Based on surveys conducted during seven teaching sessions in 2019/20, 40% of the attendees representing 137,000 acres of irrigated soybean production in western NE reported to chemigate some level of N fertilizer during the soybean reproductive stages (25-50 lbs N /ac most common rate). Main reasons for adoption of this practice is based on the notion that biological fixation and residual soil N are often not able to meet N demand of high yielding soybean (> ~70 bu/ac).

Mortzinis et al. (2017) composed data from 207 environments (site-years) across the 16 land-grant universities to find only a minimal effect of N (0-2 bu/ac yield increase) on soybean. In addition, authors suggested that variability in response to N can depend on differences in soybean cultivars, soil properties, weather conditions, and/or agronomic practices (Mortzinis et al. 2017). There are, however, a few studies from eastern parts of Nebraska (Cafaro La Menza et al. 2018, Wortmann et al. 2012) and Kansas (Wesley et al. 2014) where researchers quantified yield responses to N fertilizer sufficient to justify the additional expense associated with the application. In one study, application of 27 lb/ac N at R3 caused average yield increase of 1.1. bu/ac across 44 locations with > 60 bu/ac yield, while no additional yield increase was observed with the application of 54 lbs of N/ac.

Soybean Response to Late Season N in Western NE

In 2017, on-farm research studies were conducted in three southwest counties (Perkins, Chase and Lincoln) with no increase in yield or grain protein with the application of 80 lbs of N/ac at R3 (beginning pod) soybeans. In addition, large inefficiency of N fertilizer utilization (~13%) was observed, with the only notable difference being a 5-14 lbs of N/ac increase in crop residue. The study protocol was changed in 2018 to chemigate less fertilizer (50 lbs of N/ac) at later growth stages (R5-beginning seed) growth stages, but similar outcomes were observed. Leaf tissue analysis in all studies tested low on magnesium (Mg), while N and all other macronutrients and micronutrients remained within the sufficiency range for the most of the soybean growing season.

Biologicals and Compost in Soybean - Why not Try?

When Jimmy Frederick from Rulo, NE (eastern NE) raised 138 bu/ac soybeans to win 2017 state and national dryland soybean yield competition, many farmers wanted to learn from his approach. Mr. Frederick did not apply any synthetic N fertilizers and based his entire fertility program on biological products. In our 2018 fertility study in western NE, early planted soybeans in 15 inch rows benefited from pre-plant application of compost at 5 ton/ac, yielding as much as 107 bu/ac (results of one year at one location). Such outcome initiated a need to repeat the study and look more into the other fertility treatments to that might increase soybean yield.

Soybean Fertility Research in 2019

Public universities and private companies were contacted early in 2019 to provide input on the study protocol that would compare multiple fertility programs for soybeans in western NE. Total of 12 fertility programs were compared including: inoculant only, high N control (400 lbs of N/ac), cow and chicken manure applied both broadcast and in-furrow, two levels of Mg fertilizer, and four programs based on seed and foliar treatments recommended by our local fertilizer suppliers (Table 1). All treatments were planted to same soybean variety, GH2499X (Golden Harvest 2.4 maturity) and replicated three times with individual plots being 20 ft wide (8 rows) x 60 ft long. The study was conducted only at one location (Grant, NE) and the following data was collected in each experimental plot: bi-weekly soil and tissue samples, yield, grain quality (protein and oil), and yield components (plants/acre, nodes/plant, branches/plant, pods/plant, seed weight).

Table 1. The list of 12 soybean fertility treatments and their application time, product rates, and application method evaluated during the 2019 growing season at Grant, NE.

Trt	Fertility treatment	Application time	Application method	Products and rates used
1	Control - Inoculant	Planting	seed treatment	Inoculant - Verdesian  @ 2x rate
2	Control - High N	Pre-plant Planting	broadcast seed treatment	Urea (46-0-0) @ 869 lbs /ac (400 lbs of N/ac) Inoculant - Verdesian @ 2x rate
3	Chicken manure broadcast	Pre-plant Planting	broadcast seed treatment	Beju chicken manure  (pelleted) @ 12 ton/ac Inoculant - Verdesian @ 2x rate
4	Cow manure broadcast	Pre-plant Planting	broadcast seed treatment	Beju cow manure  (pelleted) @ 12 ton/ac Inoculant - Verdesian @ 2x rate
5	Chicken manure in-furrow	Planting	seed treatment insecticide box	Inoculant - Verdesian @ 2x rate Beju chicken manure (powdered) @ 25 lba/ac
6	Cow manure in-furrow	Planting	seed treatment insecticide box	Inoculant - Verdesian @ 2x rate Beju cow manure (powdered) @ 25 lba/ac
7	Mg High Rate	Pre-plant Planting	broadcast seed treatment	MgSO4 @ 750 lbs/ac (270 lbs Mg/ac) Inoculant - Verdesian @ 2x rate
8	Mg Low Rate	Pre-plant Planting	broadcast seed treatment	MgSO4 @ 388 lbs/ac (140 lbs Mg/ac) Inoculant - Verdesian @ 2x rate
9	Aurora Starter + Foliar	Planting Planting V3 V6 R2 R5	seed treatment seed treatment foliar foliar foliar foliar	Inoculant - Verdesian @ 2x rate Hustle  @ 1 gal/ac Heighten @ 8 oz/ac (no product label) + Realize @ 2 oz/ac Heighten @ 8 oz/ac (no product label) + Realize @ 2 oz/ac Realize @ 4 oz/ac + N-cline  @ 1 gal/ac + Evito @ 2 oz/ac Boron + Molybdenum (no product label) @ 1 qt/ac
10	Nutrien 1 Starter only	Pre-plant Planting	broadcast seed treatment	Micro Starter @ 0.3 gal/ac (no product label) Inoculant - Verdesian @ 2x rate
11	Nutrien 2 Starter + Foliar	Pre-plant Planting R2	broadcast seed treatment foliar	Micro Starter @ 0.3 gal/ac (no product label) Inoculant - Verdesian @ 2x rate NutriSyncD @ 10 oz/ac
12	Kugler Foliar products	Planting V2 V3 R2 R5	seed treatment foliar foliar foliar foliar	Inoculant - Verdesian @ 2x rate KS178c  @ 1 gal/ac + LS624  @ 1 gal/ac + FA20  @ 1 gal/ac KS2075  @ 1.5 gal/ac + LS624 @ 0.5 gal/ac + FA20 @ 1 pt/ac KS2075 @ 2 gal/ac + FA20 @ 1 pt/ac KS2075 @ 1 gal/ac + LS624 @ 1 gal/ac + FA20 @ 1 pt/ac

Key takeaways

Fertility treatments had no impact on soybean

Fertility treatments had no impact on soybean yield, grain protein, seed oil content, or any of the other yield components measured in the study (Table 2). Furthermore, no differences between the fertility treatments were observed in any of the soil and tissue analysis performed at seven different growth stages. The only notable difference was the higher levels of sulfur (S) in the soil where cow and chicken manure were applied.

Table 2. Soil pH, yield (bu/ac), grain protein (%), grain oil (%), N concentration in crop residue (lbs/ac), residual N in top soil 8 inches of soil (lbs/ac), and yield components (plants/ac, nodes/plant, branches/plant, pods/plant, seeds/pod, and 1000 seed weight) for 12 soybean fertility treatments evaluated during 2019 growing season at Grant, NE

Trt	Treatment name	Soil pH	Yield (bu/ac)	Grain Protein (%)	Grain Oil (%)	Plants/ac	Nodes/plant	Branch/plant	Pods//plant	Seed/pod	1000 Seed weight (g)
1	Control - Inoculant	6.4	77	35.7	21.1	111078	15	3.1	41	2.5	149
2	Control - High N	6.6	74	35.7	21	83345	15	3.8	62	2.5	162
3	Chicken manure - broadcast	6.8	78	34.9	20.8	96122	15	3.2	46	2.5	168
4	Cow manure - broadcast	6.4	73	35.5	20.9	102511	15	3.5	50	2.5	154
5	Chicken manure - in-furrow	6.5	74	35.5	21	117903	15	3.3	50	2.3	154
6	Cow manure - in furrow	7.2	76	35.3	21.4	114345	15	3.2	45	2.3	160
7	Mg - High rate	6.4	78	35.3	20.9	124582	15	3.1	47	2.4	151
8	Mg - Low rate	6.4	78	35.7	20.7	112965	15	3.3	51	2.5	150
9	Aurora - Starter + Foliar	6.8	74	34.5	20.6	116044	15	3	44	2.3	153
10	Nutrien 1 - Starter only	6.2	77	36	20.9	121097	14	3.4	47	2.4	155
11	Nutrien 2 - Starter + Foliar	6.1	76	36	21	111804	15	2.9	43	2.4	162
12	Kugler - Foliar products	6.8	76	35.5	21	116450	15	3	44	2.4	155
	Average of all treatments	6.6	75.9	35.4	20.9	111057	15	3.2	47	2.4	155
	Difference at 5% significance	1.1	12	1.4	0.94	22863	1.5	0.9	11	0.2	7
	Coefficient of variation	10.4	11	2.5	2.83	13	6.6	16.7	15	4.8	18

N supply was not a yield-limiting factor in soybeans (73-77 bu range)

The High N treatment (400 lbs of N/ac) had 53 lbs of N/ac left in top 24 inches of the soil at the end of season, which was 23 lbs of N/ac more than what was left behind the soybeans inoculated with rhizobia (data not shown). With plenty of N left in the soil and yields similar to those of other treatments we can conclude that N supply was not a yield limiting factor in this study (Table 2).

Soybeans will compensate for the loss of stand.

Although range of harvest populations were observed in the study plots (64,000 to 147,000 plants/ac) no yield increase was observed at higher populations due to lower number of nodes, branches and pods on individual soybean plants (Figure 3). These findings are in agreement with our 2018 research results and with the results of many other on-farm research trials conducted in Nebraska.

Figure 2. Effects of soil pH on soybean grain yield (bu/ac) and protein content (%) averaged over 12 soybean fertility treatments; study conducted during 2019 growing season at Grant, NE

Figure 3. No yield (bu/ac) response of soybean to increasing harvest population (70,000 to 150,000 plants/ac) due to lower number of nodes, branches and pods on individual soybean plants; study conducted during 2019 growing season at Grant, NE.

Soil pH was the main factor limiting soybean yield and grain protein

Despite the relatively flat topography, uniform soil type, similar soil organic matter (OM) and cation exchange capacity across the study area, soil pH varied significantly from plot to plot (5.6 to 8.2). Correlation analysis in Table 3 suggests that significant portion of variability in soybean yield and grain protein can be explained by changes in soil pH. For example, 10 bu reduction in yield and 1.5% reduction in grain protein was observed when soil pH > 7.5 (Figure 2).

Table 3. Pearson correlation (r) between soybean grain yield (bu/ac), grain protein (%), grain oil (%), plants/ac, branches/plant, nodes/plant, pods/plant, seeds/pod, seed weight (1000 seeds) in soybean fertility field experiments at Grant, NE (2019)

Parameter	Soil pH	Yield	Grain protein (%)	Grain oil (%)	Plants/ac	Nodes/plant	Branch/plant	Pods/plant	Seed/pod
Yield	-0.55*
Grain protein	-0.67*	0.35*
Grain oil	0.05	0.09	-0.06
plants/ac	0.24	-0.15	-0.1	-0.04
nodes/plant	-0.03	0.12	-0.01	-0.15	-0.3
branches/plant	-0.26	0.07	0.29	-0.28	-0.36*	0.39*
pods/plant	0.17	-0.06	-0.09	-0.1	-0.42*	0.56*	0.47*
seeds/pod	-0.22	0.28	0.14	-0.08	-0.21	-0.01	0.21	0.07
1000 seed weight (g)	-0.16	0.22	0.26	-0.02	-0.39*	-0.04	-0.09	0.08	0.03

* Correlation coefficient significant at 5% level. The sign of coefficient indicates the nature of relationship (either positive + or negative -) while the magnitude of coefficient (ranging from 0 to 1) represents the strength of the linear relationship.

Magnesium (Mg) uptake hindered

Magnesium levels in tissue samples were at the lower end of sufficiency range in the early reproductive (R1-R3) and were approaching the critical level during late reproductive stages (R3-R7; Figure 3). The decrease in Mg levels at late reproductive stages (R3-R4) coincided with the sharp increase of Ca concentration in the soybean tissue, suggesting a plausible inhibitory (i.e. unlikely preferential) uptake of Ca over Mg (Figure 3). As soil pH increased so did the concentration of both Ca and Mg in the soil solution (Figure 4). However, Ca concentration in soil solution increased disproportionally compared to Mg, which caused Ca:Mg saturation ratio (and K:Mg) to increase, especially at soil pH levels > 6.7 (Figure 5). We hypothesize that Mg uptake in soybean, especially in late reproductive stages, may be hindered due to soil replenishing with disproportional amount of Ca into the soil solution.

Figure 4. Change in concentration of macronutrients (N, P, K, S, Ca, Mg) in soybean leaf tissue during the reproductive growth stages (R1-R7) averaged over 12 fertility treatments; study conducted during 2019 growing season at Grant, NE.

Figure 5. Concentration of macronutrients (N, P, K, S, Ca, Mg) in soybean leaf tissue as affected by soil pH; study conducted during 2019 growing season at Grant, NE.

Molybdenum (Mo) levels low at late reproductive stages

Molybdenum levels in soybean tissue were at the lower end of sufficiency range during the seed filling period (Figure 6 and 7). As Mo is not included in standard soil and tissue testing for soybeans, we lack data on soil levels and early reproductive tissue analysis. Molybdenum pays an important role in soybean N-fixation and increasing its availability to the plant at late reproductive stages may help soybean maintain high rates of N-fixation. All other micronutrients were within the sufficiency range throughout the growing season.

Figure 6. Concentration of Ca and Mg in the soil solution and their Saturation ratio as affected by soil pH; study conducted during 2019 growing season at Grant, NE.

Figure 7. Change in concentration of micronutrients (Zn, Fe, Mn, Cu, B, Mo) in soybean leaf tissue during the reproductive growth stages (R1-R7) averaged over 12 fertility treatments; study conducted during 2019 growing season at Grant, NE.

Recommendations

The results of this study validate the current UNL fertilizer recommendations for soybean, especially when it comes to avoiding the routine application of N fertilizer. The need for N fertilizer cannot be predicted by soil tests; therefore, an in-season tissue sampling as well as inspecting root nodule number, spread and activity is advised prior to making such decision. Soybean response to N is, however, more likely to occur in high-yielding environments (> 80 bu/ac) and in certain conditions such as soil pH Wortmann et al. 2018). When it comes to applications of compost, manure, and other specialty products (micronutrients, foliar products, etc.), more research is needed to identify specific environmental conditions and soybean varieties where those applications might be warranted.

Soil pH was found to be the main factor influencing soybean yield and grain protein content. Significant decline in soybean yield and grain protein was observed at pH > 7.5., which is outside the optimal pH range (5.5-7.0) for soybean nutrient uptake and biological N-fixation. Recent advances in on-the-go field mapping for various soil properties and areal/satellite imagery can help farmers identify areas of the field with high soil pH and treat them as site-specific zones. Improving soybean management when soil is calcareous with pH > 7.5, especially if symptoms of lime induced chlorosis have been previously observed, may include: careful use of herbicides, planting iron-deficiency tolerant varieties at higher seeding rates in 30 inch rows, avoiding high soil nitrate levels, and applying chelated-iron in-furrow.

Tissue analysis in the past few years have consistently showed low/critical levels of magnesium both in our soybean research studies and on-farm samples. Soil analysis, however, always indicated adequate supply of Mg and range of Ca:Mg saturation ratio that is considered optimal for soybean producing. Significant amounts of Mg are applied in irrigation water. Furthermore, we observed no yield increase (or increase in levels of Mg in tissue) with the application of 140 and 270 lbs of Mg/ac; approximately 30 lbs of Mg/ac was applied through the irrigation water. Although it appears that Mg uptake is somewhat hindered by large amounts of Ca, soybeans had no yield response to applied Mg fertilizer in western NE.

References

Cafaro La Menza N, Monzon JP, Specht JE, Grassini P. 2017. Is soybean yield limited by nitrogen supply?. Field Crops Res. 213, 204-212.

Balboa G. R., D.R. Hodgins, I.A. Ciampitti. 2015. Late-Season Nitrogen Fertilizer Application in Soybean. Kansas Agricultural Experiment Station Research Reports: Vol. 1: Iss. 2. https://doi.org/10.4148/2378-5977.1016

Cafaro La Menza N., P. J. Specht, J. Rees, A. Timmerman, T. Whitney, and K. Glewen. 2018. Is Soybean Yield Limited by Nitrogen Supply? Nebraska Extension Crop Watch (January 8, 2018)

Grassini P., J. Rees N. Cafaro La Menza, and J. Specht. 2016. What does it take to produce 80+ bu/ac soybean? (EC 3000) UNL Extension Circular

Jimmy Frederick Booms 163 Bu. 2017. AgWeb Farm Journal (Feb 27, 2018)

Mortzinis S., G. Kaur, J. Orlowski, C. Shapiro, C. Lee, C. Wortmann, D. Holshouser, E. Nafziger, H. Kandel, J. Niekamp, J. Ross, J. Lofton, J. Vonk, K. Roozeboom, K. Thelen, L. Lindsey, M. Staton, S. Naeve, S. Casteel, W. Wiebold, and S. Conley. 2017. Soybean response to nitrogen application across the United States: a synthesis-analysis. Field Crops Res. 215:74-82. doi.org/10.1016/j.fcr.2017.09.035

Stepanovic S. N. Arsenijevic, Z. Ugljic. 2018a. Is Late Season N Fertilization Warranted for Irrigated Soybean in Western Nebraska? Nebraska Extension Crop Watch (November 2, 2018)

Stepanovic S., N. Arsenijevic, Z. Ugljic. 2018b. Seeding Practices and Nitrogen Management for Western Nebraska Soybean: What Matters and Why. Nebraska Extension Crop Watch (December 18, 2018)

Wesley, R. E. Lamond, V. L. Martin and S. R. Duncan. 2013. Effects of Late-Season Nitrogen Fertilizer on Irrigated Soybean Yield and Composition. J. Prod. Agric., Vol. 11 No. 3, p. 331-336

Wortmann, C.S., B.T. Krienke, R.B. Ferguson, B. Maharjan. 2018. Fertilizer recommendations for soybean. G859, revised, link: http://extensionpublications.unl.edu/assets/pdf/g859.pdf

Wortmann, C.S., C.A. Shapiro, R. Ferguson, and M. Mainz. 2012. Irrigated soybean has a small response to nitrogen applied during early reproductive growth. J. Crop Manage.

Balboa G. R., D.R. Hodgins, I.A. Ciampitti. 2015. Late-Season Nitrogen Fertilizer Application in Soybean. Kansas Agricultural Experiment Station Research Reports: Vol. 1: Iss. 2. https://doi.org/10.4148/2378-5977.1016

Figure 8. Concentration of micronutrients (Zn, Fe, Mn, Cu, B, Mo) in soybean leaf tissue as affected by soil pH; study conducted during 2019 growing season at Grant, NE.