UMN Extension Nutrient Management Podcast

Advancing Nitrogen Smart, from the University of Minnesota Nutrient Management Podcast:

“Nitrogen transformation up close: Key details for efficient fertilizer management”

August 8, 2024
Written transcripts are generated using a combination of speech recognition software and human transcribers, and may contain errors. Please check the corresponding audio before referencing content in print.

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Jack Wilcox:
Welcome back to Advancing Nitrogen Smart. I’m Jack Wilcox, in communications here at University of Minnesota Extension. Today we’re going to talk in greater detail about nitrogen transformation. And just like your leftovers, why you need to be careful about leaving N out of the fridge for too long.

As always, here to explain the science of N’s behavior in the environment for MN farmers we have Brad Carlson, Extension educator and Dan Kaiser, Extension nutrient management specialist.

Let’s just start off Brad, there’s a lot of science related to nitrogen fertilizer management, but really it all comes down to the nitrogen cycle, doesn’t it?

Brad Carlson:
That's right, Jack. We've tried to emphasize with a lot of our nitrogen education over the last several years that while you may want to simplify how you manage nitrogen fertilizer, really actually understanding how nitrogen behaves in the environment is key if you want to really fine-tune your management and adapt and adjust your management based on the climate and the situation.

And so that really comes down to the nitrogen cycle and how nitrogen changes in the environment. And so particularly we are focusing on the transformation as we move from one ionic form to another. And I think really the key is the presence of nitrate in the soil. Our loss processes derive from nitrate. It's probably worth mentioning or explaining the significance of the nitrate form in the soil, and that is nitrate is a negatively charged ion. The soil particles, clay particles, are also naturally negatively charged and therefore nitrate does not adsorb to the soil. So when nitrogen is present in its ammonium form, that's positively charged and we know opposites attract and so it's almost like a magnet. The ammonium does actually attach itself to clay particles because of that electrical charge. However, the processes of the nitrogen cycle will naturally change the ammonium into nitrate, and therefore at that point it's subject to be lost.

And like all processes that are biological, in this case, we have microbes in the soil that are doing this, they're time and temperature dependent. It's one of the reasons why we make recommendations not to be applying fertilizer until the soil gets cool in the fall. Because when we're applying anhydrous ammonia in the fall, it turns into the ammonium form in the soil, and then the processes of the nitrogen cycle, the process of nitrification will turn it into nitrate. But however, if it's cool in the fall, that process slows.

One of the analogies that I use for this frequently is the fact that if you think about just eating your meals at home and you've got leftovers, you don't leave those sit on the table, you put them in the refrigerator. And most of our refrigerators are probably 50 degrees or cooler, probably cooler than that. We recognize that if you leave stuff out at room temperature, bad things happen to it. And similarly with nitrification in the soil, we really like to see that temperature at 50, but the transformation still happens at 50, it just is slower. It really doesn't come to a near halt until we freeze the soil.

But recognize in the fall that the days are getting shorter and colder as we go so every day there's less potential for nitrification to happen. We've talked a lot about some of the averages that come from the research station at Waseca because we have such a extensive database there. And so just to put some averages on it, the average date that we get down to 50 degrees where we would recommend being able to apply fall anhydrous, with a nitrification inhibitor I should add, is October 18th and the average day that the soil freezes at Waseca is December 9th.

Dan Kaiser:
So one of the things too that, Brad, I know you'd mentioned this, I think that refrigerator analogy comes in because as you mentioned before, is really the process doesn't completely stop until we hit freezing. So it's the same thing about food in your refrigerator. If you look at actual numbers, I mean, there's been a few studies that have looked at this, looking at the amount of nitrification based on temperature. And just looking at the numbers, you might be looking at maybe 10/20% or just a very small amount of accumulation of nitrate or conversion of ammonium to nitrate at those cooler temperatures. I mean, really, it doesn't really ramp up until we get beyond 50 degrees. I mean, there are some other factors that can impact some of your nitrification of specifically soil pH. We know that the lower the pH generally doesn't favor the recovery of nitrifying bacteria or the activity of nitrifying bacteria.

Brad Carlson:
Another factor here is, as we talked about the dates is some of the time too as far as just how much time that nitrogen is present in the soil. We talk about risk factors as far as water moving through the soil or the soil being saturated. And we've also talked a lot about side-dressing nitrogen and the fact that what you're really counting on with that practice is just simply not having the nitrogen present in the soil. And that gets back to this whole time factor.

So if we look at some of the other average dates, the date that we typically would freeze up where you're no longer able to put nitrogen in the soil is December 9th and the average date that we thaw it is about March 8th. And of course we recognize in most situations is pretty muddy, then we're probably not out doing an application. However, that process of nitrification starts going at an ever accelerating rate as temperatures warm at that point in time.

Let's talk a little bit about nitrogen loss, because once the fertilizer is in the soil and in its ionic form, really the loss processes of nitrogen are water-based. And so that's why that nitrification process is so important because not only are they water-based, but they happen when the nitrogen turns itself into nitrate. And so there's really two main ways that we would lose nitrogen in the soil.

One is leaching, and that is just traveling downward as we already talked about. Whether that nitrate being in its negatively charged ion and not being absorbed in the soil or onto clay particles, it's free to move with water. Some may refer to it as being dissolved in water, I guess technically that's okay to say that, it's not really quite the same, but regardless, water's going down through the soil, nitrate is going to go down through the soil.

And as long as it's still in that rooting profile of the corn within about three feet, it's still recoverable by the plant, what we really want to avoid of course is it leaving the system all together and I think we recognize some of the issues related to nitrates in water, whether it's groundwater or surface water, there's separate issues there.

And so particularly, one, because corn roots to about three feet, and number two, because most of our drain tile that's been installed in our heavier soils across southern Minnesota and western Minnesota the last 20 years or so is about three feet, that's kind of a critical depth. I know there's been a lot of people have blamed a lot of the water quality problems on drain tile. I guess I kind of look at it differently, I view that drain tile as being the conduit through which that water travels. But frankly, once the nitrate gets below the rooting zone of the crop, it's kind of lost whether it moves through drain tile or shallow groundwater. And shallow groundwater is the other path that it could end up. In our surface waters, we don't typically see a lot of denitrification losses once we get down past that rooting zone, there's not a lot of organic matter down there and so forth to help drive that process.

So the question is what are we losing? Well, on average what we would see is some of our research at our research stations over the past several decades shows that we lose roughly about 10 pounds of nitrogen per acre regardless of whether we apply fertilizer. A lot of that is what we would consider the background nitrogen that's from soil organic matter or mineralizes. In a lot of cases, we're talking stuff that accumulates after the growing season and then is just sitting there. And then if we're following best management practices, kind of where we've settled in with a lot of our research over the last several years is about 15 pounds of nitrogen per acre. I mean, it's not insignificant, that's a fair amount. If you calculate the whole landscape, there's a lot of nitrogen that moves out that way, and therefore one of the reasons why we focus on this.

Another question that comes up is how fast does it move? Can we calculate this? Well, sort of. We've got kind of a rule of thumb that if you have an inch of drainage, that is an inch of water moving out through the drain tile, it'll move nitrogen five to six inches in the soil profile. So we've kind of been able to ascertain that based on research over the years as far as applied fertilizer and how much water is moving through the drain tile when the nitrate numbers start picking up. And so early in the season we're picking up nitrate in that tile, but physically that could not be the applied fertilizer because not enough water has moved through at that point.

Dan Kaiser:
So Brad, I think one of the key points here is we're talking an inch of drainage, not an inch of rainfall.

Brad Carlson:
Right.

Dan Kaiser:
And a lot of times I think people kind of think about it since we're measuring the rainfall coming in, and the thing that you got to know is that not all that rainfall goes into the soil. So if you're saturated, you're going to be likely getting more surface runoff so it isn't that you're getting six inches of movement in a silt or a clayloam soil with every inch of rainfall, it's an inch of actual drainage water moving out of the system.

Brad Carlson:
That's right, Dan. And the way we would calculate this for research is we would actually collect a water sample out of the outlet of the drain tile, you can use a stopwatch and time that and collect the volume, and then if you know your area that the water came from, you can multiply that volume towards the area. So what we're talking about is like an acre inch of water. The guys who irrigate talk about an acre inch of water falling from the sky or coming out of an irrigation rig, you can talk about an acre inch coming out of the drain tile also.

And that's very difficult to calculate because as I think people also recognize, there's pockets in their field that are wetter than others. And so if you got an outlet, even if you're trying to calculate that, the water did not come proportionally equal from all parts of the soil, so some areas contribute more and some areas contribute less. But that's why this stuff is really is just a rule of thumb because there's going to be a lot of differences. If you've got compacted areas that's going to affect this. There's going to be areas related to macropore flow, cracks in the soil, earthworm activity that's going to speed things up and so forth. And so it's just kind of in general.

The other thing that we have to keep in mind is that on sandy or coarse textured soils, water movement is much more extreme. There we're looking at moving more like a foot with an inch. And in a lot of those cases that can be a lot more common, as I think Dan you talked about the runoff, I think we all recognize there's typically not a lot of runoff on sandy soils, most of that water is going down through there. And so that's why our nitrogen recommendations, our recommended practices in sandy soils and irrigated soils is so drastically different than on our finer textured soils.

Dan Kaiser:
And that's where a lot of our time comes in too. If you look at the best management practice regions for the state of Minnesota, the southeast, our sands, we don't recommend fall application just because the overall potential risk for movement of water is greater, and also some of those areas like the southeast tend to get more on average rainfall than the rest of the state. So it's something to think about with that is that while we don't talk too much about from the holding capacity standpoint for nitrogen for many of these soils, a lot of this does boil down to when we talk about before as the timing aspect, just because of that overall tendency for potentially getting more transformation of ammonium to nitrate and potential for loss due to leaching.

Brad Carlson:
Yeah, and while we don't really define the southeast as coarse textured soils, it is siltier soils, and so technically by definition it is coarser than our glacial soils in south-central and western Minnesota are. But the other aspect to that is they're shallower also. And we talk about the fact that once it moves below the rooting zone of the crop, even if that rooting zone is a layer of limestone, that's the constraining layer for the crop roots. Once it gets to that point, it's pretty well lost from the system.

Dan Kaiser:
Yeah, and one of the other things, when we start talking about a lot of our glacial soils, one of the loss pathways that's pretty key is denitrification, which if you look at it from a nitrification standpoint, we need oxygen and ammonium to transfer or to convert from ammonium to nitrate. Denitrification is the opposite process by which we have saturation for a significantly long period of time where we deplete the oxygen and when we deplete the oxygen, one thing that happens is the microbes are looking for another oxygen source, which then it moves to nitrate. And we start to look at actually the reverse process where we're going from the dissolved, the nitrate form of the soil over to a gaseous form in the end, nitrous oxide is the end product. When we start talking about denitrification, I mean, in the end, if we have full denitrification, it'll actually move to N2 gas, which is what composes about 78% of our atmosphere. But generally we get to some intermediary there when we get to a loss potential.

The thing about denitrification that it is a biological process, it does require saturation for roughly about four days before we start to see these things tend to kick in. So it isn't immediate that this is going to occur after a rainfall and it is temperature dependent. It does also require a carbon source, and Brad mentioned that, we don't see a lot of deep denitrification because there isn't a source of carbon for the bacteria to feed on where they can be active in those zones, where it might be low in oxygen where we can get denitrification. So that's one of the big things that does come into play.

So if you look at saturation, again, about four days is typically kind of the point at which we see loss really start to kick in due to denitrification because it takes a while to deplete the oxygen that's in the soil. If you look at some of the numbers, there's been some studies on this looking at different temperatures that it is a temperature-dependent process. So if we have saturated soils early in the growing season, say April, maybe into March, if they're colder, we see some denitrification, but it isn't to the amount that we see later on. So you might expect 5% or less of the amount of nitrate is lost at that given point. It's really when we start getting into late May, early June, specifically in June where if we're saturated for extended periods that we can start seeing some fairly high loss potentials. Some of the research would say at about 70 degrees, about 12% loss for a four-day saturation and up 25% for 10-day saturation, you get close to 80 degrees and we're looking at 20% at four days and about 40 to 50% at 10 days.

So I mean, really it's those extended periods where we're saturated in June where it becomes the problem where we can see a substantial amount of loss. Now, the question then with a lot of those fields is those areas, do you have any corn left if it's saturated for that long period of time that you want to make that decision? But it is really the biggest when we start talking about denitrification, really the issue is June when it comes to a lot of loss potential.

Brad Carlson:
That's right, Dan. And particularly as we've seen some shifts in weather patterns over the last 20 years, I think most folks are used to it being wet in the spring, you got snow melt, it's muddy, by the time we're able to get the corn planted towards the end of April, from there we kind of think of that the weather "straightens out", but as it stayed wetter, and as we realize as you get later in the year, the soil temperatures get warmer.

And plus, we've had more time since the time that we applied our fertilizer, and so the risk just gets higher as we get later on. We don't typically see those issues with big losses because it was wet in March because the soil temperature is pretty cold at that point in time. So, A, okay, so if you apply to anhydrous in the fall, particularly if you use a nitrification inhibitor, which is not the topic of conversation for today, but in a lot of cases a lot of that nitrogen is not nitrate, it's got to be nitrate for it to denitrify, so it's not going to be lost from that standpoint. Number two, it's cold, the process is slow.

And so that's why when we get these years where it just stays wet and rains way into the growing season, that's where we're really at risk and we start seeing some of the unevenness. It is one of the areas we've talked a lot about when it's really dry because the loss processes of nitrogen are water-based, when it's really dry, you can get away with a lot of things. And so we will talk about some of our recommended practices and we'll have people say, "Well, I did that and it was fine." Well, you probably had a dry year. It's really in the wet years that we start seeing that. And I can even say from myself, from straight experience with the phone calls we get when guys call and they say, "Well, my corn looks really bad," and you start asking what the practices were, it's on these extreme years, you start finding out like, "Well, I put on urea in the fall," well, we don't really recommend that and stuff like that. That's when this stuff really starts to rear its head.

Dan Kaiser:
And one thing too that should be noted is even with tile drainage, I mean, we're still going to get some denitrification in fields. Tile drainage, we're draining away what we call free or gravitational water and what we have then is when we talk about what's left is at field capacity or available water, and there's some areas that are just overly saturated. You have some very small pores that might stay more saturated versus some of the other areas that denitrification will continue on. We have what we call microsite denitrification, which will generally occur year-long, even in situations where you wouldn't consider your soil to be saturated. And when we look at loss potentials, we generally see that most soils that we're going to see some given loss de to denitrification annually, even if the soils are dry and it's just because there are areas that are more saturated than others. And if you look at loss potentials like leaching or volatility, we can see years where we don't see much potential loss because there isn't the conditions for that.

So denitrification is one of those sneaky ones that you're going to get some loss no matter what, it's just where it's really apparent is these years where we get a lot of rainfall and the soils are saturated for a long period of time when you can see yellowing out there, because some of this microsite denitrification might be something that isn't going to necessarily be caught visually out in the field when you're looking at scouting your fields in June or July.

Brad Carlson:
And Dan, with the microsite denitrification, it's also important to recognize, I mean, this is largely a effect and effect that happens in a lot of our clay and loam soils, particularly our glacial soils are prone to this. It's kind of like perching water in places where you may have some clay accumulations and so forth. And so it can be kind of soil type specific, and it is kind of more of a regional thing also. I think if we look at some of the data that we've got, there's some of the data from the drainage plots at Waseca have that Charles Randall originally worked with going back decades provided some great insight in this.

And there's one classic study, this one runs from 2000 to 2003, and we looked at some timing and some nitrogen rate effects and what we find is that at that point in time, we were recommending about 120 pounds of nitrogen. And if you looked at the fall application of 120 pounds of anhydrous ammonia, what we ended up with as a four-year average yield was 166 bushels an acre. And if you looked at the spring, applied at the same exact same rate, it was 180 bushels an acre. So we're talking 14 bushel difference by changing the timing. And if you look at the nitrate concentration that came through the tile lines, in those instances, you didn't see a lot of difference. So we only saw the average concentration difference at 13.2 for the fall and 13.7 for the spring. Or if you look at the total nitrogen loss over the four years, now, this isn't an average, this is adding it up, it came to 220 pounds per acre for the fall applied in 219 for the spring. It was almost identical.

So the fact that we had a big yield hit but we didn't pick up that nitrogen in the tile lines really indicates that that nitrogen is going up to the atmosphere, is denitrifying. So particularly we need to be thinking about as we think about our best nitrogen management practices, if we've got these soil types that are inclined to have this effect, and I can say being familiar with those plots, in a lot of cases, we weren't talking about water standing, we weren't talking about pooled water out there, it's just wet soils, and there was water moving through the drain tile. And so while it drained and there was no water standing, clearly there was a lot of denitrification, even though we also had water moving through the drain tile.

Dan Kaiser:
And one thing that you should note is with a lot of this denitrification it's what we would typically deem as microsite denitrification, this is a natural process, again, that's going to happen in all soils all year. So if you're wondering, "Do I need to be factoring this in with my nitrogen rate?" When we start looking at our recommended rates with, say, the corn N rate calculator, that since the numbers are derived from values we've collected from actual field studies, that's all factored in with that recommended rate. So it's really those excessive years, and that's when we're talking about maybe a rescue treatment where we get a lot of loss that some of the stuff has to be factored in because some of this natural processes, some of these loss processes, particularly denitrification, if it's just more of an average year, that's already factored in with the rate guidelines. So you don't really need to worry about making any adjustments for that.

Brad Carlson:
That's right. There's one last point I want to make here before we're done, Dan, related to denitrification. And that is, and we aren't going to go into detail about this here, but the process of denitrification can actually be harnessed for our benefit too. So if we're looking at having high nitrates in water, for instance, coming out of drain tile, that water can be moved into, for instance, bioreactors or saturated buffers and so forth. And in the instances where you can actually drive denitrification in the water to lose the nitrogen out of it so it doesn't end up flowing downstream similarly, like some of our treatment wetlands water just sits in them for a long enough period of time, we can actually denitrify some of that nitrate. So this process can be harnessed also for the sake of cleaning up the water. So it's just a case of knowing the science, we can start innovating ways to deal with it.

I think the last thing is, Dan, Dan alluded to deep denitrification, we'll occasionally hear people talk about that, that nitrate will denitrify deep in the soil profile, but if you start digging into it, like a lot of things related to nitrogen, you find out that it's very region-specific. And I don't mean region within Minnesota, I mean region nationally. It requires a carbon source to denitrify. And so while we don't usually find that carbon source deep in the soil profile, there are some soils, particularly out in the Dakotas, that have sulfur. And in the absence of carbon sulfur, in this case, pyrite, which is fool's gold, and iron sulfate, that can actually also help drive denitrification. So there are some places where they don't really worry about nitrates getting into the groundwater because it will denitrify deep. There may be some pockets in Western Minnesota where this happens. In general though, this is not in effect for Minnesota.

Jack Wilcox:
Have a question about something you’re seeing on your farm and how it relates to what we talked about today? Send an email to Brad Carlson or Dan Kaiser 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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