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How will soybeans respond if you accidently put down nearly 200 lbs. of nitrogen (N) prior to planting? That was the question a grower in eastern Iowa asked me early in May when he called and said his co-op applied 180-125-200 lbs. actual of N-P-K on cornstalks going to soybeans. 

He explained that the mix was intended to go on soybean stubble in a field going to corn and was a 2-year spread for corn followed by soybeans. He added that the co-op was willing to cover the cost of the N, but not the P and K since they said it could be banked for later use. He was concerned about the 180 lbs. of N applied and its impact on soybeans, nodulation, nitrogen fixation and rank vegetative growth followed by lodging and yield loss.

I decided to conduct a straw poll among some of the Soy Envoys (present and past) to find out what they thought the impact would be on his soybean crop. As expected, there are a variety of opinions and strategies he could follow.

Dan Davidson, ISA: My response was that all that extra N up front wouldn’t impact nitrogen fixation, that he could expect more vegetative growth and lodging, and that the crop would benefit from the freshly available P and K. My advice was that going forward he should fertilize corn and soybeans independently and that 30 lbs. of actual N preplant with early planting was a good strategy until N fixation kicked in during June.

Samantha Schmidgall, GROWMARK, Inc.: “Negative—too much nitrogen. I agree with the grower’s concern. Lazy beans may put on more pods and vegetative growth, but rarely stand until the end of the season for the grower to reap the benefits.”

Kris Ehler, Ehler Bros. Co.: “Minimal. I’ve seen this scenario before. Didn’t help and didn’t hurt. If worried about rank growth, throw in a burner at post application to slow them down. Also, look at using Nachurs® KTS foliar product that will help limit internode growth.”

Todd Thumma, Golden Harvest: “My question would be: Is it an option to go corn/corn again yet? The 180 units of free N looks like a good start to a great corn crop even today. Is a herbicide on that prevents planting corn or are there ruts and wet conditions and standing cornstalks that are not easily manageable? If that is an option that is my first choice, bar none. If going to beans is the last and only option find a shorter stature soybean with great standability. Run a 3-4X rate of inoculate to drive nodulation when the bean thinks it has plenty of N and won’t support nodulation early. Reduce population by 20% or so as they will get big fast and this should help drive branching and reduce height.  Narrower rows would be preferred. ALS herbicides reduce internodal elongation and increase stem diameter with a weed control bonus. I am not the biological guy to ask, but there are more than a few on the market that reduce internodal elongation and could be added multiple times to help manage excess N when making other applications. And depending on location and history, white mold may be a huge concern. Adding a Flexstar®, Cobra® or Ultra Blazer® in a second pass application can help trigger a defensive component in the plant as well. If plant growth is driven as much as expected and weather has favored white mold, I would be adding an aggressive and effective fungicide application for white mold.”

Dave Rahe, RPM Soils, LLC: “First thing would be that he might not have as much N left as he thinks based on the weather this spring. Depending on temperatures it may be denitrified big time. Second, there has been research on using N on soybeans. It generally does not pay, but any interference with nodulation should be offset by the fertilizer N remaining. I would be asking if he can afford to waste the N if it is still there. Instead of guessing at least use a model to see how much N is left.”

Lance Tarochione, Bayer: “I would plant the farm to soybeans as planned at a seeding rate of 140,000 or less (depending on variety, planting date and soil type). In my experience the dangers of nitrate interfering with nodulation are overstated. I have seen many trials (both accidental and intentional) where nitrogen or high rates of manure were fall or spring applied ahead of soybeans and in none of those cases have I seen a yield decrease or a problem with nitrogen fixation. The only situations where I have seen soybeans fail to nodulate and fix nitrogen are fields that have never (or not for a very long time) had soybeans planted in them. As others have mentioned there is rarely enough of a yield increase to pay for the nitrogen, but yields are typically equal or slightly better in my experience. Lodging and excessive growth are a possible side effect so I would recommend a moderate to low seeding rate. Dr. Fred Below (University of Illinois) has also been surprised that his biggest yield increase from applied N to soybeans has come from preplant applications of UAN or Urea.  Not only has there not been a negative effect of preplant N in his studies, from those applications have come the best N responses he has seen.”