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 bacteria.

Hans Kandel
Ext. Agronomist
hans.kandel@ndsu.edu

R. Jay Goos
Professor - Soil Science
rj.goos@ndsu.edu

COVER CROPS FOR "PREVENTED PLANTING" ACRES

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:

1. Leaving crop residue after mechanical cultivation or chemical fallow.
2. Solid seeding of a crop that is seeded early enough to provide an adequate cover.
3. Rye and winter wheat seeded during the normal fall planting date for harvest the following year.
4. Approved wildlife food plots which may include corn windbreaks.
5. 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.
6. Winter wheat or fall rye strips up to 18 inches in width to be used the following spring for sugar beet seeding.
7. Spreading of straw-type manure and/or crop residue can be substituted in place of cover crop or crop residue.
8. 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.
9. 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 1^st. Given the above guidelines and restrictions, the following are some suggestions on potential cover crops and other management practices.

1. 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.
2. 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.
3. 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 1^st, 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 1^st if producing fodder is one of your objectives. The growth of these grasses is retarded when temperatures dip below 60 degrees.
4. 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.
5. 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.
6. 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.
7. 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.

Joel Ransom
Extension Agronomist for Cereal Crops
joel.ransom@ndsu.edu