A difference in degrees: Fall application efficiency at 50 Fahrenheit
Advancing Nitrogen Smart, from the University of Minnesota Nutrient Management Podcast:
“A difference in degrees: Fall application efficiency at 50 Fahrenheit”
October 16, 2024
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Jack Wilcox:
Welcome back to episode 11 in our special series “Advancing Nitrogen Smart”, from University of Minnesota Extension. I’m Jack Wilcox, in Extension communications. Today we’re talking all about fall application with Extension educator Brad Carlson and Extension nutrient management specialist Dan Kaiser.
Brad, what's the first thing farmers should know about applying in fall? What’s the difference in N efficiency at 50 the degree threshold versus even a 55 degree threshold, and then when have those degree dates happened historically?
Brad Carlson:
Well, the thing we've been trying to drill home for years is that the soil temperature should be at 50 degrees. The conversion to nitrate is a microbial activity. If you think about the actions of microbes, think about your refrigerator at home. You certainly don't want it above 50 degrees or food's not going to get preserved in it, and the cooler it is, the longer it's going to stay. The soil's just like that. And so when we look at applying nitrogen, we want it to be at that point where microbial life really slows down.
That 50 degrees, there's been some questions about what that means when you drill into it. We're talking about at a six-inch depth, so the depth of an anhydrous application. And so it's worth noting that the National Weather Service's standard reporting stations across the United States are actually at four inches and eight inches. They're not at six. And so what we often say is you get the data for four and eight, just take the average between the two. There's typically in the fall, the eight's going to be maybe a little warmer than the four because it stays warmer deeper. It takes a lot of energy to change the temperature of water, and so there's that buffering, but the temperature's not going to be that far apart between the two. But that's actually the 50 degrees.
And the other thing about it is we're looking for it to be 50 and stay below. Okay, you don't have a crystal ball. However, we know based on some of our climate averages that if we get a little cold snap and it's say early October and the soil gets to 50, it's probably going to get back above it again. You've probably got a pretty good feeling by the time we get to the third week of October, look at the temperature. If it's 50, you'll get the seven-day forecast and it's going to stay cool, it's probably going to stay there. If the seven-day forecast says we're going to get an unusually warm stretch and it's going to get up to 90 some day, it's probably not going to stay there. So even though you can't predict the future, you can usually get a pretty good idea of when that is.
Dan Kaiser:
And really to be clear is that the nitrification process doesn't stop at 50. It slows down considerably at 50. So really if you start looking at where we get no nitrate accumulation, that's really not until we get to freezing and the soils are completely frozen. But again at 50 degrees, we're just less concerned that the accumulation is going to be such where we're going to get a vast majority of our ammonia converted over to nitrate. So that's where that comes into play is that it slows it down substantially at that point, but it doesn't necessarily eliminate.
And that can be important because we do see some years where we'll get some warm snaps in late November, and really there's always some questions that from growers on what kind of loss potential we should expect or what kind of conversion potential we should expect. And really even in some of those years where we may not get our ground completely frozen, I'm not really as concerned as long as the nitrogen was applied below 50.
Brad Carlson:
Well, the other aspect about it, Dan, is we look at the loss of nitrate. The percentage of the nitrogen that's converted to nitrate is a portion of that, but then also how long of time it's present in the soil. So if you're applying when the soil temperature's too warm, not only is it going to happen faster, it's got more time to lose because you're talking a week or two earlier, and so you're adding to that time length where there's risk also. So that's something to consider also with that.
Farmers ask sometimes how do we get this? Our research stations, a lot of them have soil temperature data you can find on the website. I know the SROC is probably the, Waseca Southern Research and Outreach Center, Waseca is the one that's probably hit the most. I think the other ones do have that available. Also, I'm pretty sure Lamberton and Morris do, at least we look at in the corn production zone in Minnesota.
But Minnesota Department of Agriculture also has that available on their website. They used to have a really neat feature that was part of their runoff risk advisory website. Now, last year when I looked at it, that was disabled, so I don't know if that's back and functional or not right now, but that actually used a map across the state to look at how the temperature was different from one part of the state to the next.
So you can go on the Minnesota Department of Agriculture website, which is mda.state.mn.us, and you can just use their search and look for soil temperatures. You can look for the runoff risk advisory and look for that. I know the soil temperature one just simply listed soil temperatures across the state. It didn't show the pretty maps we did.
But when I put together this information for our advanced session on climate, I had gone in to the mapping feature that was on the runoff risk advisory, and I pulled some maps for different days. And so I started on that one year, I started on October 7th, and we saw that the temperatures were pretty warm across the state. By October 15th, we saw that the 50 degree temperature had migrated to about halfway down in the state. Although also recognize that when you draw a line halfway across the state, most of us north of that doesn't have a lot of ag in it other than the Red River Valley, although that was pretty warm too.
But we had a warm patch after that. And so I pulled it three days later on the 18th, and most of that area then had gone back above 50 again. So I guess that's a point worth noting is that there's some instability in the first half of October regarding that.
By the 20th, it moved so that about the northern one-third of the state, again, a lot of that area is not even much of an ag production area. However, one day later it got cold. On October 21st, it had gone down halfway across the state and down the western side of the state. And then just a couple of days later, again on October, the entire state was below. So it tends to happen pretty fast. When we get late into October, that's what we see.
Typically speaking, the first day below 50 degrees when you're about halfway across the state or maybe roughly about St. Cloud north, we're typically looking at somewhere around between the 10th and the 15th, 16th, 17th. A lot of Southern Minnesota, South-Central Minnesota, we're probably more into the 20s, 22 to 25, 27 on average. But of course, things are rarely average. It's just an idea of getting an idea on what that looks like.
The actual average date at Waseca for the last, well, last 30 years or so is October 18th. But again, that's going to vary a lot. It's been as early as the 7th of October, and it's been as late as the 28th or the 29th, so it really does depend on the year.
Now the other thing to remember, and I've talked about this in the past, is it takes a lot more energy to change the temperature of water than it does to air. So if the soils are wet, that change in soil temperature is going to be a lot slower. However, it's going to be more stable too. You're not going to see it bouncing around on a warm day or a cold day, and so that's also something to think about.
One of the things that I've did as part of our advanced nitrogen smart curriculum looking at adapting to climate is trying to calculate based on some of the published research and some of the tabular value, and of course this is all just approximations because each situation is going to be on its own, but what the effects were of applying nitrogen early. And we get this all the time. It's amazing, Dan, I know we've talked about this before, how often we'll get these phone calls. Like, "Oh, I was so-and-so on and they're already putting nitrogen in the field." Well, yeah, we know that's going on, but we're not nitrogen police, so there's nothing we're really going to do about that.
But let's take a look at that. So if you look at applying the nitrogen in the fall at 55 degrees instead of 50, so which that doesn't seem like it should be that big of a deal, but if you actually look at the rates of nitrification based on soil temperature, it is. And so there's some data that looks at rates of conversion to nitrate. And so what we would see happen, if it's at 55 degrees for 10 days, if you apply it at 10 days and it was 55 and then 10 days later finally got to 50, you'd see that approximately 25% of that nitrogen was already converted to nitrate in that 10 days. So you've already had about fourth of that nitrogen move.
The amount that changes because the rate of conversion to nitrate slows so much from 50 degrees down to the point of freezing, the actual amount that changes between 50 and freezing is only more like 15%. And so that's pretty significant that 10 day early got you 25%, whereas you only had 15% change the whole rest of the year until the soils froze. Roughly speaking, you're looking at the difference of 40% versus only 15% converted to nitrate by when you applied that 10 days earlier at the temperature of 55 degrees.
The other aspect of this is because our recommendations typically include applying a nitrification inhibitor with that hydrous in the fall, that stuff also wears off over time. And so Dan did some calculations on that looking at some of the old research on nitrapyrin and the time and its degradation and so forth.
And so roughly speaking, if you calculate the application rates of nitrapyrin based on its concentration in the soil, it ends up being at about two parts per million. The literature indicates that it still works at about one part per million. So if we hit the point where about 50% of the nitrapyrin that was applied is degraded, it's probably not functioning real well anymore. So if you applied with the nitrapyrin and you applied 10 days earlier, you thought, well, I'm going to put on a nitrification inhibitor, you actually saw about a 20% loss of that nitrification inhibitor in that time too. And so that didn't really buy your way out. You just lost some of the period of effectiveness in that product.
Dan Kaiser:
And I'm going to put just something in here. The fact that Brad's talking about nitrapyrin specifically, I mean there are other nitrification inhibitors. CENTURO is the one that's out there now that it's not nitrapyrin, it's a different product. We don't know a lot about that one. That's the issue in terms of the conversion with that. So if you take the numbers that we're giving, it isn't the same for those particular products.
And there's some other products out there that just if we look at what does tend to work, it's nitrapyrin has been the main one. So there's some other things out there that I just don't put a whole lot of stock in because we just don't have a lot of data with that. So just be careful with that if you're looking at a lot of these numbers that they're really specific to a specific material and something that we know that works.
And nitrification is the key when it comes to a lot of these issues when we talk about soil temperature. We also have another inhibitor with nitrogen, which is urease inhibitors, and that's a whole different process, and that actually isn't affected by temperature as much as nitrification. So looking at it, if you're looking at products, you're looking at materials, you have to know what you're applying and know what you're trying to do.
And specifically here, what we're talking about with fall application really is delayed nitrification because I think we've talked about this before, is what we want to try to do is maintain the most ammonia or the greatest amount of ammonia applied from the fertilizer, since most fertilizers that are applied are going to be in the ammonia form in the fall and into the spring as much as possible because we know that those, at least that ammonia form isn't going to be as subject to leaching.
Brad Carlson:
Right, and so that's really the key and the loss process of nitrogen, while they water-based, they also are dependent on the nitrogen turning into nitrate because either leaching or denitrification in both cases, those processes happen when the nitrogen is turned into nitrate. And so really what we're trying to do is just simply avoid it being nitrate because then you can avoid it being lost.
And so in addition to the soil temperature in the fall, the soil temperature in the spring also plays a key. So when we look at there's some parts of the US where they say, "You guys actually apply nitrogen in the fall, really?" And it's like, "Well, yeah, but it's so cold and it's frozen a big part of the year."
If you look at the temperature averages in Minnesota, and if we say that the conversion to nitrate, the nitrification in essence, it stops. It doesn't technically stop, but in essence it stops when the soil's frozen, that also then happens in the spring. So if we look at the averages from when the frost comes out in the soil and then the soil temperature finally hits 50 degrees, that's 53 days typically. So there's another 53 days where we're not really rapidly nitrifying. However, the point being if you apply it early in the fall and it's already nitrate, well it doesn't matter now it's subject to be lost. So that's a big deal too.
And so on average using some of the tabular data, if you apply anhydrous in the fall at 50 degrees, what we would typically see in the, we talked about that roughly about 15% would convert to nitrate between the time it's applied at 50 degrees and the time of freezes, that extra 53 days in the spring then is about another 20%. So you're looking at about 35% conversion just on your own if you're following best practices.
Now the thing about it is if you apply that 10 days early, now instead of it being 35%, it's 60%, and we look at how wet it typically is in the springtime. That's what really puts the loss of that fall applied nitrogen at risk.
Dan Kaiser:
Remember, you're spending that additional cost for the nitrapyrin, so you want to make it work as much possible. And it's one thing we're not talking specifically about in this podcast, but we do have some data that we do show during the Nitrogen Smart program talking about degradation of nitrapyrin, which is microbial degradation which is impacted by temperature.
So I mean, just like when it comes to the nitrogen is when you start looking at degradation is that it's going to slow the cooler the soils are. So that's one of the things if you start looking at a product like nitrapyrin, that early application, why we tend to see greater losses, is it just because it degrades a lot quicker and you just don't get the actual benefit for it. So that's one of the things looking at this with some of these numbers that Brad's throwing out there is you've got to remember is that there is a temperature component when it comes to nitrapyrin degradation.
The other thing is a lot of these numbers really are focused on higher clay soils because we know that, especially with how nitrapyrin adsorbs the soil, that it does better with the adsorption on higher clay soils versus silt loam soils. Still looking at areas that, we know some of our silt loams in the Southeast, we don't obviously recommend fall application in those areas due to a lot of other issues, but you're not going to get ahold of products like nitrapyrin as well.
So it is not just a simple, and a lot of these numbers we throw out there are really contingent on certain things when it comes to soil types or application timing, for sure.
Brad Carlson:
And we're doing some calculations just based on some published research. Obviously, circumstances are going to be different as far as compaction, the dynamics of the soil, of the temperature, the soil moisture, there's going to be all kinds of other factors.
Dan Kaiser:
pH will affect it as well.
Brad Carlson:
For sure. And so we're only talking about rough approximations, but this is not like, this is exactly how this happens everywhere. We're just doing some calculations so you can get a feel for what this looks like.
We looked at, and I mentioned earlier, so they're looking at nitrapyrin needing to be present at above one part per million to be effective. And if it's applied at two parts per million, having a 50% degradation at that point isn't quite as effective as it used to be. If you use that tabular data from some of that basic research, that calculates out to if you apply it at 50 degrees temperature and you use the climate averages and how long the temperature stays cool and so forth, that keeps it effective until about April 17th.
Well, that's right in the sweet spot of when we start planting corn, and that's one of the reasons why it's part of our recommendations for Southern Minnesota. So that's, it's really good news really provided it didn't get warm early or something else that happened. But the thing about it is though, as I mentioned earlier, in our scenario if we applied 10 days early because we ended up losing so much of that in the fall because the soil temperatures are warm, then at that point the data wears off. It's more like March 28th, and that's several weeks before we're out in the field. And so now all of a sudden we are at risk because we know how wet it can be that time of year, and so that's something to think about.
Now, one of the other things that's been part of this stuff is how long the soil stays frozen. We know that nitrification doesn't really approach zero until the soil temperature is freezing. The number of days that the soil stays frozen, or I should say the date that the soils freeze and the number it stays frozen has been changing a lot because of how wet it's been getting in the fall. Maybe not so much the last few years. We've had some strange dynamics with fall soil moisture or moisture overall really the last several years.
But we had about a good ten-year stretch there before that where it was fairly consistent, it was fairly wet in the fall. And so the thing you have to realize then when it's wet in the fall is, one, is that the soil is going to freeze later because it just takes more energy for the energy to be extracted, I guess you should say, for the soil to freeze.
But the other trend that we've seen with that is then the soil stays frozen longer because it takes more energy and more heat in the spring to melt that frozen water, the ice, than it would if you're just simply heating up the air and the soil particles.
And so how many days are there between 50 degrees and freezing? Typically what we're looking at, at least with that Waseca, is about 50 days. That going to vary a little bit. The other thing to think about is what's the average amount of precip we get in the fall. So we oftentimes have people say, "Well, we got to do this fall application because I just don't have time to get my nitrogen applied." Well, on average there's 50 days. Actually, that's a pretty good window. We have been getting on average about five and a half inches of rainfall during that window also. So I guess, yeah, that's something to think about. But in general, we just don't see the great urgency to have to be out there a couple of weeks early, just in general with 51 days average there's plenty of time to get that done.
But there has been a trend line of the soil staying frozen later in the spring. That also, there may be some play in there with the recent trend to more urea. It is a little bit more difficult to get anhydrous put on in the spring if it's staying frozen later. However, we still see a fair amount of spring anhydrous and a lot of the producers report that it's just fine. Maybe it puts you in a pinch for time management, but we aren't really seeing any complications with those spring applications.
So the typical day that the frost comes out in the spring on average, at least at Waseca, is on March 17th, on St. Patrick's Day. That bounces around. But if you go back the last 30 years or so, the day the frost comes out at the S-Rock, at least, has always been in March. It's been as early as the first and as late as the 31st, but it's never been in February and it's never been in April, at least for the last 30 years. So that's at least something for reference to be thinking about.
Once that frost is out, particularly if we're looking at an anhydrous application, we'll be thinking about that we're getting towards our application window. And so then also the question is, well, how many working days do we have after that to do our nitrogen application? So if we say that the application window for putting on nitrogen in the spring prior to planting roughly from about March 20th to April 30th, we don't really know in March. That's a bonus if you're able to get out there in March. However, based on some of the ways we define working days, well, part of that is just the USDA crop reports because that's part of the USDA crop reporting.
Typically speaking, you have three to five working days the first two weeks in April. There's about a 40% chance that 40% of the time it's zero, but half the time it's actually five or higher. Then if we move the last two weeks of April, it goes to about three to four working days. There's a 30% chance one and three is zero, but again, another 50% chance that it's at least five or more. But typically speaking, we get about six and a half days then between May 1st and May 15th to do an application.
That's where it plays in as far as putting on urea because we can cover so much ground so fast with urea with a spin spreader or with a floater. And so in a lot of cases that's been working. We will get the people who say, "Well, we just don't have time in the spring." Well, all the nitrogen in Southeast Minnesota goes on in the spring and they do get it on, so there's usually enough working days.
Dan Kaiser:
And we've got a lot of flexibility with urea. That's one of the things with these, especially with the urease inhibitors, just the surface application post-planning does work. I think where some people do get caught is they just wait too long and then think that they need to delay those applications to later in the growing season.
When it comes to working days, I think that's really the only way we're going to be able to make this work. We would have to go to shift more towards a spring application system across the state is really looking at some of these in-season options. And I said with urea, that's one thing that we do tend to have more flexibility.
So there's some options out there. It's just urea is more problematic, especially for fall application. I just do not recommend it for any applications in the fall just because of the lost pathways and the way it converts. It's far more of a risk than anhydrous ammonia. So just one thing you've got to think about them with that, especially we start talking about working days, is that we don't necessarily have to have all our nitrogen on right before, especially if you're corn following soybeans right at planting. So looking at some application windows, stretching and post-planting, it would give us some flexibility, particularly with a product like urea.
Brad Carlson:
I guess just to take this up to planting time, we've talked a lot about the average days and how long it is. The average day we hit 50 degrees soil temperature, at least at Waseca is May 1st. So you're basically right there at corn planting time. And at that point, we're starting to convert a lot of our nitrogen to nitrate, but that's fine. If you look at the dynamics of nitrogen loss, unless it's saturated, typically now we're getting to the point where the plant's going to start taking it up, and so we're in pretty good shape.
So just to wrap things up with our scenario previously, if you applied 10 days early in the fall, if you used nitrapyrin, it's probably gone on the 28th of March, and it's been long gone. But on the other hand, if you just simply waited until the soil temperature got to be 50 degrees, we do look at that product as working fairly well.
Jack Wilcox:
Brad Carlson, extension educator, and Dan Kaiser, extension nutrient management specialist, thank you both very much for this information.
If you have a question or comment for either Brad or Dan, please email us at nutmgmt@umn.edu. Thank you for listening and we look forward to seeing you next time.
Advancing Nitrogen Smart is proud to be supported by the farm families of Minnesota and their corn check-off investment through Minnesota Corn.
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