By Daniel Kaiser, University of Minnesota Soil Fertility Extension Specialist
With spring upon us, there have been questions regarding sulfur application for corn for the upcoming year. Our current Minnesota recommendations focus on sulfur application to sandy soils that are low in organic matter. This is mainly due to the fact that sulfate-sulfur is mobile and may leach out of the soil, and that the organic matter is a large storehouse of sulfur and through mineralization this sulfur can become available for uptake in plants. In the past, sulfur was added through atmospheric deposition, applied (but not accounted for) with other nutrients in some commercial fertilizer sources, and in animal manures. Over time most of these indirect additions have lessened and it is reasonable to assume that there may be deficiencies showing up more prevalent today then in the past. However, a large research focus has been placed on determining how widespread this problem is and if only certain soils or regions in the area are impacted by potential sulfur deficiencies. While much of our research is ongoing we have tried to identify key areas to look for in the upcoming cropping year.
Sulfur Deficiency: What to Look For
Sulfur deficiency from a distance can resemble plant nitrogen stress. Upon closer look nitrogen deficiency will show up as chlorosis beginning from the leaf tip and progressing down the midrib. Sulfur deficiency is similar in that it also produces chlorosis, but the symptoms produce striping on the leaf similar to the picture on the right. The other differing factor is that nitrogen is mobile in the plant and sulfur is not so deficiency symptoms will show up in the lower canopy for nitrogen while sulfur will show up in the newest leaves. It is important to be able to differentiate the two because adding more N to the yellow areas of the field will not help if the problem is actually sulfur. However, it must be remembered that either deficiency is not mutually exclusive so both symptoms may show up on the plant making a diagnosis challenging. This is especailly true in areas low in organic matter since organic matter mineralization can provide both nitrogen and sulfur to the plant as well as other nutrients such as phosphorus. It also should be noted that temporary sulfur deficiencies may occur in cool years, but may go away and not affect yield as temperatures warm. This occurred in many of our plots in 2008 that still yielded 220+ bu/ac at the end of the season. Sulfur applications will likely produce greener plants but this effect may not translate to yield. Therefore it can be important to identify problem areas in the field to focus applications if you are trying to get the maximum return for your investment.
Field areas to watch
Sulfur is still an important plant nutrient to apply in sandy coarse-textured soils low in organic matter. However, yellowing on areas within the field have been increasingly noticed in the past few years. A research study began a few years ago in NE Iowa when areas within corn and alfalfa fields showed yellowing and it did not appear to be nitrogen stress. Many similar areas exist especially in southeastern and south central Minnesota. The picture on the left shows one such scene from Freeborn County from a research plot in 2009 in which alternating 8 row strips were applied with and without sulfur either as a broadcast or starter fertilizer. Note the greeness of the corn in the lower areas of the field and the yellowing on the hill tops. In the hill top areas, we would expect lower soil organic matter levels due to erosion or low native levels and thus a potential lower capacity to supply sulfur to corn, while the higher organic matter soils in the low areas are likely supplying enough sulfur to the crop. These types of fields or field areas low in organic matter should be watched closely. The picture on the right is a comparison late in the season on one of the eroded knolls between rows receiving 5 gpa of 10-34-0 on the left versus 25 lbs of S banded 2x2 as ammonium thiosulfate on the right. The greening effect is still visible late in the season and is a very extreme case of sulfur deficiency from recent trials. While factors such as moisture stress my be present in these areas as well our research work in 2008 and 2009 has shown that significant yield increases can be achieved with an application of sulfur in these fields and specifically on the eroded knolls. At this point it is a question of what rate to apply, what fertilizer source, what nitrogen management needs to be considered, and if there is enough area in the field to justify the application.
Strip Trial Work 2008-2009
Strip trials were conducted at two locations in 2008 and 2009 in fields with eroded knolls and variable topography and soil organic matter levels in Freeborn County. In addition one location was selected in Yellow Medicine County that had relatively higher soil organic matter levels and another location on non-irrigated sands in Isanti County in 2008. Sulfur was applied at 25 lbs of S per acre either as broadcast potassium sulfate (0-0-51-18s) or ammonium thiosulfate (12-0-0-26s) liquid banded with the planter two inches beside and below the seed. Starter sulfur was applied alone or with 5 gpa of 10-34-0. Two additional starter treatments included nitrogen only (28 percent UAN solution) and 10-34-0 alone. Each individual nutrient was applied at the same rate in all circumstances and an additional rate of 70 lbs of K2O was applied to strips that did not receive broadcast K-sulfate.
In 2008 there was little yield benefit to any starter treatment except for the ATS treatment alone or with 10-34-0 at the Freeborn Co. location. At Isanti Co., there was a small yield increase when S was applied with P mainly due to low soil P levels at this site (average 8 ppm Olsen-P), and the only response was to starter P at the Yellow Medicine Co. location. At all locations the 10-34-0 treatments did produce larger plants early in the season, but yield ultimately was affected by the most limiting nutrient at each individual location which for the low organic matter sites tended to be sulfur. The only surprising affect was little response to broadcast S in 2008 at the Freeborn Co. location. It is plausible that the additional N with S benefited the plants more, but could not be determined. In 2009 a separate field in Freeborn Co. was studied and again corn responded to sulfur application by an average of 14 bu/ac increase. Unlike Freeborn Co. 2008, there was a clear increase in yield from N applied in the starter in 2009. This likely reflects differences between N rates applied at the field, loss potential for N, or potential climatic conditions. However, the broadcast S treatment did increase yields but not as much as potentially could have been gained by applying both nutrients together. At both locations the highly responsive areas were lowest in organic matter in the top 6 inches. In general the yield increases across the fields were large enough to pay for the cost of the fertilizer applied; however, the full economics for an individual field will likely depend on the relative amount of field area with low organic matter levels. Variable rate applications of sulfur may be economically justified in some circumstances to maximize the economic return within a field. Nitrogen best management practices should also be followed, especially if the soils are well leached, to eliminate the response to this nutrient as much as possible to maximize the potential yield increase from sulfur applications.
Soil Organic Matter: What is the Correlation to Crop Response?
Our strip trial work has attempted to identify key soil factors that may indicate a higher potential for crop response to sulfur. Across each individual location multiple soil factors were related to crop response. For determining response to sulfur the sulfate sulfur level (extracted with KCl) and organic matter levels were compared from 0-6, 6-12, and 12-24" depths. When compared, the level of organic matter in the 0-6" depth appeared to be a better indicator of crop response. The poor relationship between sulfur soil test was not surprising since this issue has been identified in numerous studies in the region in fine textured soils. Therefore, use of a sulfur soil test is not recommended outside of sandy soils low in organic matter. In our sites in 2008 and 2009 the biggest yield increases were seen in field areas that were less than 2.0 percent soil organic matter in the top six inches. Across years the critical values for organic matter was 2.5 percent, above which yield increase from sulfur would be expected. However, when looking at individual site within different years the critical value seemed to range from 2.5 percent to 3.5 percent. Bear in mind this data only includes three corn fields from Minnesota which were chosen because they were determined to have a higher likelihood of a response to sulfur. This data still needs to be evaluated and may change based on future research. Although limited, this data may indicate a potential response in areas ranging from 2-4 percent organic matter, but this may depend on climatic affects in a particular year. This is especially true if mineralization of soil organic matter is limited thereby limiting the supply of sulfur to the plant. Research in south central Minnesota by Gyles Randall and Jeffrey Vetsch in research in southern Minnesota has seen instances of yield increases in soils with higher organic matter levels, which may be indicative of variation in response do to climatic impacts. A series of replicated strip trials encompassing a number of fields, crop rotations, or fertilizer programs may need to be established to determine how individual management systems affect the response to sulfur, we just do not fully know if the 2 percent to 4 percent range is a good median range for determining whether sulfur application is warranted.
Rates and Timing
Much of our current research in Minnesota has not focused on what rate is needed to get a response. Previous to this, time trials conducted showed no response to sulfur when different rates were applied. Research over the past five years in Iowa has looked at optimum sulfur rates and found that around 15 lbs of sulfur applied as a broadcast was all that is needed in fine textured soils when there was a response to sulfur (a link to this work will be included at the bottom). Three small plot research trials were established in southern Minnesota for 2009 to look specifically at rates of sulfur (0, 10, 20, 30, and 40 lbs S/ac) applied at two times, on the surface at planting and V3 to V4 as a side dress. The three locations were near Theilman (Wabasha Co.), New Ulm (Brown Co.), and Renville (Renville Co.). Yield responses were nearly 20 bu/ac. at Theilman and Renville Co. locations in 2009. At both sites there was no difference between application timing or sulfur rate, indicating that 10 lbs was enough to get the maximum yield increase. The response at Theilman was not unexpected since the organic matter level was generally lower (around 2.0 percent), but organic matter levels at the Renville location were higher (~5.0 percent). Based on past research a response would not have been expected at this location. However, this location was third year corn and sulfur or manure had not been applied in the past two years. This type of response as well deficiency symptoms noted in a few other continuous corn fields in south central Minnesota beings up the question of how significant of an impact does cropping rotation have on the potential response to sulfur? We do not have enough information at this time to make a determination if sulfur should be recommended for continuous corn, but future research should help in our understanding of the potential inpact across the state. On a positive note, our research showed that there was no yield difference when sulfur was surface broadcast at planting versus applied as a side dress application at V3 to V4. This may indicate some flexibility in when sulfur can be applied, and sulfur can be side-dress applied if deficiency symptoms are seen in the field if it was not applied early in the season.
Implications for 2010
The observed response at the Renville Co. site in 2009 raises these questions 1) is there a factor that needs to be addressed concerning crop rotations in our sulfur recommendations; and 2) what is the relative role of the climatic conditions in relation to sulfur mineralization and crop response to applied sulfur. We do know that sulfur mineralization can be affected by cooler soil temperatures and that surface residue can insulate the soil keeping it cooler throughout the growing season. Therefore in continuous corn it may be plausible that there may be a higher likelihood of response to sulfur if there is high amounts of surface residue that cools the soil. Looking forward to 2010 it will be important to make sure that everything can be done to keep rows clear of residue to make sure the full mineralization potential of the soil. Small amounts of starter P may be beneficial to aid with stand establishment, and nitrogen and sulfur may help the appearance of the crop early in the season. Remember that high rates of fertilizer should not be placed on the seed to lessen the risk of stand loss from excessive rates of N or K placed with the seed.
Looking at the current recommendations, we still would recommend 25lbs of sulfur banded in sandy soils with low organic matter. In the future recommendations we will identify areas in fine textured soils that will benefit from the application of sulfur. Based on our current research findings, in fields that do not have large levels of residue on the surface, there appears to be the largest benefit to sulfur application when soil organic matter levels are below 2%. If there are any areas within fields with organic matters matching these levels it is up to the producer to decide if sulfur fertilization is worth the cost of the fertilizer? On average the yield benefit was low to zero when the organic matter levels reached around 2.5 percent at these locations, therefore I would consider there to still be a moderate chance for response between 2 percent to 3 percent organic matter to potentially 4 percent in some years. At this point we do not have a large enough database to know where that cutoff point lies. In most of these cases the best rate data available would indicate that 10-15 lbs of S broadcast would be adequate in fine textured soils less than 3 percent organic matter, but this is still a work in progress and rate studies have not been conducted on soils less than 2.0 percent soil organic matter. Rate responses from band applications also have not established if lower rates can be used. As we move forward with research we should have a better information on rates under these circumstances. It is also important to note that we have not evaluated much of this work in Northwest Minnesota. There have been trials conducted in areas of Northwest Minnesota, but yield increases to sulfur on corn have not been seen and the data is not available to us to see if much of our responses in southern Minnesota translate to similar soil conditions in the Northwest. It is encourage that producers leave some strips without sulfur in their fields to see what the potential response is in these areas of the state.
Remember a few key points 1) the year or location in the state may dictate in a large part the potential response to sulfur regardless of previous crop or organic matter level; 2) application of manure may lessen the benefits of additional sulfur since S is applied with manure; 3) while banded sulfur can be used effectively, high rates of some fertilizer sources placed with the seed as a "pop-up" starter can lower or delay germination so alternative placements should be considered; and 4) while we have conducted multiple trials on sulfur fertilization of corn over the past few years there still are questions remaining on appropriate rates, placement, and conditions where the greatest benefit will occur. While a few of these points have been covered, future research projects are planned to gather data to provide an updated sulfur recommendation for the state of Minnesota.
The author would like to thank the Minnesota Soybean Research and Promotion Council for providing much of the funding for the sulfur work conducted in 2008 and 2009.