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Assessing Lighter Green Soybeans

In Whitley County, as in other parts of Indiana, some of the soybean fields look quite variable due to rainfall patterns and ponding. Generally, the issue of variability in many fields is due to excess rain events, but that is not the case everywhere. A Purdue expert recently discussed “wet feet” in soybeans, and whether to let recovery take place over time or to attempt a rescue treatment.

“Chances are you will see the off-green to highlighter green soybeans throughout the fields where the field has been saturated to ponded,” said Dr. Shaun Casteel, Purdue Extension soybean specialist. “Saturated soils will limit oxygen supply to soybean roots and bradyrhizobia.” He said that soybeans grow and develop by burning plant energy (photosynthates) with the assistance of oxygen. Thus, soybean (and rhizobia) growth is limited under wet conditions. In fact, nodules can die under prolonged saturation and limit nitrogen (N) supply.

Dr. Casteel suggested digging up the plants in these fields to assess if the nodules are white, red, or dead. “White nodules are immature, but developing,” he said. “Red or pinkish interior of the nodules indicates that N fixation has started. Brown to mushy nodules are dead and will not supply N.” He added that young soybeans, V2 to V3, may only have three to five actively-fixing nodules (pink to red interior). Soybeans from V4 and onward should have eight or more actively-fixing nodules with more nodules developing. Nutritional analyses of the most recent mature leaves of soybean will help to pinpoint if N or another nutrient is deficient, he said.

“Under normal growing conditions, soybeans accumulate ~10 lb N/acre by V4 then accumulate another ~3 lb N/acre daily until R2 (full bloom),” he said. “Thus, soybeans normally accumulate another 60 lb N/acre by R2 (full bloom). Nitrogen stress during this period will impact yield.”

Dr. Casteel described Purdue studies they conducted in 2015 to soybeans after heavy June rains. “We evaluated N-based treatments; individual foliar sprays of nutrients, fungicides, and growth regulators; and tank mixes of some of those foliar sprays,” he said. “The N-based treatments were to provide the shot in the arm since we were compromised on nodule and N supply.”

They studied urea applications and granular ammonium sulfate (AMS) applications against untreated control. In this particular study, “Granular AMS (21-0-0-24S) was applied at two different rates 20 and 10 lb N/ac, which were the only treatments to increase yield above the untreated control (UTC),” he said.        

“Please note this was our first glimpse into soybean response to sulfur (S),” he said. “In fact, this site has become our S-deficient site to evaluate S management of soybean.” He said it is difficult to tease out if the rescue treatment for soybeans under saturated conditions was beneficial because of the saturated conditions that compromised root development, nodulation, and N fixation OR if it was beneficial because the field is responsive to sulfur treatments year in and year out.

“If you are considering any rescue treatments of soybean, please be sure to assess the root system and nodule activity (or the lack thereof),” he said. “If you deem a field worthy of rescuing or trying to rescue some of it, I would start with a S and/or N+S based approach to stimulate nodulation (S is needed as a co-factor) and to provide a shot in the arm for the limited N supply until N fixation takes over.” He said that it is reasonable to use 10 to 20 lb S/ac from a soluble source like granular AMS or pelletized gypsum. If you use granular AMS, 8.8 to 17.5 lb N/ac would also be supplied. “If you would like to increase the N portion applied, I suggest using urea to reach a total of 40 lb N/ac,” he said. “We do not want to overload the field with N to the point of delaying nodulation and N fixation.”

Find Dr. Casteel’s original Jul 9, 2021 article in the Purdue Pest & Crop newsletter,

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