There are still questions on the application of sulfur for soybean. Daniel Kaiser and a number of other researchers in Minnesota have been working on a number of projects focusing on sulfur management on corn, soybean, and spring wheat.
Recently the soybean research has been fully summarized so here are some highlights of the findings to outline where the researchers are at with the current guidelines for fertilizer management on soybean.
The Extension publication on fertilizer guidelines for soybean was recently updated and one of the major changes deals with sulfur application to soybean. In 2008 and 2009, through funding provided by the Minnesota Soybean Research and Promotion Council, Kaiser and other researchers conducted a series of strip trials looking at combinations of nitrogen, phosphorus, and sulfur banded to the side of the row. One of the major findings of this work was a significant yield response that occurred at one location.

The researchers have been mainly focused on researching sulfur response for corn. One thing that seems to come to the forefront in this research is the overall importance of soil organic matter when considering where a yield increase will occur. For soybean, it is not that simple. If you go through much of the research from around the country there is not much supporting evidence for recommendations for sulfur application to soybean. Kaiser believes much of any potential for response of soybean to sulfur is related to a few factors, the most important of which revolved around climatic conditions. Where he saw the response was in 2009, which was a year that saw very large responses to sulfur in many of his corn studies. With temperatures being cool and with most areas being relatively dry the potential for mineralization of sulfur from organic matter was low. Two other factors that were important at the locations were the soil and the previous history of sulfur deficiency symptoms in other crops (alfalfa and corn). The site was on a silt loam soil in southeastern Minnesota with an organic matter concentration that averaged less than 2.0% in the top six inches. All these factors together created a perfect storm in terms of creating conditions were a sulfur application would benefit soybean.

Other than the field location in 2009, there was one other instance where sulfur benefitted soybean at the Southwest Research and Outreach Center at Lamberton. In this instance, soybean yielded higher with sulfur, but the sulfur was applied before the previous corn crop. To follow up on this response, Kaiser set up trials to look further into the sulfur cycling within corn-soybean rotations to better understand if there are greater benefits from application before the corn crop. One thing that he found in the studies in 2008 and 2009 is that he increased the amount of soybean plant material relative to the total grain produced, meaning he had more plant material per acre. When sulfur and phosphorus were added with N, there was a large increase in soybean vegetative growth at all locations. This concerned Kaiser due to extra water usage in dry years and potential for disease pressure to increase.  

So what are the overall benefits? Increased yield is the overall goal but Kaiser does not think that there is a widespread need for sulfur to be applied on all soybean acres. He thinks the best bet would be to look at fields that are in a areas prone to sulfur deficiency on corn or alfalfa, soil organic matter is around 2.0% or less, and the field has not had any sulfur (including manure) applied to it for several years. In this case, 10-15 lbs of sulfur broadcast per acre should be enough for soybean. He would suggest not applying high rates of nitrogen and phosphorus with sulfur. In his field trials he broadcast potassium sulfate and did not see the large increases in plant mass. Another source that may work is gypsum.  

He does not think sulfur needs to be applied to soybean if it was applied the previous year.  Based on all of the data he has, there does appear to be enough sulfur cycling from year to year that the soybeans crop should have enough. Sulfur does leach out of the soil but the relative rate it will leach is lower than some other anions. In coarse sand, his data indicates that given enough rainfall, any sulfate applied will be below the 2 foot depth in the soil by the middle of the growing season. Even with a small increase in clay content that rate can be reduced. At another site with a loam soil texture, he just started to see elevated levels of sulfate by the end of the growing season. Thus, there will likely be some sulfate still in the profile for the soybean crop as long as rainfall is not too excessive.

The other aspect he has been studying is sulfur cycling in corn residue. Based on the information he had a few months ago, there did not appear to be much, if any, sulfur that would mineralize from the breakdown of corn stover. However, a more recent set of data shows that it may be possible. From the data he has now, without sulfur, the ratio of carbon to sulfur tends to maintain at a point where the breakdown of corn stalks will not release or tie up sulfur. However, when he looked at the data from 10 and 20 lbs of sulfur applied to corn, the ratio became smaller indicating a potential for a small amount of sulfur to be potentially released. The amount released will again depend on soil temperature, moisture, and tillage to incorporate the residue. This cycling mechanism may explain some of the response he saw in 2009. He also studied higher rates, but the effect appeared to not extend past 20 lbs of sulfur.

To reiterate, if sulfur is applied before another crop, Kaiser does not think it needs to be applied before soybean, especially corn where the chance of a profitable return is much greater. He thinks yield can still be increased without a direct application and he also has evidence to increases in grain protein concentration when sulfur was applied before corn. These increases have only been identified in our work in Southeastern Minnesota at this time. While most of the evidence does not support widespread use of sulfur on soybean, some of the isolated responses indicated that some changes needed to be made to his recommendations. Moving forward, he will continue to study the cycling mechanisms in order to provide the most up to date information for the current guidelines.

Fertilizing Soybean in Minnesota