Low Residual Soil Nitrate Tests: Should Corn Growers Adjust Nitrogen Rates?

By Dan Kaiser and Fabian Fernandez

Recent wet growing seasons have put a greater emphasis on nitrogen management. Summary data form MVTL labs sent by Brian Williams shows that nearly two-thirds of the basal stalk N tests were in the low category in 2018 and 2019. The closest years before that were 2014 and 2016, with nearly 50% of the samples in the low category, but the remaining years dating back to 2011 had only 25-33% of samples in the low category. If we factor in the marginal categories for 2018 and 2019, roughly three-fourths of fields tested likely had some N deficiencies while only 10% of fields came back with excessive N levels, which clearly illustrates that N loss was a significant issue the past two growing seasons.

Residual nitrate levels in the soil post-harvest

Jeff Vetsch, a soil scientist at the Southern Research and Outreach Center (SROC) in Waseca, documented 2019 residual soil test levels lower than he has seen in the last 25 years. Similarly, a consultant at one of our Nitrogen Smart events in western Minnesota this past winter mentioned that residual soil nitrate values were low in fall 2019.

Based on this data, should growers adjust N applications to account for low residual nitrate levels? Before we can answer this question, there are two main considerations to keep in mind.

1. Did you account for residual N in your nitrogen applications in previous seasons? If you did not account for residual N then it is hard to tell whether more N will be needed because you don’t have a baseline to make this judgment. If you have been using residual soil test data, then you can do adaptive management because you have the information needed to know how to adjust your nitrogen management based on the current conditions.
2. The university-recommended nitrogen application rates for corn were increased last year. Some of this increase may already account for less residual nitrate in the soil profile. Since our nitrogen guidelines are based on field studies, any residual nitrate that may impact the amount of fertilizer nitrogen required is automatically factored into the guidelines.

Where are the trends going for soil nitrate and should corn growers be concerned?

We can answer these questions in part thanks to John Breker and Richard Jenny from Agvise Labs and Brian Williams from MVTL. They provided us with summarized averaged soil nitrate data from recent samples across Minnesota.

Averaged by zip code, Agvise’s samples from northern Minnesota in 2019 showed that residual N ranged from 24 to34 lbs. per acre for corn-following-corn and 18-23 lbs. for corn-following-soybean. Data from Agvise’s Benson lab found average nitrate values of 27 for corn-following-corn and 22 for corn-following-soybean, which is roughly a 35-40% reduction compared to the previous five-year average. Data from MVTL, reported as an average across cropping sequence, indicated a similar value of 24 lbs. N per acre in the top two feet.

It is clear that there is a decreasing trend for residual N, likely caused by the wetter conditions we have observed in recent years. Unfortunately, lower residual N levels often mean that additional N fertilizer is needed to make the difference. As mentioned, current university guidelines already account for this automatically, as new field trials are added to the database used to make the rate calculation.

Where did the nitrate go?

It would be logical to assume that the wet fall may have reduced mineralization or resulted in denitrification of nitrate in the soil, which otherwise could be carried over to the next crop.

Agvise released a recent article showing trends in fall soil nitrate over the last several years. Their data show a gradual decrease in nitrate in the top two feet of the soil profile and an increase at a depth of two to four feet. This indicates that nitrate may not have been lost and instead moved deeper in the soil profile. Deeper rooted crops like sugarbeet are more likely to take advantage of nitrate deeper in the profile. For this reason, the long-standing suggestion for N management in sugarbeet is to sample deeper than two feet. For corn, there is more uncertainty, but that does not preclude corn from utilizing nitrate deeper in the soil profile.

3 things to consider for 2020

1. To see where things are at, it may be a good idea to take a deeper soil nitrate sample if you are in a situation where residual nitrate may normally be present. This will only work if you take the sample before fertilizer is applied. Situations where two-foot nitrate values are less than 20 lbs. per acre, and particularly 10 lbs. or less, likely warrant making some adjustments to N application.
2. Should I automatically increase the amount of N applied? To answer this question, the data provided by MVTL is illustrative. Mean soil nitrate N was 24 lbs. N per acre, the median value was near 10 lbs., and the values ranged from 2 lbs. up to 348 lbs. This indicates that there is considerable variability across the area. The decision of whether or not to increase N application rates needs to be determined on a case-by-case basis. Having some information, like a soil test, can help make the decision.
3. Can I make a decision in-season? Decision tools to help make in-season nitrogen rate adjustments are becoming more popular, but our research has not been able to identify the gold standard when it comes to in-season nitrogen stress detection. Often, difficulty in detecting nitrogen stress early in the season hinges on the fact that there is sufficient nitrogen from residual nitrate and mineralization to support the crop early in the growing season. In situations with low residual nitrate levels, deficiency detection might be more feasible. Regardless of which instrument or approach is used, it is best to try to make a decision early in the growing season, by around V6, as a large amount of nitrogen is taken up by corn between the time the crop is roughly a foot tall and silking. Making a decision early in the growing season can make a big difference in getting some value out of your fertilizer application, especially if there is less residual nitrogen than usual.