What are those Yellow Spots in my Soybean Field?

September 22, 2020

Dale Longwell, Certified Crop Advisor & Seed Specialist

“What are those yellow spots in my soybean fields?”

That’s been the question I have been getting from farmers and salesmen alike this late summer.

I started noticing these in the countryside about the first of August. My first thought was that these were stress spots showing up and they could lead to a yield limiting area in the field. Soybean yield is usually reduced by stress through the growing season, be it by moisture, sunlight, nutrients, bugs, or disease. We can use this information for management decisions in the future.

I walked several fields and noticed that it was occurring on the hills and other stressed areas of the field. So, I wondered, “Is Covid-19 stressing the beans, too, this year?” Probably not… It looked like brown spot occurring on the lower leaves. With a lot of interest in the problem, I started to investigate what could be going on. It appeared to be yellowing in the lower canopy, with notably less canopy in these stressed areas. I didn’t see any disease showing up, so I wanted to look at pest issues or nutrient deficiencies that may be present.

I focused on three farms that I was going to dig into. I knew that all three farms have had intensive grid sampling programs for many years. I also knew that I could capture yield data off of two of these farms from their yield monitors to further layer our investigative tools.

I started by pulling soil tests. I took two samples in each “good” area and two samples in each “bad” area on each of the three farms. The first sample I sent off was for nematode testing. Nematode testing is usually done in the fall after harvest, but since we wanted to compare good and bad areas, I wanted to take them “in crop” to capture the visual differences in the canopy color and volume. I sent the nematode test to Michigan State for analysis. Interestingly enough, they did find some nematodes in both the good and bad samples, but there was no difference between the two areas on all three farms.

With nematodes ruled out as the cause of the problem, I then looked to the soil test to see if there were clues to why the soybeans were yellowing. The second sample I sent off from each good and bad area, I sent for a complete soil test. On all three farms the P (phosphorus) and K (potassium) numbers came back close between the good and bad areas, as I would expect from long term grid sampling. The next column on the soil test report is magnesium. On all three, the bad areas came back higher. I don’t think high Mg is good, but I also don’t think it is a yield limiting factor. Besides, there is not much we can do to lower it. The boron levels on all three farms came back the same, or close, in each sample set. So, we can rule out boron deficiency.  Lastly, we come to sulfur. On all three sample sets, the sulfur level in the bad areas was almost half of that of the good areas of the fields.

Let’s talk about sulfur for a minute… We usually think about sulfur when we are planning our corn program, and sometimes wheat, but not much when it comes to soybeans. Two of the farms have used 2-3 gal. of sulfur in their corn programs in the past, but that (apparently) isn’t enough to sustain a crop rotation, even in our typical corn/soybeans.

I also started noticing farms that didn’t seem to have yellow spots. I asked questions of these growers and found that most, if not all of them, had something in common. They were using ammonium sulfate in their dry fertilizer program. This is a great way to get sulfur for your crop.  With an analysis of 21-0-0-24S, you get a good amount of sulfur, and the nitrogen is a slow release form. You are getting more sulfur for your money and nitrogen to boot. Some were using 150-200 pounds with their corn fertilizer. Some were doing what I have been doing, which is something I learned about at the Certified Crop Advisor convention a number of years ago: putting on 100# of 21-0-0-24S with at least 100# of 0-0-60, as close to planting a possible.

According to the university experts, applying ammonium sulfate and potash together gives a greater yield bump than applying the nutrients individually. I would recommend to each of you, that are trying to achieve greater soybean yields, to look at mixing in some sulfur as part of your management plan.

I feel it is important to keep scouting your crops, even after the sprayers are parked for the year, to look for signs from your crop telling you what it is lacking. Most times, these stress spots show up and mature early in the fall and are usually associated with lower yielding spots in the field. Many times, we won’t be able to use this information for this growing season, but we can use it for management decisions for next year’s crop. I like to look at these lower yield environments in our fields, because It will be easier to raise the bottom end of our yield brackets than to push up the top yielding areas higher, which will in turn raise over all field averages.

Be safe through this harvest season.

Dale Longwell, CCA