One reason why many farmers do not inoculate their soybeans is because they feel that their fields already have high numbers of rhizobia that inoculating isn’t worth it. They feel that the rhizobia population has been built up sufficiently over years of soybean inoculation that enough are already present to provide adequate root nodulation. While it is true that rhizobia levels can build up over time, the nitrogen-fixing quality of those rhizobia will not be as capable as they once were.
When inoculated seed is first added to the soil, it is coated with a strain of fresh rhizobia bacteria that has been selected for optimum nitrogen fixation. These bacteria are healthy and robust and readily available to begin forming nodules. Maximum benefit is provided to that season’s soybean crop. However, if the rhizobia are to continue to survive in the soil, they must spend more effort competing for food. This can then have a negative impact on their future ability to fix nitrogen.
Microscopic life in the soil is infinite and incredibly competitive. Everything is constantly fighting against each other for space and food. For the new rhizobia to continue to survive, they must learn to successfully compete for resources. This can lead to a shift in the abilities of the rhizobia, causing them to become more effective at competing for food. However, this can also result in a weakening of their ability to fix and generate nitrogen for future soybean crops. So while years of previous soybean inoculation can lead to a large background population of rhizobia, the ability of these bacteria to fix nitrogen will likely not be as strong as they once were.
The advantage of inoculating soybeans every time is that it will add fresh, robust rhizobia to the field to provide optimum nitrogen fixation to the crop. With the relatively inexpensive cost of using inoculants, using them is usually worth it regardless of field history.
SOURCE: Growing Possibilities Blog