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Making sense of Minnesota’s corn fertilizer guidelines: Frequently asked questions and answers

Tractor distributing fertilizer

Dan Kaiser, University of Minnesota Extension nutrient management specialist, recently updated the publication Fertilizer Guidelines for Agronomic Crops in Minnesota for the first time since 2011. Here are some common questions he gets about the guidelines for corn and his answers.

How should growers use the fertilizer rate guidelines?

We know there can be variability from field to field, and even within a field, in terms of fertility requirements. What our fertilizer rate guidelines provide is a starting point for growers and advisors, backed up by real world data. One of the main focuses of our Nitrogen Smart program is to start with the university-suggested rate and adjust a little bit up or down based on what you know about your field. For example, this year, following a dry 2021 growing season, would have been a good year to take a pre-plant nitrate test (PPNT) to see if there’s some carryover nitrogen (N) left in the soil that you could credit for this year’s crop. Then, you could apply a lower overall N rate this spring and summer.

Another option is on-farm research. I’m not a huge fan of plus-or-minus trials (where you apply the university-suggested rate in some strips, 30 pounds more in a few spots, and 30 pounds less in others) because the data cannot be used to completely determine how much N was required within a year. But, for a grower, they aren't the worst thing in the world since they're low risk; you're not putting no nitrogen in some areas like we do with our research plots, and it would provide some idea of whether you are too high or too low on your N rate needs. I don't think I would take data from a single year of an on-farm plus-or-minus trial and make a decision, but if you have enough data over time that says, for example, that you don’t need that additional 30 pounds of N, you may want to look at cutting back.

Why do the nitrogen rate guidelines for corn bring economics into the mix instead of just focusing on yield?

Many growers focus on the maximum yield produced, but with nitrogen we can look at the “marginal return,” which is the return for each pound of N applied, and calculate the point where the last dollar invested in nitrogen returns a dollar in crop value. At some point, as the rate increases, the cost of the nitrogen fertilizer overtakes the additional yield the fertilizer adds. With the current Maximum Return to Nitrogen (MRTN) approach, the amount of nitrogen suggested should produce near-maximum yield. Crop response to nitrogen near the inflection point of a nitrogen response curve is relatively flat, and while it is possible to increase yield further beyond the MRTN suggested rate, for every dollar in N invested, you will be getting less than a dollar in return.

The other nice thing about the regional corn N rate calculator is that the price of nitrogen fertilizer and the price of corn can change over time. With the calculator, you can put the most up-to-date prices into the calculator and it will give you the N rate, a range of rates, that our data show maximize profit. The adjustments aren't huge, but it gives you an idea of what the most profitable rate is.

Nitrogen rate and return graph

A lot of corn growers are getting significantly higher yields than they did 10, 20 years ago. Why hasn't the N rate increased in sync with rising yields?

Nitrogen needs vary a lot from field to field. I’ve worked on corn-following-soybean fields that required as little as 50 pounds of nitrogen to get 250 bushels of corn and fields that have required 250 pounds of N to get the same yield. It’s all about how much nitrogen your soil is supplying before you apply N fertilizer, which can be difficult to predict.

It isn't just the fertilizer N that's supplying those high yields. A lot of times, we see higher yields in situations where we get higher mineralization rates. Conditions that favor N mineralization also favor crop growth, so the two go hand-in-hand. It seems like fields where we get high rates of N mineralization late are fields where we get high yields and the high yield potential is not related to the amount of fertilizer applied in the spring.

Why are suggested nitrogen rates for corn-following-corn increasing faster than for corn-following-soybean?

It's a good question. If you look back at some of the factors that affect the difference, a lot of it relates to the residue and the nitrogen being tied up with the residue. Since around 2000, we've seen more traited corn and that's leading to more residue. Before, you'd have a lot of insect damage, so the quality of the residue wasn't very good by the end of the season. With the new varieties, we're seeing less crop damage during the growing season, which means more residue in fields after harvest. With more corn residue, greater amounts of inorganic N in the soil can be tied up, leading to more nitrogen fertilizer needed by the crop.

Another factor is the wet conditions we’ve seen in recent years. I know there's circumstances out there where we see a substantial amount of nitrogen being required for continuous corn, particularly in poorly drained situations. During wet years in poorly drained fields, growers are losing more nitrogen to denitrification. The last two or three years we have actually seen the average economic optimum nitrogen rate (EONR) in our dataset decline. So, while the trend has generally pointed towards a greater need for nitrogen over time, there can still be significant variation from year to year.

Corn-corn yield net return graph

Why does the corn-following-soybean data go all the way back to 1990?

When I update the calculator, I go through a few scenarios and look at different time sections within it. So, I'll go back 10 years, 15 years, 20 years. With continuous corn, it makes a difference if you include, say, back to 1990 versus including data only back to 2000. That’s why we took out some of the early continuous corn years in our dataset. But if you do the same thing for corn-following-soybean, it doesn't matter. I looked at 20 years of data versus 30 years of data versus 10 years of data for corn-following-soybean and it basically gives you the same exact value. With that stability in mind, it's better to have more data points when you do the calculation. There’s an advantage of having more locations spread out across the state. I hate to throw out data if there's no reason to.

Soybean-corn yield return graph

What role does soil organic matter play in corn’s nitrogen requirements?

One thing I’ve heard is that some growers who do variable rate nitrogen are using organic matter maps as a way to try to look at reducing their nitrogen requirements. But most of the poorly drained fields where we see high nitrogen requirements have higher organic matter. And the reason they have higher organic matter is because they have drainage issues. So, more so than organic matter, it seems like leaching potential, soil saturation potential, is the main factor driving whether a section of a field could use more or less N than the standard rate. Calculating the amount of nitrogen mineralized from soil organic matter is difficult as mineralization rate is impacted by many factors.

With wetter conditions, won’t higher suggested nitrogen rates make our water quality issues worse?

So, the two main pathways for nitrogen loss are denitrification to the atmosphere and leaching to groundwater or waterways. Most of the increase in N rates in the dataset is associated with denitrification during wet years in poorly drained fields. Denitrification is still N loss, so it’s not good for farmers’ bottom line, and nitrous oxide is a greenhouse gas, but it’s not going to lead to water quality issues like nitrate leaching is.

Another thing to consider is that some of our recent research indicates that drained fields may require less fertilizer N due to less denitrification. People, I think, suspect that having drainage would lead to a higher nitrogen requirement and would lead to more nitrate loss, but that hasn't been the case in Fabian Fernandez’s study near Wells, Minnesota. His data shows that the undrained fields require more nitrogen than the drained fields. That’s likely because there’s more denitrification in the undrained fields during wet conditions.

If you start looking at the university-suggested N rates in terms of water quality, we generally see that at or slightly below our guidelines are going to be the best rate when it comes to preventing nitrate loss to water. In general, the amount of residual nitrate left in the soil after harvest increases rapidly once the optimal N rate within a field is exceeded. Thus, applying the optimal amount of N is critical to reduce the amount of nitrate left in the soil profile at a time when there is no crop to take it up. We're not going to completely get rid of nitrate loss from agriculture, but the big thing we're really trying to do is not to leave too much fertilizer out there where you've got a lot of nitrate in the soil in the fall and early spring, when it can be flushed out of the soil profile because there's no active crop growing out there that can take it up.

Another thing to note is that reducing the N rate below the economic optimum normally has no benefit in reducing residual soil N. In other words, crops will use more or less all of the applied N up to the EONR. Rates above the EONR are the problem because the excess N is not used by the crop and it does not contribute to increasing grain yield either. What we really need to focus more on is nitrogen best management practices and trying to get to a situation where we don't have that “insurance N” assumption in play and where logistics isn't the key factor that's dictating when nitrogen is applied or how it's being applied.


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