Latest / The Illinois Nutrient Loss Reduction Podcast / Episode 86 | Making Every Nutrient Count: Fall Fertilizer Decisions for Productivity, Profitability, and Water Quality
Transcript
- Episode 86 – September 2026 **Making Every Nutrient Count: Fall Fertilizer Decisions for Productivity, Profitability, and Water Quality** University of Illinois Extension soil fertility specialist John Jones advises farmers to delay fall nitrogen applications until soil temperatures at a four-inch depth fall to 50 degrees Fahrenheit and continue cooling. To further prevent nitrogen loss through leaching, he recommends using nitrification inhibitors to delay the microbial conversion of ammonium to vulnerable nitrate. While fall applications are common, there is evidence spring anhydrous ammonia applications require 16 to 20 fewer pounds of nitrogen per acre to achieve similar yields. For phosphorus and potassium, producers should rely on recent soil tests to avoid unnecessary applications in high-testing areas, keeping in mind that dry fall conditions can artificially lower potassium and pH readings. Finally, Jones advocates using the online Nitrogen Rate Calculator to determine application rates that maximize economic profit per acre rather than absolute maximum yield, ensuring that the cost of additional fertilizer does not outpace the value of the resulting crop. John Jones, Soil Scientist, Extension Soil Fertility University of Illinois --- Todd Gleason: This is Episode 86 of the Illinois Nutrient Loss Reduction Podcast. I'm University of Illinois Extension's Todd Gleason. Todd Gleason: Farmers across the upper two-thirds of Illinois are gearing up to make their fall nitrogen application decisions. To help navigate these choices, we are joined by John Jones, an Assistant Professor of Agronomy and the State Soil Fertility Specialist at the University of Illinois. In this segment, Jones outlines the critical factors to consider before applying fall fertilizer, emphasizing the rule to wait until soil temperatures at a four-inch depth reach 50 degrees Fahrenheit and are actively cooling. He explains the science behind nitrate vulnerability, the role of nitrification inhibitors in delaying the microbial conversion of ammonium to nitrate, and how dry conditions can skew phosphorus and potassium soil test results. Jones also breaks down how producers can use the Nitrogen Rate Calculator to maximize economic profit per acre rather than just pushing for maximum yield. Let's listen to the conversation. John Jones: My name is John Jones, I’m an assistant professor of agronomy within the Department of Crop Sciences at the University of Illinois and the state soil fertility specialist. My research program evaluates and studies soil and fertilizer nutrient availability to our cropping systems around Illinois. There’s also a component that we evaluate the ability for nutrients to stay in the field and reduce losses with management decisions, evaluating diagnostic tools like soil and plant analysis. A lot of this is fueled by on-farm research that we conduct with many growers around the state of Illinois, and in some small plot research we’re able to dive into some fairly advanced and complicated levels of treatments and measurements, really all to fuel nutrient management guidelines for the state of Illinois. Todd Gleason: Farmers in the upper two thirds of the state of Illinois will be making their fall nitrogen application decisions they may have already. What things should they keep in mind? John Jones: Well, if we’re talking about nitrogen, the most important thing to keep in mind right now is the calendar, and what we’re seeing relative to soil moisture conditions and soil temperature conditions. The governing information that we like to really focus on is soil temperature. Specifically trying to wait to apply anhydrous ammonia or nitrogen fertilizer in the fall until after the soil at about a 4-inch depth is at 50 degrees Fahrenheit and cooling. And the and cooling part is important so that one, if we experience one day of cool soil temperatures or cool air temperatures leading to that, and then we get a warm-up period after that, that’s not necessarily ideal. And so we want to really see that soil temperature trend cooling at 50 degrees and past 50 degrees. More often than not that ends up being in the later part of October into early November for again as you mentioned the northern two-thirds of the state. One of the things that we can do to extend that period is look at using nitrification inhibitors. These would be products that would have a focus of trying to delay the conversion from ammonium to nitrate after anhydrous has been applied to the soil. Right now, we’re fortunate where the majority of the state has received ample fall moisture, and so I expect as long as we’re not really mudding it in, the physical efficiency of our anhydrous applications, proper slit closure, and considerations about the physical condition of the soil should be pretty good compared to some of the dry falls we’ve had. Todd Gleason: You mentioned nitrate this is the form of nitrogen and you can tell me more about that that is subject to loss. Why is it vulnerable? John Jones: Well nitrate in the soil is not going to bond as tightly or have as many places to bond and be attracted to soil as cations such as potassium or calcium. Nitrate is negatively charged and the majority of our soil surfaces if they have charge are negative as well. And two negatives are not going to attract to each other, and so you generally have a movement to a higher degree of negatively charged ions and that’s the form that nitrogen is in as nitrate, also be comparable to things like sulfate, chloride as well. So what we expect our soils to do is not be able to retain nitrate to the degree it could retain ammonium, which is a positively charged cation. The goal then is to keep nitrogen applied in the fall if it is applied in the fall in the ammonium form as long as we can before we get microbial conversion and that’s the main conversion process of ammonium to nitrate. We certainly can see ample amounts or large amounts of precipitation events in the fall at times, but particularly we’re trying to avoid a large amount of the N we apply in the fall, if it is applied in the fall, being nitrate form coming out of winter into spring when we get flushes of tile drainage that can carry nitrogen away. Todd Gleason: This is what the in inhibitor is supposed to prevent, that’s why it’s called an in inhibitor particularly in these fall applications. Is there any recent work on these inhibitors and how well they perform and what’s that data saying if that is the case? John Jones: Large amount of inhibitor and we’re talking about nitrification inhibitors so we’re really focusing on preventing, more realistically delaying the nitrification process or that conversion of ammonium to nitrate. The focus of a lot of nitrification inhibitor research has been on yield response, so trying to evaluate corn yields with and without nitrification inhibitor application. That research has been around for certainly longer than I’ve been alive, but for a handful of decades. And the general benefit to those inhibitors again is in the time frame of days or weeks, and so the process of trying to utilize that or use that technology is to essentially give you a little buffer before again soils start to really cool. Using a nitrification inhibitor is not necessarily a replacement for applying fall anhydrous at proper soil temperatures. So we want to use both of those to your benefit, not trying to use one to replace the other. There are questions about using inhibitor products with spring applied and even into sidedress applications with liquid nitrogen application, and the yield response is a little mixed. Certainly in soils that are a little coarser textured where we would expect there to be a greater propensity of nitrogen to transform and move throughout the soil and essentially not being able to hold ammonium as well, then you usually see a larger benefit to those nitrification inhibitor products. There’s new research that’s going to be going on in the state of Illinois that our lab is a part of but led by Dr. Andrew Margenot funded by the Illinois NREC program to evaluate nitrification inhibitor performance relative to nitrogen loss, but also yield response as well. So I’ll say with that answer there’s also more data to come. Todd Gleason: Tell me about the nitrogen rate calculator or in rate calculator that you ask and many other states across the Midwest ask producers to use as it points them towards the maximum return to nitrogen. What is it and how does it work? John Jones: So the philosophy around identifying the maximum return to nitrogen or the maximum economic return to a nitrogen application really comes from needing to evaluate first, yield response to nitrogen rates. That evaluation or that process does the lion’s share of the work getting from what rate do we need to apply to what rate leads to the most economic profit from that nitrogen application or that decision to apply a rate. So we look at hundreds of data points representing on-farm trials around the state that looks at what is the maximum yield we can obtain and then what’s the N rate that led to or produced that. We then take that relationship between the maximum bushels produced and the rate that provided that, and then we really just adjust for pricing conditions so that the last pound of nitrogen applied pays for itself. Or you can think of the last bushel produced paid for the last pound of N applied. Now you may think that those two philosophies of maybe going for max bushels versus max ROI we’ll call it are contrasting or conflicting with each other, but we really find that they’re not across over 800 trials and for example even last year across 73 trials we ran around the state, the difference between the yield that was the maximum bushels versus the yield that maximized profit was about 1.3 bushels. So it not a massive sacrifice in bushels to have a more profit focused decision of nitrogen rate, and usually the difference in that N rate was around 14, 18 to 20 pounds of N that it was reduced. Todd Gleason: So what you’re really talking about is optimizing the profit per acre and once you get to that point every pound of in after that doesn’t pay for itself. Despite the fact that yield may actually go still up. John Jones: Yield may go up, but it goes up in a very, we’re talking about about 1% of the total maximum yield. Maybe 2%. The only thing that changes that relationship is when pricing scenarios get less favorable, not too different than they are right now although our crop prices have rebounded a little bit, the difference between going for max bushels versus max profit, that’ll certainly widen or that gap widens if prices are less favorable. But in general if we consider about a 10 to 12 to 1 crop price to nitrogen price ratio, we’re generally within that point where you maybe sacrifice 1 or less than 1% of the bushels to move to a profit maximizing nitrogen rate. Todd Gleason: Turn your attention to other nutrients fall applied that you have been watching and that are very, very expensive at this point. Tell me about P and K applications and what producers should be considering for the fall. John Jones: The most important thing to consider is soil test levels. And the environment or scenario in which you’re dealing with soil test data perhaps as well. I will say up until this year, which is a good thing, we’ve had some fairly dry falls that soil samples have been taken. One thing that we do notice fairly consistently is a skew or a slight adjustment that happens to the amount of specifically potassium, but then also our value of pH that’s measured in dry conditions. K might be a little lower, pH might be a little lower as well. So if you are dealing with soil test data from dry falls, you might need to adjust up a little bit on those values. That would mean that you’d have more available. But you should let soil test levels guide whether or not even to apply nutrients. If you’re in a very high testing scenario, I certainly would recommend thinking about cutting back on on P and K because likely you’re not going to be finding a yield response and definitely not an economic return response. This is also a great time though to get a high ROI to soil testing this fall following the combine, because it can tell you where you don’t need to apply, and places where likely you’ll start to see you would see a yield response. So another component to this is considering in your philosophy of managing within an optimum category or around the critical level at the lower end of the maintenance range. At that point, we generally expect around a 20 to 30% chance of economic response to P or K, that may change with soils, that may change with pricing scenarios. But if you live within that space I’ll say, then you can buffer these high prices and dry fertilizer by letting things go a year or two and then maybe building back or getting back to a maintenance application once prices are more favorable. But let soil tests rule the day so to speak in terms of deciding whether or not to apply, we don’t see a difference in when you apply within the rotation, and so applying before corn or soybeans is certainly both options. I know some farmers are running to an annual application of both, so maybe it’s one removal application one year and one the next year, we don’t really see a large difference in yield response to nutrients over the rotation when you apply it within the rotation. That’s good news because it means you have some flexibility. Todd Gleason: Clearly year in and year out there are many variables that come into play when making these fall fertilizer decisions, there may be some other variables this year. What is the most important factor do you suppose before making those fall applications? What should producers think about as it relates to management? John Jones: I would certainly think about the production levels that some of some of your fields are have have certainly you’ve seen in the last few years which likely around the state based on numbers we’ve seen some very good production levels. This year might be a little more variable or down depending on where you are in the state. If you’re thinking about fall applications of nitrogen for example, again think about what we’re trying to do in terms of raise the efficiency of that application or maximize the efficiency of that application. So soil temperature, nitrification inhibitors in the fall, and then if you can, wait, you know, splitting that application even within some in the fall if that’s what your system is going to do, but applying some in the spring would certainly see a benefit. We do see a difference of about 16 to 20 pounds of N per acre lower optimum N rate of anhydrous used in the spring compared to the fall, we don’t see a large yield difference though. So that’s something to certainly consider on the nitrogen side, on the P and K side, try to use recent soil test levels, if you can get to a point where your grid or zone sampling to identify which areas of the field you can certainly cut back on that would be very useful. Overall though, what you hopefully are set up for is an expectation to have some flexibility. So there are multiple ways that you can deliver N, get P and K out there if needed, and so you’re not necessarily pigeonholed into one option or the other. Certainly though when we think about pricing scenarios, we don’t want to move to a scenario where we’re sacrificing efficiency just because of some price, that may kind of bite us in the end if if we’re we’re using a components of the source rate time and placement of nutrients that really drop our efficiency. Because and that’s going to be different for every farming operation, so evaluate how we can maximize our efficiency of these inputs that are very expensive in an annual setting. Todd Gleason: Again can you remind me of something you just said related to the difference between fall application rates and spring application rates and how farmers adjust those? John Jones: Well what we’ve seen with published research is around a 16 to 20 pounds of nitrogen per acre lower optimum N rate with spring applied anhydrous compared to fall applied anhydrous. Now this is with the majority of the N for that corn crop coming from the anhydrous application. So things shift when you start to split maybe use sidedress after a base rate, but if you’re just relying on anhydrous ammonia for the primary N source, we do see a shift towards higher efficiency for spring applied anhydrous ammonia as not as for nitrogen compared to fall. We do not see a big difference in yield, so the benefits are slightly leaning towards toward spring application for yield but not a large differential, but when we look at the amount of N needed, there is a significant difference between spring and fall applied anhydrous. Todd Gleason: Finally John can you return to the in rate calculator online. What is it that farmers should consider as they look at the number it produces? John Jones: The in rate calculator looks at a total N regardless of your source applied or used, so the main main output is a total nitrogen value. What’s important about that is it does consider, it does include, things like fall applied N with your phosphorus fertilizers, any N that’s applied within that system broadcast pre-emerge applications, we we want to make sure to do a a detailed accounting for any N that’s applied before that corn crop because we’re looking at total N that’s been applied. Todd Gleason: Thank you very much I appreciate it John. John Jones: Great, thanks Todd. Hope everyone has a safe harvest. Todd Gleason: You’ve been listening to University of Illinois Soil Fertility Specialist John Jones discuss fall fertilizer recommendations on the Illinois Nutrient Loss Reduction Podcast. Our program is produced in collaboration with University of Illinois Extension’s Rachel Curry, Nicole Haverback, and Luke Zwilling. Until next time, I’m your host, Todd Gleason.