Soybeans are a major global crop, valued for their ability to acquire nitrogen naturally. An inoculant is a preparation containing beneficial bacteria applied to the seed or soil to ensure this natural process occurs effectively. Deciding whether to use this bacterial preparation is an important decision for any soybean grower.
The Symbiotic Relationship and Nitrogen Fixation
Soybeans establish a partnership with the bacterium Bradyrhizobium japonicum. Since this microbe is not native to many North American soils, inoculation is often necessary. The plant releases chemical signals that attract the bacteria, which enter the root hairs and trigger the formation of specialized structures called nodules.
Within these nodules, the bacteria perform nitrogen fixation, converting inert nitrogen gas from the atmosphere into usable ammonia for the plant. This biological process can supply the soybean plant with 40 to 75% of the nitrogen it needs to thrive and produce a high-protein crop. This natural nitrogen source significantly reduces the plant’s reliance on synthetic nitrogen fertilizer.
Nodules that are actively fixing nitrogen have a pink or reddish color inside, which comes from a compound called leghemoglobin. This compound controls the oxygen level within the nodule, which is necessary because the enzyme that fixes nitrogen is sensitive to oxygen. The plant provides the bacteria with carbohydrates for energy, completing this mutually beneficial exchange.
Determining the Necessity of Inoculation
The need for inoculation depends on the field’s history of soybean cultivation. If a field is being planted with soybeans for the first time, applying an inoculant is necessary to introduce the required Bradyrhizobium japonicum bacteria. Without this introduction, the plants will suffer from severe nitrogen deficiency.
In fields that have successfully grown soybeans before, the need for inoculation is less certain. University guidelines often suggest re-inoculating if it has been three to five years or longer since the last soybean crop, as the native bacterial population may have declined. However, in tight rotations, the residual bacterial population may be sufficient, and an inoculant may not provide a yield benefit.
Soil and environmental conditions can reduce the existing bacterial population, making inoculation advisable even in short rotations. Factors such as low soil pH, excessive flooding, severe drought, or sandy soil texture can lower the number of viable Bradyrhizobium cells. Reintroducing the bacteria through an inoculant acts as insurance when there is a history of poor nodulation or stressful conditions.
Selecting and Applying Soybean Inoculants
Once the decision is made to inoculate, growers must choose a product and application method. Inoculants are available in different forms, including peat-based powders, liquid formulations, and granular products. Peat-based inoculants are mixed directly with the seed, while liquid inoculants are applied to the seed as it is loaded into the planter.
Granular inoculants are placed directly into the seed furrow during planting, providing a high concentration of bacteria near the emerging root. For fields with no history of soybeans, a “double inoculation” strategy is employed. This uses two different forms, such as liquid on the seed and granular in the furrow, to maximize the chance of successful nodulation.
Since the inoculant contains living organisms, proper handling is important to maintain viability. The bacteria are sensitive to heat, direct sunlight, and dehydration, so treated seed should be planted soon after application. Some seed treatments, particularly certain fungicides and insecticides, can be toxic to the bacteria, requiring careful product selection and application timing to prevent the inoculant from being killed.
Risks of Skipping Inoculation
When a field requires inoculation and the bacteria are not supplied, the consequences are significant. Without a sufficient population of Bradyrhizobium japonicum, the soybean plant cannot form enough functional nodules for nitrogen fixation. This forces the plant to rely solely on the nitrogen already present in the soil, which is usually inadequate for a high-yielding crop.
The first visible sign of nitrogen deficiency is often chlorosis, appearing as yellowing of the leaves, especially when the plant’s nitrogen demand increases later in the season. This deficiency leads to stunted growth, poor plant health, and failure to reach full yield potential. In fields with no prior soybean history, skipping inoculation can result in a yield loss of 10 bushels per acre or more.
Even in fields with a history of soybeans, environmental stress like drought or flooding can reduce the native bacteria to insufficient levels. The cost of inoculant is a small percentage of the total production expense, but the potential yield reduction from a nitrogen deficit can lead to substantial economic loss. The risk of a poor crop due to nitrogen deficiency outweighs the minor cost of the application.