With spring almost here, 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 (N) stress. Upon closer look, N 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 N is mobile in the plant and sulfur is not so deficiency symptoms will show up in the lower canopy for N 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, remember that either deficiency is not mutually exclusive so both symptoms may show up on the plant making a diagnosis challenging. This is especially true in areas low in organic matter. 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. Sulfur applications will likely produce greener plants but this 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.
These types of fields or field areas low in organic matter should be watched closely.
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/acre) applied at two times, on the surface at planting and V3-V4 as a sidedress. The three locations were near Theilman, New Ulm and Renville. Yield responses were nearly 20 bu./acre at Theilman and Renville 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%), but organic matter levels at the Renville location were higher (5%). 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 brings 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 impact across the state. On a positive note, our research showed that there was no yield difference when sulfur was surface broadcast at planting vs. applied as a sidedress application at V3-V4. This may indicate some flexibility in when sulfur can be applied, and sulfur can be sidedress applied if deficiency symptoms are seen in the field if it was not applied early in the season.
Implications for 2010
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 are high amounts of surface residue that cool 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 N 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.
We still recommend 25 lbs. 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%.
On average the yield benefit was low to zero when the organic matter levels reached around 2.5% at these locations, therefore I would consider there to still be a moderate chance for response between 2-3% organic matter to potentially 4% in some years.
In most of these cases the best rate data available would indicate that 10-15 lbs. S broadcast would be adequate in fine textured soils less than 3% organic matter. Rate responses from band applications also have not established if lower rates can be used.