Seed & Chemical
Cold temperatures hamper soybean nodulation
By Horst Bohner OMAF
Sept. 4, 2014, Ontario – The 2014 growing season was the worst year in recent memory for poor root nodulation and nitrogen (N) fixation in soybeans. Cool, wet conditions cause numerous problems, including slow growth, low pod set, increased diseases, and lower yields. One significant problem that may be overlooked is that cool soil temperatures will delay or even inhibit nitrogen fixation.
Soybeans are a subtropical species. For optimal symbiotic activity the soil temperature should be between 25 C and 30 C. There were numerous first time fields where inoculant was applied, but nodulation did not occur. In other cases, nodulation did occur but not until early-August. Problems with poor nodulation happened across a wide geography and occurred with several different inoculant products, so it was not a product failure. In a few cases, even second time soybean fields failed to nodulate properly. Biological nitrogen fixation is essential for both first time fields and fields with a history of soybeans, because it converts gaseous nitrogen in the air (N2) to a form of nitrogen the plant can use.
How does nodulation occur?
Soybean plants secrete chemical signals (flavanoids) into the soil from the roots when the plant needs nitrogen. These signals are picked up by the rhizobia, which in return send a chemical signal back to the root. The signals sent back are called Nod factors and elicit nodulation in the plant. Within 10 to 14 days of colonization, a nodule will become visible. The return signal prepares the root for infection by the bacterium. Infection can only occur where root hairs are present. The nod factor causes root hairs to curl and pick up rhizobia and allows them to invade the root. As the bacterial cells divide, they form a small tumor like structure called a nodule.
Why was nodulation poor this year?
There are a number of factors that influence nodulation, nodual growth, and nitrogen fixation. These factors include too much or too little moisture, soil nitrate levels, soil pH, diseases, organic matter, soil temperature, and rhizobial quality.
This year, cool temperatures are to blame for poor nodulation. In some cases, soil conditions also turned dry immediately after seeding causing the bacteria to dry out and die before they could invade the roots.
Experiments conducted at McGill University by Zhang, Lynch, and Smith1 showed that between 17 C and 25 C, the onset of N2 fixation was delayed by 2.5 days for each degree decrease in temperature. Below 17 C, each degree delayed the onset of N2 fixation by 7.5 days. A root zone temperature of approximately 15 to 17 C seems to be the critical temperature for soybean nodulation and N fixation. By 49 days after inoculation, plants at temperatures between 17 C and 25 C were fixing some nitrogen, but plants at 15 C were not fixing any nitrogen. They also observed that a decrease of only 2 C, from 21 C to 19 C, made an important difference in the time to onset of N2 fixation, total N accumulation within the plant and overall growth.
Matthews and Hayes2 showed that nodulation can cease when temperatures fall to 10 C. Lynch and Smith3 showed that a root zone temperature of 15 C restricted both infection and nodule development and delayed the onset of N2 fixation by four to six weeks. Plants with a root zone temperature of 15 C had only fixed nine per cent of the nitrogen fixed by plants at 25 C six weeks after inoculation.
This helps us understand why in some cases soybeans did not nodulate until late-July or early-August this year. No-till fields, especially those with large amounts of crop residue, also suffered more from a lack of nodulation because these soils are generally cooler by a few degrees C.
Soil nitrate and N fixation
High nitrate levels also caused some problems. Nodule formation is inhibited by the presence of high nitrate levels in the soil. If the soybean plant picks up too much nitrogen early in the season, it will delay or prevent nodulation. The reduction of atmospheric N2 to ammonia is energetically expensive, and costs more photosynthate than simply taking up nitrate, so the plant will naturally consume nitrates before attempting to nodulate. This fundamental inability to develop and sustain N2 fixation in the presence of soil nitrates at greater than very small “starter” fertilizer rates is one of the reasons why nitrogen fertilization does not pay in soybeans. Applying nitrogen fertilizer simply reduces the amount of N2 fixed from the air.
What about next year?
Temperatures in Ontario in June and July are generally sufficient for proper nodulation, so under average conditions this problem will not be significant. In first time soybean fields, use two inoculant products such as a peat and a liquid at the high rate with good coverage. Some first time fields that used only one product or a pre-inoculant had complete nodulation failures in 2014. Using two products will help to increase the number of live bacteria available for nodulation. It is also essential to consider the bacterial viability with pesticide seed treatments. The only remedy to a nodulation failure is to apply N fertilizer at first flower or early pod set.
1) Zhang F, Lynch D. H, and Smith D.L. (1995) Impact of low root temperatures in soybean on nodulation and nitrogen fixation. Env. And Exp. Botany, Vol 35, no3 pp. 279-285.
2) Matthews D.J. and Hayes P. (1982) Effect of root zone temperature on early growth, nodulation and nitrogen fixation in soya beans. F. Agric. Sci 98, 371-376.
3) Lynch D.H. and Smith D. L. (1993) Soybean nodulation and N2 fixation as affected by period of exposure to a low root zone temperature. Physiol. Plant. 88, 212-220.