The Story of Nitrogen

�We had a saying that we used in Illinois through the end of the 20th century that said, 1.2 is the most you should do, what that meant was your expected yield times 1.2 is the amount of nitrogen you should use��
�We know very well how much nitrogen a crop needs to produce yield. What we don't know very well is how much of that nitrogen will be coming out of the soil, versus how much will be needed through supplemental N with the fertilizer that we apply��

�Nitrogen is the elusive nutrient, you know, we can never, you know, it's it's, it's very, it's impossible to say you put on X and you're gonna get Y, you know, it just doesn't happen��

Welcome back to this 6th, bonus, episode of �The Story of Nitrogen�! This episode is all about nitrogen fertilizers and how we decide what rate of fertilizer to apply to our ground. This is a question I and many others have tried to tackle in our careers, and one whose answers regularly surprise and humble us. This episode is also about another question, one which anyone can relate to regardless of whether they work in agriculture or not, one which every person will have a different answer to: �How good is good enough?�
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My name is Greg Klinger and I'm an agronomist and educator at the University of Minnesota Extension.� Together with my friend and colleague Shane Bugeja, I've spent the last few months interviewing experts in agronomy, biology, nutrient management, and ecology, trying to understand the story of nitrogen, in the hopes of explaining the phenomena we see out in the field, woods, and water.� Join us as we explore the different facets of this complex issue.

Part Six, The Rule of 1.2.

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I want to start with a question: how do you- or anyone- decide how much nitrogen fertilizer a cornfield will need this year? There�s no easy answer, because Nitrogen doesn�t stay put. The weather drives how much nitrogen is available to a crop from the soil or from fertilizer, but no-one can foresee a seasons-worth of weather when they�re out planting and fertilizing in the spring. When nitrogen fertilizer decisions need to be made, it�s not even possible to know how good yields will be, how much nitrogen will be taken up by the plant.

In the face of this uncertainty, there are many tools, many products, many ideas that try to determine that perfect fertilizer rate. They include complex models that try and estimate how much nitrogen a soil provides or loses during a growing season along with how much nitrogen a crop needs at different points in time, sensors that measure how dark green the plants are (a good indicator of how much nitrogen the corn has), and sampling-intensive approaches that involve taking many soil samples across a field. But I want to focus on two specific methods here, the yield goal approach and the maximum return to nitrogen, or M.R.T.N., approach, because these two, which are both relatively simple but otherwise quite different in their philosophy, are the bedrock on which almost all other nitrogen fertilizer rate tools or products are constructed.

�I'm Emerson Nafziger. And I'm was a extension specialist in crops and the University of Illinois for nearly 40 years.�

I reached out to Emerson to learn more about making nitrogen fertilizer rate decisions, because he�s been in the field long enough to see a lot of changes in agriculture, including some major changes in how researchers think about nitrogen management.

�it's been a very interesting journey with nitrogen on corn, in part because the changes have been so large�
that's probably the most notable thing, if you came and looked at the records in the Corn Belt, and didn't know anything else, you would notice that the yield is just progressing steadily upwards, with some reverses due to, you know, widespread drought and this sort of thing. But that, that has been pretty much uninterrupted for the last 50 to 60 years�
but one of the things that that yield increases brought is this idea, based on the very logical understanding that high corn yields take more nitrogen than low corn yields. And so as corn yields continued to go up, people logically understood that that means that nitrogen rates have to go up- fertilizer nitrogen rates-�
in the, from the 1970s, through the 2000s, the idea was, you know, our recommendations for nitrogen were based on yield- we call it yield goal or expected yield.�

It helps to know some history here. The ABILITY to apply our current industrially manufactured nitrogen fertilizers has existed since the early 1900s, but it wasn�t until the 1960s that there was wide-scale use of these nitrogen fertilizers on crops in the US. As those fertilizers became popular, there needed to be some guidelines to help people decide how much to apply.

The yield goal concept is a wonderfully simple calculation that was one of the first, certainly the most enduring, tools that University researchers used to advise farmers. Here�s the idea: you have a yield you hope or expect to get for next year�s corn harvest. You then take that expected yield in bushels per acre and multiply it by a factor, and (with some occasional small adjustments) that�s how much nitrogen you should apply in lbs per acre. Boom, simple. Usually that multiplier is close to 1.2 lbs of nitrogen per bushel of yield, and so I�ve taken to calling it the �Rule of 1.2�. The Rule of 1.2 is based on the concept that a larger yielding crop takes up more nitrogen, and so should need more nitrogen fertilizer than a smaller yielding crop.

�So that worked pretty well, it was better than just use plenty that had sort of held before that, and sort of rationalized it and said, you know, don't be putting 200 pounds of nitrogen on a field where you expect 80 bushel yields.�
There are all sorts of maps you can find on social media that categorize U.S. states by any number of things- red vs. blue, whether people generally say �coke� or �pop� or �soda�, �casserole� vs. �hot dish�. One you AREN�T likely to see is which states use a yield goal vs. a maximum return to nitrogen, or M.R.T.N., approach in their nitrogen fertilizer guidelines. This map would show you some interesting patterns: for one, far more STATES use a yield goal set of guidelines, but despite this, far more CORN is grown in states that use the MRTN approach. I�ll explain the MRTN later, but for now, let�s just say that considerations of soil and climate are a part of the puzzle that distinguishes these two approaches.
In reality, of course, nitrogen need is driven by the corn plant, not a number on a table or calculator. The growth stage of the plant, what times of year nitrogen stress tends to happen, how it LOOKS when it�s nitrogen-stressed- these clues can be very helpful for nitrogen management. I wanted to learn more.
�So I am Fabian Fernandez, I am a nutrient management specialist at the University of Minnesota in the department of soil, water and climate, I do research and extension, as my role as nutrient management specialist. And I focus primarily almost exclusively on nitrogen management for corn cropping systems.�

�when we talk about nitrogen availability and our ability to figure out how much nitrogen we need, or if a crop has adequate amounts of nitrogen, visually sometimes we find it a bit difficult to predict, simply because as I mentioned early on during the growing season, the plants don't really need a lot of nitrogen. And then they kind of deal with the rapid increase in nitrogen demand. And we can compare that to like two cars starting a trip where one has a full tank and the other one has only a quarter tank. When you start the trip, both of them will be going as fast and as smoothly as each other, there will be no difference, no way to tell that there�s any difference. But then as the trip progresses, or as the plant continues to grow, and it starts to demand more nitrogen, then you will notice that one of the cars starts to not do as well or completely die, whereas the other will continue going. Because he has- the soil supply is adequate and the other kind of died out.�

If you were out in a corn field in the first days after it was planted, there�d be little evidence to show for it other than the straight lines cut in the earth by the planter discs and some wheel tracks. But under the ground, the seeds would be stirring. By about a week after planting, the first small, ghostly white roots would have burst out the side of the kernels, along with the first wrapped leaves, poking their way up through the earth towards the sun. For a while after these first leaves emerge and unfold, most of the plant�s growth is aboveground, as each new leaf emerges from the top of the plant, stretching up towards the sky and then out towards its neighbors in the next row. But by the time the plant has 3 or 4 leaves, the roots start to take off, each one growing as much as an inch a day, down towards the water in the ground, or out to the middle of the row between plants. Sometime in middle or late June, in what I refer to as corn�s �teenage years�, the aboveground growth again dominates, and the plants seem to grow before your very eyes, going from a knee-high plant to a plant that towers above you, seemingly overnight. And then, in its last stage of life, the corn plant shifts gears as its tassels and silks emerge, moving its energy and nutrients from leaves and stalk to developing grain, its kernels turning yellow and indented, while the rest of the plant turns tan and withers.

During the entire growing season, when corn is going from a seed in the ground to a growing plant, to an ear of corn filled with the next generation of seed, nitrogen is also moving. It is being released from or tied up in the soil organic matter, pulled into the plant�s roots, and moved to the leaves to enable the photosynthesis that is the engine of the plant�s growth. And at different points in time, too little nitrogen can limit the plant�s growth and the amount of grain it can produce, just as too little or too much water, a frost that burns the young plants leaves, or an outbreak of disease or insects can. Fabian used the analogy of two cars driving down the road with different amounts of gas in the tank, likening this to different corn fields, where they might start out the season looking the same, but ultimately one field might have its yields limited by too little nitrogen while the other doesn�t. Keeping with this analogy, you could also say that at the beginning of the trip, the destination is unknown- it could be that the car with a full tank AND the one with a quarter tank both have enough gas to reach their destination, or neither does, in which case the one with more gas will just stall out further down the road. You might also see one or the other start to slow down and think it�s run out of gas- only to see it speed back up again and realize the driver was just rubbernecking at a roadside accident or fiddling with their phone. Similarly, in a corn plant, FIRST, sometimes a visual sign of nitrogen stress in a cornfield is short-lived and doesn�t seem to limit yield potential, and SECOND, because it�s impossible to predict at the beginning of the season how much nitrogen the soil will provide, how much will be lost, how much the plant will need to take up, the cornfield�s destination- how much fertilizer it will need to reach its full yield potential- is unknown. But we do know that nitrogen stresses that limit yields are more likely at certain times during the plant�s life.

�there are different stages in the corn plant development, where different portions of the yield potential are set, you know, the number of rows, the number of kernels, and then the, the weight per kernel. And, and so if there are stresses, during that those specific periods, the yield potential gets reduced, and the plant is able to always compensate to a certain degree with the next kind of set of yield potential variables. But there is also a limit, right? Like if, for instance, the plant all of a sudden, gets substantially reducing the number of kernels per ear, it will compensate in the weight per kernel, but you won't have a pound kernel, you know, there is a limit of how much that that kernel can grow, how much weight it can accumulate.�

As an analogy, think of a human being over their lifetime- if you were to cut their food supply in half, the effects would be far more damaging to a growing teenager than to the same person in middle-age, with a sluggish metabolism and a weaker appetite. The same is true with corn- early in its life, it mostly relies on the nutrients contained within its own seed. And while it�s doing that, the soil is often building up concentrations of plant-usable nitrogen, both from any applied fertilizer and from organic matter in the soil, a process called mineralization.

�as the plants start to take up nitrogen that build up that sometimes we see early in the spring starts to deplete because now the plant it starts to take up more and more nitrogen and, and it typically happens at around the V6 stage of the plant so about knee higher, a little bit lower. That's where you start to see the plants, the corn plants taking up a lot of nitrogen. And that kind of continues through about the reproductive stages of the plant.�

As it starts to put out more leaves, a young corn plant shifts from relying on the nutrients in its seed to using the soil as a source of nutrients, and when it hits its teenage years, it becomes heavily dependent on the soil for nutrients like nitrogen. At this point, the plant and the soil are in a race- the plant is starting to suck large amounts of nitrogen out of the soil while the soil is replenishing that nitrogen from broken-down organic matter. And generally speaking, the corn plant slowly wins, leading to a slow drawdown of the plant-available nitrogen in the soil. One factor that can throw a wrench in this pattern is the possibility of large nitrogen losses from the soil, which usually happen early in the season. These big losses generally happen when conditions are too wet, allowing nitrogen to be leached down through the soil or burned off into the air; or sometimes when conditions are too dry for too long, allowing shallowly or surface applied fertilizers to burn off into the air.

Because of the ease with which nitrogen fertilizer can be lost from soil as well as the increasing rate with which organic matter releases nitrogen as soils warm up during the growing season, research has found that while most of the nitrogen a plant takes up in the first half of its lifecycle comes from fertilizer, later in the growing season, most of that nitrogen comes from organic matter. These �teenage years� are when long-lasting, yield-damaging nitrogen stresses become most likely, because the plants are using up a lot of nitrogen, they�re still pretty dependent on fertilizer, and bad weather conditions earlier in the growing season could have already caused a lot of that fertilizer to be lost.

�we find that actually the fertilizer contributes a minor portion of the total nitrogen. Early in the season�
the majority of that nitrogen that is taken out by the plant comes from the fertilizer. But soon after the majority actually comes through the process of mineralization of organic nitrogen that is in the soil that is not from the fertilizer.�

�Right, and we should always keep in mind that, you know, this, this nitrogen that is coming from fertilizer is in addition to what the soil supplies, but the timings are different. And that's that can be pretty important. We don't start to produce significant amounts of nitrogen from soil organic matter until soils warm up. And so at the middle or end of April, they haven't warmed up and they have not produced very much, it's really important, we've found that the crop have some nitrogen there when it's starting to establish. And if it doesn't, we can limit- if we limit nitrogen availability, you know, when the crop is six or eight inches tall, it can have a lasting effect on yield potential.�

�During that time, of course, the mineralization process is still producing some nitrogen. And but the crop is also taking it up pretty rapidly. And that's you know, that's a contest that happens and we've measured this during the development from corn, when it's about knee high up to pollination tasseling you know, that's when most of the uptake takes place. And these uptake rates can be very rapid, four or five pounds per acre per day can be taken up during that period. And so that's provided by a combination of fertilizer and mineralization. And under warm conditions with adequate soil moisture, mineralization can provide almost that much, I mean in a higher organic matter soil.�

Then, as a corn plant flowers and starts to develop its grain, the focus changes again- it�s still somewhat reliant on the soil for nitrogen, but less than before, as it begins to move nitrogen from other parts of the plant into the grain.

�And then after that is when it will continue to take up nitrogen from the soil, but then it will also mobilize nitrogen within the plant. And that's where we normally see these things that we normally called firing- the lower leaves of the plant tend to get yellow. That happens because well two things. One is that these very shady, they're the plants have a lot of leaves above that that portion of the plant that shaded the lower parts of the plant. And so those those leaves have very little function anymore. And so and then the other part is that the plant uses that nitrogen base in those part materials, plant materials, and moves it into the grain�
the plant puts all the resources into creating that seed because that's the whole purpose of the plant is to generate viable seed for for the next generation.�

The changing needs of the plant for nitrogen combined with changing soil nitrogen levels over the growing season has REAL-WORLD management implications. For instance, if you know that your soil generally comes into the growing season with a decent amount of nitrogen because it has high organic matter levels, with dry falls and frozen winters that prevent much nitrogen loss from occurring, you can prioritize planting over fertilization in a wet spring that delays fieldwork because you know you have at LEAST 2 or 3 weeks where a corn seedling can rely on its own nutrients and those provided by the soil before it needs any fertilizer. Or, have a low organic matter, loss-prone sandy field? You might split your nitrogen fertilizer, and apply some later in the season than you otherwise would, because that particular soil wouldn�t be releasing the 4 or 5 lbs of nitrogen per acre per day during midsummer that a high-yield-potential crop might be taking up.

�what does a plant look like when it's not getting enough nitrogen? Can you visually see that?�

�Yes, when, when you have a plant that is deficient in nitrogen, the first thing that happens is nitrogen is very important in chlorophyll, so in the green that we see in the plant, and so when you start having nitrogen deficiency, the first thing that you will see is that the leaves tend to go more pale, lighter green. If it gets extreme, then you will see dieback, leaves will become yellow and eventually die or start withering. And then the other thing that is very typical of nitrogen is that within the plant, nitrogen is mobile so it can move up and down the plant. And so obviously the plant will when, it's limited by that nutrient, will use whatever resources it has to keep growing So obviously the new leaves the top leaves will be the more important ones so the plant will translocate nitrogen from lower leaves to upper leaves and so that will create the deficiency to show up first in the lower leaves of the plant or the older leaves of the plant.�

When do you most often start to see significant nitrogen deficiencies occurring in Minnesota corn fields?

�even if you're not able to apply nitrogen in a timely way, most of the soils in Minnesota, because of the high organic matter content that they have, they produce quite a bit of nitrogen on their own. In fact, we've seen in our studies where we purposely apply no nitrogen, and then different rates of nitrogen to do studies of rate response. A lot of times, we don't see any, any difference in the plants�

�we don't see any of those differences in the plant until pretty late in kind of in the reproductive stages, or towards the end of the vegetative starting reproductive stages, when like we talked earlier, there is a lot of nitrogen being taken up, it's like the plants have sufficient nitrogen just from what is available naturally in the soil to supply all the needs. And so nitrogen is typically not the limiting factor for a while.�

�Are there scenarios where- I guess to use your analogy of a car running out of gas, this would be like rubbernecking or stopping to go to the bathroom- where you see nitrogen deficiencies that are temporary, or just not related to a lack of nitrogen at all?�

�The other time where sometimes we see nitrogen deficiency in plants is early in the spring when it's cool or wet or both. And that is more related to the plants that take up the nutrients. Maybe there's plenty of nitrogen in the system, but the roots are not growing very well. Maybe the roots are growing in really wet soils and for roots to take up nutrients, they need to have oxygen, so it's not enough oxygen, the plants will not really be able to take up the nutrients. And so those are other situations where you can start to see some nitrogen deficiency, those typically tend to be temporary, because normally we don't have those really wet or cold conditions for very long in the spring. But, but that is one situation where we see nitrogen deficiency sometimes.�

�Okay, kind of on the flip side, can you see when a plant has too much nitrogen, if there is such a thing?�

�Yeah, it's very hard because plants in general will will take up if there is nitrogen, they will take it up whether they need it or not physiologically. And the the way that you can see that is typically with more leaf proliferation. For instance, if we go to a garden situation with tomatoes, I personally like to grow tomatoes in my garden if you have too much nitrogen actually it's detrimental because the plant will just grow it will become a humongous plant, very green full of leaves and very few tomatoes or not as many tomatoes or the tomatoes will take longer to mature because there's so much shading by that plant. And so that's typically what happens. In other situations like corn where I do most of my research what you find is that the plants will be completely green from top to bottom and that's actually a good thing at some point in the growing season but if you see that the let's say the lower leaves in the plants are dark green when the grain is pretty much filled that's telling you that that plant had too much nitrogen.�

I asked Emerson what things he looked for in a cornfield to know it was supplied with enough nitrogen.

�The thing I look at most intently in the field is at around the middle of the season, is the sunlight all being intercepted. And so I like it to be so dark under the canopy at noon, that, you know, you can, you can barely read a newspaper there. I mean, that's, to me, that's the mark of having done exactly what it took to get this crop at the right density with the right nitrogen rate, you know, when the right row spacing and everything, if we can do that, you know, the correlation between percent light interception, after pollination, and yield is very high in most cases. And we ought to be intercepting 97% of the sunlight, if possible. Now, we don't carry meters around, but you can look at the ground and see sunflecks We call it. You know, little spaces that sunlight got through the leaves. And we don't like to see very many of those zeros a good number. People talk about, you know, getting sunlight down to the lower leaves, and I've never understood that idea, you know, I don't care where on the plant that lights getting intercepted, as long as it's getting intercepted, all of the leaves feed into the stock and all the stock feeds into the ear. And that's a system that works really well in corn, it's really good at it.�

Let�s get back to a comment Fabian made earlier, that late enough in the season, if all the corn leaves at the bottom of the plant are still dark green, then the plant probably contains more nitrogen than it needed. Of all the different visual clues to whether a plant has too much or not enough nitrogen, clues that seem to sometimes work and sometimes not, at least in the quick Minnesota growing seasons, this has always seemed one of the most reliable to me.

�that idea, green to the ground is the way we say it, you know, and people are really seem to be enamored of that, especially when it's pretty green till very late, almost app. And over the decades, I remember seeing, you know, maybe even some ads at one time, you know, your corn ought to look green, way down to those lower leaves, because that's a mark that it's really got plenty of nitrogen. It may be but it's not a it's not a way to get high, you know, efficiency out of nitrogen. I mean, if it's there in the leaves, at maturity, what good is that doing anything, you know, that nitrogen?...
I'll admit, you know, if you want to mark that this thing has never lacked for nitrogen, just have it be dark green. From the time it's knee high to the time it's combined, you know, it's black layered. Because that means that never lacked nitrogen. Did it need it? Almost Surely not.�

If you�ve ever broken a bone and had to have an arm or leg immobilized for a while, you have no doubt experienced this: the muscles under the cast start to shrink and degenerate really, really fast- within weeks. This is for a very good reason- it costs energy to maintain muscle. If muscles aren�t being used, why would the human body waste its precious energy maintaining them? A similar thing happens with a corn plant-if it�s well-fertilized, very little sunlight will hit the bottom leaves, and if it�s not getting sunlight to PRODUCE energy, why should the plant EXPEND energy to maintain that leaf?

Regardless, while there�s a lot we know about when nitrogen shortages are most likely to occur, and how to interpret what we�re seeing in the field, there�s a lot we still just don�t know. And so often, those things we don�t understand relate to things we can�t control, like weather.

�Interesting anecdote. Some of the highest yields we've ever had, at least in a field basis in Illinois came from a year, I think it was 2017. I'd have to go back and check. But it was a year when people started planting really early in northwestern Illinois particularly, and it froze, and they had to replant. And many of them didn't replant it until the middle or even the third week of May or something like that. And they were getting 300 bushel yields. You know, they were seeing things on their yield monitor they had never seen before. And it wasn't due to early planting. But it was due to some combination and I can't help but think that temperature is more important than we think. People don't like to hear that because we don't control temperature. We've had- I think that we've lowered yields, in some cases, just due to bad luck of planting the crop in late April, getting a period of cold, wet weather after that, the crop struggles, eventually it may have good stands. But I suspect that there are mechanisms by which we lose yield potential, you know, when the crop is this tall, that we don't have any understanding of.�

Which begs a question: how do you make decisions about something you can�t fully understand and can�t control?

Imagine you owned a paper mill. Every day, trucks filled with woodchips and sawdust would pull up to your door, dump their loads, you�d probably mix the wood with some goopy chemicals (okay, I�m a little out of my element here) and boom, you have some paper. As the owner, you could make a good guess how much paper you could produce based on how many truckloads of wood arrived; you could also know, based on how much paper had been produced, how much wood had been used. Because in a standardized factory process, the amount of wood in equates to the amount of paper out. The yield goal for nitrogen management works this way: wood in, paper out; nitrogen fertilizer in, yield out. It makes intuitive sense, wiich is why it was so widely used.
In the early 2000s, agronomists from Midwestern universities got together, looked at the data they had acquired over the years, and decided it was time to abandon the wood-in-paper-out yield goal approach in favor of something new. The question is: why? What was it that they saw in the data?
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�There were a couple of problems that started to become more visible in the 90s. One was that soils, say in Southern Illinois, for example, which had lower organic matter, a lower supply of water and generally lower yields. But just the fact that the yields were all did not mean that these had a significantly lower requirement for nitrogen�
When we looked at that we started to see that the nitrogen rates that were being calculated by the yield based system in Northern Illinois were considerably higher than the crop actually needed. And the opposite was true in Southern Illinois with the lower organic matter soils, they were actually less than the crop needed.�
Here�s the problem the researchers saw: some fields needed lots of nitrogen to get high yields; others needed very little. Some fields never had high yields, no matter what, but needed lots of nitrogen to maximize the yields they could get; other low-yielding fields hardly needed any nitrogen to maximize their yields. And the same field could bounce from category to category over time. Confusing, right? What they encountered was RANDOMNESS- it was impossible to CONSISTENTLY predict how much nitrogen they would need in any year based on its yield potential. I�ve personally seen this in research trials, where the same farm managed the same way needed 118 lbs/acre of nitrogen to get yields of 208 bushels per acre one year, only to need 240 lbs to get 228 bushels a few years later.
The approach the researchers came up to with to handle this randomness, the maximum return to nitrogen or M.R.T.N., abandoned the idea that anyone COULD predict the right rate of nitrogen fertilizer ahead of time. If the yield goal is like a paper mill, where wood in correlates to paper out, the MRTN is like�well, maybe I�ll quit with the analogies while I�m ahead and just say that the MRTN suggests the field was never a factory to begin with. It�s not just fertilizer inputs, but unknown, unseen soil organic matter inputs; it�s not just crop yield outputs but unseen soil nitrogen losses. Our crops grow in a living, breathing soil, not a sterile machine whose every process we can control and every part we can see.
The researchers asked: if we CAN�T consistently predict the right nitrogen rate, can we instead limit the cost to the farmer, so that they�re not losing money on extra fertilizer that�s not adding to yield, or on lost corn yield because there wasn�t enough fertilizer? And the answer they settled on was probability. In other words, if I�m heading into the growing season and about to make a fertilizer application, the MRTN asks: what sort of rates are MOST LIKELY to make me the most money on MOST fields?
�I always talk about nitrogen management as risk management. And its there are so many variables that impact what happens that it is difficult to know for sure what will be the outcome, what you did last year, and he worked perfect doesn't mean that it will work perfectly this next growing season or the one after. And so even though we don't like to just say, well, I managed for the average, because the average is maybe people tend to think that the average is bad, you know, I don't want to be average, I want to be above average, I want to be better than than the next guy. In reality, when we think about chance, or the probability of something working, when there's so many variables that we have very little control over and that we can not predict very well, the average is actually not that bad.�

Playing probabilities and averages- that�s how the MRTN works. Here�s the strategy: researchers across the Corn Belt states run lots of trials every year. They apply different rates of nitrogen fertilizer across a field, they figure out how much fertilizer it takes to maximize its yield. And because nitrogen fertilizer has diminishing returns- meaning that the first small amount of nitrogen you apply gives a huge yield bump, but the last bit of nitrogen you apply to get to full yield potential only adds a small amount of yield, and usually costs a farmer money- they also consider economics. They calculate, based on how much fertilizer costs and how much a bushel of corn is worth, what the most PROFITABLE nitrogen fertilizer rate is. They combine the results from a bunch of different trials over a number of years, since some years can have greater or lesser nitrogen demanding, and figure out what RANGE of rates is most profitable MOST of the time- that range is what forms the basis of their guidelines.

�we don't talk about recommendations from the university, but more as guidelines simply because that's what they are, they're guidelines. We don't have a magic ball that nobody knows about the we can predict exactly how much we you will need them, we just kind of tell you, whatever we feel like, you know, we really don't know, we have very good idea of what could be happening. But ultimately, we don't know, I wish we would, but we don't. And so those are guidelines, we are saying, Okay, this is typically what you would expect under the situations, just like when whenever you meet with a financial advisor, they don't know what is going to happen with the market, they can give you their best estimates, and based on what they know, but ultimately, the the outcome, it's it's uncertain.�

The comparison to stock markets is an interesting one. Every day, billions of stocks for thousands of companies are bought and sold for constantly changing values based on what people think all these individual companies are worth. The moods of the stock buyers and sellers, corporate assets and debts, all these complicated things exist in a complicated world, and the timing of the next major shock to the system- a natural disaster, viral pandemic, terrorist attack, war- remains unknown and unknowable. In the face of this complicated, unpredictable financial ecosystem, research often shows that those who spread their risk around by investing in the whole market rather than individual stocks and who invest consistently rather than trying to time the markets, generally make the most money. Taking human decisionmaking, with all of its emotional ups and downs, out of the equation, and just relying on probability seems to be the best answer to the financial system. For nitrogen, on the spectrum from simple-and-controllable, like a factory, to wild and just-along-for-the-ride, like a stock market, nitrogen in the soil is somewhere in the middle, and the most lucrative ways to manage it generally are too. A pure probability approach where you don�t take any other considerations into effect wouldn�t make sense, since we can have some understanding of which fields seem to need a little more or less fertilizer, and how things like weather and tillage can impact nitrogen fertilizer needs. This is why University guidelines are simply that- guidelines, a road map that�s likely to lead you to your destination. But a true factory approach that discounts the random probability aspect of nitrogen might not make sense either.

�What would you say are kind of like the the main strengths or, and weaknesses of the MRTN approach?�

�Yeah, so the the main advantages, I think of the MRTN are that they, the guidelines move with as things change. As we talked earlier, if you have newer hybrids that are different in their utilization of nitrogen, or if you have weather changing with climate changes, things change over time, or practices, weather stillage, or other management practices change, those things get reflected into the database. Another benefit is the fact that it takes into account the economics�

The limitation that we have with this approach, of course, is what we were talking a little bit earlier in terms of separating the database into different regions or different conditions, that it requires a lot of data. If you don't have adequate amount of data, then it becomes becomes a problem. The more and more data that you have, the more robust the tool becomes.�

�But yeah, and that always strikes me is one of the strengths is, you know, human nature is to kind of lend outside influence to kind of things that are extreme, you know, and so, if you have a database that encompasses, you know, a number of years of a lot of different weather, well, that that helps keep people from looking at say, like, what happened in 2019, where probably everyone needed a lot more nitrogen, you know, or most people did than recommended. And saying, Well, every year should be like that, when you also have years like 2016, where it was, I mean, it was wet later in the season, but it was dry in the spring. And so the nitrogen needs were, if you were to look at that individual year would be much lower. And so you kind of mix it up. And kind of I think it helps, to me some of the advantages psychological so that you're not putting too much emphasis on things that end up probably costing you in the long run.�

�That's right, yes. And that's, again, one of the advantages of this approach is that you build a database, and the more data that you have in that database, the better it becomes at buffering some of these extremes.�

Because the MRTN relies on balancing the cost of fertilizer against the price of the grain being produced, I wanted to know: what is good enough? How close do we need to be to that perfect nitrogen rate that we can only know after the season is over?

�So there is no perfect approach. I don't like to be mediocre, I like to do the best possible job. But when it comes to nitrogen management, what I see in reality is that if we can manage nitrogen within about 30 or 40 pounds, we're doing a great job. Because, again, simply there's just so much variability, so much uncertainty around how much nitrogen is needed that if we are within that 30 to 40 pound, I feel like we are doing that really good job.�

�Yeah, and that I mean, 30 to 40 pounds of like, what's the best rate? I mean, ultimately, is not too much of a cost in either direction in terms of loss yield or or overpaying for fertilizer. I mean, to my mind, at least, you know, you're at that range you, you're certainly never losing more than like $5 an acre or so. I mean, I guess I shouldn't say never. But you know, it's small amounts, compared to a lot of things.�

�Yeah, I am not a gambler. But these decisions is like going to Las Vegas, going to a casino. And it's like, when do you call it quits? You know? Yeah, keep putting money into this thing and hope that it will turn out or do I call it quits? Or, what is the probability that the number that I selected or whatever choice I'm making is going to pan out? Yeah, that's, that's, that's the reality. Unfortunately, in terms of nitrogen management, we have a lot of knowledge of certain things that we know very well how they work, but then there are other things that make it very difficult to pinpoint exactly.�

�Yeah.�
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�We have one producer that's done 10 years of trials. And he's just about 30 miles from here, southwest from here. And he'd started doing this just because he was interested and he keeps doing them just because he's interested and he sends me the data every year and over those 10 years His average optimum nitrogen rate has been I think, 155 pounds, his average yield has been like 245. And he hasn't told me what he's done with his rate that he was using, which I'm sure was pretty high early on. But I'm also sure that he's lowered it. Because he can see with his own eyes, you know, this makes no sense to keep using- the highest he's ever needed is 175. And the lowest is, I don't know, 120 or something like that.�

�That�s nice. I mean, it seems like a lot of variability, but in the grand scheme of things, it really isn�t.�

�It's, it's, those are very, very tight. I mean, those things just line up right there in the middle of the graph. And I said, one group I talked to, I said, if every field is like this, we could just quit now, and just tell everybody what rate to use. They're not. But the highly productive fields tend to look that way, much of the time. And we can't really afford to be managing nitrogen for outlier years. The one where it rained eight inches in June, and, you know, water stood for a while and this sort of thing. And that's what you get into, you know, how much does managing for outlier years cost you on average? And the answer is quite a bit.�

I�d like to stop here for a moment to talk about RESILIENCE. Not so much the type that keeps people going through hard times, but the type that plants have. If you want to think about the resilience that a corn plant has, ask yourself: why does one field have really good yields with a relatively small amount of nitrogen while another nearby field has poor yields no matter how much nitrogen is poured on? Research has started to show that different fields, different soils, different managements can be more or less resilient to nitrogen stress. In other words, some soils are just better at supplying plants with nitrogen than others.
�A soil that doesn't have an adequate supply of water, or has a clay pan or somehow restricts the root growth tends to restrict the amount of that soil supplied nitrogen that the crop can take up as well...
I think that the health and the functioning of the root system is just key to this crop.�
Picture a corn field at the start of some fine spring morning, its straight lines of seedlings rolling out over every hill and hollow towards the horizon. The ground is well-filled with snowmelt water, and the plants are well on their way towards a very productive existence. As the days roll by, the weather stays fairly dry, punctuated by the occasional soaker of a rainstorm, so that the ground never dries to concrete or becomes a muddy mess, and the crop prospers, maintaining a deep green color as it grows up towards the sky. Tassels emerge to shed their sticky pollen on the silks of the corn ear, and as the plant starts to build its grain in late summer, it does so under warm, sunny skies, with cool nights and enough rainfall to keep the plant�s leaves from wilting under the noon sun. These kind of conditions lead to a lot of photosynthesis during the day, with little energy being burned off during the overnight hours, and would make for a bumper corn crop. And if the field happened to have a spot where there was a fertilizer application error, where a strip of plants didn�t have any nitrogen applied, you could see how much nitrogen had been supplied from the soil itself rather than from fertilizer. In many fields, you would find that in a very high yielding year, the supply of nitrogen from the soil would also be very high, which is the mark of a nitrogen-resilient soil. In order to understand the reasons for this resilience, you have to dive into the soil itself.
In years like this, the abundant sunshine means a lot of photosynthesis on the crop�s part. Because there�s enough water stored in the ground, leaves don�t wilt and are able to photosynthesize all day long. And because the nights are cool, the plant�s metabolism slows and it doesn�t burn up as much energy while it�s not photosynthesizing. Taken together, this all means that the plant can produce a lot of biomass, a lot of grain, and it will exude more sugars from its roots. The sugars fuel the soil microbes that break down organic matter, releasing nitrogen that the plant can use. So more photosynthesis means more food for soil microbes that then leads to more nitrogen release which means the plant can photosynthesize more- a self-perpetuating cycle. An additional factor is that a large, healthy plant with a large, healthy root system can take up a lot of water, and that water being pulled into plant roots from deeper in the soil brings with it nitrogen that had been pushed deep into the ground. So, in a strip of this field where nitrogen fertilizer wasn�t applied, the plant would still be able to derive a good amount of nitrogen from the soil.
Imagine that the same cornfield didn�t have these ideal growing conditions- let�s say the field started to get a little dry by the time the plants were tasseling, and spent the grain filling period in a drought, so that fewer kernels were produced, they were lightweight, and overall yields were low. A strip of the field that didn�t get nitrogen fertilizer under these conditions would generally have lower nitrogen supplied from the soil, for several related reasons. First, when the soil is dry, microbes can�t release much nitrogen from organic matter, which is compounded by the fact that a drought-stressed plant will photosynthesize less and release fewer root exudates to help these microbes work. Additionally, without much water in the soil to carry the nutrients along, roots wouldn�t even be able to take up much of the nitrogen that WAS present in the soil.
What I�ve just described is what�s referred to as a nitrogen-buffering or nitrogen-resilient soil. Soils like this tend to release more nitrogen for the crop when it needs it to obtain high yields, and less when it doesn�t because of poor yields. Because of this, the nitrogen FERTILIZER needs of the crop will be similar in high and low yielding years. And I can�t emphasize this enough- this resilience SAVES FARMERS MONEY, in terms of fertilizer and yield, because it takes some of the guesswork out of nitrogen management.
Not every soil has this sort of resilience, sometimes because of how it�s managed and sometimes because of natural soil characteristics. Take a field that has a dense layer of compacted glacial clay 2 feet below the surface. Because of this layer, rainwater infiltrating the soil tends to pool right above that compacted layer, and plant roots can�t easily break through it. Perhaps the soil becomes saturated under even moderate rains, easily losing its oxygen and burning off its nitrogen into the air. Or perhaps the tough conditions for plant roots mean they don�t explore much of the soil, making a lot of the nitrogen in or below the clay layer inaccessible. A field like this, or one that�s been heavily compacted or lost its organic-matter-rich topsoil, would probably have very little nitrogen supplied by the soil regardless of whether its yields were high or low. And it would be more dependent on nitrogen fertilizer, which is one key aspect of making a yield goal approach more useful.

Why did so many universities abandon the yield goal approach in their guidelines on fertilizer rates? As someone who grew up in a state that still uses a yield goal approach with corn, I wanted to understand why the Corn Belt states had shifted away from this approach while others hadn�t. Were their soils fundamentally different than elsewhere in the country? Had something profoundly changed in the corn plant through decades of breeding, or was what changed just US, the way we THINK about fertilizer decisions?
I asked Emerson to reflect on what had changed in Midwestern corn production, why he thought the yield goal no longer worked as effectively.

�people still talk like we've still not given up the yield goal, basically, the yield sets the nitrogen requirement�

we say, well, it does set the nitrogen need but not the fertilizer rate. And also the crop gets nitrogen that's not fertilizer and that was one of the faults of that 1.2 factor you know it says- it treats the crop, the whole system as if the soil is not going to give anything. So its as if, if you don't put it on as fertilizer the crop doesn't get it.�

Why did it not do that? Because if you look back at the history of this, it wasn�t like people didn�t know that soil organic matter provided nitrogen, or that nitrogen losses could change that dynamic, or that legumes could change that dynamic, crop rotation. So was it just that it was so complicated?

�It actually was based on, you know, limited experimentation back in the 60s that showed that at the yields you were getting, it probably did need 1.2 pounds of nitrogen. So the hybrids were not very good at, they didn't have very good root systems. They didn't take nitrogen out very well, the nitrogen that was there, their yield limitations came from other things, you know, they didn't have good tolerance to dryness and this sort of thing. And so it was never, it was never done with- I mean, it was based on N responses, just like what we're doing is based on N responses, but I think the responses were just very different.�

What were some of those differences?

�in some cases, with all those corn hybrids back in the 60s, if you put too much nitrogen, too much nitrogen on, their yields would come down�
As the genetics have improved, I think that picture has also changed. Corn hybrids are simply more aggressive. By that, I mean, they photosynthesize faster, they grow faster, they have better root systems. When you breed for yield you, you bring all these things along. And and so I think hybrids are simply better at taking up water and the nutrients that come with water than they've ever been before�
And when they take up water during the middle of the day in June and in the last half of July and into August, generally they're bringing some nitrate with it that went deeper with earlier rains and now is available to come back up.�

So decades of plant breeding HAVE changed the corn plant, essentially making it more resilient to nitrogen stress. And that change has led to changes in nitrogen recommendations, like the MRTN. Why do most of the non-Corn Belt states still use a yield goal? One factor you can�t discount is simply how important corn is as a crop in the state. Research takes time and money, and an approach like the MRTN takes a large number of research sites every year to work well. So in places that can�t devote that kind of money to building a large database of research sites, a yield goal might just be the best thing they have to work with. The other factor is how soils interact with climate. Because, keep in mind, the main factors that make the yield goal less effective are the highly variable supply of nitrogen from organic matter to the crop, and the highly variable loss of nitrogen from the soil. So, imagine a situation where organic matter is very low and wet winters flush the soil of nitrogen during the off-season, like the warm, wet, long-cultivated soils of the southeast US. Low nitrogen supply from the soil means more has to come from fertilizer, and makes higher yields dependent on higher fertilizer rates. Or a situation where rainfall is consistently quite low, and organic matter low due to the dry climate, like the western Dakotas or Montana. These drylands have low nitrogen losses, low nitrogen supply from the soil, and a more stable relationship between grain yields and fertilizer needs. The point is that an approach that works well in one region might not work as well in another.

While the corn plant has changed through breeding, it�s hard to ignore the feeling that we�ve changed, too. We see this with the switch from the yield goal, which discounts the role of uncertainty, to the MRTN, which focuses on it; we see it with the increasing concerns expressed about the impacts of nitrogen on the environment; and we see it with the variety of tools out there to help make nitrogen decisions. I wanted to walk through the decision-making process of someone who has worked directly with farmers for 40 years, recently retired agronomist Paul Tritchka.

�let's say okay, you have a year where it's a little wet in May and June, or in May, and you get a grower that comes in to you and is not sure about whether to do some sort of additional application. What was kind of your process for helping them make those decisions?�

�Well, okay, so like you said, like, in your example, Greg, you're talking about, we had a, we had a year that was wetter than normal, you know, through the growing season, like, you know, like you said, June, maybe into July. And now we're sitting there with the question or the dilemma, you know, do we need to apply extra. And so, I used to do a lot of the soil nitrate tests�
there's still a lot of unreliability in that test. I mean, it's not perfect,�

�for sure��

�And it just got to a point where there's too much uneasiness about am I getting a decent answer? Yeah. So what I did towards the end of my career, what I was doing, Greg was, I was saying, Okay, let's look at the weather conditions...
how much rain did we get? Was it more than normal, was less than normal? How much nitrogen did we put out? Did I put on just the bare minimum?...
So what I what I could start doing is, I could start saying, let's look at this, let's do a prediction of what, what what we knew what happened, what has happened. What's the conditions today? Where what do we think the conditions are going to be tomorrow, or over the next in the near future?...
now we sit there and look and say, We probably lost more N than we, we wanted so maybe we need to come in there and make an additional application.�

Um, a couple things I was thinking about when, when you're talking about that. One is just that information always comes at a cost, right? Like, that's kind of what you're getting at with this soil sample is that you know, to collect this information, there is a cost and so you're always balancing what does it how much does it cost me to get this information versus how much does it potentially save me?

�Well, and that's a good point, Greg, I mean, you know, let's just use your example�
pulling soil samples and doing that nitrate test....
growers, they say, Okay, how much does it cost? For me on a per acre basis to pull a soil sample to do a nitrate sample? Well, the cost started to getting in. Agriculture has never been where, you know, it's always been, it's never been in I've done this for 40 years, where it's there's margins are always tight for the producers. So and they tend to look to say, how do I best get my return on investment?...
Is that six times out of 10 years, that it was a good decision? Yeah. Or do you say, is it only three times out of 10 years? Well�
Now, the other thing too, is, there's your time, you know, at different times of the year, there is you have, like, Okay, so let's, let's say we're talking about when those plants are small, let's talk about when they're about V4 to V5, there's a lot of things going on ag retail outlet, I mean, a lot of a lot of places to be, a lot of decisions are being made, growers are looking at weeds, and should I spray, you know, what kind of kind of weeds do I have, you know, what type of products should I be using. So that becomes a dilemma that you face�
at the end of the day, you know, the producer has to look and say, does this make sense from an overall perspective? You know, and does it fit into their time schedule, or what they're doing?�
When I talk to people about nitrogen management, nitrogen research, there's kind of a spectrum of opinions that some people are kind of have a thought, we, it is always just good guesswork as to what good nitrogen management is going to be. And we're probably never going to get beyond just good probabilities and guesswork. You know, that's kind of one opinion. And then there's on the other end of the spectrum, I think there's a sort of an opinion that, you know, with modeling or other things, we will get to a point where we can very precisely make nitrogen decisions for a specific field, and I'm just kind of curious, based on your experience, and maybe even just your kind of outlook on things. Are you kind of a pessimist, optimist, neither, a little bit of both in terms of our ability to use technology to get to better nitrogen Management?

Well, I, I am, you know, I mean, there's been there's been a lot of technology that's come out to answer that questions�
I'm always an optimist, I look at this and saying, you know, and I'm probably more looking at, you know, how all these tools can be incorporated with as little cost to the producer. And again, the producer is going to ultimately, at the end of the day, they're going to come in and say, I'm going to spend this, but that doesn't necessarily guarantee that they're going to get the result, you know, ultimately Mother Nature is going to dictate what happens�
yes, you can always take the word, the route, you know, good. This is as good as it gets gonna get, you know, I'm just going to do my guesswork. And we're going to hope, hope and pray that everything cooperates�
being the fact that nitrogen is an elusive nutrient, it makes it really hard to pin down and say, Yes, this is what's going to happen. I mean, if we could only look into a crystal ball and say, What does this year what is 2022s conditions going to be like, you would know what to do, but we don't. Yeah.

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�I have one more question, and in some ways, it�s a little less agronomic in some ways and almost more philosophical, I guess. My question is, okay, I�ve heard you talk a lot about complexity, like a lot of the ways we�re managing nitrogen now are increasingly making it complicated, with the implication that there�s some level of trade-off there, right?...
I guess what I�m curious about, thinking about how you predict nitrogen rates, because you know there are crop models, there are all these different ways you could try to model organic matter dynamics, nitrogen loss dynamics, plant uptake, all sorts of things. Or you could do a basic probability thing, or you could do probability plus some sort of correction- there�s a spectrum of complexity in how you make those decisions. And I guess my question is, in terms of making nitrogen rate decisions, what is good enough?�

�Yeah, I think that's a good question. And I would say that what we've done is, is good enough.
Not a single thing that affects the N responses, the variability in N responses, is- That's a strong statement. Nothing, basically nothing is predictable. In terms of, you know, knowing something at the beginning of the season, that would help us know what that end response is going to be, I thought 20-30 years ago that we'd have such good weather forecasts now that, you know, we might be able to say, well, June is going to be wet. So let's put a little more on or even wait to put some of it on. Even if we had forecasts like that now, I'm not sure they'd help very much. Because I don't think we know the fine detail very well. The things that make the real difference. That, like I say that on one hour of one day, at some point in time, this thing, you know, needed 10 pounds of nitrogen that it didn't have, and that set the tone for the rest of us. So I think that's, you know, it's a comfort in a way that we don't see a relationship between yield and and rate required. Because it tells us that unless we can predict what the yield is going to be, and even if we could predict what the yield is going to be, we still wouldn't know what N rate to use��
�the MRTN is one way to, to at least incorporate uncertainty without being able to solve it. Because the solution to uncertainty is too expensive, economically and environmentally.�

�do I believe that it is perfect, and that there is nothing else to do there? No, I, I continue to look at things, I continue to look at different variables, different approaches to to try to improve this tool or maybe eventually come up with a different tool. You know, I'm not saying that this will be the, the tool forever. I hope that it is not, I hope that we can improve upon, or the if, if it becomes obsolete completely in the something that is completely different, so be it. To me, the most important thing is that we manage nitrogen and crops in the best possible way so that we are profitable, and that we are protecting the environment.�

�but you know, hiding behind the data that we have in this, we will always acknowledge that there's a possibility that your field is going to need more. But until we know when that is, and unless we can afford to head that possibility off we can�t afford to avoid it. And someday, this might all change, but I'm not very optimistic that we're going to get a lot better at it. It's just too, it's just too, too uncertain. There's too many points of uncertainty along the whole course of the season. And we don't understand them and I don't know that we ever will.�

I once heard someone define the word �faith� as maintaining a sense of reverence and humility in the face of something we cannot completely understand, whose certainty cannot be measured. In the face of something as everchanging and unpredictable as nitrogen, something that, if we�re honest with ourselves, constantly humbles us by behaving in ways we don�t expect, this definition seems appropriate.

While everyone had different perspectives, different ways they approached nitrogen management, in all the conversations I had in this episode, one thing that struck me was the faith people had in accepting and adapting to uncertainty, whether that took the shape of abandoning one method of making fertilizer decisions that data showed was no longer working well, EVEN THOUGH at face value it made sense; or relying on probability as the antidote for uncertainty; or recognizing that your advice to farmers could never be perfect and using a career�s worth of lessons to refine that advice.

Whenever I�m walking over a field in my area of southeastern Minnesota, fields whose incredible fertility was born out of the combination of wind-blown glacial silt with the deep roots of prairie plants and oak trees, I�m reminded of something Emerson said in our conversation, something I increasingly put a lot of faith in:

�we can think of fertilizer nitrogen as a supplement to the nitrogen that the soil is already providing to the crop��
�Trust the soil to give you your first 100 pounds of nitrogen and then fertilize, like, that's all you need to add is the difference.�

Thanks for listening.