Why Inoculate or Double Inoculate
WHAT YOU'LL LEARN
- Effective nodulation is essential for nitrogen fixation, particularly on soils where soybeans have not been planted recently
- Seed-applied inoculant can be profitable on fields with a history of soybean planting
- Double inoculation can help quickly establish high populations of rhizobia bacteria to ensure optimal nodulation and soybean performance
BIOLOGICAL NITROGEN FIXATION
Effective nodulation of soybean and other leguminous crops is the key factor in the complex process of nitrogen (N) fixation. The success of N fixation is highly dependant on the presence of high populations of the specific bacteria for each crop in the soil near crop roots or applied to seed.
Bradyrhizobia japonicumis the rhizobia bacteria associated with soybean. About 70% of the total N requirement for soybean is generated by the symbiotic process of N fixation. A large percentage of this nitrogen is fixed during soybean reproductive stages, which is the period of greatest nitrogen demand. The bacteria receive
a signal (flavonoids) from soybean roots and return a signal (lipo-chitooligosaccharide - LCO) back to a plant to initiate N fixation. The bacteria enter soybean roots causing a nodule to develop. Around the V2 to V3
soybean growth stage, viable nodulation begin to fix N. A plant can have 8 to 10 healthy nodules at the V3 to V4
growth stage, with the maximum number of nodules typically occurring by the R5.5 growth stage.1,2 Viable
nodules have a pink to red interior color (Figure 2). Nodules that are green, brown, or mushy are not fixing N. New nodules are formed into the pod-filling growth stage.
Figure 2. Root nodules and a dissected, pink nodule (insert).
Inoculants can have a positive impact in many growing situations including the following:
- Field has no previous history of soybean production
- Soil pH is below 6.0. Greater response from inoculants
can be expected in fields with a pH below 6.0.
- Fields that have been flooded for several days can
have anaerobic conditions that inhibit rhizobia.
- Compaction and cool soil temperatures can reduce nodulation.
- Sandy soils with low organic matter (less than 1%) generally have very low populations of rhizobia bacteria and need to be inoculated every year.2
Several universities throughout the Midwest have found a long-term average yield advantage for a soybean seed-
applied inoculant on fields with a history of soybean planting.3,4,5 The 11 year average yield advantage in Ohio State University inoculant testing was 1.94 bu/acre when
using inoculants versus no inoculation, over an 11 year period in fields with a corn/soybean rotation, with excellent drainage, fertility, and pH.3 Inoculant product improvements, custom application, and better
compatibility with multiple seed-
applied products have contributed to performance advantages.3
Figure 1. Well nodulated plant (left) versus poorly nodulated plant (right). Photo courtesy of Stu Duncan, Kansas State University.
Applying several microbial products to soybean seed can have distinct advantages for soybean performance.
Adequate availability of phosphate is essential for N fixation. Penicillium bilaii is a soil fungus is a phosphate solubilizer that can be used in conjunction with Bradyrhizobia japonicum. Used together, these products can provide increased phosphate availablility, more fixed nitrogen, better root and shoot growth, and yield potential. Some products include nod factor compounds
(LCO). These compounds are a bacterial signal to the plant to initiate the
nodulation process to promote early plant vigor, under stress or
normal conditions, which helps support
Land that will be planted to soybean for the first time, or that has been in the Conservation Reserve Program
(CRP), pasture, or not been planted to soybean in several years may require a special inoculation plan. Double
inoculation combines seed-applied inoculants with a sequential in-furrow application of a granular or liquid
inoculant to quickly establish high populations of rhizobia bacteria to ensure optimal nodulation and soybean
performance. Recommended rates for in-furrow products are based on soybean row width (Table 1).
The following conditions are most likely to cause poor nodulation and reduce N fixation:6
- Planting soybeans in a field for the first time.
- Soil pH below 5.5 and above 8.0.
- Residual N levels greater than 40 lbs/acre.
- Anaerobic conditions in flooded soils that are saturated
for at least three days can cause nodules to rot, turn brown, and die.
- Fields with low soil rhizobia bacteria populations and/
or fields with a high residual of soil N from a previous forage legume, such as alfalfa, clover, or other crop, or
due to manure application.
- Hail damage, root diseases, or iron deficiency chlorosis
symptoms early in the season.
- Soil compaction can limit root growth and root hair development reducing rhizobia colonization of roots
and nodule development.
- Improper application of fungicide and inoculant to seed. Product suppliers publish compatibility guides for inoculant products for use with other products.
- Poor quality inoculant due to improper storage.
Symptoms of inadequate nodulation include yellowing and stunting of soybean plants. These same symptoms
can be caused by soybean cyst nematode, other nutrient deficiencies, or environmental conditions such as drought, compaction or excessive rainfall. If poor nodulation is
suspected, the best way to identify the problem is to dig up plants in the affected areas, wash the roots, and count the root nodules. If fewer than 7 nodules are found per plant, the plants will probably be nitrogen deficient. Count only nodules that are pink or red when split open.6
Fields new to soybean should always be double inoculated. Follow the inoculant expiration date and proper storage conditions to preserve the inoculant’s viability. Avoid exposing inoculants to direct sunlight or
Inoculants can have a positive impact in many growing situations such as fields with no previous history of
soybean, soil pH below 6.0, compaction and cool soil temperatures, and sandy soils with low organic matter. Applying several microbial products to soybean seed can have distinct advantages for soybean performance. Double inoculation combines seed-applied inoculants with
a sequential in-furrow application of a granular or liquid inoculant to quickly establish high populations of rhizobia.
Sources: 1 Pedersen, P. 2004. When do we need to inoculate our soybean seeds? Integrated Crop Management. Iowa State University. 2 Pedersen, P. 2003. Soybean
seed inoculation. Integrated Crop Management. Iowa State University. 3 Beuerlein, J. 2009. Soybean inoculation; Its science, use and performance. Ohio State University. http://agcrops.osu.edu. 4Thelen, K., Schultz, T. 2011. Soybean seed applied inoculation. Soybean Management and Research Technology. Michigan State University.5 Conley, S.P., and Christmas, E.P. 2005. Utilizing inoculants in a cornsoybean
rotation SPS-100-W. Purdue University.6 Staton, M. 2014. Identifying and responding to soybean inoculation failures. Michigan State University. http://msue.anr.msu.edu. Web sources verified 08/11/2015. 150727134522
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Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible.
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