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    Unlocking Soybean Yield Potential with Crop Nutrition

    There are bushels out in your soybean acres just waiting for an artful hand to bring them forth. Agronomic strategies could boost the take by 10 bushels an acre, with half of those coming through proper nutrient application, according to research conducted by Dr. Fred Below and Dr. Ross Bender at the University of Illinois.

    For three years, Dr. Below and Dr. Bender (then a senior researcher on Below’s team, now a Director, New Product Development) ran 20 trials at three locations (representing three maturity zones) across the state of Illinois. They studied how the use of phosphorus, potassium and the micronutrients in MicroEssentials® SZ® can be used to maximize soybean yield.

    With producers poised to plant 4 million more soybean acres this year (for a record 87.5 million acres; source: Informa Economics), this soybean tune-up is timely information.

    “It’s time to forge new possibilities for soybeans, which essentially have been a forgotten crop,” says Curt Woolfolk, Sr. Manager Crop Nutrition Technologoies for The Mosaic Company. “When you pay attention to your soybeans, they will repay the effort. A great place to start is Dr. Fred Below’s ‘Six Secrets of Soybean Success.‘”

    Though it is the “second secret,” fertility is the management factor farmers can use to exert the most profound effect. The first secret is the weather, and as Dr. Below writes, “We cannot control the weather, but it influences the success of each of our management factors.”

    Woolfolk points out that a typical fertility program for a corn-soybean rotation involves putting down enough nutrients at the start of the corn crop to cover its needs, and the needs of the following soybean crop.

    “One of the challenges that we run into with that approach is whether or not we are truly giving enough credit to what our soybean crop is going to do,” Woolfolk says. “Are we really taking account of our actual removal from the field? Are we actually applying what that soybean crop is going to need, in order to maximize its output?”

    Below and Bender’s trials prove that this approach may end up “mining” the nutrients out of the field, and essentially putting the soybeans on a poor diet.

    “We think that growers are not adequately fertilizing phosphorus because they don’t know how much the corn is removing, and they’re not actually fertilizing their soybeans,” Below says. “That’s what we’ve demonstrated in our trials over the past few years. Also, we only started to look at potassium this year. This leads to the idea of balanced crop nutrition. For some reason, when the potassium is adequate, the plant seems to use the phosphorus better — it can squeeze another bushel or two out of it.”

    Bender was lead author on their study of how soybeans take up nutrients and where they end up in the plant. One of the key findings is that most of the phosphorus ends up in the grain. Leaving residue on the field may not contribute as much phosphorus to the next crop as once thought. Potassium, on the other hand, is deposited in leaves, stems and other stover, so residue helps replenish K, though Below cautions that this will vary from operation to operation, and K should not be overlooked in the nutrient management plan.

    Soil testing is the first step in developing the right nutrient management plan, so that none of the key macro- or micronutrients are overlooked, Woolfolk advises.

    “Then we need to make a pretty good estimate as to what our crop is going to do,” Woolfolk says. “Weather is a factor we can’t completely plan for, but it’s also important to make sure that we don’t underestimate our yield potential, especially when it comes to soybeans. Look at the removal rate of nutrients for a 60-bushel soybean crop. Especially when we look at something like potassium, the removal could be as high if not higher than our standard corn crop.

    “Forgotten things like this really come into play with soybeans,” Woolfolk adds. “Compare a 60-bushel soybean crop to a 230-bushel corn crop: That corn crop is going to remove about 80 pounds of P₂O₅, while the soybean crop is going to remove about 40 pounds. But when we look at potassium, that 230-bushel corn crop is going to remove about 58 pounds of K₂O, and that soybean crop is actually going to remove about 75 pounds of K₂O. So with crop nutrients like potassium, we need to make sure we have adequate amounts. Sometimes that can be tough, especially when we’re soil testing every two to four years. In that case, we’re applying that fertilizer a year-and-a-half prior to when that soybean crop is actually going to use it.”

    Bender advises that even-splitting the application, so that some nutrients are put down just before the soybean crop, can be a helpful approach.

    “Of the phosphate that the corn and soybean accumulate, 80 percent of it is removed with the grain,” Below explains. “It has the highest removal percentage of any nutrient. When 80 percent of what the plant takes up is taken off with the seed or the grain, that means that you’ve got to manage it. And for sulfur, approximately 60 percent of what the crop takes up is actually removed with the grain. Let’s face it, when you take that grain off, you take that nutrient off the field. That’s what you have to replace with fertilizer. When the growers are counting on fertilizing their soybeans at the same time as the corn, and the corn is taking 80 percent of the nutrients, that means there’s not much left.”

    Another micronutrient producers should pay attention to is boron, according to Woolfolk.

    “Boron is very important to both corn and soybeans,” Woolfolk says. “We need close to 0.3 ounces per acre of boron for a 60-bushel soybean crop, as compared to corn, for which we only need 1.8 ounces per acre for a 230-bushel crop. So there’s a big difference in boron needs between soybeans and corn. Even though removal rates aren’t as high, we still have to make sure we’re accounting for the boron, because boron is mobile within the soil. P and K typically can be held fairly well, and we’re pretty confident in most Midwestern soils that what we apply will be available for that crop. But boron is mobile, and it can be leached through the soil profile.”

    According to Woolfolk, although foliar application can be part of a fertilizer program, the plant can’t get all it needs without adequate amounts in the soil.

    Bender, who also runs his own for-profit farm operation, adds, “I would suspect that the average farmer probably has 4 or 5 bushels sitting out in the field that could be obtained with better plant nutrition management. We see that fairly consistently across our trials throughout the state of Illinois.”

    According to Bender, their soybean research trials have taken a comprehensive look at agronomic and nutritional considerations that can be used to improve soybean productivity. In addition to P and K, as well as micronutrients, they have looked at the application of foliar insecticides and fungicides, row spacing, variety selection and seed treatments.

    “When you have a whole suite of agronomic management in place, the biological yield improvement is around 10 bushels per acre,” Bender says.

    “Ross’ research also dived into how better nutritional management improves soybean yield potential throughout the different plant regions,” Below adds. “How can we complement the gains in fertility with other important management practices? Soybean is an indeterminate plant. It flowers and grows over a long period of time. There’s different podding of seeds at different positions on the plant. It’s not like corn.”

    In contrast, corn’s sequence of growth and reproduction follows a stepwise pattern.

    Because of this attribute of soybean production, season-long fertility, leaf health and other factors work in concert to boost yield.

    Foliar protection from disease and insects boosts yield in the middle of the plant, Bender’s research has shown, because this is where 50 to 60 percent of soybean yield comes from.

    “Those leaves in the middle portion of the plant are the ones you are protecting if you make an R3 foliar-protection application, which is the most typical one,” Below says. “Another thing that goes along with the fertility is choosing the fullest relative maturity for the region. That maturity changes with the latitude you are at.

    “So this is a matter of variety selection,” Dr. Below says. “What we’ve noticed is that if you make full use of the season, especially if you plant soybeans earlier, and especially if they are well-fed and protected, we often see a 2- to 3-bushel yield gain simply by changing the variety from the normal maturity for the region to one that’s a little bit fuller maturity.”