Soybean Nodulation

Why are fields a Pale Colour?

Soybeans naturally go through a period when leaves are light green or even pale yellow. This is the period just before the nodules start to supply adequate nitrogen to the leaves. Once the nodules provide nitrogen the leaves turn a dark green colour. 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. In first time soybean fields nodulation must be watched closely, inadequate nodulation does happen on occasion.

Check Roots for Nodulation

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 3-4 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. When checking roots dig them out carefully to avoid sloughed off the nodules. Use a shovel to check at least 10 sites in the field. Adequate nodulation requires about 7 - 14 nodules per plant. If less than 5 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. After about 50 - 60 days from the time of formation the nodules start to degenerate. Nodules that are fixing nitrogen are pink or red inside. White, green or brown nodules indicate that little or no fixation is occurring.

Why does Nodulation Sometimes Fail?

Nodules form when a single bacteria infects a root hair. Infection can only occur if both the bacteria and root hairs are present. Root hairs are only formed on new root growth and in first time fields bacteria is only present were the seed was placed. That's why on first time fields nodules form on the tap root. That is where the initial inoculant was placed at seeding time. Soils that have previously grown soybeans will have bacteria spread throughout the soil so infection will occur across the whole root system. If the roots on first time fields do not nodulate within a few weeks after emergence the roots will have grown past where the inoculant was placed. There are two main reasons why a field may not have nodulated:

1. There is a problem with the bacteria. A lack of viable bacteria may have been placed on the seed. This could be due to improper storage of the product or the inoculant not sticking to the seed. Or in fields that have previously grown soybeans the bacteria in the soil have not survived well because of low pH or sandy conditions.
2. There is a problem with infection of the plant. Excess mineral N in the soil at planting time will inhibit infection. If there is excess N 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 N level in the field may have delayed root infection.

Corrective Action

If plants remain pale green and no nodules are present nitrogen should be applied. Although it's impossible to get the same amount of nitrogen to the plant as the nodules would have, a profitable response has been found up to 50 kg/ha. Broadcasting urea or calcium ammonium nitrate at early flowering, when the foliage is dry, is the best timing. Higher rates of N can be applied but are not usually profitable.

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