Megan Bourns, Agronomist – On-Farm Network, MPSG – Fall/Winter (December) Pulse Beat 2021
This spring, as soil test results were rolling in from trial fields, an unusual number of fields tested high to very high for residual nitrate in the top 24 inches. In most cases, the farmers could not explain the high nitrate, and there were even fields testing high in the spring that had been within the normal range for pulses and soybeans when sampled in the fall of 2020. Asking around and gathering information as to why this might be occurring, it became clear that we do not have reliable answers about why nitrate was so high in these fields and what might happen to those nitrate levels over the growing season. So, we set out to investigate this through the On-Farm Network (OFN). We picked 11 trial fields where we established a microplot to track nitrate in the top 24 inches every two to three weeks during the growing season. The idea was to gather some preliminary information about these high nitrate conditions to hopefully develop more targeted questions about gaps in our knowledge moving forward.
While there are a lot of data points to digest, the main takeaway message is that nitrate levels in the top 24 inches did decline over the growing season (Figure 1). The decreases at each microplot ranged in magnitude from 20 to 149 lbs nitrate/ ac. While this is good news in and of itself, another layer of this investigation involves asking questions about agronomic effects of these high nitrate conditions on soybean production. Spring soil test nitrate in the top 24 inches ranged from 35 to 456 lbs/ac across the 11 fields, and average yields ranged from 10 to 45 bu/ ac. Interestingly, there was no statistically significant or agronomically meaningful relationship between average yield and spring nitrate in the top 24 inches (Figure 2). In other words, across the fields where we had microplots, the high nitrate levels were not likely the main determining factor for yield outcomes, and we did see yields exceeding 40 bu/ac where spring nitrate was greater than 100 lbs/ac.
In addition to the effect on yield, another pertinent question is the effect of high nitrate on soybean nodulation. Generally, we observed that nodulation was less than agronomically ideal leading up to R1–R2 (<10 active nodules per plant). However, for some fields, nodules seemed to develop later than normal. While this could certainly be an effect of high nitrates, we also observed this in other fields where nitrate levels were within normal ranges — likely in response to the dry conditions this season.
So, where does this leave us? The OFN team has some further data investigation to do, exploring the relationship between changes in nitrate and precipitation through the season. The Birch effect is one of the main theories about where the high nitrates came from in the first place. Essentially, the Birch effect is a phenomenon whereby there is a mass death of microbes in the soil, which leads to the release of nutrients from the microbial cells. In this case, a mass microbial death could have been caused by prolonged dry conditions. If this were the case (which we are still investigating, working with our data), we would expect to see more of a change in soil nitrate in the top six inches, where microbial activity dominates, as moisture and heat help the microbial population re-establish. Looking at nitrate changes in the top six inches over the season and how that relates to precipitation events (although they were few and far between in 2021 for the most part) could inform whether the Birch effect was a primary contributing factor to the high nitrate.
Additionally, we want to conduct a more detailed exploration of field histories. Our initial takeaway from reviewing tillage practices, soil organic matter, soil type, manure history and crop history suggest there is not an obvious link between those factors and high spring nitrate or how much nitrate decreased over the growing season for these fields.
When scenarios like these inexplicably high nitrate levels, pop up as a result of certain conditions, there are always a lot of questions that follow any initial investigation. We will keep you updated on what we learn as data analysis continues!