Available or Not: The Nitrogen Guessing Game in Manure Planning

In this episode, Dan Andersen discusses nitrogen availability in livestock manure. He dives into PMR 1003, Using Manure Nutrients for Crop Production, which estimates nitrogen availability and the need for updated research to reflect changing manure handling methods and storage practices. 

Show Notes

PMR 1003 Using Manure Nutrients for Crop Production

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Dan Andersen: 0:06

Hello, and welcome to Talkin' Crap, a podcast by Iowa State University Extension and Outreach. This institution is an equal opportunity provider for the full non discrimination statement or accommodation inquiries, go to www.extension.iastate.edu/legal. In this podcast, we discuss insights into the science, technology, and best practices surrounding manure management. Our objectives are to build awareness about the challenges farmers and the broader agricultural industry face around manure and to demonstrate solutions and areas of innovation. Hello, and welcome back for another episode of Talkin' Crap. I'm your host, Dan Andersen, associate professor and extension specialist in the ag and biosystems engineering department and go by the name Dr. Manure. This month's episode is Available or Not, the Nitrogen Guessing Game in Manure Planning. So this is a topic that I really love. It's in Manure Management 101, and then we get to zoom into maybe the senior level version of what availability is, what we're trying to talk about, where the science is taking us. And what the heck are we doing about this, which is exactly what I love. It seems simple, straightforward at the start. And then we get to make it rocket science. And then hopefully at the end, we can zoom back and say, alright, now that we heard all that, thought about it, tried to digest some of that information, what does it mean for us when we actually make some of these decisions? So I wanted to start out with, what is nitrogen availability? What do we mean when we say that? And to me, I think the important first thing is not all nitrogen in manure acts like fertilizer nitrogen. Available nitrogen refers to how much N the crop can use in the applied season or in subsequent growing seasons. So a definition I like is available N is nitrogen the plant can use. Total N is mostly organic N and ammonium N, though in some manure systems, nitrate may be present at relatively small levels, organic N is slow releasing nitrogen. Ammonium N is basically what we're getting in many commercial fertilizers, and except for what is lost to the air, can be used directly in that application year. On the other hand, organic nitrogen generally must be released or turned into smaller compounds that plants can use, and the two we think of most of the time are ammonium and nitrate. There might be a few others you could think of, like nitrites, but for the most part, we're talking ammonium and nitrate when we think about what plants can use. So historically, we've always said that organic nitrogen must be mineralized to ammonium and nitrate, and I still pretty much believe that. But I do want to admit there has been some research that say plants may take up simple amino acids or even short peptides. So small parts of basically protein. However, that is generally most common in things like forest tundra or extremely low nitrogen systems, not typical of agricultural soils, probably not something we have to worry about in less we we suddenly have brilliant ideas about how to get really specialized, operate at low levels of nitrate in the soil and still maintain production. In most agricultural soils, microbes tend to be much faster than plants at finding that nitrogen breaking it down because they're getting energy from it and using it for nitrogen, so it's their entire life support, whereas plants are really thinking about it as I just need it for nitrogen and the rest is sort of a benefit. So oftentimes microbes will out compete plants. And that's going to be a theme that I try and bring up a few times throughout this when we think about, why is it available? Why does it get tied up? So an opposite idea of availability, and I didn't use the term yet, but one that I think is important as we set this context, would be immobilization. And what I mean when I say immobilization is that some portion of the nitrogen that gets turned into organic compounds, whether that be cellular biomass, enzymes, some people use it for plant biomass. I tend to not do that. I just call that plant uptake, but it's getting turned into a type of nitrogen that isn't just sitting in the soil mineral pool, and it can't be used, right? It has to re break down. And there's sort of contrasting ideas. The more that's available, the less that we've had immobilized. If we're getting mobilization, that means we don't have as much availability at the moment. The other thing that I wanted to talk a little bit about, and we'll get to this in more detail as we get to PMR 1003, is sort of the idea of availability versus supply. And some states use this differently than I do, and that's why I wanted to bring it up. When in Iowa, we say the word availability what we really are trying to refer to is that it's in a form that plants can use. That is it's available to help support plant growth, not that it will. When we say supply, we're really talking about how much nitrogen is in that soil altogether, right? It's, it's related to this addition loss question. So supplies related to things like leaching, losses, denitrification, maybe ammonia, volatilization. When we talk about availability, at least in the context of Iowa, we're saying it's in a form that plants can use, other things can happen to it too. But that doesn't ultimately change what was available in the manure. It changes how much that manure might be capable of supplying. All right, in my notes here, it says I'm supposed to talk a little bit more about forms of nitrogen. I feel like we've been hitting on that pretty good. But really, when we think about manure, there's three types of nitrogen. There's ammonium nitrogen, so that acts just like most ammonia fertilizers. I want to say, like most ammonia fertilizers, it's not perfectly similar. For instance, if you're using anhydrous ammonia and inject that into the soil, you get a zone that's really basic in pH. Maybe I've killed some microbes in that, that near vicinity with manure. We don't tend to see that reaction. We might still inject the manure, get it into a zone, but we don't get the same PH effect, right? We don't get nearly as basic. We're probably not killing microbes in the soil. We might even be firing up or inspiring them to do a little bit more activity, because I fed them a really nice, available carbon source. But for the most part, it's at least relatively similar to something like urea. The other form that we think about the most is organic nitrogen, and that's tied to some sort of organic matter. It's coming with carbon attached to it, and that means it needs some time to break down. The final form is nitrate, and in most manure sources, there's extremely low levels of nitrate. Oftentimes, I don't even think about it or consider it every now and then, if you're working with the solidmanure system, one that might get aerobic, you can sometimes see some nitrate, but it does tend to be a relatively minimal portion of the nitrogen that we're putting on the soil. Now that isn't to say it stays that way once we put it on the soil, once we're putting it in the soil, and those aerobic conditions, all bets are off. So why this matters, or at least why I think about it a lot, is it's complicated. It really changes our fertility picture. And what we're trying to do is take advantage of manure as a resource on every acre we have that in the state. So avoid over applying. So trying to avoid places where man, that yield, that corn, looks terrible because I didn't have enough nitrogen there, and also to avoid over applying right, because that leads to more leaching losses, volatilization losses, or at least ways that it's getting into the atmosphere, whether that be denitrification or movement with water. We have a great history of knowing that matching manure and crop demand and trying to get those two numbers right improves nutrient use efficiency and environmental outcomes. So we're on the same page, right? If we're doing the best by our crop, that also probably means we're doing the best by water and air quality. But it is complicated. Every year is different, and I don't want to make light of that. And when we think about how variable manure is, how uncertain manure is, it's a difficult, difficult subject, one that I'm 10 years in 12 years into this job, and still wake up every day excited. And get to say there are a lot of things that I don't know enough about to really give everyone the recommendations they want, but I do think we're learning more about how to point in the right direction. So all that to say it's not just about how much nitrogen is in the manure, it's whether the microbes want to share it with your crop. The other day, I was sitting in a defense and it was about nitrogen mineralization. So inspiring. This this talk a little bit, but he talked about how fertilizers are often feeding the plant and manures are feeding the soil. And I like that analogy to some level. But really, with manures, we're doing some of both, right, and that makes it more complicated. And when he says that it's not so much for necessarily feeding the soil, when we think of just about the nitrogen forms that has more to do with carbon. But it is an analogy that I think does some justice here, that this is really, really complicated. So when we think about nitrogen availability, or what's happening in soils, generally, soil microbes tend to run the show. Right when it comes to breaking down organic matter in soils, we're pretty dependent on microbes. When it comes to nitrogen processing, the microbes are determining what's happening. So when you think about what microbes are doing when they find that organic matter, they're breaking the carbon down. Some of it gets converted into CO2and some of it gets converted into microbial biomass. Oftentimes we think of that as cell biomass, but it could be other things, like enzymes that they're making or whatever, right? And then the nitrogen was just there along for the ride for the first part. But then they have a choice. Now I've broken down this carbon. I freed up this molecule of nitrogen. If I'm growing cell biomass, making proteins, making enzymes, I might need that nitrogen. I'm gonna say, oh, that's precious to me. Gonna hold on to it, put it in my cell and use it if I have more than what I need, well, then it's a waste product, and I start throwing it back into the environment, right? Essentially throwing it away. Now that's a good thing for us. It's not a waste, right? We still want it to help support crop production. That tends to be how we're really using a lot of the organic nitrogen in the manure. It gets cycled through that microbe, and they make a decision of, should I turn it into microbial biomass, or put it back in the soil? And it's a waste to me, and then when it's a waste of them, it becomes our crop food. So some rules of thumb on this, the C to N ratio of things often, are a nice first indicator of how the microbial pendulum of whether release or mobilization is going to occur. And this isn't perfect, especially when I talk coarsely about carbon to nitrogen ratio. What I'm really talking about is bioavailable carbon to nitrogen ratio. But that gets complicated to say every time, so it's sort of implied. And then, because of that, we use some rules of thumb to understand maybe how bioavailable that carbon might be in general. So if we're talking about low carbon to nitrogen ratios, and by that, I tend to mean something below 15 carbons to 1 nitrogen. That really means there's enough and there for the microbes to have excess. So as they break down stuff, some of that nitrogen is released in the form of ammonium, back into the environment. They're mineralizing that nitrogen, taking it from an organic form, turning it into a mineral form. So essentially that they're consuming that carbon, they're releasing CO2 and they can't use it all for cell mass and enzymes. Something has to go and they get rid of it. And that's really typical for many of our liquid manure systems, swine manure, liquid dairy manure, liquid beef manure. When we get the opposite of that, something with a high C to N ratio. And to me, that's generally above 30 to 1. The microbes need to pull nitrogen from the environment to support themselves, so there's not enough carbon to handle sort of what cell mass they're going to make as they're breaking this material down, and they're like, I need more nitrogen. I'm in a good environment. I'm growing well, I have plenty of energy. There's one. Take it in, I'm going to turn that into something. So they're really scavenging for nitrogen in the environment. And that's that situation where we think about immobilization. Now, microbes don't last forever, right? They're going to break down. Enzymes are going to break down. So just because it was immobilized doesn't mean it no longer can ever be plant available again. It just means it needs to go through a cycle where that cell dies. Eventually, something else comes along and breaks that cellular material down, and eventually it might lead to some release. Now you might notice that I had a gap in there 15 to 1 to 30 to 1 What happens in between? And the answer is, I don't know. That's when sort of that bioavailability of the carbon question gets to matter a lot, or what sort of conditions do we have, and how rapid can the microbes process these things? And that really is an unknown zone, right? We could really dive in and try and do better, but essentially, to me, what it's saying is, well, we probably won't see a lot of nitrogen release, a lot of availability. We probably won't see a lot of immobilization. We're sort of hanging out in So you've all probably had some experience with this. And if we between. think through some of our manure types, it's probably pretty easy to say, why? So when we talked about swine manure earlier, I said, Well, that's a low C to N ratio. It's broken down pretty good in the pit and we put it on the soil. And oftentimes we're putting it on the soil, about 80% of the nitrogen is already in the ammonium form, and only 20% in the organic form. So that's one easy to say, well, even if I have some tie up from breaking down those organic solids, I probably don't. They have low C to N ratios, probably in the neighborhood of 10 to 1, 12, to 1, generally in soy manure. So we're probably getting even released from those. But even if I wasn't, there's enough ammonium that I've added to the soil to really cover what I'm breaking down. Corn is gonna look green, gonna look good. I don't have to worry as much about early season event and availability. If you contrast that with something like solid beef cattle manure, maybe you've had this experience where, wherever the chunks of bedding fall, my corn looks the worst in that part of the field, and that's really because of that N high up, right? We're getting an immobilization. And that's the rule of thumb that we really want to think about and remember, from this section high carbon to nitrogen ratio manures tend to lead to nitrogen tie up early in the season. Maybe we'll get some release later low carbon manures, especially low carbon nitrogen ratio manures, we tend to get that nitrogen release. And that's why two manure samples with the same total N can behave very, very differently in practice, right? It's all about that availability. So if one's high with bedding or straw, you might actually be getting some immobilization or tie up to start, even though you're like, I put on how much nitrogen I actually think the crop needs? Well, it might not be available at the right times. All right. That takes us to the next portion of what my notes say I'm supposed to talk about, and that's really the Iowa guide to this discussion, or at least the starting point to the Iowa guide to this discussion, that's PMR 1003, Using Manure Nutrients for Crop Production. And it's not the end all be all, right, but it's a great starting point for what we're doing. And a couple things on this. The recommendations here are based on limited field and lab data that was collected quite a while, a while ago, now, in the early 2000s and the data is probably getting more dated. Storage message methods are changing. For instance, in poultry litter, we went from high rise facilities to maybe belt facilities where we're doing some manure drying. Well, hopefully that system is holding on to more of the urea, ammonia nitrogen in the manure. And one would think that increases availability. I think we've probably seen that. I think I've seen that on some of the trials that I do with poultry litter, but that's one example. We've also seen a lot more people looking towards bed pack and deep pit beef barns rather than open lot facilities. And while as a starting point that one says all beef manures are the same, I think when you look at the samples, you do. See some differentiation, and it's probably about time that we start catching up with that. And that's before I get into well, manure processing has really taken off. We see many dairy farmers doing different types of solid liquid separation for various reasons. We've seen some farms think about anaerobic digestion systems. There's just more variation in the types of systems that we're building. And it's not manures, manures, manure, it really is time to dive into some of the science that and start trying to maybe refine some of these estimates a little bit more to meet what production systems are actually doing these days. One thing I do want to talk about, and we rarely state this when we talk about that availability table from PMR 1003, but it's true is this method was at least picked in part for simplicity. It only needs to know the total nitrogen content of the manure, and you simply apply a coefficient, and boom, you get your answer. The method was picked in at least in part because many labs at the time weren't measuring ammonical nitrogen. They were only reporting total nitrogen back to the user. And if you only have total nitrogen, it's nice to have one coefficient to measure. It, say that's good and move on with life. But I think a lot of the more recent work, one that comes to mind is some work that I saw out of Wisconsin has shown that generally, by having both the ammonium nitrogen and the total total nitrogen, organic nitrogen fraction, you can do a better job of estimating because we know that or that inorganic that ammonium fraction is plant available, and we can focus on refining maybe for that organic fraction estimate. So when we think about what that PMR 1003, says, I just wanted to give you some ranges. When we're dealing with cattle manures, whether liquid or solid, beef or dairy, we're grouping them all the same. And I don't particularly love that, especially as we've we've learned to feed different it probably matters, right? A beef cow probably gets fed a fair amount different than a finishing steer. Dairy cattle probably have the protein cranked through them, and they're not eating maybe the forage that they were when we developed this, or at least in the same form, right? They're eating a lot more corn silage, a lot less alfalfa hay, and that means it's probably time to update. But for the for the starting point on this, at least it estimated 30 to 50% first year available. With our pig manures, we were at that 90 to 100% first year available. And then with poultry, all the species got grouped together. It was at 50 to 60% first year availability. And then it does provide some mineralization coefficients for year two and in some cases, year three, as footnotes on that. I'm not going to take the time to read them out, but it does put them in there. I think the important part to note is that they don't always add up to 100% and there's reasons for that, and we'll try and talk to some of those a little bit later. A second way to estimate availability, and one that in many ways I'm partial to, is the old Midwest Plan Service method. And it starts out more sophisticated. It says, Hey, that ammonium is already plant available. We don't have to do much about that. Just take the amount you have there and say that's 100% available by definition. It's in a plant available form. And then it says, let's focus on the organic and fraction next, right? So how much of this is going to mineralize is related to how the manure has been previously stored and handled, at least to some respect, because it's kind of about the losses or breakdown that it's had during the storage. And I'm not trying to say we should treat these methods, these numbers, like gospel, but I think that first breakdown, it at least intuitively, tells us that there's some reasons to believe that it might be a little bit better, at least pointing us in the right direction. One thing in particular I enjoy is it's sort of a tuning factor for for the variation of manure, right? So if I go get a swine manure that comes back with only 50% of the nitrogen in the ammonium form, I might say, well, that doesn't look quite like a typical manure. And if I'm just applying that Iowa number, I'm still gonna say 90 - 100% first year availability, whereas in this case, I get to say, well, something was weird about that manure. Let's think about it a little bit more deeply. How is it stored? How is it used? And go from there. Now what they tend to do is they came up with a table that basically has two factors to it, the manure type, so basically the animal species that you're using, and then a manure handling system. And you can find what those numbers are in the show notes, and they're not the gospel truths. As a matter of fact, I can't find where much of that number, how they derived it comes from, but it does fit at least relatively well with some of the data we see from some of the mineralization studies that you can find in literature, and it seems directionally correct. So for example, you might look at the swine manure. And for swine, they have basically three different types of manure. They have fresh manure, anaerobic liquid and aerobic liquid, and for the organic nitrogen in those they give them three different mineralization factors. For first year availability for the fresh manure, it says 50% first year available for anaerobic liquid, it says 35% of the organic nitrogen will break down. And for the aerobic liquid, it says 30%. So why those numbers? And in many cases, it's sort of what you'd expect with fresh manure, it hasn't broken down much yet, but there are more compounds that are easy to break down still remaining in the manure. So because of that, they're saying, well, it has to break down. It didn't break down if I stored it, it's going to break down when I put it in the soil. That availability has to be. Higher. And the same sort of follows for that anaerobic liquid being higher than the aerobic liquid. In this case, I often will tell students, there's a reason elephants breathe oxygen, right? And it's that oxygen reactions tend to be exothermic. They make lots of energy. That's why fires are cool and fun, also dangerous, but cool and fun, right? And so that means that when I go from an anaerobic environment and put that thing in an aerobic environment, like soil, those microbes are going to get after it and get stuff that in an anaerobic environment, there just wasn't enough energy available to keep chasing do the same reaction. In an aerobic environment, it's worth your while to get that last juice from the squeeze, all right. And then after that, we sort of hit the, my favorite part of this method, and that's the part where it looks like we needed to come up with an answer and and they said, That'll do because we don't actually have the data we need. So they also give you a method to estimate availability in years two, three and four. And it does feel a little bit to me like they said, who knows, but it'd be nice to have a sequence like, how about 50% 25% and then 12.5% of what was available in year one, sort of a curve that's slowly getting towards that 100% saying you don't have to take as much credit for what's left. And then someone else at the meeting probably said, I don't know either, but that at least sounds like a reasonable first approximation. And given that not that much of the nitrogen is organic, probably minimizes some of what we're doing. It also means that if I have a manure that had a lot of organic nitrogen, I might have less of it becoming available, and it'll take a long, long time to get to 100% and they said, you get to stop at four years. You don't have to keep doing this, right? And at some point that nitrogen just acts like it's in the soil nitrogen pool. Now, at this point, my show notes say I am supposed to talk a little bit about how you'd do some experience experiments to figure out how to measure nitrogen availability, or how to figure it out. And there's a couple approaches to this, but the one I wanted to highlight first was doing it at a field scale, and why getting those numbers from year two and year three and year four might be really tough to do, or not, something that we've done enough of in the past. So to do this type of work, what you do is you put a manure onto a plot at some reasonable rate, or two reasonable rates. Now it has to be reasonable, because if I put on too much manure, too much nitrogen, it won't work for me. I have to be a little short on nitrogen. If I'm if I'm optimal to short, I'm probably okay, if I put on too much nitrogen I don't get any information out of the study at all. And you'll see why in a second. And then, right next to where I did that manure trial, I have to generate a nitrogen response curve. So I have to go and say, What happens if I had no nitrogen? What happens if I put on 25, 50, 75, 100 pounds an acre, all the way up to whatever I think is optimal. And ideally I'd say one more rate or two more rates, so that we really see that we've maximized yield, and we know the shape of that curve. And then what we do is we look back at the yield of that manured plot and say, Well, if that would have been commercial fertilizer, we'd fall right here on the yield response curve. And then you read the x axis and say, Well, that was worth 170 units of nitrogen. And in the plot, I put on 200 units of total nitrogen from the manure. So that means it's 85% available. And that's great. Year one went fantastic. We got her done. Now I say that's great and easy. There's a lot of variability in some of those numbers, so deciding where you are on the curve can be difficult. And lead to saying, I should just pick it was in this range. But that also means that if I want to do this for year two, three or four availability, I have to take that same manure plot and then the next year generate a nitrogen response curve near that plot, putting nothing on my manure plot, and say, Well, where did I fall on that curve now? And as you might guess, some of that corn might look a little yellow because there isn't much nitrogen supply there, but that's what we'd have to do. And then year three and year four get even tougher, right? Because we're getting less and less nitrogen essentially back from that manure. But if you want to get good carry over numbers, that's really the experiment you're going to do, or you have to work with some modeling to try and get at that number. The other way to look at it is, if I had a manure with really high C to N ratio, if I apply in the fall, maybe I had some time for that stuff to break down. I went through that initial immobilization cycle, and by the time I get to next spring, I hope we haven't lost a whole lot, but I've already done that tie up, and we're starting to get on the lower nitrogen at least portion of that leg, because it effectively changed the carbon nitrogen ratio of the manure by the time the crop will be growing. All that to say they're complicated things, right? But I think sometimes this nuance matters, and oftentimes when we have the discussion at manure 101 level, we don't get into some of those details, and I think they're worth having. They're worth thinking about, how do they impact how we're making decisions? Where do we go from here? And I think that's that's important for Iowa. It's important for better manure management and better environmental outcomes. So a takeaway, when you apply affects not how much nitrogen it. Affects how much nitrogen is available, in theory, but to a small level, what we really probably need to focus on is how much is actually sticking around to be available or supplied to that plant. Alright, so at the end of the day, not all nitrogen and manure is created equal. Some of it feeds the crop right away. Some of it feeds the soil. And microbial activity has a pit stop essentially, along the way, and some of it we never really see show up. Maybe it stabilizes soil organic nitrogen. Maybe it is lost during those shoulder seasons. It remains to be seen, the carbon to nitrogen ratio, the ammonium content and how we apply the manure all shape whether the manure becomes a nutrient that we can use, or if it's lost to the environment. Tools like PMR 1003, give us a place to start, but we need better more current data to guide real world decisions. That means more research, more field trials, and better ways to predict nitrogen behavior in modern systems. If we want to unlock the full value of manure, we've got to keep digging into the science, into the field and into the soil. Thank you for joining this installment of Talkin' Crap. Be sure to take a look at the show notes on our website for links and materials mentioned in the episode. For more information, or to get in touch, go to our website, www.extension.iastate.edu/immag. If you found what you heard today useful, or it made you think, we hope you subscribe to the show on your podcast app of choice. Signing off from a job that sometimes smells but never stinks, keep on talking crap.