Is Late Season N Fertilization Warranted for Irrigated Soybean in Western Nebraska?

Figure 1. Final soybean stand count was not necessarily an indicator of yield in this 2017 study of the effects of late-season (R3) nitrogen application in soybean. The Lincoln County plot (left) had a final stand count of 72,000 plants per acre and yielded 72.7-73 bu/ac. The Perkins County plot (right) had a final stand count of 161,000 plants per acre and yielded 74.4-77 bu/ac.

Is Late Season N Fertilization Warranted for Irrigated Soybean in Western Nebraska? November 2, 2018

Soybean absorbs 60% of total N uptake during the pod setting and seed filling phases. Is N supplied from soil N and biological fixation sufficient to meet this demand, or do soybeans need supplemental N to reach their full yield potential?

On-Farm Research Trials in Southwest Nebraska

To answer farmer questions as to whether the practice of late-season N application increases yield and enhances profit in western Nebraska soybeans, we conducted on-farm research studies in three southwest counties (Perkins, Chase and Lincoln) in 2017.

Layout for research plots on soybean late-season N application (R3)
Figure 1. Satellite image taken Aug 23, 2017 at Chase County site shows differences between pie-shaped pivot areas with and without application of 65 lbs of N/ac.
Photos of soybean nodulation at the three research sites
Figure 3. Visual soybean nodulation inspection at V7 growth stage in 2017 on-farm research studies in (from left) Perkins County (excellent), Lincoln County (excellent), and Chase County (poor).

The plot layout consisted of alternating pie-shaped sections over an entire pivot. Some segments received N through the pivot and some were left as untreated checks (Figure 1). A foliar application of 65-85 lb/acre of N was made through an irrigation application at the R3 (beginning pod) growth stage.

 
Graphs tracking total N available at planting, R2, and harvest.
Figure 2. Soil available N (NO3 + NH4) from 0-24 inches for two nitrogen treatments (High-N vs No-N) sampled at planting, R2 (prior to N chemigation), and after harvest at on-farm research studies in Perkins, Lincoln, and Chase counties during the 2017 growing season.
 

Surface (0-8 inch depth) and sub-surface (8-24 inch depth) soil samples were collected in each pie at three times: prior to planting, at the R2 growth stage, and after harvest. This was to identify changes in both NO3-N and NH4-N soil concentrations throughout the growing season (Figure 2). Visual nodulation inspection was conducted at V7 growth stage (Figure 3) and plant tissue samples were taken at the R2 and R5-R6 growth stages to monitor nutrient content within the plants (Figures 4 and 5). Plant residue was analyzed for residual N content (Table 1, below). In addition to yield, grain samples were analyzed for protein and oil content (Table 1).

Main Findings

  • Soybean plants at Perkins County and Lincoln County had many active nodules well spread on the roots. Nodulation at Chase County was very poor, possibly due to excessive mineral N in the top 24 inches of soil (Figure 3). The amount of soil N available at planting (62-106 lbs/ac) had decreased by more than 50% when measured at the R2 growth stage (30-40 lbs/ac), suggesting that the plant was taking up soil N as it entered the reproductive stages.
  • Nutrient concentrations of both micro- and macronutrients in plant tissue did not differ between the N treatments (Figures 4-5). The nutrient levels in plant tissue at all three sites were within the sufficiency range for all macronutrients and micronutrients, except for magnesium (Mg). A positive correlation was found between N supplementation and manganese (Mn) uptake and metabolism. 
  • Supplementing N to soybeans at R3 did not increase yield, grain protein, oil content, or marginal net return at any of the three locations (Table 1). A slight yield increase of 2.6 bu/ac was observed at Perkins County, but due to large field variability this increase cannot be attributed to additional N fertilizer. The post-harvest soil N was similar in both N treatments. The only notable difference in end-of-season N balance was N content in plant residue, which was 5, 8, and 14 lbs of N/ac higher in the N treatments at Chase County, Lincoln County, and Perkins County, respectively.
 
Charts of macro- and micronutrients at R2
Figure 4. Leaf tissue sample analyses were conducted at R2 stage soybean (prior to chemigation) for macronutrients (N, P, K, S, Ca, Mg) and micronutrients (Zn, Fe, Mn, Cu, B) at on-farm research trials in Perkins, Lincoln, and Chase counties. Red squares represent sufficiency range for nutrients.
 
 
Charts of macro- and micronutrients at R5
Figure 5. Leaf tissue sample analysis at R5 soybeans (two to three weeks after chemigation) for macronutrients (Zn, Fe, Mn, Cu, B) and micronutrients (Zn, Fe, Mn, Cu, and B) in Perkins, Lincoln, and Chase counties. Red squares represent sufficiency range for nutrients.
 

Take-Home Message

More Research on N in Soybean

The article, “Is Soybean Yield Limited by Nitrogen Supply,” reports on field experiments in eastern Nebraska and Argentina. The authors’ findings indicate soybean yield was limited by N supply, especially in high-yield environments, and that N supplementation slightly increased yield and seed protein content. See the full research report in Field Crops Research.

An article from retired University of Nebraska-Lincoln Soil Scientist Charles Shapiro titled “Nitrogen on Soybeans – the Hope Never Dies” looks at two recent research studies and searches for the answer to the lack of response by soybeans to N in cases when N balance (supply vs. demand) clearly suggests that supplementation is needed. The papers being compared both suggest "that in high-yielding environments, nitrogen is likely to have a marginal beneficial effect." Shapiro points to the need for future research on N timing, application, source, and management for high-yielding soybeans.

  • Late season (R3) N fertilization did not increase yield or profit of soybeans grown at three on-farm trials in southwest Nebraska in 2017. Supplementing N to soybeans is more likely to be beneficial in higher yielding environments, perhaps higher than yields achieved in these studies (up to 77 bu/ac).
  • More research is needed to fine-tune soybean fertility management. Based on our findings, we suggest further work relative to:
    • spoon-feeding lighter rates of N to soybean during the reproductive stages (rather than applying one large amount at one time),
    • applying lower rates of N later in the season (R5-R6 growth stages),
    • using manure or compost as a form of slow release N,
    • consideration of foliar micro- and macronutrients, and
    • using fertilizers containing nutrients other than N (e.g., magnesium, sulfur, molybdenum, boron, etc.)

We highly encourage farmers in western Nebraska to test fertilizer practices in their own fields, and more importantly, to consider implementing other practices that are critical in increasing soybean yield potential. These practices include ensuring an adequate water supply and early planting.

For more information on practices recommended for high-yield soybean production, see the Nebraska Extension publication, What Does it Take to Produce 80+ bu/ac Soybean? (EC3000).

Acknowledgment

We would like to thank the Nebraska Soybean Board for funding this research project and continuing to support research on soybeans in western Nebraska. We also thank Conrad Nelson of Wallace, Stacy Friesen of Grant, and Dan Reeves of Imperial for collaborating on this research project.

Table 1. Site description, agronomic information and data on grain yield (bu/ac), oil content (%), protein content (%), soil N after harvest, N in the plant residue (lbs/ac), and marginal net return ($/ac) for irrigated soybeans at three western Nebraska sites in 2017.
Perkins CountyLincoln CountyChase County
Site description
Soil organic material Keith silt loam Holdrege fine sandy loam Valent loamy sand
Previous crop corn corn corn
Planting date 15-May 25-May 15-May
Harvest date 20-Oct 15-Oct 16-Oct
Rainfall (inches) 12.0 15.0 11.5
Irrigation (inches) 10.5 13.5 11.0
Hail minor injury no hail 40% damage on 10/01/18
Agronomic information
Tillage Vertical till No-till Vertical till
Row spacing 10-inch 15-inch 10-inch
Variety Pioneer 22T41 Chanel 2402 NK S30C1
Maturity group 2.6 2.4 2.6
Final stand (plants/ac) 161,000 72,000 168,000
Nodulation excellent excellent poor
Nitrogen applied (32-0-0) at R3 70 lbs N/ac 85 lbs N/ac 65 lbs N/ac
Study results
Treatment (N applied at R3) High N No N High N No N High N No N
Yield (bu/acre)† 77.0 74.4 73.0 72.7 65.1 64.4
Oil content (%) 34.5 34.4 36.0 35.9 34.5 34.4
Protein content (%) 20.4 19.9 20.2 19.7 21.7 18.6
Soil N after harvest at 0-24 in (lbs N/ac) 44.7 47.0 34.0 27.8.0 28.8 30.5
N in the plant residue (lbs of N/ac) 47.0 39.0 34.0 28.8 52.3 38.0
Marginal net return‡ ($/ac) $656 $646 $614 $646 $573 $552
*Values with the same letter are not significantly different at a 90% confidence level.
†Bushels per acre corrected to 13% moisture.
‡Marginal net return based on $8.90/bu soybean.