The 5 main pathways of nitrogen loss: What are they, and how can you increase efficiency?
Hello, and welcome back to Advancing Nitrogen Smart, the special podcast series from University of Minnesota Extension. I'm Jack Wilcox in Extension Communications. I'm here with Brad Carlson, Extension Educator, and Daniel Kaiser, Extension Nutrient Management Specialist.
Jack Wilcox:Dan, explain for us some ways that nitrogen can be lost from the soil.
Daniel Kaiser:Well, you know, Jack, it really kinda depends on what form we're talking about. If we're looking at different forms, there are different ways that we tend to lose them, and we've talked about some of these before in advancing Nitrogen Smart. But there are five main ways if we start talking about loss pathways, volatilization of ammonia from the soil surface, which tends to happen, with ammonia forms of fertilizer like urea and anhydrous. I mean, as well can, lead some loss loss from ammonia volatilization. Leaching of nitrate below the ridding zone, you know, I'll certainly we we talk about loss, and we we'll talk about this a little bit more.
Daniel Kaiser:But, you know, deeper in the profile, it does become a problem because the plant tends to take nitrate up with water. And if it gets below that, then the plant can't capture it, then there's really nothing we can do to prevent it from going any further and either going out to the tile line or getting into some of the water below the surface. Denitrification, you know, it's a big one in Minnesota, which is the loss of n two gas or some sort of n o x, what we call NOx or nitrous oxide forms through d or through the reduction of nitrate to eventually, hopefully, to dinitrogen gas. Runoff, we'll talk a little bit maybe about that. It's not as much what we're concerned about runoff of water containing nitrate or erosion of soil organic matter.
Daniel Kaiser:I mean, these are all, you know, ways in general that we tend to lose nitrate potentially from the soil.
Brad Carlson:I think really the key for for this stuff, and we we kinda really stress this a lot when we teach Nitrogen Smart, is that these are all water based processes of well, maybe not so much volatilization that's a little bit different. But but in general, when we talk about, losing applied fertilizer or losing nitrogen that was mineralized out of soil organic matter, most of this stuff is going to be water based. And so, you know, particularly when you look at denitrification, you look at leaching, those things happen when the soil is saturated. Runoff, as we'll mention here a little bit later, which isn't a significant process, but, you know, if the water is running off, you also assume that obviously then it's not infiltrating, and that's also a factor of of, being saturated. And so that's that's something to keep in mind is that a lot of this stuff really is is water based.
Brad Carlson:So let's talk a little bit about denitrification. You know, that's the one that we tend to worry a lot about in Minnesota, and and that happens again when the soil is saturated. And so if you think about living organisms wanting oxygen, when the soil is completely saturated, there's no oxygen. There's water is taking up, all the pore space in there. And so there are microbes down there that will try and source oxygen out of nitrate.
Brad Carlson:So nitrate n o three has oxygen in it, and so this process is is biological. Maybe some people don't think it makes sense for there to be a biological process happening when there's no oxygen, but, there's microbes that do all sorts of things. And so this is the process whereby those microbes steal the oxygen out of the nitrate molecule. And if the process is complete, it goes all the way to n two gas, which, of course, is the major component of the atmosphere. If it's not complete, it ends up as, as NOx or some sort of nitrous oxide gas, which is a a concern these days, with respect to climate change.
Brad Carlson:But, but particularly, you know, because it's biological, it it does what happens with a lot well, a lot of things that are biological speeds up when it's warm, slows down when it's cool because life depends on warmth. You know? And and so in Minnesota and then because it's depending on the soil being saturated with our heavier textured glacial soils in in Southern Minnesota, we tend to see this condition setting up frequently.
Daniel Kaiser:So the speed at which this happens, it doesn't happen right away, and that's one of the things to, you know, I I think understand about this issue with denitrification is that, you know, if you get a rainfall event, if your soils are saturated, it tends to really, you know, take about twenty four to forty eight hours after flooding for this denitrification process to really occur, and it is temperature dependent. We'll we'll talk about that here in a moment. Compacted soil, it's it's more of an issue because they can stay waterlogged longer because the more you compact the soil, the smaller the pore space, so there's less pores to saturate and fill. The main thing, though, about denitrification, there there are a couple things. You know?
Daniel Kaiser:One is when you start looking at it, it does require a carbon source. So it's one of the questions why if we've got groundwater deep in the profile that's saturated constantly, why doesn't the nitrate in that groundwater denitrify? And it just simply that there isn't a source of food for the microorganisms to be active. So there isn't a whole lot of biological activity deeper in the profile for this to occur. But as Brad said, it is a microbial process, and nitrate, it's, you know, the easiest when it comes to organisms when we start talking about reducing, elements, or reducing compounds in the soil.
Daniel Kaiser:So that's why nitrate typically is acted upon first. And, you know, it is temperature dependent, so that's kinda one of the keys. And, you know, again, we'll talk about that here in a moment because I think it's kind of an important point. If you're you're why we don't get or don't expect a lot of denitrification early in the spring is because of what we see with temperatures of the soils at those points in time.
Brad Carlson:Yeah. And and I guess it's important to remember it does require carbon, the catalyst in this process. And so when we get below topsoil, when we start losing the color of the soil, the the colors associated with organic matter and humic acids and carbon sources, this process is gonna become less important until, you know, by the time you get down below the rooting zone. In general, it just sort of stops or maybe small amounts of it with some carbon down there, but there's not a lot of carbon once we lose the color in the soil and that it it turns either yellow or gray. It's worth noting that and and I guess it's not significant in Minnesota, but but pyrite, which is iron sulfate, that sulfur and pyrite will actually also serve as a surrogate for carbon and drive denitrification.
Brad Carlson:So when you get out west, particularly like in the Dakotas, there is free pyrite in the soil. And so they will actually see deep denitrification, you know, because that mineral is occurring throughout the soil profile. That's not something we see happening in Minnesota, but, it's one of the reasons why that we don't see a lot of significant groundwater issues, for instance, in the Dakotas, because, there will be some denitrification in much deeper in the profile than in Minnesota where we typically see you gotta have carbon there or else it's not happening. I think another aspect about denitrification that's that's interesting is, you know, we we can actually harness some of these natural processes for the sake of trying to clean up the water. And so denitrification will remove nitrate.
Brad Carlson:Now, obviously, when it's in the soil profile, we want the we want the nitrate there left in the soil for the plants to pick up. However, if it's already lost to water, you know, which we'll mention leaching here in a bit, but it's gone in the water, denitrification is a process that can actually be harnessed to get the nitrate back out of the water. And so some of our conservation drainage practices or or edge, what we might call edge of field practices, such as bioreactors or constructed wetlands and so forth, we'll actually use denitrification as a natural process to try and get nitrate out of the water. And so while it's not something that we necessarily want to happen out in the field, it is a process that we can harness, for our own purposes, under the right circumstance.
Daniel Kaiser:So I mentioned this before on the temperature side, and this is really a key component to this because you will get questions every once in a while when the soils are cool whether or not we see much denitrification. And if you look at some of the historical data, there's a study that's been done a number of years ago where they looked at soil saturation for four or ten days and looked at the amount of nitrate loss due to denitrification. If you look at a situation where soils are cool at around 50 degrees Fahrenheit, they were looking at four days, 3% loss, at six or at ten days, 6% loss. Compared to 77 degrees Fahrenheit, they're looking at 20% loss after four days and 43% after ten days. And that's really one of the things to look at here in Minnesota is where we tend to see more issues with losses in the growing season due to de electrification.
Daniel Kaiser:It's generally when we start seeing soil temps get into that 70 to, you know, upwards of maybe 80 degree. So you're looking at really June being kind of that target point where we tend to get a lot of water, then we get a lot of saturation, which can result in a fair amount of loss. So that's one of the if you're you're getting a lot of saturation early in the spring, well, we might see a a potential for a small amount of the the nitrate converted, it's not gonna be as bad as it would be later in the growing season. Wouldn't really be as concerned, if you're in those with some of those really early spring rainfall events. And and in fact, a lot of times in those cases, hopefully, the nitrogen's in not in the nitrate form at that point anyway from the fertilizer we applied, so the risk tends to be low.
Daniel Kaiser:So just when it comes down to it, really, I mean, really just remember that it takes an excessive amount of saturation. And if your soils are saturated for two weeks and you're they're warm, you're gonna have a a pretty significant potential for denitrification. Now we do expect some on a yearly basis. We just cannot get around denitrification losses. There's gonna be some in about every soil every year.
Daniel Kaiser:It's just really a question about how much, and and, you know, really looking at whether or not we need to do anything about it later on really depends on on the time period when when that saturation occurs.
Brad Carlson:And I think it's an important point to remember for those who don't deal with this stuff every day is denitrification happens to nitrate. And so, Dan, you already mentioned the amount of nitrogen that's converted to nitrate from the applied fertilizer. And so if we're applying anhydrous ammonia in the fall, that is turning into ammonium in the soil, and then it's gotta go through nitrification. So it's one of the reasons why we wanted we want to delay application timing until it gets below 50 degrees because, obviously, that process is slow, but also denitrification is slow at that point also. Or we would use a night for a or I should say and, not or, and a nitrification inhibitor, all intended to prevent the the nitrogen from from being in the nitrate form because this process doesn't process doesn't happen if it's not nitrate.
Brad Carlson:And so then furthermore, I guess, we also think about, you know, typically speaking, it's it's drier in the fall, at least historically. Now the last decade or two, we've had some very wet falls, and that can really change the dynamics. But as, you know, as we said, this this requires the soil to be saturated. So if we are relatively dry in the fall, we've got capacity in the soil to absorb water. Then in the spring, when the snow starts melting, the frost goes out of the soil.
Brad Carlson:You know, first, that water is going to infiltrate into the soil, and and it's gonna get soaked up to some extent before it gets saturated. So the the whole process of the, needing it to convert to nitrate, needing it to be saturated, and then also wanting to be warm, all lead to not seeing significant denitrification losses in the early early spring. Now but it's worth noting, though, that that some of the research that we had on the drainage plots at Waseca do show some significant yield differences between fall applied and spring applied, fertilizer. However, they don't translate to water quality issues. And so, you know, we will assume that on those heavy soils in Waseca that the difference there in yield was probably associated with denitrification.
Brad Carlson:So we don't wanna say that it doesn't happen through the winter. It definitely can and and will, at least to some small extent. And like everything, that all is gonna vary depending on the weather conditions through the winter. And so it's one of the reasons why if you talk about, for instance, split application, the reason that you take advantage is that that split application works is fertilizer, it's not even out there to be lost. So in the same situation, while we don't look at seeing significant amounts of denitrification happening in the late fall and the early spring, if it's not even out there, then it's not gonna happen either.
Brad Carlson:So, you know, that is one way of hedging, using timing to to hedge on loss, avoiding loss from denitrification.
Daniel Kaiser:So the other form of gaseous loss is volatilization. So while denitrification involves the loss of nitrous oxide gas, volatilization is the loss of ammonia gas from the soil surface from ammonium fertilizer sources, which are the bulk of what we apply our ammonium. We don't have very many nitrate sources, just mainly because of timing issues and the leachability of nitrate. So a few things that can affect volatilization, surface applied and fertilizers. Urea is really the one that's of more concern.
Daniel Kaiser:We'll talk about why that is here in a moment. Surface applied manure, we get ammonia volatilization from a manure. If you can smell ammonia in the field after application, that's what volatilization is. Then there also can be some loss during storage and handling of manure. I've you know, just working with turkey litter and some poultry litter in the past.
Daniel Kaiser:I mean, I remember from some of my days as a younger person doing the research in Iowa, just digging into those piles and the amount of white clouds coming out of it, which was the ammonia escaping as the piles were being stirred. Ammonia volatility is I mean, it's it's an issue, and it's one thing that I've been kind of a major concern, particularly with urea fertilizer, and I'll talk about, you know, urea timing here in a moment that, you know, we could be getting more than we think we are. That could be the reason why we see some underperformance of urea. So what tends to increase volatilization of liquid versus solid fertilizers, mean the the fertilizer has to be solubilized into into more of a kind of a liquid mix with the with the water in the soil to have some volatilization of the fertilizer. One of the things about urea is when it comes to volatility, it's more of an issue when you can't see it versus when you can.
Daniel Kaiser:If you can see the material, the material isn't dissolving, we know that there's likely not any any volatilization of material occurring. Broadcast surface applications without incorporation, we like to see incorporation at about three to four inches with ureas. Yeah. The deeper you can go, the the less issue with volatility. I mean, the same thing with anhydrous injection.
Daniel Kaiser:You know, you wanna be looking at five, six inch injection zones to reduce the risk of any gaseous loss of of of the ammonia post anhydrous application. One of the things that affects volatility more than anything is moist soils, especially if you if you're putting it right directly in the soil surface and there's some water there and you get a lot of evaporation because you see upward or upward movement of air, which tends to carry the ammonia as it's being converted. High temperatures and high soil pH can also impact volatility. And, you know, the reason for this really is because of how urea specifically is converted to plant available forms. If you look at urea, it involves a enzyme called urease, which is in all of our soils.
Daniel Kaiser:It's actually if you look at where the highest concentration of urease, it's actually in the residue that sits on top of your soil. So if you apply urea specifically in something like continuous corn where there's a lot of residue, we can really, kick start the process by which, that hydrolysis, which essentially is splitting the urea molecule, which is organic, into ammonia and carbon dioxide. So it's said in that splitting process, the the urea has to go through a gaseous phase, that NH three, before it becomes NH four. And to become NH four, it needs acidity and and water in the soil. So if it's near the soil surface, and that can't be captured, we that's where we tend to see the loss.
Daniel Kaiser:The way we get around it is with some of the urease inhibitors, which, you know, essentially delay or they affect they kinda jam up the process. If you you kinda think of urease, they call it kind of a kind of picture it as like a as like a key in a door that the urease unlocks the process, and these these inhibitors kinda jam it up that process to prevent the urease from acting and slow the process down. And what that does essentially is to give more ability specifically of urea to be leached deeper in the profile. And urea is a neutral molecule itself, so it is water soluble, and it will move with rainfall. So if you can't get it incorporated, what we, you know, typically say is we can get a quarter inch of rainfall within four days of application.
Daniel Kaiser:Typically, the loss potential is not as significant. But, if you can't, then incorporation is some of an issue. Now one thing about this issue is we said temperature will kick start it, but it doesn't stop it. Since it is an enzyme process, this loss pathway can happen into situations where the soils are cold or even frozen, that we can get some volatilization of urea over the winter, which is one of the reasons why I think, we're underestimating this loss pathway specifically for areas that are utilizing fall ureas where we might be getting a lot of our losses and underperformance of that particular product. So it is, pretty significant, particularly if you're managing urea, UAN, those sources that have urea in it.
Daniel Kaiser:Revolatilization is is a big thing you need to be thinking about in terms of what you're doing for applications to make sure we're not seeing ex an excessive amount of the the nitrogen in these these urea based materials lost before it can be converted into a plant available form.
Brad Carlson:Yeah. I know there there's we get questions every year about top dressing urea, You know, what what should be done as far as incorporation, or is it is it safe? You know? Or if I just bowl it out there on top of a standing crop, am I gonna lose it? You know, obviously, a lot of that is just gonna depend on the weather forecast.
Brad Carlson:You you really need to be paying attention to that. And, you know, in general, we get a fair amount of rain in early June, so timing that, isn't usually an issue. The ability of farmers to cultivate that and and get it into the into the profile isn't so great. There are some guys who do do that. But, you know, I've I I think years ago, a lot of people used to say, well, we get a heavy dew. That'll work it in. Well, no. It won't. It just dissolves the prills, spreads it thin, and then it'll blow off. You know?
Brad Carlson:And so, you know, the the other thing, I guess, it's important to realize that that, you know, as Dan, as you mentioned, that this will happen when it's cold. This can also happen, really at an accelerated rate when it's hot too. I remember we had our nutrient management group had a meeting at the experiment station in Waseca, and that that seemed to vaguely remember it was, like, on a Monday or Tuesday, but it was in June, and it had been extremely hot. It had been, like, approaching a 100 degrees, and we were seeing severe signs of of of nitrogen deficiency in some of those plots just simply because the even though a urease inhibitor was used on the urea that was put on, them high, high temperatures still drove a lot of nitrogen loss through volatilization.
Daniel Kaiser:Well, it wasn't as much deficiency as it was burning on the leaf edges, and that's one of the things that's interesting you can see is and the worst thing for it is with warm temperatures is when you what I said before, evaporation. I mean, if we we tend to get a lot of upward movement of water and a lot of evapotranspiration, which includes evaporation from the soil surface, which tends to drive this process. And even with an inhibitor, you can see it. In those years, you could see the, we're applying directly in the soil surface, not applying trying to keep it off the leaves, but you're seeing a lot of burning on the leaf edges from ammonia being volatilized. So it's it's a sneaky one because it's one thing with urea.
Daniel Kaiser:I said, when you don't see it, it's it's not when you do see it. It's when you don't see it when the problem can occur. And it's not like anhydrous where you can always smell it out there. If you go out into the field, if you're getting some loss after application with urea, it can be nondetectable and happen over a longer period of time and, you know, happen at times of the year when you're you're not really expecting it. So it's a little bit more of an issue.
Daniel Kaiser:It's why I worry more about urea based. When we start talking more urea based applications, with timing, I mean, really, it needs to be timed closer when that crop is out there in the field, and fall applications of urea really are at a a greater risk, which we could go on for another half hour probably about if we had the time.
Brad Carlson:And and, Dan, we also get questions every year about Dribbble applying 28%, 32% to UAM, and so some of that is is urea also. What's been your experiences with with volatility loss of of that product?
Daniel Kaiser:So certainly, knife or coulter injection is probably a better option for prevent venoming loss. But if you look at some of the data, I mean, really, that that small band, you don't have a whole lot of surface area for it to evaporate off of, so it isn't as much of an issue. One thing I would say though is get it away from residue because you don't wanna be laying UAN directly on residue because of what I talked about before with the amount of urease being high in residue, that we can see more volatility of it if it's directly on the residue. So there are some options. I just try to keep, like, with the Y drop system.
Daniel Kaiser:I know that was one of the big things that, you know, putting bands on both sides of the plant. I don't necessarily think that's a good idea. I think it's better the fewer bands you have, the better because that's just less surface area to volatilize your your material from versus the Coulter injection, which is your better option.
Brad Carlson:And so we do get questions frequently about how is nitrogen lost in the soil. And, obviously, there's all all the dynamics that we've talked about throughout the nitrogen smart series. But we we do think that on our heavier clay loam soils in Southern Minnesota, it's the most significant process. But, of course, it's gonna depend a lot on the year and the timing of rainfall and, you know, and then, of course, as we move into the lighter textured cells in the Southeast or the Southwest or anywhere, of course, there's coarse textured cells, it's probably gonna be more leaching. So let's talk a little bit about leaching.
Brad Carlson:You know, it's it's we've talked about this many times that the nitrate molecule is negatively charged, that that it the clay particles are also negatively charged. And so while they don't necessarily repel each other, they don't attract. And so the nitrate just kinda sits free in the soil when it's when it's converted through the nitrogen cycle into nitrate. And and so that will move with water. And and so as you've already mentioned once, Dion, that that's the main process by which it's taken up by the plant.
Brad Carlson:It's it's pulling in water, and that nitrate's free in the soil. It comes along with the water, and it's picking up the nitrogen. It'll also go down with the water. And and so that's, you know, that's obviously a big concern. I think a lot of the water quality issues that we that we hear about these days are nitrate related.
Brad Carlson:There's certainly there's some other stuff. There's phosphorus, and occasionally, pesticide issue will come up. But for the most part, really, nitrate is kinda where the action's at these days. It's it's, the the main driver with our state nutrient reduction strategy, which is kind of trickled down from the federal effort on on trying to reduce the hypoxic zone in the in The Gulf. And and so that's because nitrate is a limiting nutrient saltwater, and it's going down the Mississippi River.
Brad Carlson:So we've got this target of a 45% reduction in the total amount of nitrogen going down the river. Furthermore, it's also the focus on a lot of the groundwater issues in Southeast Minnesota, and think a lot of people have been paying attention to that more the last couple of years. There's been some petitions and lawsuits and other things related to the amount of contaminated wells in Southeast Minnesota. So in all those cases, we're dealing with nitrate that moved with the water, either moved into tile drainage and into a surface water, moved with shallow groundwater into surface water, or it just leached straight down into groundwater, you know, as particularly is the case in the in the Southeastern part of the state, but it's also an issue in the the far southwestern tip of the state that's in the Missouri watershed. That also has some less soils, that are quickly, easily permeable for nitrate to move also.
Brad Carlson:And so, you know, the the the one thing we need to think about when it comes to nitrate loss is the total amount of nitrogen we're losing as a factor of how much nitrate is free in the soil and how much water is moving. And so from that standpoint, we are going to see differences in this from one year to the next just simply based on the amount of precipitation and excess water. So another another thing, that we get asked quite often is is how fast does this happen? So if if it takes excess water to move nitrate, then what's the speed at which this happens? And while there's no real perfect measurement of this, a lot of the data indicates that nitrate will move itself down, you know, roughly five to six inches in our heavier textured soils, and maybe as much as a foot in coarse textured soils with about an inch of free free water movement.
Brad Carlson:And so if you think about the water the soil being completely saturated with water, if one extra inch, if it rains an inch and that inch goes down, it doesn't run off the surface, that's probably going to move nitrate down about six inches, or maybe about a foot if it's sandier soil. So, you know, that's really the key reason why we see so many issues with sandy soils and nitrogen availability, as well as water quality concerns in in some of those soils. So let's talk a little bit about how we minimize leaching. So, obviously, we've talked many times about when we talk about rate, we talk about only applying what the crop really needs. And so, you know, we've said this before that if we over apply for a particular site or circumstance, it's more or less left behind pound for pound, in the field, and so that's left and subject to leaching.
Brad Carlson:This is not a process that depends on temperature, so water moving with free nitrate will move it regardless as long as it's not frozen. The water is moving, and it will continue to go. Applying the fertilizer closer to when the plants need nitrogen, again, that just gets back to the hole. You can't lose it if it isn't there. And so, you you know, in most circumstances, on our average conditions, we don't experience significant loss, but of course, you can get those big catastrophic rainfalls.
Brad Carlson:They're hard to predict more than a couple days in advance. And so, you know, if if you choose to or are able to manage your fertilizer in a way that delays application, that will avoid that risk. You know, how many years out of 10 does that happen? I can't really say exactly, but it will happen occasionally, and that is that is a strategy to avoid that. Using nitrification inhibitors, does have some effect, of course, if we're just simply delaying the conversion of ammonium to nitrate.
Brad Carlson:Ammonium is a cation. It does not leach through the soil like nitrate does. So as long as it's, the nitrogen is is maintained in that form, we're not gonna see it moving through the sole pro soil profile the way we do with nitrate, using cover crops. So just simply taking up that nitrogen, we talk about the vulnerability that we have in a field once we've reached physiological maturity of our crop, which is generally happening in early September. You know, we continue to see nitrogen mineralized out of the soil organic matter throughout the fall.
Brad Carlson:It's just sitting there because there's no plant out there growing to take that up. Furthermore, we talked about the the, amount of fertilizer that's left behind. If it if more was applied than the crop needed, that's also just sitting there. So cover crops are a potential way to take that nitrogen up and kinda hold it to lock it up into organic matter until the next year. So that's something that can happen.
Brad Carlson:And I guess the last is just simply reducing the amount of water flowing off the land. You know, that's that's a little bit tricky because we understand the importance of drainage and and aerated root zones on growing plants. But, you know, in some circumstances, if if you don't need that much drainage, just simply reducing the amount of water going through the tile is one way of reducing the amount of nitrate ending up in the water. So let's talk a little bit about how this all adds up. So we mentioned the fact that that that nitrate can move itself roughly about six inches with an inch of free drainage.
Brad Carlson:So if we think about our drainage systems, how they're designed, those that have, you know, have experienced dealing with drainage systems know that they have a drainage coefficient. That's the design standard. And and so what it means is how much water can be removed in twenty four hours. So most of our our our subsurface tile drainage systems across Southern Minnesota have a half inch drainage coefficient. When we get over towards the Western side of the state, it's, more often three ace.
Brad Carlson:And when you get up towards the Northwest, it's often only a quarter of an inch. But but in essence, and what that says is with a half inch drainage coefficient, if you got an inch rainfall and then the soil was completely saturated, it's gonna take about two days to get that water off of the out of the profile to where it's down to the, you know, below the point of saturation again. If we think about the the nitrate needing to move about three feet to get down to the tile line, it's got a flow capacity for about twelve days. Well, maybe 10 if it's applied at six inches deep. But regardless, you know, the condition where that's flowing at capacity, doesn't happen that often.
Brad Carlson:I mean, that'd be like running like a fire hose, and so it's gonna be some gradient below that that it's running. And and but farmers can kinda keep track of and and kinda look at how much drainage they're getting out of drain tile and come up with somewhat of an estimate on how much they might have moved. You can also do that roughly speaking with a bit of a water budget. Now, of course, realize that when you're saturated, a lot of that water is gonna run off over the surface. It's not all, going down through the the the tile, but it's at least a thumbnail way of kinda keeping track of how far the nitrate has moved.
Brad Carlson:You know, in that same token, though, it's it's how we know that a lot of the nitrate that we're losing comes from the previous year's carryover nitrogen or mineralized nitrogen. Because if you look at it needing to drain that far, you know, in that amount of time, a lot of our fertilizer that we applied physically can't get out the drain tile quite that fast. And so if you look at some of the data from Waseca and Giles Randle looked at this many years ago, decades ago, really, most of the nitrate leaching is happening in the springtime. And and as I said, the the water can't get itself out quite that fast. And so if you look at the the the total amount of rainfall broken down on a monthly basis, the amount of drainage and then the amount of rainfall on a monthly basis, what you'll see is is that drainage will continue to to kinda tick up in the spring until when the crop starts growing.
Brad Carlson:And, of course, we know that corn really uses a lot of water. And what happens is while the rainfall continues to increase, the drainage starts to decrease. And so what we see is that the majority of our drainage is actually happening, you know, prior to the plant being out of the ground and growing. And so we see most of the nitrate that's being lost through tile lines is also happening at that point in time. And so, you know, from that standpoint, you know, it's it's very difficult to manage because a lot of people are advocating changes in fertilizer application practices for the sake of cleaning up drainage water, you'll hear people say, well, apply substandard rates or so forth.
Brad Carlson:When a lot of this is because of what happened the previous year, how you manage fertilizer this year isn't gonna change that a whole lot. And so, you know, it's also a reason why we need to avoid overapplying because it'll carry over from the previous year, you know, and then it'll be coming into a year we got soybeans oftentimes, and they'll just be sitting there, and then you're gonna lose that in in April and May.
Daniel Kaiser:So we mentioned before, I mean, when we start talking about loss pathways, I mean, our our our main concerns really are nitrate loss either through denitrification or leaching or through volatility. I mean, runoff and erosion can be an issue, for total nitrogen. We talk about nitrates. Certainly, I mean, there'll be some nitrate in the soil that's carried away. But if you look at some of the data that's out there, some things that were put together, I think the University of Nebraska did did some of this to look at this.
Daniel Kaiser:You know, at 10 tons per acre per year at 2% organic matter, that's about 20 pounds of end per acre per year. So, I mean, certainly, it's something, being that it's not as nitrate. It's of less initial concern. It's a question of whether or not any of that can turn to nitrate once you get into water bodies, and that's kinda beyond what we're gonna talk about. But if you look at just on the nitrate side, some of the Nebraska data would you're kinda estimating about 6.7 pounds of nitrate per acre per year.
Daniel Kaiser:So it's I mean, like I said, it's something. Certainly, if you're not getting erosion, though, in fields, it's not as much of an issue. And for at least for groundwater, I mean, erosion's not gonna be as big of an issue as it would be for nitrate in surface water. So it's it's not something that we'd be as concerned about. Really, our issue more on the runoff side comes from rainfall events that occur immediately after within a a certain time frame of fertilizer applications, which are more of an issue, you know, particularly as we get later into the fall.
Daniel Kaiser:Frozen ground, that's one of the thing reasons why we really stress to not apply nitrogen on frozen ground because if we get situations where we have here in Minnesota in December getting some rainfall that occurring if you get runoff, it can solubilize fertilizer. And the fertilizer, since it is highly soluble, especially the nitrate source nitrogen sources will move with the water. So that's really where, you know, certainly, there's some loss of organic matter that can cover we can see some loss of nitrogen. Fertilizer is really a bigger issue when it comes to runoff and erosion, particularly after recent applications.
Brad Carlson:You know, one of the things that we'll we'll occasionally look at or hear talked about is is, atmospheric deposition of nitrogen. So some of that's coming in, with with dust and and wind erosion and so forth and and, regardless of whatever the source may be. We oftentimes look at atmospheric deposition of nitrogen being about about the the same rate as the runoff loss. And so from a nitrogen budget standpoint, we usually figure those two things cancel each other off. You know?
Brad Carlson:As far as from a water quality standpoint, we do, of course, have to pay some attention. If some nitrogen is running off, it's not really significant compared to the, the amount that's leached. And and, of course, the the denitrification part is not necessarily always a water quality concern, but it's an air quality concern. And so you will see it discussed at least to an extent in the states nutrient reduction strategy. So the probably the only really important thing to realize about potential runoff of nitrogen off the soil surfaces, and we're also paying a lot of attention to water quality issues related to phosphorus.
Brad Carlson:And so a lot of our practices that are intended to reduce phosphorus loss, which are going to be drive driven by preventing erosion, and so forth, and and just stopping the water, letting particulates settle out, and and so forth. Those are also going to help reduce the amount of nitrogen lost in runoff. So probably in the grand scheme of things, we've got bigger fish to fry than to worry about about runoff and and erosion.
Daniel Kaiser:So if we look at positives, I mean and we talk about loss potential crop harvest is is really the one, and that's really what we're trying to do with the nitrogen is convert it into the harvested product. We know that large amounts of nitrogen are removed in our harvest yet, No. We can't completely calculate everything in for potential when it comes to budgeting. And if we look at what we call nitrogen use efficiency, which essentially is the utilization of the fertilizer we apply, I mean, we know that, you know, generally that, you know, I think some of the best numbers I've seen are about 50% getting about what we've applied into the the harvest material. And, you know, that doesn't certainly mean that we're losing it all through some of these other processes that we've talked about here, within this podcast.
Daniel Kaiser:But, you know, we look at it you know, we can cycle around somewhere within that nitrogen cycle that might be recovered in future years. So, I mean, harvest is our positive. It's really what we're trying to do, although it does represent what we would call a loss since, the nitrogen's being taken up and being exported off the soil following either the fertilizer application or just what's being mineralized in soil organic matter itself.
Brad Carlson:It's one of the the the principles by which the whole that whole, thumbnail calculation of 1.2 times of you know, which is the amount of nitrogen in a bushel of corn, and then you're trying to add the credits and so forth. Our research just shows there's there's too many other things going on than simply looking at what your yield is to calculate a nitrogen budget and then and then calculate it back to how much fertilizer you needed to apply. However, I will say that, you know, we've worked a lot with with crop models in the past. They're not really under favor so much at the moment. But but really, a crop model is trying to come up with a nitrogen budget and keep track of all of these factors that are driving availability and loss.
Brad Carlson:And so in the long run, you know, we may be able to use some sort of, you know, computer aided system that does run a nitrogen budget to come up with a more accurate, determination of how much nitrogen fertilizer we need to apply. And and that would in in, you know, in the long run, that would be based on yield. But at the moment, there's just there's just too many variables, the bugs aren't worked out on that stuff.
Jack Wilcox:Daniel Kaiser, Extension nutrient management specialist, and Brad Carlson, Extension educator. Thank you both for being here.
Brad Carlson:Thanks, Jack.
Daniel Kaiser:Thanks.
Jack Wilcox:Do you have a question about something on your farm? Just send us an email here at nutmgmt@umn.edu. Thanks a lot for listening, and we look forward to seeing you next time.
Jack Wilcox:Advancing Nitrogen Smart is proud to be supported by the farm families of Minnesota and their corn checkoff investment through Minnesota Corn.
