Yield improvement in soybean by seed inoculation

Seed inoculation is the practice of covering the seed surface with a nitrogen-fixing bacteria (Rhizobium or Brady rhizobium) prior to planting. The bacteria penetrates the root, resulting in the formation of root nodules that fix nitrogen from the air and make it readily available to the plant.

The soybean plant gets nitrogen from the bacteria, while providing the bacteria with carbohydrates. When soybean seed germinates, the bacteria invade the root hairs of the seedling and begin to multiply. Nodules, which house the bacteria, form on the roots. Under field conditions, the first nodules form within a week after seedling emergence and become visible as they increase in size. Active fixation begins in the V2 to V3 stage, after which the number of nodules and the amount of nitrogen fixed continue to increase. The soybean demand for nitrogen is highest from the R5 to R8 stages.

Brady rhizobium species, a bacterium within the nodules of soybean roots. Soybeans get needed nitrogen and the bacteria get some carbohydrates in return. This process provides soybean plants 46%-74% of the N they need; the remaining amount is taken up from the soil .A 70-bushel-per-acre soybean crop takes up roughly 330 lb N/acre in the aboveground portion of the plant, roughly equivalent to the nitrogen demand of a 245-bushel-per-acre corn crop.

It has been more than three to five years since soybeans were grown or soybeans have been grown in rotation every two to three years, but an environmental factor (e.g., flood, drought) may have negatively impacted survival of bacteria. This would include documented N deficiency or poor nodulation in a previous soybean crop.

Soybeans plant that have been grown in the rotation of every two or three years with environmental conditions have been good.

In situations where soybeans have never been grown, yield increases of as much as 49 bushels per acre were measured in Nebraska fields with the addition of inoculant . However, more modest yield increases of 1 to 10 bushels per acre are expected. Yield effects are largely dependent on the nitrogen supply from soils and the relative increase in nitrogen fixation. For example, a 2009 Iowa study reported inoculation increased soybean yield 2 bushels per acre on land coming out of pasture.

The decision on whether or not to inoculate depends on whether the field has a recent history of healthy-looking soybean. Most soils in Iowa have a good population of B. japonicum if soybean has been grown in recent years. However, if a field is new to soybean, or has been out of soybean for more than three to five years, it is good insurance to inoculate.

Current recommendations for Iowa are to inoculate the seed if:

1) the field has never been planted to soybean
2) soybean have not been grown in the field in the past three to five years
3) the soil pH is below 6.0
4) the soil has a high sand content
5) the field has been flooded for more than a week, creating anaerobic conditions

Factors Affecting Soybean Nodulation

If your field falls in the second situation, several environmental factors can cause poor nodulation, decrease nitrogen fixation, and reduce survival of these bacteria, including:

Infographic asking 3 key questions to consider before inoculating soybeans pNon-optimal soil pH. Low soil pH can decrease nodulation, nitrogen fixation, and survival of various strains. Ideal pH is near 6.8. Soil pH less than 6.0 can decrease root hair modification needed for the formation of nodules (Duzan 2004). Therefore, following economic thresholds for lime application when soil pH decreases to 5.6-5.8 also manages soil bacteria. High soil pH and salinity also decrease nodulation, nitrogen fixation, and survival of these bacteria.

Floods and droughts. Fields flooded for more than one week decrease nitrogen fixation and can decrease survival of bacteria because bacteria need oxygen. However, two recent research studies suggest not to be overly confident in assuming a yield response to inoculants after floods. Sandy soils and droughts in years when soybeans are not being grown also can lower bacteria populations.

Increasing number of years since soybeans were last planted. Most university guidelines suggest inoculating if it has been more than three to five years since soybeans were last grown in the field. These bacteria have been found in soils 30 years after being planted back to perennial grasses, just at a much lower population. Two more common situations in Nebraska where soybeans may benefit from inoculation are when soybeans are planted after continuous corn or after corn that followed four years of alfalfa—alfalfa utilizes a different bacterial species for successful inoculation. The probability of a yield response in fields with a recent history of soybeans is extremely low. An eight-year study (2000-2008) testing 51 inoculant products in 73 experiments conducted in Wisconsin, Iowa, Indiana, Minnesota, and Nebraska resulted in average yield response of 0 bu/ac.

If your field falls into the third situation described above and environmental factors (bulleted list) were not an issue, some still advocate for inoculating with more effective nitrogen-fixing strains of these bacteria over common indigenous strains, e.g., Brady rhizobium japonicum Bj123 in the Midwest even when environmental conditions have been conducive for nitrogen fixation. In the 2000 to 2008 study, new inoculant products and strains were used. Of the 10 most widely tested products, yields were not influenced in fields with a recent history of soybean. This is likely due to highly competitive indigenous strains diminishing potential gains from applying more effective strains.

For fields with no soybean history, inoculant treatments increased nodulation and yields. For fields with a history of soybean, there was no difference in yield or nodulation between inoculant treatments and the non-inoculated control.

In both types of fields, the product, formulation and rate did not affect yields or nodulation. The rate comparison was single versus double inoculation. A double inoculation involves using two formulations or placement techniques. For instance, a grower might use a liquid inoculant on the seed and a granular inoculant in-furrow as a way to ensure successful inoculation.

Writer: Mamata Dhungana (Student, Bsc Ag 4th Sem)