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Soybeans Need for Supplemental Nitrogen

Categories: GROWING, SOYBEANS
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  • In high-yielding environments with conditions conducive to nitrogen loss, the potential for soybean yield response to nitrogen (N) is greatest.
  • Biologicals could have potential to supplement N in soybeans without reducing nodulation and N fixation.

Soybeans have a high demand for nitrogen (N) and must accumulate 4.8 lbs. of N per bushel. It has been documented that biological N fixation from Bradyrhizobium can supply roughly 60% of the N requirement for soybeans.1 The other 40% must come from the soil through mineralization or synthetic N fertilizers. At low yield levels, it is likely the soil can supply the remaining N requirement for soybeans. However, in high-yield environments the soil may not be able to mineralize enough incremental N above what is supplied by Bradyrhizobium to meet the total demand of soybeans.

Graph 1. Influence of soybean yield on N in the crop, N supplied by fixation and required soil N
Adopted from: Salvagiotti et al., 20084

The presence of plant-available N (nitrate or ammonium) has been shown to reduce nodule formation, growth and activity in soybeans.2,3,4 The reduction is directly proportional to the soil level of N supply so applications of synthetic N fertilizers may not be an effective method to fill the N requirement gap. A slow-release form of N through biological fixation from other bacteria could be a promising concept to meet this need. Alternatively, synthetic N fertilizer applied later in the season, after peak nodulation has occurred, may be a way to provide N without reducing nodulation.

Agronomy in Action Trials

The Golden Harvest® Agronomy in Action research team implemented trials at 8 locations across the Midwest designed to supplement N to soybeans without reducing nodulation. 2 different Golden Harvest soybean varieties, either GH2102XF and GH2329X, or GH2788X and GH3088X, or GH3475X and GH3546X, were grown at each location.

3 biological products were evaluated, including BlueN™, Envita™ and New Tech SI-IF™. BlueN is an endophytic bacterium (Methylobacterium symbioticum) developed by Symborg that is foliar-applied at the V4 growth stage. Recently, Symborg reached a multi-year agreement providing an exclusive distribution license of the bacteria to Corteva Agriscience under the brand name Utrisha™ N. Envita is a naturally occurring bacteria (Gluconacetobacter diazotrophicus) from Azotic North America that is applied in-furrow or foliar at the V4 growth stage. New Tech SI-IF from TerraMax is a soybean inoculant (Bradyrhizobium japonicum) combined with a bacteria known to fix N and enhance root hair growth and development (Azospirillum brasilense) that is applied in-furrow.

In addition, 3 synthetic N fertilizer treatments were included in the trial. Either 30, 60 or 90 lbs. of N/acre was broadcasted as Agrotain®-coated urea at the R3 growth stage. The R3 growth stage is beyond peak nodulation, minimizing the negative effect of soil nitrate concentration on nodulation.

Nitrogen Management Effect on Soybean Yield  

On average, across all locations and varieties, there was little yield difference between the biological or synthetic N treatments compared to the check (Graph 2). New Tech SI-IF was the highest yielding treatment averaging 73.6 bu/A. Any potential yield responses to the different rates of synthetic N may have been mitigated by leaf burn from the broadcast Agrotain®

Graph 2. Effect of biological or synthetic N fertilizer on grain yield averaged across 8 locations and 2 varieties in 2021
LSD (0.10) = NS

Clinton and Elwood, IL, experienced environmental conditions that were conducive for soybean N limitations. Both locations were high yielding, with Clinton averaging 91 bu/A and Elwood yielding 77 bu/A. At those yield levels, the crop would require 437 and 370 lbs. of N/acre, respectively. If N fixation supplies 60% of the plant N requirement, the remaining N gap that would need to be supplied from the soil would be 175 lbs. of N/acre at Clinton and 148 lbs. of N/acre at Elwood. Clinton received nearly 8 in. of precipitation in June and Elwood received 7 in., which is close to double the 30-year average during June at both locations. Nitrogen loss through leaching was likely during this time.

Tech SI-IF significantly increased yield by 5.1 bu/A at Elwood and numerically by 2.0 bu/A at Clinton compared to the check (Table 1). Soybeans tended to respond by 1.6 bu/A when Envita was applied either in-furrow or with a foliar application at V4 in Elwood. BlueN also tended to provide a 2.4 bu/A yield increase at Elwood. The 60 lbs./A of N rate significantly increased yields at Clinton. At Elwood, all rates of synthetic N fertilizer tended to increase yield with the response being greatest at the 30 lbs./A of N rate. This rate likely supplemented N with minimum leaf burn compared to the higher rates.

Table 1. Effect of biological or synthetic N fertilizer on grain yield averaged across 2 varieties

In these high-yield environments that were subject to N loss, the slow-release form of N through biological fixation from bacteria likely supplemented the N requirement for soybeans and reduced the N demand gap.

Effect on Nodulation

Roots from plants in the border rows of the check, Envita and New Tech SI-IF in-furrow treatment plots were evaluated for nodulation at the R2 growth stage. The degree of nodulation was rated on a scale from 0-3. A score of 0 would mean no nodules were present. Nodules present on only the taproot would be a score of 1. A score of 2 would have nodules present on the taproot and 1-3 lateral roots while a score of 3 would mean nodules were present on the taproot along with 4 or more lateral roots. Nodules were removed from the roots and split open, and the percentage of nodules that were pink, or healthy and active, on the inside was recorded.

Photo 1. Soybean root with many nodules on the taproot and lateral roots in left image; Right image illustrates the inside of nodule actively fixing N on the top and nodule no longer fixing N on the bottom.

There was no effect of either biological application on nodulation or nodule activity at any location. These results indicate that N fixed from the bacteria in these products does not reduce nodulation. Also, any yield responses from New Tech SI-IF were likely driven by N fixed from the Azospirillum brasilense bacteria and potentially increased root hair development, rather than the Bradyrhizobium japonicum increasing nodulation.

Summary

Results from this study show that soybean response to synthetic N fertilizer and N-enhancing biologicals are inconsistent and environment dependent. Many factors contribute to the potential of soybean yield to be limited by N. In high-yielding environments with conditions conducive to N loss, the potential for soybean yield response to N is greatest.

To learn more about strategies to reduce nitrogen limitations in soybeans, watch this video.

References


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