Don't Take Shortcuts with B. japonicum

Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension

Soybean, being a legume plant, has the ability to "fix" its own nitrogen. Similar to having 4-wheel drive on your pick-up truck, this is a nice added feature considering that a soybean crop incorporates more than 300 pounds per acre of nitrogen in the above ground plant parts. The nitrogen fixation process is accomplished with a lot of help from a specific population of bacteria called Bradyrhizobium japonicum.

For a soybean crop to realize its full yield potential, B. japonicum must be abundant in the soil or introduced to the soil at planting in the form of seed inoculant. In Wisconsin, the general recommendation has been to inoculate every acre of soybeans that are planted unless a grower knows for sure that adequate bacteria populations already exist in the soil from a long history of soybean production. The cost of inoculant is minimal compared to the potential yield loss associated with inadequate nitrogen fixation.

In 1996, replicated field trials were done in Fond du Lac County where we evaluated inoculant use on two first-year soybean fields that were planted no-till. In one field, soybean yields increased from 34 to 49 bushels per acre by using an inoculant (see figure) while the other trial showed a five bushel per acre increase.

In doing these trials, we were careful to apply the recommended amount of inoculant and insure uniform coverage on every seed. Soybean seed inoculants need to be applied at a minimum of 100,000 cells per seed or about 2 billion cells per pound of seed. Although inoculants can now be purchased in both a liquid and granular form, the most common method of application is using a peat-based product that is mixed directly with the seed as it is put into the grain drill. Sterile (versus non-sterile) peat products provide the most stable and greatest number of viable cells.

In the rush of planting it is sometimes easy just to load the grain drill with seed and simply dump the peat product on top assuming it will work its way through the seed and into the soil as you move across the field. This approach can prove to be a costly mistake. In reality, the peat works its way down to the bottom of the seed box and moves little beyond that point. To obtain uniform seed coverage, the most effective method is to use a "sticker" and mix the inoculant with a volume of seed that is manageable to accomplish good coverage. This may mean mixing at three or four different times during the course of filling the seed box.

Many products can be used as a sticker. One cheap and effective approach is to purchase an off-brand two liter bottle of citrus soda and punch several pin holes in the top of the cap. The container can then be used similar to a spray bottle. Apply a small amount of soda over the top of the seed prior to adding the inoculant and then mix thoroughly using an elbow length latex glove making sure every seed is covered. By doing so, the problem of having the inoculant sit at the bottom of the seed box is eliminated.

Both local and university research trials in Wisconsin point to the importance of soybean seed inoculation for obtaining maximum yields. The keys to proper inoculation are to purchase sterile peat products, apply them at the proper rate, and mix to obtain uniform seed coverage. In our part of the world, where soils tend to cooler and crop rotation to soybeans is longer, it is still recommended to use inoculant on every soybean acre. For first-year soybean fields, it is especially important to follow proper rate and application procedures.

For more information contact Mike Rankin