Four Things I’ve Learned About Nitrogen Management for Corn Based on a Decade of Research in Minnesota

By Fabian Fernandez

Taking advantage of the cold days on which I prefer to stay inside, I am taking a few minutes to share four things I have learned over more than a decade of research on nitrogen management for corn in Minnesota.

1. Spring applications are better than fall applications.

While there are logistical and practical reasons why nitrogen fertilizers are sometimes applied in the fall, it is important to recognize that those applications are seldom better than doing the same application (same rate, placement, and N source) in the spring.

IN a 25-site-year study across Minnesota, encompassing corn-after corn and corn after soybean cropping systems and N rates with fall and spring applications of urea, I calculated that a spring application of urea reduced the amount of N needed to achieve the economic optimum N rate (EONR) by 28 pounds of N/acre and it produced 9 bushels/acre more grain than a fall application. Clearly, fall applications of urea are a lose-lose situation.

Last year, I looked at various N sources and placements at a sub-optimum N rate. The reason to use a sub-optimum N rate is to more easily see differences resulting from the treatment. In Waseca, in south-central Minnesota, regardless of the N source or placement I used, a spring application produced anywhere from 9 to 34 bushels/acre more than the same application done in the fall. And our fall applications always followed best management practices, applying after soil temperatures at 4 inches were below 50°F. In Lamberton, in southwest Minnesota, there were a few treatments that showed little difference between fall and spring, but two of the treatments showed a substantial advantage (29 to 30 bushels/acre) with spring applications.

In other words, with fall applications, there is a slim chance to do about as well as you would do with a spring application, but with a spring application, more often than not, you will see a substantial increase in yield compared to fall. I also suspect some of the smaller differences I saw in Lamberton between fall and spring applications last growing season are from the fact that there were really wet conditions all the way to the end of June or start of July, which likely resulted in greater than normal N losses, even for spring applications.

Table 1. 2024 study results: Spring nitrogen application versus fall nitrogen application.

2. If you do apply in the fall, anhydrous ammonia is the only source to consider.

In the past, anhydrous ammonia was the number one source of N in Minnesota. Over the last decade or so, urea has become the primary N source. It is critical to understand that these two sources are not the same when it comes to potential for N loss. Urea is more susceptible to loss than anhydrous ammonia because it nitrifies quicker than anhydrous ammonia.

Comparing responses over 30 site years, I have observed a 60% chance of getting statistically better yields (an average 49 bushel/acre improvement) with a fall anhydrous ammonia application versus a fall urea application. If we look at yield improvements that were numerically greater, though not sufficient to established statistical differences, the percentage chance of getting more yield with anhydrous ammonia than urea with a fall application is much greater than 60%. In other words, if you are considering a fall application (though I already mentioned it’s not going to be as efficient as a spring application), anhydrous ammonia is the only source I would feel confident using. Anything else will lose yield and profitability, to say nothing of the negative environmental and water quality impacts.

Banding urea, or polymer-coated urea (ESN), tends to perform better than a broadcast-and-incorporated application in the fall, but again, it is rarely better than any kind of application done in the spring. When compared with anhydrous ammonia, anhydrous is still a better option. Based on 20 site-years of data, a sub-surface band application of urea was better than a fall application of anhydrous ammonia only 30% of the time.

The fact that urea nitrifies faster than anhydrous ammonia is important even for pre-plant applications, especially in wet springs, when the potential for N loss is greater, like we saw during the last growing season. In 2024, across two locations, the EONR for a spring application of anhydrous ammonia was 7 pounds of N/acre lower and the yield was 5 bushels/acre greater than the same application done with urea.

3. In wet years, split applying N can be better than a single preplant application.

I have done a substantial amount of research looking at the question of pre-plant vs split applications of nitrogen. In general, there is little difference between the two times of application, but there are two exceptions.

Consistently I see better results with two or, even better, three or four applications in sandy soils. This is because any N applied before planting is highly susceptible to loss in these kinds of soils. For this reason, I actually prefer not to apply any N until after emergence. I apply a small amount of N (30-40 lb N/ac) around the V2 growth stage and most of the N goes on after the middle of June, when the potential for N loss goes down, and before corn needs a lot of N.

The other exception, though not as consistent as with sandy soils, is when spring conditions are wet in fine-textured soils. In this situation, the chance of seeing better yields and N use efficiency by the crop is improved, but it is not a given. I have seen plenty of situations where I would have expected to see better results with a split vs pre-plant application, and that was not the case. While the chance of seeing an improvement with split applications is better in wet springs, the practical challenge is that we have very limited ability to accurately forecast weather conditions more than 4 to 5 days ahead. In wet springs, one would think that there will be greater potential for loss of N from pre-plant applications. There are at least a couple of reasons why we don’t see such strong consistency as we see in sandy soils. It is likely that strong mineralization later in the season may overshadow any early-season N loss, which does not happen in sandy soils where the amount of N mineralization is much smaller. The other reason is that just because we split-apply N, it doesn’t mean we are assure the latter application is less susceptible to N loss. I have observed in several studies that N loss increased for split applications simply because by happenstance, after the application we had wet conditions for a while.

4. In wet years, undrained soils lose more N than drained soil.

Nitrogen loss in wet soils tends to be driven by leaching or denitrification. Over a number of years, I have compared N management in fine-textured soils that need sub-surface drainage, where I have implemented treatments with and without installed sub-surface drainage. I have observed that in “typical” growing seasons, the different drainage conditions made little difference in terms of N needs for the crop. However, when it is wet, on a relative base, we lose more N by denitrification in the soils without sub-surface drainage than the N loss that happens by leaching in the soils with sub-surface drainage. The soil without sub-surface drainage installed not only needed more N to optimize yield, but the N use efficiency was lower, highlighting the importance of adequate soil drainage.