ISSUE 7 June 25, 2009
NITROGEN AND NODULATION IN SOYBEAN
Soybeans are legumes that form a beneficial relationship with specialized
bacteria to fix atmospheric nitrogen (N) making it available to the soybean
plant. In North Dakota soybeans do not normally require N fertilizer because of
this relationship with beneficial bacteria. The nitrogen-fixing bacteria
colonize host roots and form nodules (small swellings) on the root system (see
picture). These nodules are pink or red inside when they function properly.
Nodulation is decreased when Nitrogen is available in the soil. Limited
nodulation may also occur in fields where there is no previous soybean cropping
history (limited numbers of beneficial bacteria). Other factors that may reduce
nodulation are wet conditions early in the season or dry soil conditions. The
presence of root rots may also inhibit the ability of the bacteria to form
nodules. It is recommended to check roots for nodules. Carefully dig up plants
to avoid sloughing off the nodules and wash them in a bucket of water. Check a
number of locations in each field.
During the past winter a hands-on demonstration about the effect of nitrogen
application and nodulation was given at two locations (Grand Forks and Fargo).
Sixty-seven groups of 3 to 4 producers were asked to evaluate soybean plants at
four growth stages (unifoliolate, V1,V2, and V3) for their nodule number per
plant and compare plants which had received nitrogen to plants which received no
nitrogen. Dr. Jay Goos and his team prepared the demonstration and grew the
plants in a greenhouse.
Each pot was filled with one kg ‘Renshaw’ sandy loam soil and one kg sand.
The soil pH was neutral and had no history of soybeans. Soil was inoculated at a
level of roughly 1,000 Bradyrhizobium japonicum per gram. Nutrients (P, K, S and
Zn) were applied before inoculation and mixed completely with the soil mass. Two
Nitrogen rates were 0 and 200 mg N/pot. By surface area, the equivalent N rate
was about 100 lb N per acre.
The average of the observations made by producers is summarized in graph
below. Pots which did not receive Nitrogen had higher numbers of nodules at each
of the four development stages and the nodules where bigger. This demonstration
showed clearly that the Nitrogen available to the plant resulted in lower
numbers of nodules and smaller nodules. Most fields will have some residual
nitrogen at the beginning of the growing season and soybean plants will use this
Nitrogen. In North Dakota, we do not recommend the application of Nitrogen
fertilizer if the plant is able to normally nodulate and provide the needed
Nitrogen to the plant from the symbiotic relationship with the beneficial
R. Jay Goos
Professor - Soil Science
COVER CROPS FOR "PREVENTED
As a result of a multitude of weather related challenges beginning last fall
and continuing this spring, many acres have not been planted this cropping
season. We are now past the late planting period for practically all crops and
decisions are needed on how to manage fields that were not planted. Many will
want to plant a crop in order to deplete water from the soil so as to improve
the chances of planting a crop next spring. Additionally, in order to maintain
eligibility for Direct and Counter-cyclical Payments on land that was not
planted this spring, it must be protected from wind and water erosion and must
be maintained to control the propagation of weeds, including noxious weeds
throughout the crop year. Approved practices include:
- Leaving crop residue after mechanical cultivation or chemical fallow.
- Solid seeding of a crop that is seeded early enough to provide an adequate
- Rye and winter wheat seeded during the normal fall planting date for
harvest the following year.
- Approved wildlife food plots which may include corn windbreaks.
- Flax strips at the normal seeding rate of not more than 16 feet apart may
contain a minimum of one row of flax, with two or more rows encouraged. Two or
more rows are required on flax strips up to 25 feet apart.
- Winter wheat or fall rye strips up to 18 inches in width to be used the
following spring for sugar beet seeding.
- Spreading of straw-type manure and/or crop residue can be substituted in
place of cover crop or crop residue.
- Tillage is allowed to control weeds, wind and water erosion in a manner
consistent with erosion control measures normally carried out on other crop
land in the area.
- Weeds, including weeds on planted acreage, must be control by either
mechanical or chemical treatment.
Additionally, if a prevented planting payment was received, cover crops
cannot be grazed or hayed until after November 1st. Given the above
guidelines and restrictions, the following are some suggestions on potential
cover crops and other management practices.
- Small grains – Small grains can be planted as soon as conditions allow,
usually establish quickly and are relatively heavy water users. Because small
grains will likely head and produce seed, even when planted in June and July,
they should be handled as a green manure crop and should be incorporated into
the soil before viable seeds are formed. The advantages of using a small grain
crop as a cover are: seed is usually available on-farm, seed costs are
moderated when using retained seed, they compete well with weeds, and numerous
herbicide options are available for within crop weed control, if needed.
- Sugar beets – In the unlikely event that you can find a cheap source of
non-GMO seed, sugar beets would be an excellent crop for areas of excess
moisture and developing salt problems. Sugar beets’ deep roots are capable of
capturing nitrogen that might have moved to deeper soil profiles and are
fairly tolerant to moderate levels of salt.
- Sudangrass, sorghum-sudangrass hybrids, and millets – These warm season
grasses are capable of excellent growth and form a good cover when planted
when temperatures are warm. If planted in late June or early July, these crops
would have to be grown as a green manure crop; however, since they cannot be
hayed or grazed until after November 1st, and would be quite tall
and not particularly palatable by that date. When planted in late July or
early August, they may be in reasonable form for grazing or haying after
November 1st if producing fodder is one of your objectives. The
growth of these grasses is retarded when temperatures dip below 60 degrees.
- Warm season legumes – There are a few species of legumes that establish
and grow well in mid-summer. These include the sweet clovers, cowpea, soybean,
and Sunn Hemp. The advantage of growing a legume cover is that it is able to
fix nitrogen prior to being incorporated into the soil. The downside is the
cost of the seed. Furthermore, some of the small seeded legumes have hard seed
coats and may become a nuisance in subsequent crops. Bin run soybeans with GMO
herbicide tolerance cannot legally be grown, even if there is no plan to
harvest the seed.
- Commercial mixtures of cool season crops – A few seed companies market
mixtures of crops specifically for use as cover crops following a normal fall
harvest. Since these mixtures contain cool season crops they tend to do best
when planted in August when temperatures begin to cool. Therefore, they would
be a poor option for developing a cover in early or mid-summer if that is your
objective. Seed cost for some of these mixtures can be pricey.
- Flax and winter wheat – If you plan to plant winter wheat, planting flax
in early August to establish a residue crop in which to plant winter wheat is
probably the best available option. Recommendations on how to use flax as a
residue crop were contained in an earlier issue of this newsletter.
- Chemically fallowing previous crop residues in no-till regions – For
regions of the state where no-till is practiced and excessive moisture is
usually not a serious issue, retaining the existing residue and controlling
weeds with herbicides can be a practical option. If residues are of sufficient
height, they could also be used for establishing winter wheat this fall.
Undoubtedly there are many other viable cover crop options that might be
considered. When choosing a crop, consider the cost of seed, the objective of
establishing the cover, how the potential crop might respond to mid-summer or
fall planting, and how you plan to use the crop residue once it is established.
Extension Agronomist for Cereal Crops