate-sulfur, that could be considered. Remember that elemental sulfur is not immediately available.