Variable rate nitrogen: What are we trying to accomplish?
Hello and welcome back to Advancing Nitrogen Smart, the special podcast series from University of Minnesota Extension. I'm Jack Wilcox in communications here with Extension.
Jack Wilcox:On this episode, we're going to talk about variable rate nitrogen. Here with me as always are Dan Kaiser, nutrient management specialist here with Extension, and Brad Carlson, Extension educator. Brad, what is variable rate nitrogen generally, and what's the philosophy behind it?
Brad Carlson:Well, what we want to talk about today, Jack, is just kind of a philosophical what are we trying to accomplish when we variable rate nitrogen. I know I had written a blog post about not wanting to use yield maps for basing variable rate nitrogen decisions on it. I think it's worth kind of backing up and talking a little bit more about exactly it is we're doing if we're variable rating nitrogen. And I think the simplest way to put this is variable rate nitrogen is nothing more complicated than a sequence of fixed rate nitrogen decisions. And so if you look at what makes your total nitrogen need for your crop vary across the field, you're just simply instead of taking the average for the field, you're taking that into account in various places, within the field.
Brad Carlson:And From that standpoint, there doesn't need to be some kind of magical wizardry on everything's just rolling, sliding scale. Really, it's just a matter of every place you're making a decision what's going on in that place. From that standpoint, it is really a factor of just kind of figuring out what those conditions are that would make the nitrogen needs of the crop going up or down in any particular place in the field. I think what is really important to remember is nitrogen behaves the same everywhere. Are talking the nitrogen cycle, so the effects of the soil temperature and weather and total amount of rainfall we receive and so forth, that's not going to be different going across the field.
Brad Carlson:Some of the soil factors are going to be different based on soil type, particularly like texture. You've got sandier soils, we've got faster water movement through there. If you've higher clay content, we may be ponding and holding water and so But nitrogen itself, there is no real mystery for how it is behaving, it is just simply a matter of looking at those factors in any given place where you are in the field.
Daniel Kaiser:So one of the keys really with it is trying to figure out, you know, what do you base your decision on? And that's, I think, the thing that if you look at the research we struggle with is trying to figure out just that, you know, one piece of information or multiple pieces of information that growers can use to start looking at, you know, variable rate n. Because, I mean, essentially, what you're doing with variable rate n, it's pretty simple. I mean, you're just making a series of fixed rate n decisions in different areas of the field. And so trying to figure out then, you know, what key components are supplying nitrogen because because I think it's important to, you know, note that I would just, you know, say corn following soybean situation that, you know, roughly about 75 maybe percent of the nitrogen that's taken up by the crop is coming from something other than fertilizer.
Daniel Kaiser:So, you know, it's getting back then into the nitrogen cycle and trying to figure out where that 75% is coming from. You know, a lot of people in the past have used organic matter while we know that's a pretty significant pool of nitrogen that we can be getting, in a yearly basis. There's also issues, since organic matter mineralization is impacted by moisture temperature, just the amount of organic matter that's there, saturation. That's one of the things in Minnesota that, you know, people ask, well, why don't we use organic matter? Well, if I've got a field that's well drained, that has variable organic matter in field, it might be possible to use organic matter because the areas that would be higher organic matter probably are gonna be mineralizing more.
Daniel Kaiser:I mean, and they would be go to like a rodent knoll versus kind of a lower part of the field. But if you get into, you know, Central And Western Minnesota where we tend to have high organic matter areas that are frequently saturated, which limits the breakdown of organic matter to why they're high, but that also impacts other things like denitrification. So you've got kind of a net positive and a net negative in there impacting it where it's not a simple decision. And then, I mean, carryover and we can measure that. You've got atmospheric deposition, which really isn't that big of an issue.
Daniel Kaiser:Anything, you've got some cycling from, you know, manure, you know, which can impact things for multiple years in the future. It's what it boils really down to is it's pretty complicated in trying to model all this and predict that's really it's difficult to try to figure out where the starting point is. I think that's really the challenge is if you look at the the complexity of the nitrogen cycle, there's just so many errors involved where if we could model it, those calculation where there's just a lot of uncertainty in terms of what that target value could be in terms of the starting point for, you know, where you'd be look at starting your nitrogen applications.
Brad Carlson:Yeah. And and, you know, I I think the one area with organic matter that's important to remember is, particularly low organic matter soils are not going to mineralize as much nitrogen, realize that most of our low organic matter soils, in a lot of cases, are sandy soils, and we have also got different nitrogen recommendations for sandy soils. From that standpoint, it is a question of, are you really concerned then about the organic matter or are you concerned about managing sandy soils the way we manage sandy soils, which is probably much more important because we are also then worrying about the potential for losing nitrogen because you got sandy soils. You know? So there's multiple factors, going on there.
Brad Carlson:You know, I like to kinda think of this all as just a little bit of a math equation in terms of if you can kinda figure out what the additions are, the credit you get from your soil, which is not a simple thing to to be doing, then then it's a matter of, you know, if there's any other additions like you mentioned, Dan, atmospheric deposition, we really can't worry about that because there's no way to calibrate it and we don't really know what it's going to be or predict what it can be, although typically it's not going to be more than 10 pounds an acre anyway. However, we do know that there are circumstances where there could be unused fertilizer from the previous year in corn on corn. Obviously, if you've got unused nitrogen from the previous year and you're growing soybeans, we don't really care. But in drought situations where we've experienced soil conditions whereby we haven't really lost any nitrogen and it's just kind of sitting in the soil, that may be possible. And we also know that there's some kind of an X factor related to fields with long term manure history and their ability to mineralize higher amounts of nitrogen than kind of what we normally expect.
Brad Carlson:That is not a circumstance we can easily predict beyond an individual farmer may start to get a feel for that with their own fields. I know, Dan, you've had some experience with doing nitrogen rate trials and finding unresponsive sites.
Daniel Kaiser:Well, yeah. And it's interesting with some the N rate trials. I mean, it's not a perfect scenario because, just one example, I think it's around 2014, we were doing some studies, what I call, these variable rate and well, what essentially, we're looking at with those studies is we're looking at, drones at that point in time, but looking at, just a small rate of nitrogen, say about 40 pounds applied to corn right at planting, then coming back in and sensing and then looking at the n rate response, where we had full n rate curves at planting at v four and v eight to v 10. And, you know, it's interesting on some of that data was we had a really wet year in a site down by New Richland where if you looked at the data or if you looked at the a lot of the satellite imagery, I mean, you would have just swore we lost all the nitrogen. But at the end of the season when we did yield, it was the same rate of end it took in that area this areas that were flooded, the lower areas versus the upper end areas, but they just got lower yield potential.
Daniel Kaiser:So like I said, that's not we're there's it's and there's not a perfect scenario that across the entire state that you can use the same set of guidelines to base a variable rate in application. I mean, another example of this is if you look at historically the Southeastern part of state and a lot of the less soils, those soils, you know, Brad was talking about organic matter, you know, they can be a little bit on the lighter side, maybe more 3% ish to four. I got some at Rosemont are 4%. But since they're well drained, they're well oxygenated, a lot of times we get higher mineralization potential on those soils. So when you look at historically, some of the guidelines for some of those soils in the Southeast has been for less and even though they have lower organic matter.
Daniel Kaiser:If you looked at how we broke things out at one point in time with with organic matter. So I think there's some things you can do, but it's gotta be more regional and more soil specific to consider some of these things that are happening in the nitrogen cycle because that that's just really key is that, meanwhile, we get the same things that can happen across the entire state and all soils, it's just to a degree where some of these loss, pathways really starting to come into play and that's really affected by drainage. I think that's that's really the big thing.
Brad Carlson:Well, and so I was saying this is a little bit of a math equation. If you look at the additions, as far as as, what's being supplied by the soil and then other other potential sources, you know, then just subtract off what we're losing during the season, which of course, that gets into our old discussion about whether we should be managing our four Rs just simply with the rate because we're talking variable rate here. If you've got issues with loss, potentially changing your timing is a lot better plan of attack here than modifying your rate. But that being said, we do know that there's low areas in the fields that are more prone to flooding and therefore subject to denitrification. If you've got sandy pockets, it's more prone to leaching.
Brad Carlson:Again, we'd really like to manage those in other ways beyond just simply increasing the rate. But then of course, there's other things going on out there also, which you probably are aware of if you've been farming these fields for any period of time, whether there's a compaction problem out there, any restrictive layer that's preventing root growth deeper into the profile, poor soil structure, those sorts of things. That's really more of a factor of the roots not finding the nitrogen than it is of there not being enough nitrogen. And so from that standpoint, they really need to be managed kind of on more of a soil health perspective and improving your soils out there, not just simply by applying higher rates of nitrogen. And so that's one of the things you have to also be kind of careful of, is if you've got issues or circumstances where you think you need a higher rate, what's causing that and is really raising the rate the appropriate management technique for that.
Brad Carlson:The other things, guess, far as, know, Dan, you talked about the difficulty in kind of figuring out where your starting point is. It's scientific, however, it is very difficult to ever really know precisely what is going on. I think you can get some rough ideas based some of the stuff we have talked about in some of the other Nitrogen Smart content as far as the fate of applied nitrogen, particularly some of the stuff we do in our climate materials where we talk about the conversion ammonium into nitrate, the effects of soil temperature, the effects of moisture and so forth. Most variable rate nitrogen systems are going to apply some kind of a smaller base rate of nitrogen and then try and figure out how much extra is necessary after that. From that standpoint, you need to kind of look at the fate of that applied nitrogen and make sure that it's still there and then kind of look at what the crop demands are after that and so that can be a little bit difficult.
Brad Carlson:There are factors at work that are very difficult to predict, that gets a little bit more into the realm of are we making a rescue treatment or are we variable rating our nitrogen, but I guess it's worth talking about that if you do have highly saturated conditions and we think we lost a lot of nitrogen, you're probably going to have to pick that back up again in some way. The other area I think we also have to be kind of careful of is that immobilization is also a factor we've seen in some of our trials in corn on corn situations where the decomposition of corn residue, may create some deficiency symptoms in the field, and so we need to be kind of careful. Is that really a deficiency or is it just a very transient temporary thing that's gonna disappear? Now, again, that's a factor that's it's it's scientific, yet it's very difficult to really know exactly what's going on. You know, from my recommendation for that from a farmer is, if that's something you really care about, start paying attention to it and start getting a better feel for what's going on in your fields in that way.
Daniel Kaiser:I think the big mistake a lot of growers or some have made in the past is, I mean, you look at your yield maps and you just automatically assume these areas with higher yield need more. And that's one of the things that, you know, we're looking at now and I it's kind of the the quintessential argument. It's the, you know, feed the rich, starve the poor. You know, what's are you, you know, starve the rich, feed the poor approach, those types of things in terms of availability. And, you know, kind of, you know, from my end looking at the data, I mean, a lot of the you start talking about variable rate just simply using yield, or yield goal or something for for nitrogen.
Daniel Kaiser:It just really isn't the best thing to do because I think it's, you know, looking at a lot of the data we have, the good areas of the field are good for a reason. They're probably because they're mineralizing more in. You you probably need less than some of those areas, but it's it's challenging and that's, I know one of the the knocks a lot of people have on the MRTN or the maximum return to nitrogen, you know, approach that we use for our nitrogen guidelines is that it's a number. I mean, there is a range of numbers, yes, there, but it's I mean, it's just simply a number. And, you know, it's really you know, the challenge I think is coming up with the general guideline for people to use because Brad said, you know, you look at lower rates.
Daniel Kaiser:I think there is something to say about having some flexibility to make adjustments versus putting on too high of a rate to start with where if there's nothing to take that nitrogen out, I mean, there's really it's just gonna be left at the end of the growing season. So the thing that I guess is the challenge for most growers, I mean, since farming began is the fact that a lot of the decisions in terms of what they are being made before we know what's gonna happen environmentally. And that's really the the the big key is if we could accurately predict the weather ahead of time, I think it'd be a lot easier to, tell growers what to do for options maybe in terms of timing or or rate to avoid some of these these situations where you get a lot of excessive rainfall. That that's so the key thing is it's just it's really challenging, and, you know, I think there's some complaints of and, you know, the fact that thinking that we're behind the times and the fact that we're, not recommending a lot of these these variable rate applications because technology is there for application.
Daniel Kaiser:It's just the reasoning behind in making decisions, I think, is where we have have yet to catch up to. And, you know, the only thing that's really been looking at a lot now is we've been looking at a lot of, satellite imagery, you know, active sensors, and those types of things to help us make some of these decisions. But the problem is here in Minnesota is we've got residual nitrate that carries over from one year to the next. So a lot of times, we don't really necessarily see the plant manifest an efficiency that might be related to what we're seeing at the end of the growing season early enough to make some decisions. The whole nitrogen cycle, it's complicated.
Daniel Kaiser:You know, those inputs, outputs are hard to predict. And in many cases, we're making the decisions well ahead of time that, you know, if we could have a better snapshot of what's gonna happen, say, in June, I think we might have a better idea in terms of what to do with.
Brad Carlson:It does come down to how how are you what kind of a technology are you actually using then to to make these determinations on where you need to vary the rate? You really hit it, Dan. Remember there was the research trial at Waseca on the drainage plots that's looking at cover crops, and Jeff Vetch has some data here from a few years ago where one of the treatments where they had cover crops established needed a higher rate of nitrogen. That was evident at the end of the year that there was nitrogen deficiency where there had been that extra residue. However, they were doing chlorophyll readings and they found no difference in those treatments in July.
Brad Carlson:And that didn't show up until they did chlorophyll readings kind of towards Labor It suddenly separated itself, a very, very difficult thing to manage. Mean, We don't like people going in and putting nitrogen on in Minnesota late in the growing season, however, it wasn't until late in the growing season that that showed itself. One of the other areas that we've been looking at lately, and we've talked about this some in the past too, is the ability to predict total crop need sort of based on what the yield is at zero nitrogen rate, that that's kind of an indicator of the soil's ability to supply nitrogen. If we look at the number of nitrogen rate studies where we require higher rates of nitrogen, it does tend to correlate with places that are less capable of supplying mineralized nitrogen to the crop. However, one of the things we don't really have a feel for is the extent to which that happens within the field.
Brad Carlson:We've got those studies that are all over the state, but as far as looking at you know, applying zero to an entire field, we don't have a lot of producers who are jumping up and saying, plant my whole 40 at zero and then let's look at the yield variability.
Daniel Kaiser:And that's one of the things I am interested in because, I mean, if you wanna look at something that really integrates a lot of these things, I mean, this this full nitrogen cycle, it's it's that zero end yield. And, you know, I don't think, Brad, mean, playing the whole field really is necessary, but if you'd say, like, something like the old NMI programs that were done through the Department of Ag, the nutrient management initiative that, you know, say you pay growers to leave some areas that are maybe a quarter acre or less just of zeros at strategic points in the field just to get a general handle on what that control yield is relative to the max. I mean, might go a long way in terms of figuring out, you know, some of the the differences in terms of optimal end need. I mean, the the question I have though about that essentially is the stability over time. You know, if we did that year after year after year, would we get the same results?
Daniel Kaiser:Because there's always this this thought that if we don't apply nitrogen that deviation from the zero to the the max yield potential widens over time, and I really don't know if that's true. But it is, think, an admirable question and just for simply the fact for growers gonna do this, I mean, I think you're gonna be seldom to a situation where you're not gonna have a true zero with no nitrogen as most growers are probably putting MAPDAP or ammonium sulfate down, which are gonna supply some. So there's there's some difficulties there with that, but if I look at the data, I mean, it's pretty clear. If you wanna if you look at the optimal n rate versus yield potential within our whole dataset that we use for a nitrogen guidelines, it's a shotgun blast. But if you take the corn corn and the corn soybean, they fit both together and you plot the zero n yield as a relative potential to the max yield, it's a linear relationship with, the optimum n rate.
Daniel Kaiser:And even as Brad said, the the sites that where you see lower check yields compared to the max, I mean, clearly, those are ones where you look at needing, you know, much more more than what we generally recommend for our nitrogen applications. And we've had situations where I know, Brad, you were working with some growers around the Northfield area with corn on corn that some fields there, very high yields, you know, be a we're probably recommend one eighty, 200 pounds of N in that situation, but you were finding more closer to that one sixty range. Right. One fifty. One fifty.
Daniel Kaiser:So I mean and
Brad Carlson:280 bushel corn, corn on corn at that.
Daniel Kaiser:So, I mean, you know, we know there's some variability out there. Just it's just how do we weight that? And I've I've got some thoughts. It's just I think getting some people really amped up, but only a lot of people, like you said, are amped up putting no nitrogen in areas of the field.
Brad Carlson:You know, and again, we're not really spending a lot of time here talking about the technologies that advise, but, you know, the one area that we did find that were kind of encouraging and they're pretty much gone from the market now were the crop models. However, I personally, and I'm kind of just speculating here, I kind of tend to wonder if these things aren't going to come back instead of being a crop model more in the form of artificial intelligence, being able to weed through actual field performance and climate factors and other things relative to the hybrids that were used and so forth, which was kind of what the crop models were based on anyway, and maybe be able to pick some of that stuff out. As I said before, a farmer may get a feel for that, but it's a lot of stuff to keep a handle on, particularly when we're talking about over many years, that's just a lot of information. So computers do have the power to potentially start picking some of that out, you know, so that maybe we're able to to start managing, in that way.
Daniel Kaiser:And the I mean, the issue though, I think what the challenges with AI is the fact that for the most part, we'll be looking at maximum yield potential. And there's a difference between maximum yield and yield response. I mean, and that's really what we look at with the MRTN. It's what we call that marginal yield or it's it's a difference between no nitrogen where we do get some yield versus the maximum yield, making sure that we're getting a return on investments. But I think that's gonna be the challenge because, you know, with variable rate, you know, with the ability we right now, we have for grid sampling where you can gather a lot of data, you could feed it into a model, there's still a lot of things that go into raising crops.
Daniel Kaiser:That's one of the, you know, the thing I start looking at some people start talking about certain things and and how they get higher yields with certain things versus in the other when they're just looking kind of more arbitrarily at the maximum yield produced and you really have no reason unless you have a with or without treatment, to know that whatever you're looking at had does have an effect because it is challenging, because, you mean, even with crops, I mean, you've got optimal nutrient availability of optimal water, optimal weather conditions. I mean, it's just biological systems are complicated, and that's really what makes nitrogen management and variable rate management, I think, really difficult because of the complexities within that nitrogen cycle that you can predict a lot of these things with bits and pieces of research, but there's errors always associated with a lot of those predictions. And the more of those that you have together, the more it just compounds the overall error and the ability really to hone in on a specific rate. So, you know, you're still probably looking at a pretty wide rate window with a lot of these things, especially in those models, if you're looking at rate predictions.
Daniel Kaiser:I mean, maybe AI can do something. I mean, I don't know. We'll we'll find out. Mean, probably saying some of the same things for the last forty, fifty years. I mean, my my predecessor been saying we're still stuck here with kind of the same the same thing is that, you know, it's challenging and we're just not quite there yet in terms of some of the sites for cinematics, specifically for for nitrogen.
Daniel Kaiser:I mean, if nitrogen wasn't leaky, it'd be easy. With p and k soil tests do a good job in terms of a risk assessment tool that you're gonna be short or you're need fertilizer. With nitrogen, we just don't know. I mean, there's there's certainly a number of things that that, you know, we could predict. It's just the high accurate those predictions are gonna be.
Brad Carlson:That kind of leads back to where I kind of wanna wrap things up and it's something I've already kind of touched on. That's, you know, be very careful about using rate to compensate for some of the other 4 Rs that if it seems like it's easy, like, hey, we got this figured out. In reality, what you may have actually figured out is you got parts of the field that are losing nitrogen and you're just applying a higher rate out there. And so I think that's really the kicker for a lot of this stuff is getting back to what is going on in that, on that any given site or spot in the field that's requiring a different nitrogen rate. And if it's something that should be managed because you should be using a different application timing, if the site is just historically too wet, if it's sandy soils, variable rating nitrogen is not the solution there.
Brad Carlson:It's changing your management approach. And we talk a lot about, in general, on average, we don't see a big advantage to split applying nitrogen. But the thing that's important to remember is there definitely are sites and situations that do respond to that. Very particularly, some of our coarse textured soils are going to be some of those as well as maybe some of the poorly drained sites that are prone to just lose nitrogen through denitrification, that maybe if you can come back in in June after the crop is growing and using some water, you can be a little bit more efficient with that. There's a lot of research data that we've got from different trials and particularly the ones that I really like to use a lot are the drainage studies at Waseca because it gives us a clue as far as how much nitrogen is being lost and in what way.
Brad Carlson:Because if we look at the total amount of nitrogen that we're finding in the drain tile and it's all the same whether you apply it in the fall or you apply it in the spring, yet we see a big yield difference. We know we're denitrifying the nitrogen out there and really that site's just simply telling us it needs to have an application that's going on in the spring and not in the fall. And so in a similar way, there's just a lot of other factors that should be managed in that way and not just simply by varying the nitrogen rate because you're gonna lose a bunch of it.
Jack Wilcox:Dan Kaiser, Extension Nutrient Management Specialist and Brad Carlson, Extension Educator. Thank you very much as always.
Brad Carlson:Thanks, Jack.
Jack Wilcox:Do you have a question about something on your farm? Just send us an email here at mgmt@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.
