Factors Influencing Soybean Nodulation
WHAT YOU'LL LEARN
- Many factors, both environmental and man-made can affect the level of rhizobial nodulation on soybeans.
- Nodulation is a natural process that is initiated by the
plant, through a complex signaling relationship with
- Because it is a natural process, the signaling events
between the soybean plant and the rhizobia can
become disrupted by several factors.
Nodulation generally begins about 3 to 4 weeks after
emergence once the plant senses a need for nitrogen.
The following factors can have a dramatic effect on the
intensity, timing, and efficiency of nodule development
and nitrogen fixation. Taken alone any one of the
following factors can affect nodulation; however, it is
common to find more than one factor influencing the
extent of nodule formation on soybeans.
SOIL CHEMISTRY AND NUTRIENTS
- As soil pH drops below 6, the conditions can become
too acidic for rhizobia to effectively create nod factor
and form nodules.1 Rhizobia survival can also be
affected. Important micro nutrients, including
molybdenum, that are cofactors for nitrogen fixation
may become unavailable under low pH conditions.
- Salt content in soil could be naturally occurring or due
to irrigation. Introduction of salt can adversely affect
nodulation even in concentrations low enough to
allow for rhizobial survival and root colonization.
- As carryover nitrogen levels in the soil rise above
40 lbs/acre, nodule formation is negatively
affected.2 If plants have a source of nitrogen
readily available, there is no incentive to signal to
rhizobia to form nodules and thus the rhizobia do
not create nod factor. Once this carryover nitrogen
is used up, the plant then may signal to the
rhizobia, but the whole nodulation process then
becomes delayed or the signaling window can be
missed, resulting in little to no nodulation on the
CULTURAL AND PHYSICAL
- Fields that have never been planted to soybeans have
little to no rhizobia present, making inoculation/
nodulation difficult. In general, the more times a field
has been planted to soybeans with successful
inoculation/nodulation, the higher the level of
indigenous rhizobia. However, naturalized rhizobia
may become less infective and/or effective over time
and thus a supply of elite rhizobia, selected and
fermented for these critical attributes, are needed to
ensure effective nodulation.
- Natural differences in soybean products can also
affect the intensity of nodulation because soybean
plants control the symbiotic nitrogen fixation process
and some soybean products perform this task more
efficiently than others. In the absence of
supplemental inoculation, there can be vast
differences in presence of nodules between two
given soybean products. These differences can be
lessened by introducing elite strains of rhizobia into
the environment to counter those variances.
- Soil texture/organic matter can affect rhizobia
populations. In general, the coarser the soil, the less
rhizobia can survive year to year, negatively affecting
rhizobia populations and inoculation/nodulation.
Sandy soils can also get extremely dry and hot,
which cause the rhizobia populations to desiccate
and decrease rapidly.
- No-till conditions can create colder, wetter
conditions early in the season, which can increase
the stress levels of the plant, negatively affecting the
signaling process between the plant and the
rhizobia. These same conditions also can decrease
the activity of the rhizobia, thus delaying nodulation.
TEMPERATURE AND PRECIPITATION
- The northern range of soybean growing areas
experience more extreme seasonal temperature
fluctuations from colder winters, to hot and dry
summers, making it less likely that rhizobia can
survive from year to year. The southern range of
soybean-growing areas also can experience
extremely high temperatures and dry conditions.
- In addition to creating plant stress, soil moisture can
affect rhizobia survival. Hot, dry conditions can cause rhizobia desiccation and death, while flooding can
create anaerobic conditions which cause rhizobial
death due to low oxygen conditions.
- In addition to creating plant stress, temperature
extremes can have an effect on the efficacy of soil
rhizobia. In temperatures below 50° F (10° C),
rhizobia become mostly inactive and the nodulation
signaling process can be interrupted.3 In high
temperatures above 90° F (32° C), especially when
combined with dry conditions, rhizobial desiccation
and death can occur.4
MONSANTO BIOAG INOCULANT
PRODUCTS CAN HELP
- Often times, indigenous or native rhizobia will
compete with the elite strains in an inoculant to
occupy the infection sites on the soybean root. These
native rhizobia may then infect and form nodules, but
fix little to no nitrogen, making them parasitic to the
- Any practice that stresses the plant (disease,
herbicide injury, nutrient deficiency/poor fertility,
compaction, cold early-season temperatures) reduce
the ability of the plant to signal the rhizobia regarding
its need for nitrogen, thus delaying nodulation.
- Compounds applied to the seed and the soil such as
incompatible pesticides, fertilizers, and nutrients can
cause rhizobial death. Care should be used with
compounds such as talc (when applied during
treating causes rapid rhizobial desiccation) or
molybdenum (high toxicity) which can be
incompatible with rhizobia. Always refer to published
compatibility charts before using any unknown
materials with rhizobia inoculants.
Monsanto BioAg’s line of single-, dual-, and triple-action
inoculants help enhance the nodulation process. These
products make the crucial pieces of the nodulation
process available even in cases of environmental stress when they cannot be
in products such as
Cell-Tech® can help
mitigate many of the
stress factors they
face. The nodulation
factors delivered in
TagTeam® LCO and
(such as low pH
tillage) to support
nodulation. In cases of flooding and soil toxicity (e.g. salt
& pesticide carryover) the supply of healthy rhizobia in
these products or our single action inoculant Cell-Tech®
support quick and effective nodulation.
In conclusion, by using an inoculant from Monsanto
BioAg you can maximize opportunities for successful
initiation of nitrogen fixing nodules.
For more information please visit: www.monsantobioag.com
1 Pedersen, P. 2015. When do we need to inoculate our soybean seeds? Integrated
Crop Management. Iowa State University. Paper 1559.
2 Staton, M. 2014. Identifying and responding to soybean inoculation failures.
Michigan State University.
3 Bohner, H. 2014. Cold temperatures hamper soybean nodulation. Crop Talk.
4 Yadav, A.S. and Nehra, K. 2013. Selection/isolation of high temperature tolerant
strains of Rhizobium for management of high temperature stress on Rhizobium—
legume symbiosis. International Journal of Microbial Resource Technology. Vol.
5 Klein, R. 2013. Check soybean nodulation to determine inoculant efficiency. UNL
CropWatch. University of Nebraska, Lincoln.
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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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