Manure Hydrology

In this episode, Dan Andersen looks beyond N and P and focuses on how soil properties and equipment choices impact the volume of manure that can be applied.

Liquid Slurry Manure Movement in Soil

Transcript

Dan Andersen  00:06

Hello and welcome to Talkin' Crap, a podcast by Iowa State Extension and Outreach. This institution is an equal opportunity provider. For the full nondiscrimination statement or accommodation inquiries, go to www.extension.iastate.edu/diversity/ext. I n 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 in areas of innovation. 

Dan Andersen  00:42

I'm Dan Andersen, and today we're going to be talking about manure and hydrology, specifically how understanding some soil hydraulic properties can help us in making sure that we are getting the right application rate. Also talk about what that means for picking the right piece of equipment to make sure you are getting successful manure injection. 

Dan Andersen  01:04

So with that, why should we care about? Well, we often think about manure providing nitrogen, phosphorus, potassium, sulfur, and that those nutrient balances are really the tool that we use to set how much manure we're going to apply. And I agree with that completely, it's what we should do is what we want to do. But there are times when we might think about the water in that manure as well. Especially if we're working with liquid manures, it can be useful or important. While the total amount of nitrogen generally is what we're thinking about is impacting both loss of nutrients the most, and especially in terms of nitrate to drainage water, or nitrate potential movement below the soil profile, I think that's really important. But when it comes to the actual day of application, when we're doing our thing, oftentimes, it's really the hydrology, that water with the manure that plays the biggest role. And we're gonna break this into two or three parts as we talk about it and think about it. And the first one is going to be about selecting manure injectors for effective liquid manure application. And what do I mean by that? Well, if we think about what an injector is doing, and what we're trying to accomplish, we're really trying to cut a large enough cavity in the soil to hold all that manure that we're trying to put on. And that means that if we have a high application rate, say, dairy manure with 15,000 gallons an acre, we're going to need to cut a substantially larger cavity than when we have a lower application rate, say, two to 4,000 gallons an acre, where we might be able to get by with smaller slits. And that's really the role the injectors playing. And there's just a few things that play into this and that we can change. But to achieve successful injection, what we're really trying to do is get all that manure that we put on down into the cavity that we cut so that it doesn't bubble up to the surface of the soil, and we don't see it. Now, you might also want to try and put a little dirt over the top of that slit so that it's protected. And even that area can emit odors and ammonia. And that's a little bit different of an objective. But I think they to really play hand in hand in what we're trying to do. And the first big part of that is we know that injection is a good practice, it helps reduce ammonia volatilization. Because the manure isn't seen by the air, if it's in the soil, we aren't losing ammonia, if it's in the soil, we tend to have much lower odor emissions, if we should get a rainfall event after that manure has been applied, and it's in the soil and not sitting on top, we know that it's less prone for runoff. So injection does a lot of very environmentally friendly, beneficial things. It also helps ensure or provide insurance essentially that the nitrogen that we put on is going to be there it's going to be plant available and it's not moving up into the air. 

Dan Andersen  03:43

When we think about different in gesture options and certain things we can do the the first one to think about for the injection capacity is just the injector style. And there's a few things out there we see a lot of vertical till where we're cutting a slit, hopefully relatively deep in soil, two to three inches wide, depending on how aggressive those wavy disks are set up. And we're really trying to make a hole in the soil that way that can take the manure. We also see lots of sweeps where they're large shovels essentially, they get pulled into the soil, they're helping pull themselves down, stay in the soil. And we're cutting a big cavity with those wings on the sweeps down lower in the soil and trying to put a band of manure down there. And depending on the size we need, it'll help us pick which one of those implements is really what we want. And, and it's really a trade-off game right with the larger sweeps that oftentimes takes more horsepower to pull, or we have to go slower through the field than with some of those narrower sweeps or with vertical till implements, we might be able to drive a little faster, be able to apply more gallons per hour as we hit some of those lower application rates. So it's not a one size fits all. It's really what is my objective and how does that work together. But if you are needing to hit some of those higher application rates, making sure that you have a large enough cavity in that soil is really important. The next thing to think about is how does injector depth play into that and, and certainly, especially with some of those vertical tills, where we're basically cutting a rectangle or some quasi rectangular shape into the soil, more depth equals more capacity with something like the sweep. Sure more depth give us more capacity, but it's really the wings on the bottom that that are setting where we're at. So they sort of work together. And then sometimes making sure that we're getting more depth that is important to us right to really increase that capacity and with other other injector styles, maybe making sure that we have that depth isn't quite as important. And then the last one I wanted to talk about was injector spacing. And while oftentimes we think about putting manure on in 30 inch centers that match our cornrows, that isn't necessarily what we have to do, we could have more injectors; we could put them at 15 or 20 inches, where they don't even match up with our cornrows. And in some cases that might feel wrong or weird, but especially if you're trying to hit a higher rate, it might help you spread some of that manure out, even though the injector each injectors capacity isn't as large, because we have more of them essentially in the same width, we end up with more cavity in the ground to hold some of that manure. 

Dan Andersen  06:04

So all those things sort of work together as we're figuring out what is right for us. And the reason I say that is because I'm not trying to say this is a one-size-fits-all, you should pick this injector or that injector. But understanding really what you need and are trying to accomplish is really important. Because in addition to just making sure that we have sufficient injection capacity, you are also trying to balance a lot of other things, right? The cost of the unit, the horsepower to pull it, even things like what's it leaves the soil condition like? Am I gonna have to do another tillage operation or is this the tillage I needed? Or is it too aggressive and it's not leaving enough residue on the surface to keep me happy in terms of satisfying some of those no-till criteria. So it's not not an easy decision. And it's there's a lot of personal belief about what you want to do, how you want to farm and what you're trying to get your field to look like, that really plays into this. With that said, to try and give a science perspective of maybe what is possible with different injectors, and how much manure can we inject into the soil successfully, I'm going to work with sort of a perspective of a no overflow criterion. And what I mean by that is no overflow criterion is the cavity in the soil that we're cutting contains all the manure so that after it's in there, it doesn't bubble back out to the surface. So if there's not enough capacity, right, if we put on 20,000 gallons, and we're only cutting a cavity in the soil that can hold 10, well, we try and put those 20,000 gallons in and some of that manure is going to bubble up back to the surface, it's going to be exposed and, and I would argue that that's not being successful at what we're trying to do, we're hoping to achieve. And there's been some work on this in the past. And basically what people have have shown or found is that if you're running in an injection system, that the cross-sectional area, if you're looking at that tool, the projected area is roughly proportional to the cavity size that it's opening up in the soil. And we're just going to assume that that is true, it works reasonably well, it doesn't work perfectly in all cases, sometimes the soil will crumble or break apart and fall back in the injection trench and take up some of that space. Because of that, oftentimes, we're trying to make a little bit bigger cavity than what we're really trying to inject. But there's other things that happen as well. For instance, if you are using a vertical till wavy disc sort of unit and it's fracturing the soil or crumbling apart, you might be making some larger, mezzo-sized pores in the soil that can take in more of that manure beyond just what's in the cavity. And I think that's, that's useful to think about too, because in that no overflow sort of criterion I'm saying the cavity needs to be this big. But it's also nice to have a soil that's crumbled and takes in the manure quickly, right. And it really depends on your soil type. You're soil moisture conditions at the time. And what you're really trying to hope for think looks nice on your field to be successful. But we know from the science that if you're getting good injection, not seeing that manure, you're losing much less ammonia holding on to that nitrogen so that your crop can use it, and also helping limit some of that odor. 

Dan Andersen  09:00

All right, so where does that leave us? If we start thinking about what that means, and some different tools, and in the attachments for this episode, you can see a handout where I do some examples here and show some formulas that you can use to get at least a first-order estimate. But if we said we had manure injection toolbar spacings 30 inch on center, between each injectors and that with the down pressure that we're running, and the injector unit we're running, maybe we're making a six-inch deep by two-inch wide sort of injection area. What sort of capacities are we talking about there? And if you start doing the math and working through what that means, you end up with an implement there that that can incorporate something like 10,000 gallons per acre. And that's oftentimes what I'm thinking about when I see those vertical till injectors is are we in that 10 to 12 range, that's probably near the top of what I expect those to do, unless maybe we're being really aggressive with the wavy knives, or if we're running more than more injection units then every 30 inch on center. With that said, the soil condition that you're starting with makes a big difference. I've seen that work really well for 12,000 gallons in a field where we're getting good down pressure getting it to open up. But sometimes on the compacted headlands for nowhere close to getting that same implement to work successfully. So it's sort of understanding your field conditions and where you're at and what you really want to achieve. But just thinking about sort of what we might be able to achieve and what's reasonable, I think, with that 10 - 12,000 gallons an acre, those units generally perform reasonably well. If we start talking higher rates than that, sometimes it might take a little more cavity opening in the soil are just more knives on that unit to get us to sort of that level. On the other hand, if we're talking lower rates that we often see with the deep pit swine manure, or maybe that two to 4000 gallon an acre, well, they're opening up plenty of cavity in there in the soil. And it's really about how well is it making sure that it's getting coverage on the top of that manure since it's very ammonia-rich. But I do think there's lots of opportunities and certainly, I'm not saying that this math is perfect, right? Because it's it's merely a starting point, what the most important part of this is trying that unit out on your field, seeing sort of how it leaves your soil and and especially the day you're using it to put on manure, are we getting what we consider a successful injection, or should we be thinking about making a tweak to it does it need to be adjusted to be more aggressive in the tillage it's performing, or we can get away with something that's a little less aggressive to leave more residue. And looking behind us, inspecting how we're dealing with when the job is getting done is really the most important part of that. And the rest of this is just sort of considerations of where we're at and how we're doing. 

Dan Andersen  11:33

The second part I wanted to talk about was the water-holding capacity of the soil. And when I think about water-holding capacity of the soil, what we're normally talking about is how much water is in the soil when that soil is at this thing called field capacity. And you can define this in lots of different ways. But generally, when I think about field capacity, what it's what I'm thinking about or referring to is if we had a rainstorm that got the soil really wet, how much moisture would still be in the soil three or four days later, right. And anything above that that drained well, we didn't do a good job of holding on to that water, if it was a rainstorm and getting it to react with the soil. And if we think about what that means for manure, when we're putting manure on, we want those nutrients to stay in the red zone. And I don't necessarily want all the water that came with my manure to stay in the red zone, that's not the objective. But what's the objective is to hold on to that nitrogen, phosphorus, potassium, sulfur. And to do that, we need to give some time for the soil to react with nutrients that are in the manure, and really absorb them and hold on to them. And I think one way to think about doing that is to say, all of the water that's tagging along with our manure has to be in the root zone of the crop. And that means we don't want to put on in excess of field capacity. Because if we put on more than what field capacity is we're going to start causing drainage. And you could say, well, it's okay to have some drainage as long as I had sufficient reaction time. And I agree with that completely. Unfortunately, for different soil types, different soil textures, the reaction times that we need, are probably a little different. And we don't have the best guidance on that. So that means that trying to think about our rate so that we stay below field capacity is probably a good starting point for making sure we have that, that time period, that retention time in the soil to make sure the soil is absorbing that nitrogen, it's absorbing that phosphorus. And then if we have a rainstorm later, a week later, where we do get some drainage, it's not moving those nutrients necessarily, but just some of the water that was along for the manure. 

Dan Andersen  13:31

A few things we know about so water holding capacity. There's a couple of properties here and things that we could talk about. And the first is it's related to texture. Now, there's another property, so-called the wilting point. And that's basically the water potential that plants can extract water. And if it gets drier than that, that's when we tend to think of seeing wilty plants or plants that are dealing with water stress. And that is extremely related to soil texture. Field capacity is related to soil texture, but not nearly as tightly because lots of other factors come into play. How much organic matters in your soil? What's the structure of your soil, essentially, what is the pore distribution look like? Because big pores can hold lots of water, but it holds that water relatively loosely. And if the water is bound tightly on the surface area of the soil particle, it's held extremely tightly, but there tends to not be a large fraction of the potential water available. And the thing about field capacity is it's sort of a balance of all those things that come into play. So while soil texture is a good first estimate of what the field capacity is, it's not necessarily this is the number for my soil because it's that texture, right? It can vary a lot from field to field. So if we start thinking about what this means, there's really a couple of things that we need to know or think about in terms of how to use that to help with determining what's an appropriate application rate. And while field capacity, we can estimate it for different soil types based on texture, the other part of it is what's the current soil moisture level. And the reason that one comes into play, if you think about it, if you're starting with a really dry soil, right, that profile is almost empty of water, there's a lot more that we could add to get up to field capacity. On the other hand, if we started really wet, close to full capacity to start with, we can't add that much more water before all of a sudden, we're at field capacity. So there are some places online where you can get a first estimate of the moisture capacity of the soil, when I like to take a look at it from time to time is the Mesonet at Iowa State, I think it does a nice job of providing at least some rough data on where we are as a state, how dry or wet we are tending for a specific year. And while it's not perfect for your farm, it does give you a starting point to think about. And I think that's really when you think mostly about me, no matter what we're trying to do is get a good first starting estimate about is this wetter than normal, is it drier than normal? How do I normally do when I put on my application rate? Is it oftentimes a lot for that? So the taking, or is it no problem for that soil to take in and if you say, well, I'm drier than normal, it's generally not a problem. It's easy to say, well, I think we're in good shape this year. Again, if we're wetter than normal, and you're pushing that line of the soil be able to take it in already. That might mean we have to think about do we drop rate this time and either apply again in the spring to add the rest of our nutrients? Or do I look for another fertilizer source to supplement some of the manure and try and space my manure off over more acres? The second part of what we need to consider is how deep are we allowing the manure to go? What's the rooting zone we want those nutrients to be held in. And there's no right or wrong answer for this. Certainly corn goes down and it can root for five feet without too much trouble. But it might take us some time to get roots that deep. So generally, when I'm thinking about what's an allowable depth, I tend to think in that two-foot range just to make sure it's staying up where the plant can get it while we still need that nitrogen. 

Dan Andersen  16:50

And then I guess the last part that I wanted to mention, or at least talk about is, if we're injecting manure that manure distribution isn't really uniform across the soil, right? It's put in some bands. And unfortunately, that means we're not using the whole soil profile to help hold on to that water. It depends on our application rate, how much that manure will spread out, it depends on the soil type, how much of that manure will be whipped sideways and how big the application band really gets. But I think it's fair to say that if you're injecting on something like 30 inch centers, at best, we're probably utilizing 50% of the soil profile to start with. And again, it depends on your implement that you're using to inject manure, right? If you have something where the sweeps are pretty big, and you're getting a six or eight-inch injection trench down in that soil, that's that wide, it's easier to envision how that could be spreading out a little wider. On the other hand, if our injection is really something like two to three inches wide, you know, getting it to spread out 4, 5, 6 inches is certainly feasible under the right weather conditions. But it might not always wick that wide, especially for wet to start with. So that's the second part that we wanted to think about. And then the third part is really related to how quickly can that soil, get that water get into the soil of the manure get into the soil to start with? And there's no perfect way to do this or think about it, because every case is a little bit different, right? But it really comes down to two things. What's the chance of getting runoff when we put manure on? And then how much are we trying to put on? So if you're out there saying well, I'm gonna put on dairy manure, and I think I need to get half an inch of equivalent of of manure on and we have a soil that can only take in a 10th of an inch an hour, right, that means the manure is going to be sitting on the soil surface for five hours or my injection trench for five hours. If the fields nice and flat, in some cases that that might still be successful, right, it might not be a place where we can go incorporate their work the manure in for a while, because it's going to be pretty wet. But you could see how that could be successful getting it in. 

Dan Andersen  18:43

On the other hand, if we're in a place with some topography with some sloped to our fields, if we're trying to put on half an inch of water in one big blast, and we can only take in a 10th of an inch or two-tenths of an inch an hour, well, that's that'd be a quarter inch of manure water ponded on the surface. And it's probably not going to sit there. Even if you're trying to do the same thing in injection trenches, right, where we're not using the whole soil surface and you're saying, well, I'm injecting it to have this rectangle that I've cut thats two inches wide, six inches deep, and I'm going to put that manure in, but I'm going up and down a hill, what's the chance that the manure I put on at the on the side slope of that hill is going to be taken in right where I put it down? Or is it going to flow down the hill, because we're trying to fertilize the corn on the side of the hill to right and that provide double fertilizer at the bottom of the hill. Now, I'd like to tell you that we know how to calculate that there's lots of ways to estimate how fast it will move. And unfortunately, it's really hard because it depends on so many things. And while Manning's equation can provide estimates of flow rates in channels, like that manure injection trenching, it doesn't really account for what happens if we're crumbling soil back into and absorbing some of that manure in the soil right away, which is what we're physically having happen. Right. So it is a complicated thing to say how, how quickly will it infiltrate? How long does it take before it starts flowing or moving in the cases that we're working with. With that said, thinking about how quickly does the water move in the soil helps us at least provide a perspective on how does that compare to what I'm putting on? And is there a good chance that it's soaking into the soil fast enough to minimize how much ammonia volatilization I could get. And if you think about what the guidelines are for that, ISU Publication PMR 1003,  Using Manure Nutrients for Crop Production says if we're injecting it and getting good coverage, generally, we won't lose any ammonia or maybe at most 2%. If we're applying it and then incorporating it later, we're somewhere between that zero and 5% loss on what's coming out between that zero and 5% ammonia loss. And the truth is, it's really hard to know how much ammonia is going to lose, it's dependent on the soil temperature, the air temperature, wind speed, sort of how that transition layer, the air turbulence, right where the manure is, all make a difference. But I think it's reasonable to say that we really want to get that manure into contact with the soil as quickly as possible. If it's taking more than four or five hours to infiltrate that manure that we put on, we're going to be losing some of the nitrogen we put on, and it provides some uncertainty in that manure. So trying to match your application rate to at least be reasonable with hydraulic conductivity is a starting point. And with that said, most of the time, infiltration rates are going to be higher than the hydraulic conductivity. So it's an extremely conservative estimate. 

Dan Andersen  21:22

But if you start doing things like irrigating with your manure, that hydraulic conductivity gets to be a lot more important, right? Because matching the two, how quickly that soil continuing take water really plays a role in in making sure it's getting down into the soil and helping protects it from some of that nutrient loss or ammonia volatilization. And that was really what I wanted to talk to you all about today was just we often think about nitrogen balances, phosphorus balances, and certainly those are the most important and the ones that we're going to hopefully talk more about in the future and I think are worth spending the most time on. But making sure our equipments working, making sure that we're picking up the right type of equipment to make us successful that we want, what we want to achieve, is important for minimizing nutrient movement, right at the time of application and helping you feel like you're successfully achieving our goal of using it as that manure as a fertilizer, and not just as something that we're out there trying to get rid of. And I think we do a great job in Iowa. Certainly, as we've seen nutrient concentrations in many of our manures increase in the last decade to decade and a half. I think maybe some of the constraints about this have went down. But with that said, I think that has made the divide between maybe some of our finishing manure and dairy manure, greater right, that difference in application rates greater than they used to be. And because of that maybe every piece of equipment that we're using isn't set up to handle both in all cases and all soil conditions. So just trying to understand when it fits when it doesn't fit us. And the other things we can do to help minimize some of those nutrient losses. For instance, we talked about the flow rate and how slope plays a role in that, is there an opportunity to contour or follow field paths slightly differently to try and hold on to some of those nutrients to minimize runoff and make sure that the manure is staying where we place it? Or for doing a good job of getting the manure covered, don't have that overflow, are we actually getting the injection slipcovered so that it's not there to cause some ammonia loss and what are we doing there. So just a real quick run-through on some of the important things that we we should be thinking about as we're doing our manure application. And like I said, these math equations, I think they're a good start. They're illustrative on, on why the properties are important and how it relates to things about our equipment, how it relates to things about our soil, but they aren't the end all be all the most important part is the day we're putting our manure, checking out the job we're doing, seeing how it's going and making sure that we're looking behind us and satisfied that the manure is getting into the ground quickly that we're doing a good job of getting it all injected and covered. And that really tells more of the story than this math does. But the math I think puts us in a range where we have good perspective on if we're going to be successful at the time we're doing it. 

Dan Andersen  23:52

Thank you for joining this installment of talking 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, Iowa Manure Management Action Group, which you can find at www.extension.iastate.edu/immag. If you found what you heard today useful or 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 talkin' crap.