In the world of downstream treaters, there are many treatment options for growers, one being the inclusion of a soybean inoculant. This article will walk through what an inoculant does, why it is helpful for soybeans, and the conditions which warrant inoculant use.

First, it is important to understand the nitrogen needs of soybeans. As a rule of thumb, each bushel of soybean seed produced requires four to five pounds of nitrogen. To put it in perspective, a 60-bushel soybean yield would require a minimum 240 pounds of nitrogen per acre.

There is plenty of nitrogen available in the air, it composes 79% of the earth’s atmosphere. Unfortunately, it is present in dinitrogen form with the atoms of nitrogen triple bonded to one another. Plants cannot utilize nitrogen in this form and since it is very stable a lot of energy is needed to convert it to the appropriate form for plant uptake. Fortunately, many legumes like soybeans can fix atmospheric nitrogen through a symbiotic relationship with bacteria. Bradyrhizobium japonicum is the only bacteria that forms this relationship with soybeans.

If soybean production has taken place in a field within the last 3 years, there are roughly a half trillion cells of Bradyrhizobium already present in the soil. The total bacteria population triples within 16 days of planting. Root hairs send chemical signals to rhizobia to attract it to the plant. The root hair then engulfs the rhizobia which then sends an infection thread to the center of the root. Hormones trigger root cells to divide and form nodules on the roots. Nitrogen fixation officially takes place 14 days after soybeans reach the V2 growth stage. The soybean plant will provide essential carbohydrates to the rhizobia to keep nitrogen fixation going. Healthy nodules in soybeans will appear pink in color and nitrogen fixation will reach its peak around R5.

There are several conditions that will result in reduced nodulation of rhizobia populations. You might hear some growers applying nitrogen to soybeans, but it is important to realize that soybeans will not form nodules if they can utilize residual soil nitrogen instead of expending the energy to feed rhizobia. Yield benefits have been inconsistent when inoculants were used in fields in which soybeans were grown within the last three years. Generally, there are enough rhizobia present in the soil when soybeans have been included frequently in the rotation.

The following conditions are situations in which you want to consider reintroducing a new inoculant source when planting soybeans to help build up rhizobia populations and ensure enough nodulation occurs:

In acidic soils molybdenum, a micronutrient that is important to rhizobia in the nitrogen fixation process, becomes tightly bound to soil particles. Acidic soils also interfere with the chemical signals attracting the rhizobia to the root hairs.

Inoculant usage for fields with dry, sandy soils and low organic matter is recommended because the soil has a lower water holding capacity that leads to more frequent desiccation, causing rhizobia to die out.  On the other end of the spectrum, if fields become water logged or are flooded, anaerobic conditions become prevalent that also lead to reduced populations.