Nitrogen pools: Our experts talk timing, testing and uptake
Advancing Nitrogen Smart, from the University of Minnesota Nutrient Management Podcast:
“Nitrogen pools: Our experts talk timing, testing and uptake”
August 27, 2024
Written transcripts are generated using a combination of speech recognition software and human transcribers, and may contain errors. Please check the corresponding audio before referencing content in print.
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Jack Wilcox:
Welcome back to episode 7 in our special series “Advancing Nitrogen Smart”, from University of Minnesota Extension. I’m Jack Wilcox, University of Minnesota Extension communications. Today we’re talking in-depth about nitrogen pools with Extension educator Brad Carlson and Extension nutrient management specialist Dan Kaiser.
Dan, Let’s start off very generally. What are nitrogen pools, and where do we find them?
Dan Kaiser:
Well, when we talk about pools, what we're talking essentially about are storage, or where the nitrogen is stored at different parts of the environment. I mean, there are really two major pools or sources of nitrogen when it comes to the environment. One is the atmosphere, that's probably the largest pool. It's if you look at our atmosphere, it's 78% of it's as dinitrogen gas or N2 gas. The thing about N2 gas is it's chemically stable, and it's not readily available to plants.
When we talk about biological or industrial nitrogen fixation, the way at which we're going from N2 gas into an available form, that has to be done by some other means. So if you look at it though on a per acre basis, is that there's potentially around 117,000 tons of nitrogen above each acre of land that's potentially available, if we can get it fixed into the form that the plants can take up.
Now, if you look at soil organic matter, a lot of people would look at that as being a large pool. If you look at it, kind of rule of thumb, there's about 1,000 pounds of nitrogen per acre for every 1% organic matter, and that's at roughly about a six-inch depth. So that's a fair amount. Again, when it comes to that, that has to be converted, because organic nitrogen isn't available nitrogen, so it has to be converted through one process or another. And while we know these numbers from a rule of thumb, it's still hard to budget those values when it comes to looking at nitrogen guidelines and nitrogen availability of the plants. So looking at it, there's a lot there. If you look at plants themselves, we start talking about inorganic N.
Brad Carlson:
Well, it's kind of fascinating, Dan, and you think about the how much nitrogen there actually is in the environment, the fact that the atmosphere is primarily nitrogen and the fact that it's not reactive really is really, it's the key to life, that we think about oxygen being good, but actually, if the atmosphere had too high of a percentage of oxygen, literally, our air would be flammable, and that certainly is not acceptable. And then, in addition to that, if you think about what you just said, about the amount of nitrogen in our soil organic matter, 1,000 pounds, I don't know if people let register with them, 1,000 pounds for every percent organic matter, think about that. You got 4% organic matter in our glacial soils in southern Minnesota, that's 4,000 pounds of nitrogen in the soil.
And I think people need to really kind of register that in their brain when we talk about the high potential that we have for supplying nitrogen naturally out of the soil, that there really is a lot there. And on the flip side, if that all... Well, if it decomposed faster than it does, it wouldn't even be there, of course, it would all just be lost. But if we did speed that up greatly, we would probably really exacerbate our water quality issues too, because a lot of that's getting released when the crop's not able to take it up. And so these are some pretty significant things, but it's also important to recognize why it is the way it is.
Dan Kaiser:
And it's really interesting, because as Brad said, I mean, there's a lot of nitrogen in the organic matter. I mean, if we could accurately predict the mineralization potential, I mean, we might be able to utilize some of that information for nitrogen guidelines, but as it sits right now, we really don't. And a lot of that just stems from the fact that it's difficult to predict with some certainty how much actual nitrogen is being released on an annual basis, because again, we need to have nitrogen mineralized. It's not in the organic form that the plants take it up. When we start talking about plants themselves, we know that nitrogen, particularly in most of our cereal grains, is the most limiting nutrient in terrestrial ecosystems.
That's the nutrient, if you look at return on investment, you're more likely to get a very high return on investment from versus some of the other nutrients, because while we know there's a lot of nitrogen in the environment, that it doesn't supply everything. On average, corn, if you look at Minnesota, we know that the nitrogen we apply in fertilizer only represents about 25% of the end yield that we get. About 75%, if you look at corn following soybean of the yield produced, really comes from what we're getting from the soil itself. So part of that's mineralizable N.
Corn on corn is slightly less, because the corn residue, being that it's high carbon, low nitrogen, takes some nitrogen to break some of that down. Plants themselves, if you look at it, they're about 1% to 6%. Most of them. If you start talking, say, a lot of people might have experience with cover crops, cereal grains and stuff, you're looking at probably around 2% total nitrogen with that. So if you're looking at uptake-wise, I mean, it's generally 2% to 3%. if you multiply by your tonnage. is what you're going to get for uptake. But generally. grasses are going to be lower, are going to be closer to that 1%, 2%. If you get into your legumes that have higher protein concentrations or have the ability symbiotically to fix nitrogen, you can see upwards to about 6% nitrogen. So it really depends on the plant in terms of the amount of nitrogen that's taken up and their ability to produce nitrogen.
Brad Carlson:
And Dan, we talk about that it being the limiting nutrient in our crops, that being that it's the single biggest need for supplying as fertilizer, because of the importance in its growth. Myself, as a bit of a history nut and an ag history nut, if you've ever looked at what happened with the bonanza farms in the 1800s, how people think we've got big farms now, we had big farms back then, but they turned out to be not sustainable. And the reason for that was when we originally turned that prairie grass over, there was a big supply of nitrogen feeding all that wheat that was being grown for adequate fertility for many years. However, without the addition of fertilizer, that eventually was depleted, and the yields decreased, and the weeds came in, and suddenly you couldn't run a farm that size with 1800s technology. And so some of these factors actually are traceable with our history.
Dan Kaiser:
So if we look at the forms of nitrogen absorbed by plants, there are two main ones, ammonium or NH4+. This is what we talked, the ion that can be held by the soil or nitrate. If you look at Minnesota specifically, while we know that plants can take up ammonium, the vast majority of our nitrogen is taken up in the nitrate form. That's just simply the fact that we have a pretty rapid conversion of ammonium to nitrate that just doesn't stick around for too long in the soil itself. But both can be taken up. Some plants, at times, will prefer ammonium.
I think corn doesn't really matter one way or the other. It's just again that nitrate's mostly around. There is a limited uptake of some small organic molecules, but generally, inorganic N is really the big one, in terms of what the crop's going to take up. So we start talking about, if you're purchasing fertilizer, if you're dealing with some of these companies that like to sell amendments and things that talk a lot about forms of nitrogen, I mean, really, the main two, and I said the big one being nitrate, that's the one you're really going to be focused on, because in terms of overall efficiency, it moves in with the water. It's pretty easy for the plant to take it up. So that's really going to get into the plant.
Brad Carlson:
Frequently, Dan, I'll hear people say, "Well, the nitrogen has to turn into nitrate form for the plant to take it up." Well, that's really not true. The plant will take up ammonium through the same exchange processes through which it takes other cations out of the soil, like potassium, or calcium, or so forth. The fact of the matter is, as you said, because most of our nitrogen turns into nitrate very quickly, the majority of the nitrogen taken up by the plant is nitrate. But that doesn't mean it has to be nitrate. And so that's really, it seems like a fine line, but occasionally, I'll hear people use that for the purposes of recommending one type of fertilizer over another, or suggesting that it has some effect on plant availability and timing, and really, it probably doesn't have very much.
Dan Kaiser:
Yeah, and a lot of the nitrogen that's in the plants is in the amino acid form or the protein form. It's what actually builds proteins or amino acids. So those amines, I mean, ammonia amines, and it's in NH4. So if it's nitrate, it's got to be converted back at some point. We do see a lot of some free nitrate in the plant itself. If you would grind your plant and just do a tissue analysis, some time, at one point in season, you will see some nitrate floating around in there, because the plant does have the ability to take up excess. It's going to take it up if it's there or not. Now, it's not to the capacity that if you over-apply by 50 to 100 pounds that it's going to take all that up. And I've never seen anything where something like nitrate poisoning, where the plant gets too much. I mean, it will limit itself at some point with that, but you will see nitrate floating around in the plant.
Brad Carlson:
There has been some talk over the years about using SAP tests, taking leaf tissue, and extruding some of the liquid out of there, and testing it, and so forth. While technically, that may be scientifically valid. I know when I was a graduate student, we were working a lot on the SAP test with potatoes, it's pretty labor-intensive. I mean it's not that it can't be accurate and can't be done, it's just simply a matter of does anybody really want to go out there, and collect leaves, and take them back, and then put them in a press, and then you got to analyze it, you got to have the equipment for that. It ends up being really expensive and labor-intensive.
Dan Kaiser:
Yeah, and some plants, like you mentioned potatoes, I mean, there is some recommendation of using petiole nitrate, but a lot of other plants, I mean, really don't use it. Nitrate's fairly transient in the plant. I mean, it moves around a lot. So getting an accurate number, what does work well is something like the basal stalk nitrate test at the end of the season, if you want a report card on what you did, because it gives you a good idea if you're above or below what's deemed a critical level or enough, that it can be used effectively just because of the ability of the plant to accumulate nitrate in excess of what it needs. So the plant, I mean, it's somewhat good at telling you at certain points of time, it's just whether or not it's good enough to tell you if you're trying to make a management decision in season, where it becomes quite a bit more difficult.
Brad Carlson:
Yeah, and so we've talked in the past about the nitrogen cycle. I guess it's important to remember that because the nitrogen cycle does move nitrogen into the nitrate form fairly quickly, being time and temperature dependent, nitrate is subject to loss. And also, we want to time the presence of nitrate in the soil at the same time that the plant is able to take it up, when the roots are down there and so forth. The ammonium is not quite so mobile in the soil, and so that's why we really steer towards anhydrous ammonia only for our fall applications.
Dan Kaiser:
So one thing about nitrogen is, like some of the other macronutrients we have, that it is mobile in the plants. If you're looking at deficiencies, they're going to generally manifest themselves in older leaves. And that's just basically what the plant's trying to do, is if it is short on nitrogen, it's trying to move some of that nitrogen it has in the plant to some of the more actively growing, some of the newer growth, where it's actively growing, so near the canopy. So you're going to see nitrogen's really the easy one. It's that characteristic V pattern progressing from the leaf tip across the midrib on the older leaves. So we talk about firing, if it gets severe enough, a lot of times the leaves are just going to essentially go completely yellow and slough off with that, but it is pretty easy to see. So your deficiencies are always going to be on your first, on your lower and older leaves, then progress upward.
Now, one of the things to consider is that, when we start getting to oxygen deprivation is that you can see yellowing, and then there's a lot of other things that can cause yellowing on the plants as well: Sulfur, potassium, again, nitrogen is one of the easiest ones, just because of the pattern, just seeing that. And typically, if nitrogen is deficient, it's going to show up first more than anything else. And that's one of the things that I will caution you, is if you are using tissue analysis, if nitrogen is deficient, it's not generally uncommon to see the uptake of other nutrients to be impacted and those to be deficient as well.
So it's one of the things that, when it comes down to sorting some things out, is that typically the thing that's more likely to cause a yield loss is a thing to correct first. So nitrogen, one, is that's deficient. I don't really pay a lot of attention beyond the nitrogen number, if that's deficient, in a tissue report, because you could get reports of a lot of other low things that you go put some sprays on, it's going to do absolutely nothing, because you're not likely correcting the major problem.
Brad Carlson:
And Dan, we talk about because nitrates mobile in the plant, it'll rob it out of the oldest tissue first and take it to the new growth. So the bottom, oldest leaves are the ones we see sloughing off, and I think it's important to note that obviously I think everybody knows that those very first leaves are no longer on the plant. By the time the plant reaches maturity, that's not necessarily a nitrogen deficiency issue. When the sunlight gets blocked off, those leaves kind of lose their purpose.
It's not dissimilar to walking out in the forest and seeing no branches on the trees all the way to the ground. There were branches there at one time, but when the sunlight disappeared, they lost their function and they fell off. And so that's kind of the key, is if you're looking at the crop and looking at those dead lower leaves, is there still sunlight getting down there? If not, that might be a natural process. However, if it's early in the season, the rows aren't closed, there's not a full canopy out there, then you're probably dealing more with nitrogen deficiency.
Dan Kaiser:
You know, some people might like their corn dark green all the way to the ground, and with corn, we know that excess to nitrogen really isn't as much of an issue. And that's one of the issues when it comes to management, that it's easier to over-apply, because when you under-apply, you definitely see it. If you over-apply, it is not necessarily the case where you see it. That's not the case for some other crops. I mean, we look at, say, smaller grains, we get excess nitrogen can lead to lodging. something like potatoes, I think, Brad, excessive growth.
Brad Carlson:
Well, potatoes particularly, it stimulates disease. You get the excessive growth, and then you get a weak stalk, and then leaf diseases really start taking off. I think the thing we've talked a lot about over the years is one of the reasons why we've had issues with over-fertilizing corn, is that corn doesn't give you a penalty for that. And almost all the other commercial crops that we grow do penalize you for that. If you put on too much nitrogen on sugar beets, your sugar content goes down, and you're going to lose money on that. Like you said, Dan, you put too much on small grains, it ends up flat on the ground.
I think everybody knows that, and there's a lot of evidence, particularly, I know from my past, when we had a lot more smaller dairies, there was a lot of solid manure application, and then guys were planting oats as a nurse crop, coming in ahead of alfalfa. And you'd see those flat lines all the way through the field, where a manure spreader had passed, and there was too much nitrogen, and then the oats went flat.
Even if we go down south, cotton is highly susceptible to too much nitrogen. The term for that is bolting, it bolts. It takes off, and it grows vegetation instead of putting on the cotton bowls. They'll get 10 feet high and you'll get nothing out of it. So really, corn is an interesting crop, and it's kind of allowed us to get away with putting on higher rates, and I know there's a lot of guys that, boy, they really have an emotional response to the color of corn, and it's understandable, because of course, it is an indicator of whether you're probably going to get a good yield or not. However, we need to be a little bit careful if you're over-fertilizing, just simply so it makes you feel good. That's potentially one of the reasons why we've had some issues with some of the nitrogen loss in the environment.
Dan Kaiser:
Yeah, I can throw soybean and dry beans in there too. I mean, excessive growth, both disease issues with soybean too, excess N. You get more available N ahead of it, too. We get problems with iron chlorosis more often in the western part of Minnesota, for soils that are prone to it. So said corn, it's a lot easier to... A lot of guys do think about that insurance, and attitude, and the issues anymore as environmental conscious. We've seen with a lot of issues show up here in the state, that insurance and philosophy, really, we need to be looking at the four Rs again, and just seeing if there's kind of a way we can avoid that, because it's kind of a tough one when it comes to knowing that you're applying extra, just because you're going to be getting lost by a certain practice. There's some things we need to be considering.
Brad Carlson:
Well, yeah, and you're talking about the soybean, Dan, there has been some research done over the years from manure application on soybeans, if we've had livestock farms that were having issues with acres. And so soybeans are a good scavenger of nitrogen. That takes energy from the plant to fix nitrogen. If it's a legume, it will take up nitrogen if it's present in the soil. But I know from some of the research projects we've done, some of those beans that get a high rate of nitrogen fertilizer, they don't die. I mean, you're waiting for a hard, hard frost, because the stems are all still green, you can't combine them. They just wrap when they get in there. So that can cause a lot of issues, also.
Dan Kaiser:
And one of the things, you know Brad, I think you had mentioned too was when we talk about the corn that we can lose a few lower leaves, and we do know that. I mean, it's all kind a question of timing and the severity. Certainly, you don't want to have firing where you have a nitrogen deficiency on the leaf below up to the year. We start talking about when we get towards pollination or silking, we know that's going to be an issue, and even towards the end of the growing season.
So we know that there can be at least some loss of leaves and some firing, particularly if it starts to occur towards the end of the season, because really, you want the corn to start running out of nitrogen, I would say probably R4, R5. That kind of means that you're likely at that stage where you're most efficient, and you might be losing a few leaves on that tail end, but it shouldn't be at a point in time where you're setting yield back. And it's one of the things that as long as it's the first maybe four leaves or so and it's not severe firing, we generally know that we back things up, say with the basal stock test, I can generally-
So we generally know that when we start backing things up with the basal stock test, that when we start looking at those numbers, is that you're generally going to see low values if you start getting firing close to the year. So we know we ran out at that point in time. So visually, a lot of times, you can go and hit some sort of estimation of where you're at just by looking at where things are firing. But it's problematic, because even with the deficiency symptoms, is that we don't always catch them at the point in time that we need to know. And that's one of the reasons I've run into issues with some of my fields is that a lot of our soil, since we do have a lot of mineralized in or we're carrying over a lot of nitrate in, that we may not run out to the point where we're getting of those deficiency symptoms till later in the growing season, where it's a point at which it's too late to apply nitrogen.
So that's one of the things that we have to remember, is that when it comes down to these pools and the availability, is that there's still a timing aspect of that, and we want most of our available nitrogen at key points in time when that crop's going to be taking it up. And that's really around V5, V6 is we want to start having our maximum available nitrogen, because if you look at later in the season, that generally the amount of nitrate in the soil is relatively low, because that plant's extracted a lot of it, and it's able to extract what it needs to keep it going late in the growing season.
Brad Carlson:
Yeah, Dan, you talk about using the basal stock nitrate test, and we're not going to go into a lot of detail on those numbers right now, but you can't actually visually assess that corn crop also, when we get late in the growing season. Because nitrogen is mobile in the plant, it does start robbing the nitrogen out of the stock to fill the grain at the end. So it is very normal to see nitrogen deficiency in lower leaves, right as we're reaching maturity at that point in time.
That's not an indication that you under-fertilized. If the corn stock is and the leaves are green all the way to the ground, and it's the end of August, you probably over-fertilized. There probably was not the need for the amount of fertilizer that was applied in those soils. And so you can kind of estimate that. I think we really kind of look at if you're seeing nitrogen deficiency in the leaves, and as long as it's below the ear, you're probably fine. If those nitrogen deficiency symptoms reached up to as high as the ear, now you need to start questioning whether maybe the yield was affected.
Jack Wilcox:
Extension educator Brad Carlson and Extension nutrient management specialist Dan Kaiser, thank you for this information.
If you have a question or comment for either Brad or Dan, please email us at nutmgmt@umn.edu. Thank you for listening and we look forward to seeing you next time.
Advancing Nitrogen Smart is proud to be supported by the farm families of Minnesota and their corn check-off investment through Minnesota Corn.
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