Bradyrhizobium japonicum is the bacterium which forms a symbiotic relationship with soybean roots to fix nitrogen. These bacteria induce formation of nodules on soybean roots but will not successfully nodulate any other legume. The nodule is the site where atmospheric N2 is transformed into nitrogenous compounds that become available for plant growth and development. This biologically fixed nitrogen, along with other sources of N, provide the nitrogen needs of the crop if the soil environment is conducive to bacterial multiplication and soybean infection. Without nitrogen provided by these bacteria, soybean production costs would escalate due to the need to add N fertilizer to achieve acceptable yields.
Iowa has a bountiful population of Bradyrhizobium japonicum in most soils, if soybean have been grown in recent years on the field. Because most cultivated fields include a rotation with soybean, the need to inoculate with more bacteria rarely exists. In general, if soybean has been grown on a field within the last five years, the yield response to inoculation has not been significant. However, conditions such as floods which keep the soil under water for more than a week may reduce the bacterial population to a level that would suggest that inoculating soybean would be an inexpensive form of insurance. The cost for peat-based inoculant is less than $1.00 per acre. Other forms, such as granular inoculant, may cost up to $5.00 per acre. The granular form will supply more bacteria per acre compared to the peat-based forms that stick to the seed, but peat-based inoculant is more popular and will most likely be locally available.
Potholes that were flooded out in 1993 may have a reduced Bradyrhizobium japonicum bacterial population. If the pothole area is relatively small and is tilled with the same equipment as adjacent areas that have recently produced soybean, inoculation is probably not necessary. The bacteria move on farm equipment, in soil particles, and in dust particles. Therefore, the bacteria will spread rapidly and may eliminate the need to inoculate. Potholes may be large enough to justify seed inoculation to insure sufficient bacteria in the soil.
River floodplains may present a different situation than potholes. If the floodplain was flooded for more than a week, the bacterial population may be reduced as with potholes. However, floodplains that were covered with sand deposits probably should be planted with inoculated soybean. Sand usually does not contain enough organic matter to sustain the bacteria, so large populations will not survive from one season to the next.
The cost of inoculant is usually not a limiting factor, but the inconvenience of applying the inoculant does take time when the producer is in a hurry. Most peat-based inoculant is mixed with the seed in the planter box and requires the operator to stir the seed after the powder is placed in the box to insureuniform distribution on all seed. Granular forms of inoculant would be spread in the furrow on the seed before the furrow is closed. The bacteria are more likely to infect the roots and form nodules if they are placed in the furrow with the seed when planting.
The answer to the question, Should soybean be inoculated?, is probably no on most fields in a soybean rotation. However, inoculant is an inexpensive form of insurance to make sure sufficient bacteria are available to fix nitrogen. If fields were flooded for more than a week or covered with sand deposits, inoculation is probably worthwhile.