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Is inoculation necessary on long-term pulse land?

For best nodulation and N fixation, inoculate pulses every year. Photo by Bruce Barker.

To inoculate or not to inoculate? That is the question many pulse growers think about every year when growing pulses on land that has been previously inoculated. To answer that question, Dr. Fran Walley, professor and head of the soil science department at the University of Saskatchewan, initiated a three-year study with funding from the Saskatchewan Pulse Growers.

Many factors can influence the response to inoculation, including several soil characteristics like soil moisture, soil organic levels, soil inorganic N levels, soil pH and others. All of these factors are known to vary across landscapes. Inoculation response is also affected by the vagaries of the weather in some years.

Some earlier research suggests that under long-term pulse land there may not be a response to inoculation because there may be enough rhizobia in the soil already, and there will not be additional benefits from applying an inoculant. However, all of those research trials were conducted on very homogenous, flat land without any inherent soil variability. “Therefore to get a better understanding of whether or not inoculation was improving yields in variable landscapes, we selected two field pea sites at Davidson and Elstow on hummocky terrain,” explains Walley. “We wanted to assess the level of variability that exists within a field and to try and understand the impacts that variability might have on N2 fixation and whether or not there was a yield response to inoculation. We wanted to know where in the field we were getting a response to inoculation and what soil characteristics could be related to that response.”

Inoculant response variable across landscape, but response usually seen
Overall, the results from the field experiments demonstrated that N2 fixation and response to inoculation are highly variable within a field and both positive and negative responses can be observed on a point by point basis. It was clear that the overall response to inoculation taken over the entire landscape was generally positive even when there was a history of inoculation use. At all but one field site, inoculation resulted in an overall mean enhancement of both yield and N2 fixation parameters.

The field experiments were paired up with a growth chamber study that compared a number of different soil samples collected from across Saskatchewan. The soils came from different soil zones and different management histories. For each soil sample, the total number of rhizobia present is counted prior to the experiments. “The study confirmed that in the vast majority of cases, we were getting a response and sometimes that response was quite dramatic, although in some instances there was no response to inoculation,” says Walley. “One very interesting finding was the response to inoculation was unrelated to the number of existing rhizobia in the soil prior to inoculation.”

The research confirms the difficulty in trying to predict whether or not a field or particular parts of a field will potentially be responsive to inoculation. “Overall, we were unable to identify a single factor or factors that can be used to determine whether or not you will get a response to inoculation,” explains Walley. “Ideally, it would be great if we could run a simple soil test and predict the likelihood of an inoculation response, but our research indicates that there isn’t a single factor, including the existing population of rhizobia in the soil, that controls inoculation response. If we assume fields will not be responsive because there is a long-term history of growing pulses in that field, we run the risk of losing out on a potential inoculation response.

While it might be true that in some years there is no response, or there is only a response in some areas of the field, we really need to look at the bigger picture which tells us that there is a greater likelihood of getting a response in most years, and in most areas within a field.”
Overall, the research confirms the inoculation message has not changed. “For growers, inoculation is still required even on long-term pulse land to ensure that yield potential is not lost,” says Walley. “This does not ensure you will get an inoculation response, but you will not encounter a yield limitation if that inoculant is present.”