UMN Extension Nutrient Management Podcast

Advancing Nitrogen Smart, from the University of Minnesota Nutrient Management Podcast:

“How soil water storage, temperature, and the nitrogen cycle help MN farmers make key decisions”

July 25, 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 5 of Advancing Nitrogen Smart. I’m Jack Wilcox, in communications here at University of Minnesota Extension. As always, here to explain the science of N’s behavior in the environment for MN farmers we have Brad Carlson, Extension educator and Dan Kaiser, Extension nutrient management specialist.

Today we're talking about the ways that the soil stores water, and how nitrogen transforms in that soil under different climate conditions. We’ll discuss field saturation and the water budget, heat units and how to track them, and how these trends have changed throughout MN.

First of all Brad, remind us what are the overall factors related to climate that affect the nitrogen cycle itself?

Brad Carlson:
Well, when we talk about the nitrogen cycle, we're talking about either the conversion of nitrogen through its various forms in the soil, as well as potentially its loss processes, which of course we are concerned about both from an environmental standpoint and from a production standpoint, that if we need the nitrogen out there for the crop, we do need to worry about whether it's there and the crop is able to reach its full potential. It's one of those areas where we talk a lot about the fact that this is all science, and it most certainly is, it's just very difficult to know exactly what's going on in any one place at any one time, but it is something you keep track of. So there's really two main aspects of the climate that affect this and they're temperature and precipitation. So the temperature both affects the crop growth, but because conversion of nitrogen in the soil happens through soil microbes, the living organism, they also drive off of temperature.

They slow down when it's cold and they speed up when it gets hot. And if it gets too hot then they slow down again just like we do. And then as far as precipitation goes, we need to be concerned about whether we've got enough during the growing season for the crop, but there's also issues relative to annual precipitation because that does affect some of the environmental fate of nitrogen. We do need to think a little bit about large events, single rainfall events that can cause problems. If you look at the growing season total, and it looks like it was normal, but you had one five inch rainfall in there, well then actually your growing season wasn't very normal. So that's also of concern to us. And then the other area that's getting into a little more science-y, but I think a lot of farmers should probably pay a little bit more attention to this is, the amount of precipitation versus the amount of evapotranspiration.

Evapotranspiration is... There's really two things. The evapo part is just simply how much water evaporates. So we know that if it falls on your pickup truck and the sun comes out, it evaporates. So some of it evaporates off the soil surface and evaporates off of plants and so forth. The transpiration is the part where the plant is actually taking it up, running it through the plant, and then it's going off into the atmosphere. So if you add those two together, that's the amount of water that we lose. So it's simply what we would call our water budget. It's the amount that you get versus the amount that you lose and what is the balance of that, and that does affect the amount of nitrogen, or I should say the way nitrogen behaves in the soil and whether we might've lost it.

Dan Kaiser:
So transpiration, just to get back to that, essentially it's like sweating. Essentially. It's like us sweating, it's what the plant's doing. It's cooling itself by transpiring water through it. So you'll see that when the plant starts... Especially when you're water stressed, if it starts to curl, that's the plant trying to, especially corn, trying to reduce the amount of water being lost through the stoma of the leaves.

Brad Carlson:
The stoma is like a pore. It looks almost like a set of lips if you look at them under the microscope, and they open up wide when it's hot and wet and they close themselves up to avoid losing the moisture when it's dry. So as far as we try and assign some averages or we try and look at what usually happens, for our standpoint, we've done an awful lot of our analysis based on the numbers that came from the research station at Waseca. One of the reasons to use that is that it's a long-running very detailed data set, and so we've pretty much got all the numbers we need and a lot of folks might say, "Well, but that's not here," or "That's not close to me." The thing you have to remember about any climate data is it's always backward-looking and we can't accurately predict what it's going to be next year because always variability from one year to the next and one place to the next and so forth.

So it doesn't really matter where you're taking that climate data from if it's in the general region or area that we're talking about, because what happens in the future is going to only be a general approximation anyway. And so we've done a lot of our looking at... When we talk about our averages, we've looked at the Waseca data, so I guess you realize you're listening to this in Minnesota. If you're further north, it's going to be a little bit later, maybe a couple of days later. Can't get too much farther south when you're in Iowa, so that's going to be pretty accurate. And then from a moisture precept standpoint, we see that it gets wetter in the state as we go from southeast to northwest, so know that the farther north and west you go from Waseca, you're getting a little bit drier, the farther south and east you go, it's a little bit wetter.

But we've used a lot of that data when we've looked at our averages. One of the key elements of keeping track of all of this is to track heat units and corn over the years, we used to call them growing degree days and some people call them growing degree units, it doesn't really matter. It's all the same thing. That's a mathematical calculation that takes the maximum temperature of the day plus the minimum temperature of the day, divided by two. So basically meaning it's the average temperature, so halfway in between whatever the maximum and the minimum was and then minus 50. And that's to account for the fact that at below 50 degrees pretty much stuff stops happening and so we don't see a lot going on below 50 degrees, so we subtract 50 and then that puts it on a single basis.

Dan Kaiser:
So we know there's also a maximum in there and that's one of the things that it's on the equation that Torrent tends to shut down. I think it's around eighty-ish, somewhere in there. So it's one of the things that if you are interested in calculating this, you can go online and look at that exact equation to figure that out. I do get newsletters or get updates from Waseca from the Southern Research and Outreach Center where they tend to track some of this stuff. So it's one of the things if you are interested in that, you could follow that, because they do a good job of tracking that from the initial start of planning until we hit... It freezes where they stop calculating.

Brad Carlson:
So from a statistical standpoint in Minnesota, we start tracking this on May 1st. I'm not sure exactly where it is in other parts of the corn belt. I think we know that particularly over the last 20 years, corn planting has crept a little earlier and a little earlier now we're at that point where it was April 15th. Well now the crop insurance early date has gone backwards towards April 10th. And so depending on who you are and what your circumstance is, we're starting to see corn go in that early. So we get questions about, "Well, should we track the heat unit accumulation prior to May 1st?" The answer to that is no, because actually if you look at what the weather looks like from the 10th of April until May 1st, the total accumulated heat units on any given year is actually pretty small. And then in addition to that, your corn's not all planted on one day anyway. It's going to be different for every field on every different day you plant. And so that gets pretty complicated if you're going to keep track of it that way.

We've always used May 1st and that allows us to just keep looking at historical databases without there being an asterisk. Kind of like when the major league baseball changed their season from 156 to 162 days, and then they put asterisks on all the records. We don't need to do that with our climate records. We're just going to keep it at May 1st. And so if you look at the numbers of heat units that are necessary to hit various growth stages of the corn, it takes about 120 to germinate the corn. V-four is reached at just a little less than 400, 395. We hit V-twelve at about 945 in that ballpark, which is important. I think we think about V-four as being when we do most of our sidedress. V-twelve is the hard cut off.

After that, we've just not seen any benefit to any nitrogen application after that. Then depending on the relative maturity length of the corn, 95 day hybrid is going to hit black layer at about 2,375. The hundred-day hybrid is going to hit it at about 2,510, and the 105 day is going to at about 2,610. So if we look at the data from Waseca, like I talked about, you want to put calendar dates on that, we start accumulating on May 1st. So if you assume we start... If we plant on May 1st, we hit that 120 heat unit accumulation on average on May 16th, so about two weeks it lays in the ground. We hit the V-four stage on average on June 2nd. We hit V-twelve, on average, about June 30th. And so that's our window. I guess we know that your nitrogen pretty much needs to all be applied before the end of June, and so you can think about that.

The average date of a hundred-day relative maturity, hybrid black layering at the research and outreach center in Waseca is September 21st. We know that given the year and the circumstance, you plan early and so forth, that it could happen maybe as early as Labor Day, but on average that's the way it is. Now, the thing that's important to think about is actually two of the last 10 years, it actually froze before we hit black layer. That's black layer on paper, but the point being we play it pretty close in Minnesota as far as using that entire growing season and what we can squeeze in for a crop, so it doesn't give us a lot of extra time. The average date of the first frost in Minnesota is October 4th, so you're dealing with about two weeks before the average black layer for that a hundred-day relative maturity corn.

Some people have said, "Well, if the climate changes, maybe we're going to shift a lot of what we can do here." The thing you have to remember though, is that even if it does get warmer, it doesn't mean the days get longer. So once we start getting past the 21st, 22nd of September, when it turns into fall, the day length just starts getting shorter and shorter and so short... If you've got say instead of freezing on October 4th, we froze on November 4th. Those days in October don't amount to much because getting pretty short as far as the amount of sunlight is concerned. And so that's probably not something really to be thinking a lot about.

And if you look at how this varies from one year to the next, and we did some graphing of this, the seasonal growing degree units, it bounces around obviously from one year to the next. There's a slight trend upward going back to 1965, but it pretty well bounces anywhere from about 2,000 up to about 3,000, and we've been trending towards that 3,000 mark. I don't think we've actually hit 3,000. We've gotten very, very close to it, but on average we've been running around 2,600 in accumulated heat units a year over the last few years, and so that's really right on, like I said, for 105 day hybrid. That's one of the reasons why you're pushing your luck if you're going a longer season hybrid than that in Southern Minnesota.

If you look at that average frost date, that also bounces around, but it's anywhere from about the 12th of September up to about the 18th of October, in that range, during that period of time.

If you look at the precipitation data, that also obviously bounces around a lot from one ear to the next. We have also seen somewhat of an upward trend there too. The last three years have been fairly dry. If you look at the decade prior to that though, it was quite wet. And so if you look at the numbers, you do see a definite look at a upward trend in the total amount of precip ranging all the way up to, what, 56 inches or so I think we've had, was the record there back in about 2015. We look at the total number of high rainfall events, that seems also like it's been increasing and you go back to that last wet decade, it certainly has, but if you look at the long-term data going back to the 1960s, there's up to six or seven, one and a half inch plus rainfall events per year scattered throughout that whole time.

The one area that's been a little bit alarming for a lot of us is looking at the thirty-year rolling average of precipitation. I think I'm like a lot of people listening to this podcast, if you're from Minnesota, you probably studied Minnesota history when you were in the sixth grade in elementary. That's standard curriculum in Minnesota. And so I was in the sixth grade in 1979, and so we use the... The standard for climate data is to look at what we call a thirty-year rolling average. And so it takes the previous three decades, but it doesn't reset year per year, it resets every time there's a year with a zero. And so if I look at the number that I was taught as far as what the average annual precipitation was in 1979, it was actually the accumulated precipitation records from the 1960s, '50s and '40s.

And so that number that I was taught was just a little less than 30 inches of rainfall at Waseca at the experiment station there. As that number advances, every time we hit a year with a zero on, they reset that. They drop off the years 20 to 30 and then they add the last 10 years to that, that number is advanced. So if you look at even the period before that, which was the number used in the 1960s, the average thirty-year precip at Waseca then was 27.8 inches. The average annual precip now that reset in 2020 is 37.9. It's 10 inches more precip annually, looking at how that's changed over the years.

Dan Kaiser:
And we're not going to get into any philosophical arguments in terms of climate change. I mean you don't really care on where it's coming from and why it's coming from. The main thing I think we need to reflect is the fact that you said, Brad, that we've seen a big uptick in some of the annual precipitation with that. And that's going to be one of the things moving forward when we start talking about best management practices, that especially when we talk about the state, that it may not be even where some of these things have changed, where what we've recommended for BMPs in one of the last revisions, which is back in 2007, may not be what we recommend now.

Because we need to be looking at adapting to what we're seeing just to make sure that we're not promoting practices that are putting nitrogen at excess risk for loss. And that's really... I think the key here when looking at a lot of this data, is knowing that a lot of these things are dynamic and they need to change with what we're seeing as I think we can pretty much... As you said, look at that data, that's a pretty stark difference. That's almost 10 inches more in the span of that 50 years.

Brad Carlson:
Yeah, and every farmer knows this. I mean you know how much drain tile you've installed, if you're in southern Minnesota in the last 20 years, and you know because you've talked to your dads and your grandfathers who tell you, "Well, that didn't ever used to be wet," or, "Oh, I've never seen water stand there before." You know it's a lot wetter than it has been, and so we need to adjust for that. Our nitrogen BMPs, Dan, as you mentioned, were developed in the early 1990s that was using the climate records from the 1980s, '70s and '60s and the 1980s in particular, I think most of us at least, if you're my age, how the 1980s ended, it was bone dry and so we had precipitation averages that we were dealing with that were quite different than today. So that number of the average annual precip at Waseca that was used in '94 back when we first rolled out BMPs, my first year in extension coincidentally, was 32.4 inches of precip and the number we're dealing with now in the year 2024, which is 30 years later, is 37.9.

So we're talking five inches more in that period of time, and most of that's all during the growing season. If you look at how it shakes out statewide, and I already mentioned this once, we pretty well go from southeast to northwest as being wettest to driest. And so if you see a graphical representation of this, it really goes diagonally. If you draw just a pure diagonal line that runs northeast, southwest, the farther you advance that towards the northwest, the drier it gets as far as the amount of average annual precipitation. And so the extent to which the amount of water falling from the sky has an effect on nitrogen, that's something we need to consider and it's one of the reasons why nitrogen best management practices are different, and have historically been different, in Western Minnesota.

Now, the other aspect of this we do need to think about, is that in addition to, if we think about the water budget, in addition to the water falling from the sky, we think about that ET that use by the crop, we know that a lot of the water that we are losing out of the system running through drain tile and so forth, that's pretty much all happening in April, May, and June, that once the crop gets growing good starting in June through the rest of the summer is taking up most of the water out of the soil profile. And we're kind of done losing nitrogen at that point in time.And so that does impact how we manage nitrogen, because we need to protect it and keep it from being lost during that vulnerable point in time that's right at the very beginning of the growing season.

If you look at the precipitation versus evapotranspiration budgets, and you calculate those for the growing season, roughly speaking, if we look at the nitrogen loss season is really the part I would look at, which again, the data shows we lose most of our nitrogen in March through about the 1st of July, after that loss is pretty negligible. Historically, we don't see a lot of water movement through drain tile, meaning we probably don't see a lot going into shallow groundwater either following that period of time. If you look at that, how that shook out back in, again, in the early '90s when BMP's were first put in place, again, we've got this hard southeast to northwest, although a little bit more, there's a little bit more of a north-south or east-west pattern on there, where the east side from the Iowa border to the Canadian border just seems to be wetter.

As you move west, it gets drier and maybe a little bit on the western side, then it gets a little drier, or a little wetter, down to the south. However, if you look at what that looked like from 1991 to 2020 instead, what we actually see then is its wet just simply all the way across the state. All the way from the Mississippi River to the South Dakota border. And particularly we already referenced that wet decade that we just got done with, that really is striking. That has been wet pretty much all the way to the South Dakota border. It's one of the reasons why even though we originally had separate nitrogen best management practices for Southwest Minnesota that came out in the mid 1990s, today, we're looking at really probably needing to follow the same management practices all the way across the southern third of the state.

Dan Kaiser:
Yeah, and that's one of the points I wanted to bring up, Brad, because if you look at the data, I mean, I think it is just spot on. If you look at the '61 to 1990 where we look at that precipitation versus evapotranspiration being different in the southwest versus what it is now. And an easy way to visualize this, if you look at soil pH, we start talking about precipitation versus ET, we break it into what we call the leaching index, which is if you look at it in the western part of the state, we see pHs that are higher near the soil surface just because the soils aren't as leached. Because of that ET, essentially that more upward movement of water, tends to maintain... Even with nitrates, I mean with more upward movement, you should be able to maintain nitrate even in that upper soil profile for longer versus the southwest.

So it affects a lot of things when it comes to management as it reflects on timing of application. And that's been one of the keys that we're looking at right now with the best management practices, is some of the recommendations in the southwest where we're a little more lax and what we would have for timing, maybe not including inhibitor, and particularly with sources, with urea being more flexible for applications there, that with the numbers we have right now, if we start looking at just the overall potential for leaching, it just does not favor fertilizer sources, particularly for fall application that are at high risk for potential leaching losses. So that's one of the keys. And I mean one of the things that we need to be adaptable on a lot of these things is we see some changes or shifts in patterns over time that we need to be reflecting on some of the key management recommendations that are more reflective, being able to maintain nitrate availability across... Or at least early enough... Enough in the growing season to get the crop what it needs at maximum peak uptake periods.

Brad Carlson:
Right, and probably the only part of the state where we really haven't seen a lot of change over that period of time is the Red River Valley. The part we're talking about that's gotten increasingly wetter is basically from about the headwaters of the Minnesota River going right straight east and then cutting off that third of the state. If you look at that area, west central and Northwest Minnesota, it's pretty well been same almost.

If you look at over the last 30, 40 years, the numbers tell us it's been the same. So we probably don't look at a lot of changes there as far as what our best management practices are looking forward. However, the one thing we have seen change drastically, is crop rotations because when those things came out in 1994, there was not very much corn grown in the Crookston area and there is now. And so we have seen the march of corn going north, and therefore we do need to pay a little more attention to how we manage corn in that part of the state because frankly, there wasn't a reason to do it in the past.

Dan Kaiser:
And one of the things what we talk a lot about these changes in weather, is the thing you have to factor in is the BMP regions also factor in soils. So it's a mixture of both of them when it comes to breaking down these regions to factoring, looking at the ability to hold the nitrogen, then how quickly it converts and the potential for risk for loss. So these things all factor in if you look at our BMP regions across the state. So it isn't one single factor, but weather is a big key, because if we know that all nitrogen loss pathways focus on water, I mean it has some sort of effect with water that comes into play, that's one of the key components.

Brad Carlson:
And really the point where things become problematic is when we reach total saturation. And so that is a concept that we refer to, as soil scientists, refer to as field capacity, not very technical there. It's something everybody can probably relate to. Field capacity is the capacity of the field to hold water. And so what we typically look at in a three-foot soil profile, we think about three feet being about the same rooting depth as a corn crop, is it'll hold about 10 to 12 inches of water. Sandy soils actually have less pore space. Some people may feel like it's got more, but it actually has less. And there is some effect to that as far as bulk density, if you've got areas with compaction, of course it's collapsed, some of that pore space and it'll hold less.

But really the one area that farmers who really want to get into the weeds on managing their nitrogen might want to start doing, is thinking about the water budget in their fields. So in most years, you probably reach saturation at some point in the spring. That's very clear to everybody. When that happens, it's shiny on the surface or there's water puddled, you got water running through drain tile and so forth. Water doesn't run through tile until the field has reached field capacity. And so from that standpoint, you can start calculating a water budget based on the amount of water that falls from the sky versus the amount that the crop uses with point of saturation as your starting point.

But in general, we look at the soils being able to hold 10 to 12 inches of water during the year. And so you can start thinking about when we have water deficits, what that means as far as that total. When you get to the bottom end of that, in a lot of cases that water's held very tightly to the soil particles. It's really not plant available. But that's going to be a topic for a future podcast to get into more detail on that as far as calculating water budgets. But you can think about most of our soils in Minnesota will hold 10 to 12 inches of water.

Dan Kaiser:
And with saturated soils, I mean when we start getting more water in the profile, it does help with nutrient uptake, because obviously if you start talking about films and water that form around the soil particles, it's that the more water there is, the less it takes for an ion to go from point A to point B in the soil. So we see situations where we have optimal water availability, you're going to get optimal uptake of that water and optimal uptake, hopefully, of your nutrients.

So that's one of the key things. Again, the other thing is with a lot of this when we talk about something like sidedress is that we do know we need to have some moisture there, because if you've got nitrogen sitting in a situation where there's not a lot of available water, it's just really not going to go anywhere. So it's really important to think about this, but then oversaturation can lead to the risk of denitrification and potentially leaching. So it's challenging with that. I mean, the water budget is a key point when it comes to a lot of things when it relates back to nitrogen management.

Brad Carlson:
I think the key for most farmers is just simply to realize that temperature and precipitation are major drivers for nitrogen conversion. So if you really want to start doing a more precise job of managing your nitrogen, it does probably require that you start paying a lot closer attention to how much water is falling from the sky, what the soil moisture status is in your fields, and then also taking a look at the progression of temperatures throughout the season, throughout the year.

Dan Kaiser:
And I think a lot of times too, you start to get some tunnel vision in terms of management where you can do the same things over and over again. So I think one of the things that needs to be addressed here is looking at what options are available to you, looking at it in terms of timing. I think that's really a key to try to make sure that you can get the nitrogen applied at optimal times, but again, not apply early or too early or some suboptimal just because logistics... You're afraid you can't get back into the field, because we really shouldn't be looking at... Particularly with rates, increasing our rate, just based on what we're doing because we know we're losing nitrogen. I mean, that's one of the things that you really shouldn't have in the back of your head, although it does come up from time to time.

I know from growers, particularly with fall applications, still mentioning that, talking about loss potential and increasing the rate because of potential loss. So I think really the key is, again, getting back to looking at options and if there are multiple options, and we know that particularly with the sources that are out there, that there's a lot of flexibility. So we should be able to manage around some of this. That's not a perfect system, because again, we're really dependent on the weather. So when it comes down to application, I mean you can do everything at that key point, the point in time where you're applying it, that's optimal, then it's up to mother nature in terms of what happens later on.

So that's one of the things that gets really challenging, particularly with nitrogen, is once it hits that nitrate form, there's not a lot we can do. We can't stabilize it. We just need a plan out there to actively take it up. So once it's that point, there's not a lot we can do, and we're at the mercy of mother nature in terms of what is thrown at us, particularly for rainfall that can really throw a monkey wrench into what we're seeing in terms of management and overall nutrient availability.

Brad Carlson:
Yeah, I think a last point then, Dan is, and this is again is a topic we'll cover at some point in the future, is there have been over the last several years some computer aids that farmers have been able to purchase that did track climate data as we go through the growing season. I think those are worth investigating. Some of those have gone off the market, but there are products that are still available. I think that is something that can provide you some fairly valuable information and take a lot of the headache out of keeping track of rainfall and heat units. And I think there's a lot of guys that have a genuine interest in what the weather has been like anyway, so that could be worth your investment.

Dan Kaiser:
That's one of the challenges I think, is you start looking at... You know Brad, we've seen this initially in your area of the state and in South Central where there's a lot of small-scale variability. So that's one of the challenges with climate data is having something that's accurate down to the scale of your particular farm. And that's one of the things I think MDA is trying to do right now is increase some of their... I don't know if it's a Mesonet, they're going to, I think, replicate something like the Endon system that North Dakota has to try to get more small-scale measurements across the state. Because looking at a lot of the precipitation patterns, you just go a few miles away and it's amazing at how different it can be. I know we saw that in 2023. I know around your area, particularly in South Central Minnesota, where you didn't have to go very far. You had four inches of rain versus almost nothing.

I think the thing that we're seeing at least in some of these drier years is there's a lot of variability and it's feast or famine when it comes to some of these rainfall events. When you talk about a lot of these tools that were available, some of them, like I think the climate corp one wasn't necessarily based off of really small scale, a lot of measurements across particular area versus I think Pioneer were actually setting some up. So I think that's one of the challenges with that is some of those estimations aren't always perfect, but looking at it, it's something to at least base some of your decisions on. I think that's really key. It's really good to have at least some backing information to know why you're doing what you're doing.

Brad Carlson:
I think the National Weather Services really improved the technology on the radar too, so that they're better able to tell the density, basically the air density, based on how much water is in it as far as then how much is actually reaching the ground. And so I would look forward to over the next several years, that technology improving so we can start getting a lot more detailed rainfall coverage maps that are a lot more accurate than they've been in the past.

Jack Wilcox:
Brad Carlson, Extension educator and Dan Kaiser Extension nutrient management specialist, thank you for talking with us today about how the soil stores precipitation, how nitrogen reacts with that water when you factor in climate conditions, and what all this might mean for a MN farmer’s operation.

Brad Carlson:
Thank you.

Dan Kaiser:
Thank you.

Jack Wilcox:
Have a question about something you’re seeing on your farm and how it relates to what we talked about today? Send an email to Brad or Dan 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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