Over the last few years, we have been documenting some remarkable soybean yield responses (upwards of 13 bushels) to sulfur (S) in northwestern Indiana. I shared more details of these trials this winter and in various articles across the Midwest, says Shaun N. Casteel. 

We do have other sites across Indiana that have seen modest yield improvements (3 to 7 bushels) and other sites with no response. We have experimented with several products, rates and scenarios, but we have not tried everything.

I have received numerous calls and emails on the subject, so I want to provide some answers and leanings, says the Purdue University Extension educator.

Are my soybeans and field responsive to sulfur?

My recommendation to answer this question is based on a set rate of S at the beginning of the season compared to no application. We have tested granular AMS (ammonium sulfate, 21-0-0-24S), MES10 and elemental S at various S rates at a S-deficient site. 

The most straightforward approach is to apply 15 to 25 lb S per acre with granular AMS (62.5 to 104 lb of AMS product per acre) depending on the applicators’ performance and the option of blending other fertilizer.

We are evaluating other sources this year such as K-Mag, gypsum and ATS (ammonium thiosulfate,  12-0-0-26S). ATS is a source that we are testing in more depth this year.

I think it could be a nice option in the burndown application or in a starter band if the planter was setup for it. I am worried about the leaf damage if ATS was applied foliarly, so we are looking at various rates, timings and dilutions/carrier volumes.

Can I still apply granular AMS (or other S fertilizers) to emerged soybeans?

Ideally, the application of granular AMS (or one of the other granular S fertilizers that are moderately soluble) is made prior to soybean planting to reduce logistics and complications from tire compaction. However, the weather and field conditions were not fit for many of these applications this year.

If you had to choose between timely planting of soybean or testing the S responsiveness, I would choose timely planting every time. 

The granular application of S fertilizers such as AMS, MES10, K-mag and gypsum can still be applied to soybean plants (up to V4), but you will have to live with the tire damage. 

We have applied these materials up to V4 soybean plants and still seen good yield response. Though, the later the application of these granular materials will be less effective. Soybeans need the S early in its development.

The fertilizer granules could cause some cosmetic damage to the leaves if they stick, so do not apply these materials when there is a dew or the soybean plants are wet from a recent rain.

Can I apply a foliar S fertilizer and see the same response?

The market is full of foliar fertilizers that have S as well as other nutrients. Most of these products have use rates that are very low in comparison to the potential S need in a S-deficient site.

For example, one product has a use rate of 1 to 2 qt of product per acre which would only supply 0.25 to 0.5 lb S per acre. Our foliar S trials have been much, much higher. Our starting point was 5 lb S per acre, which is 10 to 20 times higher than most foliar fertilizer products.

Again, many of these foliar fertilizer have other nutrients that cannot be applied in high of quantity, especially the micronutrients (i.e., potential micronutrient toxicity) or it is cost-prohibitive.

We have evaluated spray-grade AMS (finer particles and purer product, 21-0-0-24S) in foliar applications to soybean at V4, R2, R4 and R6 plus various combinations at 5 lb S per acre (21 lb of spray-grade AMS dissolved in 15 gallons per acre). This mix will take time to dissolve, so slowly mix and agitate the mixture.

This application rate should also invoke concern (it is a “hot” rate). The likelihood of crop damage (leaf burn) is high if applied in hot and dry conditions and/or to older soybeans (R4 and later).  

The best responses were with the V4 and R2 applications (6 to 10 bushels in 2016, 4 to 7 bushels in 2017). Please note, these were applied as nutrient solutions alone and NOT tankmixed with herbicides, insecticides or fungicides.

We are evaluating foliar S rates this year at V4 and R3, so a more definite answer will come this fall and winter. 

If soybeans are deficient in S, my leaning is that a foliar application of 3 to 5 lb S per acre may be warranted. In severe causes, a sequential application near V4 and R2 or R3 may also be warranted. At this point, I am assuming that granular fertilizer is not an option.

Other fertilizer products are available that may be safer at higher rates (but they may be more expensive too). Again, we have not looked at ATS in foliar sprays so I cannot give a direction with it.

Other scenarios or concerns

Many other scenarios are possible: side-dressing, dribble or y-drop applications, tankmixes, fertigation and more.

Please use caution as you test and try them out, especially tankmixes. We did a few tankmixes last year with some fungicides and insecticides. We did notice crop phytotoxicity that lead to yield hits with some of those combinations.

At this point, I am not making any recommendations on tankmixes. As always, please keep untreated strips for comparison throughout the season and with the combine.

I highly suggest taking leaf samples to help document the S need/response. Soil samples will be good for site characterization. Sulfur is mobile in the soil, and there is not a routine soil analysis available to determine S needs.