The last time researchers seriously looked at nutritional needs in soybeans was in 1971, Shaun Casteel says. Richard Nixon was president, and the International 1206 was still a mainstay on many farms.
Much has happened since then, notes Casteel, Purdue University Extension soybean specialist. Work by researchers has determined that modern soybean varieties don’t behave like their predecessors in many ways.
“It doesn’t make sense to still base plant nutrition programs and fertilizer strategies for soybeans as farmers did back when the top varieties were Williams and Harasoy,” Casteel says. “The first step is to study today’s varieties vs. past varieties and realize how they’re different. That provides a clue into how we need to adjust management techniques to reach higher yields today.”
Casteel still grows out older lines such as Williams in plots for comparison purposes. “When you pull a leaflet from a Williams plant, it’s huge compared to today’s varieties,” he says. “There is also a difference in color. Most varieties today tend to be darker green than varieties from the Williams era.”
That’s a significant observation, Casteel believes. “Darker leaves usually mean there is more nitrogen in the leaves. At the same time, there is also more chlorophyll, and probably more sulfur than in the past.”
Casteel suspects the amount of chlorophyll in soybean leaves plays a role in leaf retention. The higher the chlorophyll content, the more likely plants will retain leaves longer into the season.
Nighttime temperature lows in the 50s during reproductive stages, like what occurred in many areas this year, aren’t particularly good for soybeans, he says. The photosynthetic factory slows down at those temperatures, and it takes time to get working again. In contrast, temperatures that low during reproduction were rare in 2016, when yields hit record levels.
What Casteel has discovered is that in many older lines, the R5 stage, which typically occurs in August, was indeed when the bulk of pods filled with soybeans. By R6, processes that add yield were tapering off, and it was more difficult to increase yield at that point.
“With newer soybean lines, there’s still a lot going on at R6 and even after R6,” Casteel says. “The 2016 season was a very good example of this idea. A lot continued to happen in R6 and beyond in many fields, increasing yields.
“We’ve always thought July was the critical month for corn, and August was the critical month for soybeans. With what we’re seeing from modern varieties, it may turn out that both August and September are critical months for soybean development,” Casteel says.
RETHINK SOYBEAN FERTILITY: Agronomist Steve Gauck believes higher soybean yields start with considering what soybeans’ true nutrient needs are and when they need them. There are more questions than answers so far.
5 steps toward higher yields in your soybean fields
If you get hungry while watching football and go to the refrigerator during a commercial only to find very little there, you’ll be disappointed. Unless someone stocks the fridge between commercial breaks, you’ll be just as disappointed the next time. The only way to avoid disappointment is to keep it stocked.
“It’s much the same way with soybeans,” says Mike Hannewald, a Beck’s agronomist working in Ohio and Michigan. “For soybeans, the better analogy might be a freezer, because you need to keep it stocked all season long. Just like you would fill it with half a beef and then pull out a steak to grill that evening, soybeans need their soil fertility ‘freezer’ stocked so they can pull out nutrients when they need them.”
Hannewald uses this analogy to make the point that soybeans need nutrition as much as any other crop. Yet some growers tend to overlook beans to get to corn, and then wonder why they can’t pull off 100-bushel-per-acre soybean yields.
“We’ve got to stop treating soybeans like a weed crop we grow between two crops of corn,” says Steve Gauck, a Beck’s sales agronomist, Greensburg, Ind. “We have under-estimated soil fertility needs of soybeans many times in the past.”
Change your thinking
Hannewald and Gauck recommend this five-step process to help you begin thinking about soil fertility for soybeans.
1. Approach soil fertility for soybeans like you do for corn. “Research from the University of Illinois also shows that soybeans today need nutrients later in the season than they once did,” Hannewald says. “They need phosphorus right up to the end of the season.
“The curve is steeper for potassium, meaning they take up more during reproduction, but they still need potassium all the way to the end as well,” he says.
2. Consider fertilizing before soybeans instead of applying for two years before corn. Gauck sees this adjustment as a must. A bushel of soybeans removes 0.8 pound of P and 1.4 pounds of K. Are you really applying enough of those nutrients every other year before corn to cover corn removal and what an 80- to 100-bushel soybean crop will need?
3. Look at foliar application during the season as one option. There are still lots of answers to learn here, Gauck says. If you experiment with providing nutrients for the plant during the season via foliar application, timing will likely be critical.
4. Know your soil test levels. Even if you fertilize ahead of soybeans every year, you still need to know where levels are for P and K. “If you have some areas below 15 parts per million, the critical level for P, you may make more net profit by diverting some fertilizer from where P levels are high to the levels where it’s deficient. There’s more chance of a positive yield response,” he says.
5. Do everything you can to increase seed size. Seed size is where high yields thrive, Gauck says. How you increase seed size may be part art and part science.
“One thing you can do is keep plants as healthy and productive as long as possible,” he says. That may involve a mix of management practices, including planting early and applying fungicides and insecticides.
Decision Time: Production is independently produced by Farm Progress and brought to you through the support of Case IH.