Think Twice Before Applying Nitrogen to Poorly Nodulated Soybean Fields

Over the last 3 days my phone, email and twitter account has been blowing up with questions surrounding poorly nodulated soybeans and my thoughts regarding applying foliar nitrogen to alleviate those symptoms. I have been reluctant to write this article for two reasons. 

  1. This article will contradict some of my colleagues’ thoughts (Identifying and responding to poor nodulation in soybean) however I would agree with their scenario of applying nitrogen to early-seeded, non-nodulating soybean planted on virgin ground.
  2. I like to speak from data and I don’t have a ton of data to speak from 
So with these caveats in mind here is my thought process for asking growers and crop consultants to think twice before applying nitrogen to poorly nodulated soybean fields. 
First lets start with the problem. A record number of soybean acres were planted in 2014. To get those acres both virgin ground as well as long term continuous corn acres were converted to soybean. For the most part (unless someone forgot) those acres did receive a 1x or 2x rate of inoculant. Unfortunately the perfect storm of delayed planting, poor planting conditions, compaction and poor environmental conditions all led to saturated anaerobic soil conditions that limited rhizobia infection.  These poor establishment conditions were then followed by poor early season growth conditions (cool saturated soils) delayed herbicide applications, increased herbicide rates and weed competition that further stressed the plants and limited infection. 
So with all this stress in mind why do I suggest no additional nitrogen?
  1. Once soils dry out and aerobic conditions resume the ethylene stress response in plants quickly dissipates and normal N-fixation can resume. This will lead to nodulation occurring on lateral roots as infection occurs behind the root tip of actively growing root. Furthermore once plant roots resume normal growth they will be able to take advantage of residual and mineralized soil nitrogen which will alleviate the pale green coloration.
  2. Be realistic with your yield potential. Many of the fields in question are late planted, with stunted soybeans and thin stands.  A short soybean crop will require much less N than a big one.  Salvagiotti et al; (2008) indicated in “Nitrogen uptake, fixation and response to fertilizer N in soybeans: A review” that the most likely soybean response to additional nitrogen was in high yield environments. 
  3. Be realistic with your expected yield loss in non-nodulated virgin soil environments. Somehow the idea of a 20 bu yield loss has been floating around the coffee shops. Our most recent data from a virgin soil site in 2010 showed an average +4.6 bu yield gain (range: -0.9 to 9.6 bu depending upon product) from inoculants. The untreated uninoculated check averaged 73.5 bu per acre. Also remember no history of soybean = low soil borne disease pressure and beautiful healthy roots!
  4. What are your 2014 beans marketed at $14.00 or $11.92 and dropping?
  5. What source of nitrogen are you going to apply and what is that cost per pound coupled with application cost and crop damage?
  •  Simple math equation (please insert your number for mine). 70 pounds of urea @ $0.55 per pound  + $8 application cost + $10.26 yield loss from running down soybeans (90 foot applicator = 1.9% on 45 bu $12 beans) = $56.76. At $12 soybeans you would need 4.73 bu to break even. Our average response in 2010 was 4.6 bu.
I know not everyone will agree with my thought process but understand that I am cognizant of the realities of today’s production ag world….high land rent costs coupled with high commodity prices = grower risk aversion. If you do apply nitrogen please leave at least one yield check and be fair to that yield check placement. Given our climate variability this will not be the last time we deal with this question and having data to streamline recommendations in future years makes us all better stewards and producers.

*Reviewed by Dr. Seth Naeve, Extension Soybean Agronomist, University of Minnesota.