Many of you have heard about the Six Secrets project being conducted by Dr. Fred Below and students at the University of Illinois. Dr. Below has been examining the different factors that impact soybean yield. He presented an update webinar on this project and it is available on the ILSoyAdvisor here.

During the webinar listeners asked a number of questions whose answers I am providing below:

Do soybeans uptake nitrate N or ammonia N? Plants roots can take up either the ammonium (NH4) or nitrate (NO3) forms without preference. However, if there is more NO3 in the soil, it will take up more of that form, and vice versa. One interesting fact is that NO3 in the soil will suppress nitrogen fixation, while it is thought that NH4 won’t.

What is your recommendation for soybeans on a soil with a pH of 5.4 with potatoes the main cash crop in a rotation? Should we apply Pell lime or Super U? The pH is very low and that will impact nutrient availability and nitrogen fixation. The first step is to apply enough lime to raise the pH to 6.2 or higher. Either ag lime or pelleted lime will work. A lot of work is being done on applying nitrogen on soybeans. If your yield potential is 75 bushels or higher, a slot release form of dry nitrogen seems to work well.

Controlling white mold has been a problem. We are thinking of the following program with 6 oz of Phoenix® (lactofen) when days begin to shorten (summer Solstice) or when the first flowers appear on the plant followed by Priaxor® fungicide 14 days later. Has there been any data on this approach, particularly the Priaxor? I plant 120,000 seeds/ac on 30” rows. Planting in wide rows and a lower population helps keep the canopy open and air circulating so the spores don’t germinate. Lactofen herbicides like Phoenix® and Cobra® are known to suppress white mold development in soybeans. Priaxor doesn’t have efficacy against white mold. Endura® and Aproach® are two fungicides that provide good efficacy against white mold.

What populations would you recommend in 7.5-inch rows for high yield soybeans? Planting in 7.5-inch rows means you are drilling soybeans. Generally, drilling is equivalent to pouring seed in the furrow with uncontrolled placement and precision. In this instance you probably want to drop at least 180,000 seeds per acre. If your drill is equipped with more precise seed meters where you can control placement and depth you can drop back to 150,000 to 160,000 seeds per acre. High yield soybeans don’t require lots of plants, just a good stand that emerges evenly and narrow rows are better than wider rows. You should have a jump on high yields by planting in narrow rows.

Do you have data showing how fungicide seed treatments increase yields in soybeans? Seed treatments by themselves don’t increase yield. However, the more seeds that grow into plants and the faster those seedling emerge and become healthy plants, the greater your opportunity to achieve high yields. Remember, yield is based on seeds per acre so the more nodes and pods you can produce the greater the opportunity to produce high yields.

What about root enhancements, do they work, do they increase yield? That is a very good question. There are a number of products on the market that stimulate root growth and most are biological in origin. First, be careful that the claims have been backed up by legitimate research. Second, do your own testing to see if they work and look for something measurable. Remember that most products, including root enhancements, don’t work all the time. But if they do 60 to 65% of the time, that is a good bet. And while they may stimulate root growth, there is no guarantee they will increase yield. Do your homework.

We are wondering how to get better yields even when soil and weather are good. What do we focus on? That is the quandary most soybean producers face and the questions they ask themselves or their agronomists. The first step is to select the best variety that is high yielding. The second step is to implement the best basic agronomic practices such as seedbed preparation, early planting, right planting (method and population), right soil pH, good base fertility and keeping the field weed free. The third step is enhancements such as seed treatments (can’t live without them), foliar protection against insects and disease, growth stimulants and foliar feeding. You have to vet the technologies and products available which work best in your operation.

How much nitrogen at planting time? Is there any yield response to 2 x 2 fertilizer at planting? Any work with pop-up fertilizer? Soybeans don’t begin to fix nitrogen until after mid-June, so a preplant application of 25 to 40 lbs. per acre is of benefit and particularly in a cool and wet spring when nodulation and nitrogen fixation is delayed. But too much nitrogen means too much nitrate in the soil, which can work against you and suppress nitrogen fixation. Pop-up fertilizers work, but keep the rate to 2 to 2.5 gallons per acre and top that off with another 2 to 2.5 gallons of water to dilute it out in the soil. And remember N and K are the salt risks, not P. Lastly, you can put down a 2 x 2 and increase the volume maybe to 4 gallons; a 10-34-0 would be a good choice because it provides about 4.4 lbs. nitrogen and 15 lbs. phosphate.

With about 90% of an average 2,000 pounds per acre being organically tied-up N; what is the best way to liberate (mineralize) that N for uptake by soybeans? The soil contains a lot of organic matter containing nutrients. The best way to release these nutrients is to have a biologically active soil where worms, other insects, fungi and bacteria are feeding on organic materials and releasing nutrients. To do that they need a quality soil, ample supplies of soluble carbon and nitrogen, and the right moisture and temperature. Of course, many companies are now selling humates, enzymes and microbes that supposedly will achieve the same result.

Should P and K be applied in the spring before planting? Is that more beneficial? That can be argued either way. Fall application is more convenient, soil conditions are better and fertilizer is cheaper. And over the winter P and K can be lost through non-point source runoff and it can also tie up so less is available. If you have time to do it yourself in the spring, I favor spring application because the nutrient source is fresher and more available.

You talked about importance of applying P and K and there are a lot of foliar products. How do they fit into providing these needs? Nutrients like N, P and K are needed in large amounts and there is no way that a plant can get its requirements through the foliage. It has to meet its requirement through root uptake. Nevertheless, the plant can take up small amounts of N, P, K and micronutrients through the foliage and this might just be enough to stimulate the roots to take up more nutrients.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at

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About the Author: Dan Davidson

Soybean agronomist Daniel Davidson, Ph.D., posts blogs on topics related to soybean agronomy. Feel free to contact him at or ring him at 402-649-5919.