Early-planted, high-yielding soybeans remove more nutrients from the soil than many growers expect. To realize the high-yield potential of the soybean crop the following season, growers might need to replace those nutrients. They are urged to develop comprehensive fertilizer programs to maximize soybean production, whether grown in either a monocrop or in a rotation.
Mississippi State University agronomist Bobby Golden says the U.S. average nutrient loss per soybean bushel comprises 3.3 pounds of nitrogen, 0.73 pound of phosphorus, 1.2 pounds of potassium, and 0.18 pound of sulfur. “That doesn’t sound like a lot of fertilizer loss until you put it on a per acre yield basis — particularly in high-yielding soybeans,” he says. “Additionally, keep in mind that this is a national average that is driven by Midwestern dryland production data. In Mississippi, less than 35 percent of producers are applying fertilizers other than nitrogen.”
Growers need to know how much and what nutrients they are removing from the crop in order to know how much fertilizer they need. This begins with soil testing; soil samples provide a baseline for understanding a significant part of the yield equation — the soil’s nutrient status.
Mississippi Extension research demonstrates that soybeans respond positively to potassium in the state. “Our research indicates that if soil tests show you don’t need additional potassium, then you don’t need to apply any,” Golden says. “However, if the soil test calls for potassium, we see a 15 percent yield increase. Applying 80 to 120 units per acre will get you to the maximum level required for optimum yield potential.”
Mississippi researchers also conducted similar work with phosphorus. “Again, if a soil test says you don’t need additional phosphorus, then don’t apply it,” Golden says. “When you apply it where it’s needed, you get about a 14 percent or an 8-bushel per acre yield increase. Our studies also show that applying 40 pounds per acre is need to maximize our yield potential.”
Golden and his colleagues have been working the last five to six years on calibrating and re-correlating the state’s nutrient recommendations for soybeans. Recently, they also began assessing the total amount of nutrients taken up in the soybean plant. “We’re asking how soybeans take up nutrients in the Mid-South in an irrigated environment in both low and high-yield situations,” Golden explains.
Previous research showed that nutrient partitioning occurs in three distinct phases: V4 - 30 days after emergence, R2 - full bloom, and R5 to R8 - reduced rates of nutrient accumulation. “In the plant’s early life, it takes very little nutrients,” Golden says. “After V4, we start seeing nutrient levels increase in the plant. At the R2 stage we see optimum nutrient uptake. And then we see nutrient uptake decline from the R5 stage on, because most nutrients are moving rapidly into the seed.”
Yield and planting date
In 2016, Golden’s graduate student Brian Pieralisi initiated a study to determine the effect that planting date has on soybeans yields and total nutrient uptake. The study used four varieties — Asgrow 46X7, DG 4934 LL, Asgrow 4632, which is widely grown in Mississippi, and UA 5014 — that were planted April 8 and May 10.
“One thing we wanted to see is if nutrient uptake differed among some of the Xtend, LibertyLink and Roundup herbicide trait platforms, as well as a conventional variety,” Golden says. “We found that nutrient uptake was very similar across the trait platforms, as well as the conventional variety. Additionally, the April planted soybeans averaged 80 bushels per acre, while the May planted soybeans averaged around 50 bushels.”
The study also showed the difference in nutrient uptake and removal from the soil between the April- and May-planted soybeans. As expected, May-planted soybeans with their shorter season and reduced yield took up less nutrients. April-planted soybeans removed 27 percent more nitrogen than May planted soybeans from the soil into the plant (396 pounds versus 262 pounds), 21 percent more phosphorus than May-planted soybeans (80 pounds versus 55 pounds), and 16 percent more potassium than May-planted soybeans (342 pounds versus 266 pounds).
“We’re removing a tremendous amount of nutrients from the soil, especially in our early-planted, high-yield soybeans, but sometimes we’re not replacing as fast as we’re removing,” Golden says. “Our research shows that we need to fertilize to maximize the genetic yield potential of our soybean varieties. It’s the adage: take care of the soil and it’ll take care of you.
“Some growers say they can’t afford their fertilizer bill. University/Extension can help you afford your fertilizer bill by implementing several tools, including variable rate fertilizer application, and using soil testing to determine exactly what fertilizer you need to apply, how much fertilizer you need to apply, and where you need to apply fertilizer. We want to help growers be as profitable as they can while implementing the 4R’s of fertilization.”