If your soybean fields look pale green, it may be suffering a nitrogen deficiency.

Horst Bohner, a soybean specialist from the Ontario Ministry of Agriculture Food & Rural Affairs, says a number of first-time soybean fields that were inoculated this year did not nodulate properly. First-time soybean fields should be inspected carefully, since it's not uncommon to get poor nodulation on first-time fields.

For proper nodulation to occur, a relatively high number of rhizobia must be present in the soil, Bohner says. Soybean plants secrete chemical signals (flavanoids) into the soil from the roots. These signals are picked up by the bacteria, which in return send a chemical signal back to the root. The signals sent back are lipochitooligosaccharides (nod factors), which elicit nodulation in the plant.

"Within 10 to 14 days of colonization, a nodule will become visible," Bohner says. "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."

For the enzyme which fixes nitrogen (nitrogenase) to function, the environment within the nodule must be almost oxygen-free, Bohner says. The plant forms leghaemoglobin inside the cell to absorb the oxygen. Leghemoglobin is red, so the inside of a functioning node will be red or pink in color.

"The nodule begins growing rapidly and will start to fix nitrogen at about V2 or V3," Bohner says. "The nodule will reach full size in about 4 weeks and fix nitrogen for about 6 to 7 weeks. At that time it will begin to senesce and become dark brown or black in color. The plant reaches maximum nitrogen fixation during the R5 crop stage."

Factors that influence nodulation, nodual growth and nitrogen fixation include too much or too little moisture, soil temperature, soil pH, diseases, organic matter and soil nitrate availability, as well as the rhizobial quality and bacterial strain in the soil.

"It is well recognized that nitrogen fixation in soybeans is highly sensitive to soil drying," Bohner says. "Nitrogen fixation is more sensitive to water-deficit stress than all other physiological processes in the plant. This may partially explain why soybeans do poorly in dry years compared to other crops."

Bohner says the nodule is essentially a closed system and dependent almost entirely on phloem flow for water supply, photosynthate and energy. He says the rhizobium uses this energy to extract the nitrogen from the air to make the nitrogen compounds that the plant will use to make protein.

Soybeans generally do not respond to pre-plant nitrogen fertilization, but Bohner says there are a few exceptions, including poorly drained soils, very low organic matter, very low residual nitrogen, acidic soils or very dry conditions.

"Field experiments comparing yield between irrigated and rain-fed treatments found that dependence on nitrogen fixation resulted in a significant yield decrease in the rain-fed treatments compared to irrigated treatments," Bohner says. "This is only the case when conditions are very dry.

"There would be a benefit to having soybeans with the dual capacity of using significant levels of nitrate from the soil and actively fixing nitrogen from the air at the same time. In this situation, roots could uptake nitrates during the first part of the cropping season -- before the nodules were established -- and then switch to nitrogen fixation later in the season, once the nitrates have been depleted."

Bohner says this cannot occur, however, because 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.

He says the reduction of atmospheric nitrogen 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 nitrogen fixation in the presence of soil nitrates at greater than very small starter fertilizer rates is largely why nitrogen fertilization does not pay in soybeans," Bohner says. "Applying nitrogen fertilizer simply reduces the amount of nitrogen fixed from the air."

Bohner says soybeans naturally go through a period when leaves are light green or even pale yellow just before the nodules start to supply adequate nitrogen to the leaves. This is an import phase in the development of a healthy crop. Only in the absence of nitrogen (a pale-looking crop) will the roots send the signal to the rhizobia to initiate nodulation.

He adds that once the nodules have established and start providing nitrogen, the leaves turn a dark green color. If proper nodulation, sufficient nutrients and moisture are present, soybeans will remain yellow for only a few weeks.

"Nodulation problems are very rare in fields that have previously grown a dark green crop of soybeans," Bohner says. "In first-time soybean fields, nodulation must be watched closely, because inadequate nodulation does happen on occasion."

Bohner says the symbiotic relationship between soybeans and the bacteria rhizobium japonicum can be seen shortly after emergence. Small nodules can be observed on the tap root 3 to 4 weeks after planting, but nitrogen fixation does not occur until V2 or V3.

"In first-time fields, it's important to check nodules early to allow for nitrogen application if a failure does occur," Bohner says. "When checking roots, dig them out carefully to avoid sloughing off the nodules. Use a shovel to check at least 10 sites in the field.

"Adequate nodulation requires about seven to 14 nodules per plant. If less than five nodules are present, wait for about a week and take another assessment. The number of nodules formed on the roots along with the amount of nitrogen fixed continues to increase until R5. Nodules that are fixing nitrogen are pink or red inside. White, green or brown nodules indicate that little or no fixation is occurring.

Bohner offers several reasons why a first-time soybean field may not have nodulated.

  1.  A lack of viable bacteria may have been placed on the seed. This could be due to improper storage of the product.
  2.  The inoculant did not stick to the seed or was added unevenly.
  3.  There is a problem with infection of the plant. Excess nitrate in the soil at planting time will inhibit infection. If there is excess nitrogen in the soil, the plant will first use that nitrogen before allowing proper nodules to form. In fields with a history of soybeans, the nodules can form later, but in first-time fields the opportunity may be missed. The roots may have grown past where the inoculant was placed or the bacteria may have died because of dry conditions. This may be the reason why old forage fields sometimes have nodulation failures when first seeded to beans. The initial high nitrogen level in the field may have delayed root infection.

For fields with a history of soybeans, once introduced the bacteria will survive for many years in most soils, but poor nodulation can occur even in an established field if:

  1.  The soil has a low pH or is sandy.
  2. There is excess nitrate in the soil.
  3.  The soil is very dry.

If plants remain pale green and no nodules are present, Bohner says nitrogen should be applied. Although it's impossible to get the same amount of nitrogen to the plant as the nodules would supply, a profitable response has been found up to 44 pounds per acre.

"Broadcasting urea or calcium ammonium nitrate at early flowering, when the foliage is dry, is the best timing," Bohner says. "Higher rates of nitrogen can be applied but are not usually profitable."