At the Crop Production Clinic at the Eastern Nebraska Research and Extension Center near Mead last week, I presented on “Making Data-Driven Decisions on Soybean Inoculation in Nebraska Cropping Systems.” As producers, I have the following three goals for you: Gain a better understanding of biological nitrogen fixation, learn how to place fields into categories of probability of yield response to inoculation, and use data to guide your decision on whether to inoculate.
Biological nitrogen fixation or the symbiosis between Bradyrhizobium bacteria species and soybean plants can provide up to 75% of the soybean nitrogen need. A 70 bushel per acres soybean crop will uptake about 330 pounds of nitrogen per acre, where 50 to 75 percent of that demand comes from biological nitrogen fixation and the remaining balance is taken from the soil. Soybean plants provide the bacteria carbohydrates in return for ammonia, which happens at the root nodule. There could be two different species helping your soybeans out, Bradyrhizobium japonicum and Bradyrhizobium elkanii. Abiotic stresses such as low soil pH limit the start of this relation, as root hairs do not curl or deform to create the nodule. These symbiotic bacteria can survive in the soil as a saprophyte for over 20 years.
We have created an inoculation decision tree for producers to help place fields into the five different probabilities of yield response to inoculating soybeans with these beneficial bacteria. The four categories in the decision tree are high, moderate, low to moderate, and low. High probability of the yield response to inoculation is only where soybeans have never been planted, which is uncommon anymore in this area. You might expect to see up to a 15 bushel per acre yield response. Moderate probability includes situations where you have not planted soybeans in the past five years such as after alfalfa or continuous corn. Another situation would be where you have planted soybeans in the past 5 years, but there have been abiotic stresses to the bacteria including pH less than 5.5 or more at 8.0. You might expect to see a 2 to 3 bushel per acre yield response in the moderate category. The low to moderate probability includes recently flooded fields where soil erosion or deposition occurred or extended flooding periods along the Missouri River.
The last category, low probability of yield response is where you have planted soybeans in within the last 5 years and fields have had a good soil pH, and no flooding or extreme drought, and soybean plants were well-nodulated in the last crop. In this low category, expected to zero or no yield response. This has verified out in 24 comparisons in 18 Nebraska On-Farm Research Network studies where the average yield response was negative 0.1 bushels per acre. In 21 on-farm research studies by the Iowa Soybean Association should an average response of 0 bushels per acre too.
To learn more about making data-driven decisions on soybean inoculation in Nebraska cropping systems, view my recent presentation at my local agronomy website for Saline, Jefferson, and Gage counties at croptechcafe.org. Know your crop, know your tech, know your bottom line.
