Soybean acreage in Louisiana took a big jump in 2008, reaching 1 million acres for the first time since 2004. In 2009, soybean acreage topped a million acres. Growers who will plant soybeans in fields where the crop has not been grown for several years could see an economic benefit from soybean inoculation.

Soybeans are big users of N, removing about 4 lbs. N/bu. Soybeans that are poorly nodulated will have to take up most of the N they need from the soil. Since N fertilizer is generally not applied to soybeans, a crop that is poorly nodulated will quickly use up the available N in the soil and become chlorotic from N deficiency.

Soybean inoculum contains Bradyrhizobium japonicum bacteria. The Bradyrhizobium bacteria forms nodules on soybean roots, and these nodules fix N from the atmosphere and supply it to the plants. For N fixation to occur, the N-fixing bacteria need to be readily available in the soil or must be applied to the seed or soil. When the seed germinates, the bacteria invade the root hairs of the seedling and begin to multiply, forming nodules on soybean roots.

Nodules, which house the bacteria, can be seen shortly after emergence, but active N fixation does not begin until about the V2 stage. After this, the number of nodules formed and the amount of N fixed increase with time until about R5.5 (midway between R5 and R6), when they decrease sharply.

There is a mutual benefit in the relationship between the Bradyrhizobium bacteria and the soybean plant. The plant, in turn, provides the bacteria's carbohydrate supply. A relationship such as this, where both bacteria and plant profit from the other, is a symbiotic relationship.

When does it pay to inoculate soybeans?

IF SOYBEANS HAVE been grown on the field in the past three years or so, there may be enough Bradyrhizobium bacteria in the soil to nodulate the soybeans adequately. In that case, inoculum may not benefit the crop.

But if there is not enough Bradyrhizobium in the soil, the inoculum may increase yields by 2 bu./acre or more on fields that have had soybeans in the recent past. On fields where soybeans have never been grown, inoculum has been shown to increase yields by 10 bu./acre or more.

Soybean inoculation should be considered in the following circumstances:

  • When the field has not been planted to soybeans for the past three to four years or more.

  • When the soil pH is less than 5.5 or greater than 8.5.

  • When soil organic matter levels are less than 1%.

  • When there has been severe drought or flood conditions, as in a rice rotation.

There may be several causes of poor nodulation and inoculation failure, including:

  • Poor-quality inoculum.

  • Poor storage and handling.

  • Poor seed coverage with inoculum.

Most fungicide seed treatments should not harm the inoculum if applied according to directions, but be sure to check the label of the specific fungicide seed treatment to be used.