Rice & Advice Series: S2 Ep. 2 Importance of Nitrogen Source Selection for Rice

In this episode, Drs. Jarrod Hardke and Trent Roberts discuss the pros and cons of various nitrogen (N) sources that could be used in rice production systems and the importance of selecting the right source.

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Rice and Advice Season 2, Episode 02.
Title:  Importance of Nitrogen Source Selection for Rice
Date:  Apri 26, 2022


[Music]:  Arkansas Row Crops Radio, providing up to date information and timely recommendations on row crop production in Arkansas.

Jarrod: Welcome to Arkansas Row Crops Radio. I'm Jarrod Hardke, Rice Extension Agronomist for the University of Arkansas System Division of Agriculture. Today on Rice and Advice I have with me Dr. Trent Roberts the Sultan of Soil, the Doctor of Dirt. And we're going to talk a little bit about the importance of nitrogen source selection for rice. How's it going today, Trent?

Trent: It's going good. Thanks for having me back, Jarrod.

Jarrod: Yeah. Good to get on here and talk a little bit more in the realm of soil fertility. And see if we can get pointed in the right direction as it relates to nitrogen for this year, as we're still waiting to make major progress in the rice planting situation. Hopefully that will make a turn for the better sooner rather than later. Still looking for that. But again, we're going to talk about nitrogen source selection in rice and Trent, now we were talking earlier, and where we're going here is certainly the 4-R principles of right source, right rate, right time and right place, with today focusing on the sources of nitrogen. So, Trent, you want to give us some initial thoughts on nitrogen in rice?

Trent: Yes. As we start getting ready for potential early season applications, pre-flood applications of nitrogen in rice, I think it's good for us to go back and kind of review some of these tenants that we need to keep in mind. And unfortunately, there's no easy button for nitrogen fertilization. Right? There's such a dynamic system that takes place in our flooded rice production systems or even our non-flooded rice production systems that impact nitrogen availability. And so if we take a step back and we think about the rate side, right? The rate side is influenced by the crop needs, what the soil supplies – which we can use N-StaR to determine that, and then it's impacted by the efficiency of plant uptake. And that's really where the source component comes in, right? The source is going to have a profound impact on the efficiency of plant uptake. That's why it's so important. To follow that up, we need to realize that there's really no perfect nitrogen fertilizer source, and what I mean by that is every source is going to have some benefits and every source is going to have some potential drawbacks or negatives. So, I thought it would be important for us to kind of go through and highlight the predominant nitrogen fertilizer sources that we have at our disposal in Arkansas. And kind of hit on the positive aspects of some of them, as well as the negative aspects, to help producers understand why we promote certain fertilizers for rice production and why we tend to avoid others.

Jarrod: Yeah, I think that's it, and keeping in mind, this is a dynamic biological system. A lot of work on nitrogen and rice for a very long time. And the only thing that's definite is constant change in what's happening out there in the field, what we're gaining, what we're losing all the time. Again, why there's no easy button to this deal.

Trent: Well, and you bring up a good point. You know, our predecessors have been working on nitrogen fertilization for 30, 40 years. One funny thing that always comes up is, well, “how many nitrogen rate trials do we need?” And it's kind of like, well, when you think about the complexity of the system and the improvements that we make, there's always the potential to find new avenues, right? Or new ways to do things, because that efficiency component is really what everything hinges on. I know we can't do it in a podcast, but I always like to show people just how complex the nitrogen cycle is. And when you take time to appreciate all those little complexities and intricacies, hopefully it makes it a little bit easier to understand why so much time and energy has to go into managing this particular nutrient.

Jarrod: And we'll try to do that here at some point on a video part of the video series. Again, hard to see what you're drawing on a whiteboard, on a podcast so we'll try to follow up and get that out there. But, you know, that's a fun part of the conversation. Like you said, how many nitrogen trials the need? Well one thing to keep in mind is over the past 30 or 40 years, as new cultivars are developed, yield potential keeps increasing. But also at the same time, our efficiency out there at the commercial level for managing our fields continues to improve. So, all of those things have continued to drive changes in how we recommend fertilization strategies. I mean, how we're going about it. I mean, without diving off into too much detail and get into the meat of what we want to cover today. But I mean, you go from all the way back to the three-way splits that we were using for a long time, even still putting stuff out at planting and everything and sprinkled through the year, to trimming it down more to the solid defined three-way with half inch timings, to down to the two-way split. Now, to seeing where we're efficient enough, we can do a lot of single pre-flood, with no mid-season stuff. So I mean, just constantly changing. And then now you bring in furrow irrigated rice and flip everything we were doing before on its head, because now the flood is not there. And sorry we're going to touch on some of that, but the ride will continue.

Trent: Well, everything's connected, right? So anything we do to alter water management is going to impact our nitrogen fertilizer management and so all that stuff has to continually be addressed and investigated. But when we start talking about nitrogen fertilizer sources, the one that is at the forefront of production is always urea. And so, if we just think about the positives and negatives of urea as a nitrogen and source specifically for rice production, why do we use it? Well, one, it's a dry, granular product, which is kind of essential for aerial application and those types of things. But it also has a high nitrogen analysis, right? At 46%. So, the fact that it's dry, granular and it has a high analysis is really what makes it perfect for rice production.

Jarrod: It’s relatively cheap in most years. Not this year so much, but in most years. [laughter]

Trent: Yeah, well and, and what that really comes back to is that cost per pound of N, and so when you think about that, that's really a key component that needs to be taken into account. But with rice production, we always want to use ammonium based or ammonium forming fertilizers. And urea is the top of that list as an ammonium forming fertilizer, in the sense that as soon as it's hydrolyzed, that first product is going to be ammonium. The reason we want ammonium is that in a flooded rice production system or an anaerobic production system, ammonium is going to persist in the soil with very few loss pathways. What I mean by that is if we apply ammonium, and we have reduced soil conditions or anaerobic soil conditions that are typically found in a flooded system, we eliminate a lot of the loss pathways, right? We eliminate nitrification and denitrification, we eliminate leaching. And so really the only way that ammonium can leave the system is through plant uptake. That's why we really focus on ammonium forming fertilizers or ammonium-based fertilizers. Now, what's the Achilles heel of urea? Ammonia volatilization loss.

Jarrod: Yep. Right. Every time.

Trent: So we got all these positive boxes we checked, right? But then there's this big glaring deficiency over on the other side, which is ammonium volatilization loss potential. And we have a lot of great tools, right? – to combat that now, where 15, 20 years ago we didn't. And, we see that with these improvements in nitrogen stabilizers, specifically urease inhibitors, we can see these protection things added to our urea fertilizers that give us flexibility in application timing and flood timing and all those types of things.

Jarrod: Yeah. See, I chalk up us first getting urea stabilizers for use in rice you know, again, 20 years ago. But I chalk that up as one of the most significant events in our history of rice production in terms of improving our efficiency and overall yield potential, meeting those goals because of that layer of protection that wasn't ever there before. With the time that it takes us to get flooded on so many of our fields, the amount of nitrogen loss that was occurring before we could get the urea incorporated. You know we've talked about before, highly variable. So, you could never really know how much nitrogen you actually got in the ground for the rice. And it's a guessing game throughout the year. You start adding these and suddenly we have a lot more certainty, relatively speaking, assuming it gets flooded within a week or so. It's still so low, you know you've got about what you need out there. Of course, you've got the data and you've done it. That loss doesn't continue to creep up like it does with unprotected urea. It just continues. The losses continue to mount.

Trent: Well, I completely agree. And I think we take for granted those yield gains that have occurred, but that we haven't attributed to the use of those urease inhibitors. You take a 60-acre cascaded and flooded rice field where you were losing five, ten bushels, those last three or four paddies, and all of a sudden that yield is achieved or recovered or however you want to look at it through the use of urease inhibitors. But we didn't necessarily connect those two things. 

Jarrod: Yes.

Trent: So definitely a major, major improvement. The next thing is always okay, well what about ammonium sulfate?

Jarrod: Well, hang on. Before you jump over there, while we're just on the top of urease inhibitors real quick, I know that you and I have both dealt with some questions about this over the past couple of years or so. We're not sure where this side of a conversation came from or how it kind of got out there. Though, I guess that's kind of irrelevant. But there were some comments or some thoughts out there, or at least somebody was recommending I don't know about, “don't use a urease inhibitor because it's going to slow down the ability of your rice to take up your nitrogen, so don't use it.” Which you and I both know, again, we just discussed the gains in the value of how much protection you're getting out of it. But do you want to comment a little bit on that? That other side about what it's doing in terms of affecting any rice uptake or availability of that nitrogen.

Trent: Yeah. So, I think the misconception, right? – is that if we're protecting the nitrogen, we're making it unavailable and that's not necessarily the case with the urease inhibitor. So, ammonia volatilization is a surface loss mechanism. As soon as that urea is incorporated below the soil surface, either through rainfall or irrigation, then we no longer have to worry about ammonia volatilization loss. Really with the urease inhibitor, all we're wanting to do is protect that urea until it gets incorporated. Well, the nice thing about this particular process is that once we get that incorporation, we basically separate the urea from the urease inhibitor and so then it goes ahead and once it's below that soil surface, it hydrolyzes and converts to ammonium, which is then plant available. So, it's kind of a misconception that we need to kind of myth-bust.

Jarrod: Exactly.

Trent: And hopefully when we talk a little bit more about pre-flood nitrogen and we've got the ability to use some visuals, that would be a great avenue to discuss that a little more in depth.

Jarrod: Yeah, I agree. So anyway, I jumped in there for a second, but we're going to move into ammonium sulfate, AMS talking about that if you want to jump in there.

Trent: Yeah. So ammonium sulfate is one of those that's probably as close to a perfect nitrogen fertilizer that you can get from the standpoint of it's an ammonium based fertilizer, which we need for rice production. It does have sulfur which we could argue in some cases is needed, and in some cases it is not. But what are the limitations? Unfortunately, it just has a low nitrogen analysis. So even though it's ammonium-based and we have less potential, and I would say almost no potential for ammonia volatilization losses with ammonium sulfate, we do have that low analysis, right? It's less than half of what we see for urea. And when you start talking about things like aerial application or just fertilizer application in general, if you've got half the nitrogen content, that means double the application rate. So, something's got to give there, right? In terms of time or cost and all those issues that come in with doubling the application rate.

Jarrod: You know, a year or two ago, it just so happened that I didn't fully understand why it kind of happened. But actually they got really close in the cost of ammonium sulfate, per unit of nitrogen – per r unit of N, got really close to that of urea and even when you included the additional application cost of having to run more. I mean, they were pretty close, so that’s not common, but that can happen. And then, yeah, you could be up to argue which one to run there. But, that's not been the common situation. That did happen one recent year. I think that was one out of the past decade for sure that has ever happened.

Trent: Well, I think that just emphasizes the importance, right? – of calculate the cost per pound of N or per unit of N that you're applying, because that's really the common ground that we need to compare the cost of those nitrogen fertilizer sources on.

Jarrod: Yep.

Trent: But yeah, ammonium sulfate, you know, a great nitrogen fertilizer source, it can get costly if you don't need the sulfur. But that's just something that you've got to sit down and calculate on a case-by-case basis, to see what it pencils out as.

Jarrod: Yeah, and the majority of our rice soils don't need sulfur.

Trent: Yeah. Typically if you look at what I always call a typical silt loam soil or a clay soil, is typically going to have adequate organic matter, which is really the source of our sulfur. And as long as we have adequate organic matter, we're not going to see responses to sulfur fertilization and rice. And it's really those extreme scenarios where we have beet stand, right? Either streaks or areas of the field that we see those responses to sulfur and typically we're aware of those, right? So, we know that when we grow rice in those fields or those situations, that sulfur is going to be essential.

Jarrod: Absolutely. And we're still combating some of those where we've done some more leveling. Things like that where we’ve exposed some sand veins. There's an occasional rice field on a sandy loam, that's it's going to have a problem out of the gate. But yeah, those are our areas for wanting/needing some salt, for incorporating that into our nitrogen program. But again, even if that's the case, you're just typically trying to get some sulfur out there. That doesn't mean your whole nitrogen fertilization program has to be all ammonium sulfate. You just need to get some ammonium sulfate out there as part of it, to get your sulfur needs covered for the year. Again, the remainder to get your total nitrogen units. The remainder could be made up of urea. You know, just a blend of the two. Just another avenue of approach. So what about UAN? We’ve covered urea and ammonium sulfate. What about UAN? And this has come up several times this past year.

Trent: Yeah. So UAN, just to remind everyone, is a liquid fertilizer, you know, nitrogen fertilizer source.

Jarrod: Urea, ammonium nitrate.

Trent: Urea, ammonium nitrate. And so 50% of the nitrogen comes from urea as the primary source. 25% is ammonium and 25% is nitrate. So essentially, what are we doing? We're taking granular urea and granular ammonium nitrate and dissolving it into a liquid form. And there are a lot of benefits for that, you know, ease of application, injecting right below the soil surface. So by putting those nitrogen, traditionally dry, granular sources into a liquid form, there are a lot of potential benefits that could come from that. But when we think about this, 50% is in the urea form. 25% in the ammonium form, 25% in the nitrate form. So, if we break that down, a couple of things to remember is that, if we surface supply urea, ammonium nitrate we still have significant potential for ammonia volatilization losses, because at least 50% is urea. We know the problems with that, but you've got 25% as ammonium. So, if the soil conditions with high pH and things like that persist, there's the potential that at least 50% and as much as 75% of our nitrogen could be prone to ammonia volatilization loss. So, we need to keep that in mind. And then we've got this other 25% over here that exists as nitrate. Well, early on we talked about the importance of either ammonium or ammonium forming fertilizers for rice production. And the reason, once again, for that is when we have ammonium in a reduced soil condition or a flooded soil condition, it's protected. We're eliminating lost pathways. When we look at something like nitrate – nitrate in the soil system is prone to a ton of lost pathways. We can have leaching, we can have de-nitrification, all these negatives that are going to compete with a plant for that nitrogen that we have there. So, any time that we can avoid nitrate in a rice production system, we're going to try to do that just because that nitrate is one step away from loss. In upland crops, like corn, grain, sorghum, cotton, we're not traditionally flooding on purpose, right? We're typically in a raised bed type of system. So even if it does get a lot of rainfall, it tends to drain more rapidly. The point being nitrate can be used in those production systems because it's less likely to be lost. In a rice production system, just the nature of the cultural and water management practices that we use, that nitrate, we can't count it, right? We have to assume that it's going to be lost. So, the message there is – if you use UAN, there's a quarter of your nitrogen that you can just assume is lost.

Jarrod: Yeah. So, 80% of our production acres are still flooded at this point. Whether you, and not that I know of anybody really doing this, but whether you go knife it into the soil in that rice field or it's a big volume broadcast, that liquid on the surface – we got problems in that scenario.

Trent: Yeah. And that's essentially, you know, you want to avoid it just because a quarter of the nitrogen you're putting out, you have to assume is going to be lost as soon as the flood is applied. And what I always try to use as an anecdotal example is, if you look at the efficiency of like pre-plant nitrogen sources in rice, why are they so inefficient? It's because even if we apply urea pre-plant, it converts to nitrate by the time we flood. So as soon as we flood it, the majority of that is lost to denitrification. And the purpose of this program is to focus on nitrogen sources. But you have to remember that any of that nitrate you lose, it's being lost as nitrous oxide, which is a detrimental greenhouse gas. And that's something that we want to avoid.

Jarrod: Absolutely. But going back to losing 25% of UAN. So I go back to our things we've already passed on a couple of times. Get your pencil, your calculator back out and go back to that cost per unit nitrogen. So whatever rate you think you're running of UAN, in terms of pounds of nitrogen you think you're putting out, well multiply it by .75, because that's all you got. Now redo that math. Okay, I'm actually I'm paying this much, but only for three quarters of the nitrogen I'm paying for, again, if I'm in a flooded rice system – 25% is going out the door. Now you touched on the corn and cotton and some of those parts in the raised beds, so what about UAN in a furrow irrigated rice system which is about 20% of our acreage right now.

Trent: Well so I think the difference, right? – is still you have to think about those cultural and water management practices in a furrow irrigated system versus like a corn or cotton production system. Just think about how much wetter and how much more saturated we want the soil to be in a furrow irrigated rice production system versus a corn or cotton production system. And that's where the difference comes in is, we're not trying to keep the soil saturated and corn or cotton production because we know that’s going to…

Jarrod: You don’t like dead corn? I don’t like dead corn.

Trent: Why is all that corn at the bottom of the field yellow, right? 

Jarrod: Yeah. 

Trent: I mean, that's the exact issue. That same thing happens in furrow irrigated rice. The rice doesn't turn yellow, right? 

Jarrod: Yep. 

Trent: So it's one of those types of situations that even though we feel like, okay, we're growing it on beds and we're furrow irrigating it, not flooding – you have to remember that furrow irrigated rice, the soil conditions are much different than they are for furrow irrigated corn or cotton.

Jarrod: Yeah. I think that's one of the you know – we’re meeting in the middle. But obviously going to furrow irrigated, we're all guilty of it. You get this mindset of picturing corn slash cotton production, but with rice. But we know we're watering it way more often and again, way more towards saturation or getting close to saturation then back off. We're hammering away at it. So just again, very different dynamics for that nitrogen out there, than in that corn or cotton system. So, I think that kind of covers UAN. Not that we need to spend much time on this last topic, but you bring it up because it comes up in conversation – controlled release nitrogen fertilizers. There are quite a few out there, that have different percentage nitrogen, different make up – but the idea of being a controlled release of that nitrogen. You got any comments on that one?

Trent: Yes. When we think about controlled release fertilizers, they're usually controlled release either due to solubility or some type of coding, with the idea being that if we can slow the release of nitrogen from the prill or the granule, then the plant, whatever the plant is whether it's rice, corn, whatever, will be more likely to take it up and less likely for that nitrogen to be lost to the environment. And the key part about a controlled, successful controlled release fertilizer product is that release rate really needs to match our plant demand or our crop demand with the idea being, okay we want to meter that nitrogen out to where we're not oversupplying nitrogen and it's being lost, but that we're also not undersupplying it, and the plant is starving, right? Or that the plant is hungry. And for rice production in particular, this is very difficult because if you think about the biomass accumulation that occurs from five-leaf rice until mid-season, in a three week time period, think about how rapid that biomass accumulation is. And I would guess that in no other crop, does a biomass accumulate that rapidly in such a short amount of time. And this is where the trouble with controlled release fertilizer, specifically for rice, comes in – is we really need no release and then we need very rapid and immediate release over a very short time period. And so just from a technology standpoint, we really don't have that yet for rice production. It's something that people are working on. It's something that's obviously important, but we just haven't found that timing where we can mimic the plant demands well enough to use those particular controlled release products.

Jarrod: And you mentioned, there's a bunch of people working on them and it's probably a matter of time. You know, somebody will get there to potentially line up and make us even more efficient than what we have been. But for various products that we've kind of looked at over the years, it's just what it looks like. It just doesn't quite match up with what that rice demand. So, it's hard to lean that route versus going with a urea. You know, in that scenario, still looks the most favorable because it's most available and the plant never checks up. It's got all they could want available to take off and run.

Trent: Yeah. And it's one of those types of things that everything is just a little bit too slow. So, then we're losing yield because it's starving or it's a little bit too fast. Right? And so then now all of a sudden, we've got those loss mechanisms coming into play. And finding that happy medium for rice production is just, it's been a little bit more of a struggle than it has been for other crops.

Jarrod: And I'd say some of them on the front end, you go back to some of the solubility based things like that. I mean, stuff I've seen over the past however many years – looking at some of these, well they’re not very soluble initially, so getting them to stay put and get incorporated at the initial flood, clearly another issue. So again, there's multiple facets going on to us arriving at a point where we can possibly have some control release fertilizers that I think we can dependably use, at least at this point in time. Well, at least for kind of what I had jotted down, Trent, I think we kind of walked through most of the sources unless we've left something off the list here. Certainly, I want to remind everybody if there is a source you're curious about and want to visit, reach out to Trent or I to dig into that a little bit more. Any kind of closing comments, Trent, on this?

Trent: Well, the only comment that I would make is, with any of these types of things, I just hope people understand that there's a lot of time and effort that goes into the research to promote the recommendations that we currently have. And I just hope people understand that the rice production system has been optimized in kind of all facets, and nitrogen is just one piece of that big puzzle. And so, there's a reason we promote and use urea – and hopefully we've kind of covered that all today, and there's a reason it's a superior source in rice compared to other sources. Once again, hopefully through today's podcast, we've kind of hit on those points as to why urea is up there. And the only other thing that I would mention is whether we're talking about nitrogen sources or urea stabilizers or any of those types of things, please just keep in mind that the significant amount of research goes into evaluating all of this information. Our goal is to provide you with the most accurate and up to date information as it relates to nitrogen management and fertilization. And we're going to be the first ones out there promoting a new product if it's been shown to either increase fertilizer use efficiency or improve of producer profitability.

Jarrod: That's it. I think you hit the nail on the head with that. As soon as we know if something's going to make your life better, we're going to be right out in front telling you and pushing you in that direction. But the kind of research, obviously some a little bit more information, general information geared today. But, we've referenced a lot of research that’s been going on for a lot of years and pretty much everything we've talked about, just a reminder, all of that research is supported by the Arkansas Rice Checkoff, administered by the Arkansas Rice Research and Promotion Board. Trent and I both could not do what we do without that support. And we're very grateful for that support as well as certainly from the U of A system, Division of Agriculture that we're here. So once again, on behalf of myself and Trent, we hope you enjoyed the information. We'll try to keep some more of these coming, including on some pre-flood nitrogen elements as we hopefully get more rice in the ground and get closer to that point that it's time to have that discussion. But please visit the Extension rice webpage www.uaex.uada.edu/rice for plenty more information, fact sheets and management guides that may be able to help you. Again, always feel free to reach out to Trent or I directly if you think we can help. And thanks for joining us once again for this episode of Rice and Advice on Arkansas Road Crops Radio.

Jarrod:  Thanks, everybody. So you.

Trent: See you.

[Music]: Arkansas Row Crops Radio is a production of the University of Arkansas System Division of Agriculture. For more information, please contact your local county extension agent or visit uaex.uada.edu.