Talkin' Crap

How much N is already there: Using LSNT to Fine-Tune Your Nitrogen Program

• Dan Andersen • Episode 27

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In this episode, Dan Andersen is joined by Mitch Baum to discuss the late-spring nitrate test and its role in nitrogen management. 

Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production 

Iowa Nitrogen Initiative website

N-FACT website


Dan Andersen:

Hello and welcome to Talkin' Crap, a podcast by Iowa State University Extension and Outreach. This institution is an equal opportunity provider for the full non discrimination statement or accommodation inquiries, go to www.extension.iastate.edu/legal. In this podcast, we discuss insights into the science, technology, and best practices surrounding manure management. Our objectives are to build awareness about the challenges farmers and the broader agricultural industry face around manure and to demonstrate solutions and areas of innovation. Hello and welcome back for another episode of Talkin' Crap. This time we're talking about how much nitrogen is already there, using the late spring nitrate test to fine tune your nitrogen program. Heading into 2026 many Iowa fields carried more residual nitrogen than normal after a record mineralization year in 2025 and then a relatively warm winter, that raises an important question, how much additional nitrogen does the crop actually need? In this episode, I'm sitting down with Mitch Baum to discuss the late spring nitrate test and how it can help farmers fine tune side dress nitrogen decisions. Specifically, we're exploring the new Iowa State decision support tool on using results from the late spring nitrate test and how to best reach your optimum yield potential. So with that, Mitch, thanks for joining me. Great to have you back on on the show. I know one of your last episodes was one of our most listened to podcasts. People are really interested in nitrogen management. So Mitch is in a scientist with the Iowa Nitrogen Initiative here at Iowa State. He's done some great work, and recently he's been working on using that data to help understand how good does the late spring nitrate test work, and maybe can we come up with a better prediction. So how's the Iowa Nitrogen Initiative going? What are we looking forward to this year?

Mitch Baum:

Well, once again, thank you, Dan, for having me on. It's a pleasure to be back. I had no idea it was one of your most listened to podcasts, but that's great. In terms of the Iowa Nitrogen Initiative, we are chugging right along. We have more trials being planted every single day. We already had a significant amount in the fall, and we have a little bit of a difference in our trial design this year, which I'm excited about. So this year, we're being a little bit more restrictive. We're going to have more nitrogen rates, which should further improve our ability to estimate the optimum N rate for different management practices. And I think we're going to find a lot of really cool things this fall.

Dan Andersen:

Perfect. And I know I was excited. You just released the 2025 data onto the website so we can review all last year's trials and and what we found, I haven't done it yet. It's on my to do list, but it's always nice to see where the state is, and especially last year record minitization. What that mean in terms of how the trials went?

Mitch Baum:

Yeah, that's great. So how the trials went, we saw actually the lowest average optimum N rates in our short three year history of the Iowa Nitrogen Initiative, they were about 24% lower than the 2025 values. The average was 171, maybe 172, I can't remember, but that that is due to the record mineralization. So we had very timely rainfall during July, and really what that did is that did push quite a bit of nitrate down through the soil profile, but it also boosted mineralization right at the time when plants were requiring it to fill their grain. And that really had a big impact on our optical nitrogen rates.

Dan Andersen:

Yeah, which historically, we might have been able to say the modeling really showed us that we think it was a record mineralization year. We would have had some ISU trials that said, looks like that might be true, but seeing it on farm and on a statewide level was really something unique and different. Okay, but I think we're going to jump into the late spring nitrate test, but I think that really sets the tone for why this year might be a little different, and why I'm going to suggest maybe the late spring nitrate test has a little more weight this year than it historically has. So we had this record mineralization. Where do we think that left us in terms of nitrate last year? And what should we be anticipating?

Mitch Baum:

Yeah, so what we found in general, by fall testing in 2025 we saw that the levels were three times higher than they were in 2024. Now that sounds alarming, but it's not unheard of, right? So those levels were actually on par with the levels we observed in 2023, but we also had a relatively warm winter, so the potential for a lot of nitrate to be in the soil is there, and it really emphasizes our need to measure how much is in there right now, so we can apply accordingly in season, if you choose to do that.

Dan Andersen:

And one of the things I've learned from the N FACT tool is your residual nitrogen estimate that you choose in that tool actually has a fair amount of weight on what it thinks the optimum rate will be in the coming year. And this is really our touch point to say, how do we use that number a little better? Is that fair?

Mitch Baum:

That is fair. And we're doing some ongoing work to test whether or not or really just how predictive a fall nitrate test is versus spring test versus the late spring nitrate test, which I know we're going to be talking about now. So I'm really excited to get those results in this fall, but what we can say so far is that sampling in both the spring, pre plant and post plant is predictive of what the optimum nitrogen can be, essentially saying, the more you have in your soil, the less you need to apply.

Dan Andersen:

I mean, that the easy answer. So why are you interested in, maybe that post harvest in the fall, how it relates to spring, and then how much it relates to late spring nitrate test? Why do why do we care about that?

Mitch Baum:

Yeah, that's a great question, Dan. So the thought is, is that when winters over winter, the soil freezes right, and that nitrogen isn't moving anywhere. While the further out you sample from when the crop needs it, the more uncertain that measurement is going to be. However, that doesn't mean it's meaningless, right? So we're pretty confident that the more you have in the fall is still going to be indicative of having more in the spring and having more in the summer, which we know will lower your optimum nitrogen rates. And testing in the fall has the benefit of coming at a time when you're not overly busy after harvest, right? You don't have to get all your ducks in a row to plant, till, line up a nitrogen application or a herbicide like you do in the spring,

Dan Andersen:

Absolutely, and especially in this year where maybe planting in Iowa has been a little bit more of a struggle. We get that time crunch. Having a good idea of what your plan is, super helpful, rather than trying to figure it out on the fly when I'm maybe trying to finish up the last few fields to plant. Okay, so the late spring nitrate test. What is it? What do I have to do? How do I collect it?

Mitch Baum:

Yeah, well, in its simplest terms, it is a soil nitrogen test when the plants are about six to 12 inches tall, V4 to V6 roughly. And what it does is it tells you is, is just a snapshot in time of how much nitrogen is in the soil. It's not necessarily predictive of how much will be mineralized, but very good indication of where you're at right now. It gives you a very good data point to make the best decision possible for applying additional nitrogen in season, with the thought that if you already have high levels of nitrogen in the soil, adding more probably isn't going to pay for itself in the long run adding more bushels.

Dan Andersen:

So I'm going to go take a soil test. What kind of soil probe do I use? How deep do I push? How many cores should I get? Like, how many samples should I get from a field, any thoughts or guidance on that?

Mitch Baum:

Sure. So the depth is actually very important. We recommend going one foot down using a standard soil probe. I believe that's a one inch diameter. You'd go down one foot the amount of cores you need is actually uncertain. And of course, the great PhD answer is, it depends. If you have a relatively uniform field you need less samples than if you have a lot of different field zones. The previous guidance was to get samples representative of every 10 acre grids. However, that may not be price effective. Really, what you want to do is get a representative sample of your field, or representative sample of the areas of your field you could potentially variable rate.

Dan Andersen:

Yeah. So really thinking about that variability in your field, maybe using last year's yield map to understand how variable yield is in the field as an attribute to understand where and how often I need to sample. Unfortunately, that's one we just don't have a great answer to yet.

Mitch Baum:

No.

Dan Andersen:

Really sort of related to how willing you are to change management practices within a field. If you're going to go and put the one consistent sidedress rate of nitrogen on the whole field, kind of getting a field average is probably okay.

Mitch Baum:

Yep, I agree. And let me expand on that. So we do know the optimal nitrogen rate changes in field, right? In extreme cases, it can be over 100 pounds. So it's not unreasonable to think that your field is going to have different nitrogen requirements within it. So it's good to sample.

Dan Andersen:

Yep, and it's definitely trying to use some attribute of the field to figure out how you're going to zone to use that information. Because, like you said, we want the test to pay for itself. We want it to provide valuable information to us. So think about how could you manage differently from taking that? Okay, so we know we're sampling to a one foot depth. We know when we should sample. We know roughly how we're going to try and manage it now. All right, now ISU has had a late spring nitrate test for a while. We've had a publication out about how to use the results, and it hasn't really gained a lot of traction. It was still a test that people hadn't used. What inspired you to try and look at the INI data, the Iowa Nitrogen Initiative data to say how predictive is it?

Mitch Baum:

That's a great question, and it's kind of three fold. The first is that now we actually had enough information to do it, and I'm really excited to do it. Second was we know that cropping systems have evolved over time. The previous publication uses data from 1989 to 2015, so we know during that time, yields have increased, the optimum nitrogen rate has increased, nitrogen use efficiency has improved, and overall equipment and management strategies have have been tweaked a little bit. So there's no reason to think that the late spring nitrate test couldn't also be improved to be more reflective of today's cropping systems.

Dan Andersen:

And when you think about maybe 1990 to today, that's 30 ish years, you get about two bushels a year. So that's 60 bushels more today than we had back then. So it isn't a small change, it's these are pretty drastic changes in yield, and reasons to think that calibration might be important. Okay, so you have the Iowa Nitrogen Initiative data. What? What did you pull from it? How did you start to go about your analysis?

Mitch Baum:

Sure. So the Iowa Nitrogen Initiative gets soil samples three times a year for every trial that has a true zero nitrogen rate. So we get samples after harvest, before planting, and then after planting, which is this late spring nitrate test, and we get these samples on the lowest the zero rate, the farmer average rate and the highest rate. And using that data, we can compare what the relative yield is at those plots to the measured soil nitrate. And we use that to build a generalized response where we can determine if your soil nitrate level is, let's say, 10 ppm. We can say there's with X amount of confidence that yield will improve to more nitrogen.

Dan Andersen:

Perfect. So we know how much nitrogen was there. You get the yield at the end of the season to see could have it been better, how much better it would have been? And there's really two things that you your tool really tells us. One is the probability of that yield response if you put on additional nitrogen and then the other is sort of some guidance on how much nitrogen you may need. Is that fair?

Mitch Baum:

No, that's exactly right. And this is actually kind of where it differs from the previous late spring nitrate tool. The previous late spring nitrate decision support guidance was to relate those soil values, apply a mathematical equation to it, and then you know exactly how much to apply. Here we're taking a little bit of a different approach by saying, Well, we know what the optimum nitrogen rate was. We know your soil value, and we know how much fertilizer was applied there, and we just take the difference between the optimum rate and the amount fertilizer applied to show how much you could need. And that's exactly why we supply a range in our tool. Because we're not trying to predict what it will be this year. We're using our past data and knowledge to say this is the probable range that worked in the past. And by past, I mean within the last three years.

Dan Andersen:

Short past.

Mitch Baum:

Yeah, the short past to optimize your fertilizer demand.

Dan Andersen:

And I think that's an interesting sort of note. Like the old tool, used one mathematical formula, and it felt like you put in your soil test nitrate, and it spits out how much nitrogen you should put on. And the truth is, the world's a lot messier than that.

Mitch Baum:

Exactly.

Dan Andersen:

And I think one of the things, both with N FACTS and with this tool, is you've recognized that there's a lot of variability, field to field, year to year, because of the weather pattern that we encounter. I mean, in May last year, I wouldn't have guessed it was going to be a record mineralization year, right?

Mitch Baum:

No one did.

Dan Andersen:

So I think recognizing that there's some variation in that and how that might change, how I would interpret the test to you would interpret the test, but still that there's this general trend of, if I had a field test 20, and I have a field that test 10, they probably could use different amounts of nitrogen, and I could understand something from that is extremely important. Okay, so the new tool is on the Forecast and Assessment Cropping Tool Systems page under the late spring nitrate response. And when a farmer, advisor, crop advisor goes there, what do I need for data to input, and what does it spit out at me?

Mitch Baum:

Yeah. So what you need is actually very simple. You need your soil test value, and you need what yield level you're willing to accept if you don't apply any fertilizer. So if you think applying fertilizer, well, let me run through an example for you,

Dan Andersen:

Perfect.

Mitch Baum:

So. Using today's prices, I did look this up this morning, liquid UAN cost about 97 cents per pound of nitrogen, and today's corn price was$4.52, so you would need to have a yield benefit of 11 pounds or 11 bushels for a 50 pound UAN application to pay for itself, and that's completely up to the farmer's discretion whether or not they're willing to forego five pounds, 10 pounds, 20 pounds, if they don't think the side dress application is worth it. What our tool does is it says, Okay, that's great. You're willing to forego that loss to not apply in the field. Here's the percent chance however, though it's going to pay off if you do.

Dan Andersen:

So it gives the percent chance that it would pay off. And I think that's what we all I mean, that's

Mitch Baum:

Yeah. really what we all want to know. It's it costs me time, it costs me effort. It costs me money every time I have to drive through the field in addition to the nitrogen I buy. So is it worth it for me to drive by at this point? Okay, so I go in there. I have my test results back from the lab. I took a good sample. I know what I'm going to do. So the first number I get back is sort of the odds, or the percent chance that nitrogen application is going to pay for itself. So if it says there's a 70% chance it pays for itself, I probably make a decision to go apply, right? It says a 10% chance. It pays for itself, maybe not this year. Yeah, it's completely how risk adverse you're willing to be.

Dan Andersen:

And then how do I use this information to figure out what you'd tell me I put on and then I know you give a range, so how do I think about that range?

Mitch Baum:

Sure, so keep in mind, these are independent but complementary. So the percent chance of a yield increase doesn't change your required nitrogen demand. So you can play with how many ppms are on your soil test, the yield willing to forego is not going to impact your nitrogen demand to reach the optimum. The reason we supplied the range there for that is for that very reason, right? So say there's an 80% chance you increase yields to further application. And the tool gives you a range of probable nitrogen rates from anywhere between 20 to 60. Well, you're probably going to figure this pass at 20 pounds probably isn't going to pay for itself. I might want to add 50 or 60 and ensure that I'm making profit.

Dan Andersen:

Perfect. And I think as I look at the tool we used to think about like critical thresholds in the old test of how much nitrate we wanted in the soil. So So if my soil test is above, what number would I probably say? I really probably don't need more this year. And I'm not going to hold you to that, because when you look at the tool, there's always a chance that additional nitrogen can cause a yield increase.

Mitch Baum:

Thank you. That was going to be my answer.

Dan Andersen:

There is.

Mitch Baum:

However low, yeah. There is, yeah.

Dan Andersen:

As we get above maybe something like 25, 30, the chance is pretty small. So while there's no guarantee, it could be if you test 30 and you haven't applied nitrogen yet this year, you might say, Well, I really don't need a lot, but I'm probably living in a field where maybe there's a small chance from for a yield increase if I put a little on. And your trials bore that out like no matter what your threshold was, it surprised me, you pretty much always got a yield increase, but it was on a fewer and fewer number of the trials that that sort of had had that or saw it.

Mitch Baum:

Right, right? Yeah, we did almost always see a yield increase. But keep in mind that yield increase usually didn't pay for itself when you had those high, high test values.

Dan Andersen:

Yeah. So it, you probably don't need it above a high level. But there isn't. There's a small chance, depending on the year that we could see, one and maybe depending on the side dress technology you're using, there's always chances to apply nitrogen later. If I'm above 30, I'm probably thinking, I don't need any nitrogen right now, let's see what the rest of the growing season brings to us. Is that a reasonably fair approach?

Mitch Baum:

I think so.

Dan Andersen:

And then when I looked at your tool, it looked to me if you were maybe below eight or 10, basically, it feels like there's no nitrogen on the soil, sort of asking for what a typical rate would be. Is that semi fair?

Mitch Baum:

Yeah, I think so. If you're in the eight or 10 ppm, there's a pretty high likelihood your plants can respond favorably to adding more nitrogen. It really wasn't until you get into about the 20s that it, you know, it really tapers off, you know.

Dan Andersen:

And then between sort of in that range. So if I'm below five, I'm probably thinking, whatever my normal nitrogen rate is for this field, probably is still what it looks like I need if I've applied before. Maybe I should think about why I didn't see the response that I take good samples that I make sure I get where my fertilizer band would be, et cetera. But in between, it really was sort of a tapering off between what my base rate would be, and by the time I get to 25, 30 basically no response or no fertilizer needed.

Mitch Baum:

Exactly.

Dan Andersen:

How different is this from the previous test? In terms of the response curve that we you tended to see.

Mitch Baum:

That's a great question. So we tend to see it being more responsive at lower PPM than the previous test. And once again, that goes back to how our farm systems have evolved over time, right? And I really do think it has to do with we have much more efficient crops. So the previous critical value I was trying to avoid this talk. But let's talk well, it's 25 in original late spring nitrate test tool. When we did it, it was closer to 18. And the reason it does that is because it really shoots up really quick when your soil nitrate test is that low, your plants are really hungry for more nitrogen at that point. But because of the improved nitrogen use efficiency, I suspect it tapers off quicker and it just has that lower critical value.

Dan Andersen:

Yeah, and as you pointed out, there's a few things that have changed. We grow more corn per acre. We tend to plant corn a fair amount earlier than maybe we did in 1990. I'm a little older than you, I still remember corn planting dates in 1990 and we weren't quite this early, like Mother's Day was a good target date to be planted in. Now, a lot of corn is in before that, so

Mitch Baum:

That's okay. I still hear every year, if it's knee high by the Fourth of July, you're good to go. And I always tell people like, I don't think so.

Dan Andersen:

See, when I was young, it was knee high by the Fourth of July, and that was a good goal. And I agree with you. Now, if it's shoulder high or head high by the Fourth of July, anything less than that, and I'm feeling a little disappointed in where I'm sitting. Okay? So the range, when should I decide I maybe want to apply to the high side of the range from your tool the low side? Any advice on that? Or is that still a sort of a place we need to learn more?

Mitch Baum:

I'd absolutely say it's a place we need to learn more. It goes back to really how risk adverse you are as a farmer. If you're willing to, you know, take a little bit of a hit on added nitrogen to ensure high yield, I'd apply the high side. If you're more purely profit oriented and you want to, you know, ensure you're not wasting money on extra nitrogen, apply a little bit towards the medium, maybe lower.

Dan Andersen:

Yeah. And I think one of the things I look at on that range and really respect is we sort of had that in the N FACT. Every field isn't the same, and it's going to be very impractical in the near term without tons and tons of data to sort of understand what causes field to field variation. I think this gives us some idea of what the data says it looks like and maybe where you should be. But despite that field to field variation, you still see a really clear trend. And I know in the past, I've taken some late spring nitrate tests and had trouble deciding what to do with them. Now the good news is I'm like, I don't want to do anything anyway. I just wanted to know it and see where my plots ended up at the end of the year. But having that data sometimes made it hard for me to think about, like, what did it mean? What would have I done? And this really provide some clarity of sort of the odds of getting a response. And I could have said, well, that one said I needed it this year, and I didn't see any response. Why would have that been right?

Mitch Baum:

Yeah.

Dan Andersen:

And I think that just helps me feel a little more confident about the test. So as we start to move towards our wrap up here, any tips on what are some of the biggest mistakes people sometimes make when collecting late spring nitrate samples. Or how confident should I feel like? How does this tool help me build some confidence in making a

Mitch Baum:

Right? I'd say some common mistakes would be to one sample in areas you that are not representative of your field, and to sample in bands of fertilizer.

Dan Andersen:

Yeah? So, making sure you weight that. It's okay to get one sample from a band, but it should be weighted to how much of the field literally has a band in it. You're not trying to sampleize the sample the fertilizer band. That's not the goal of the test. Yeah.

Mitch Baum:

No.

Dan Andersen:

So, and sometimes it feels like that's what I really am chasing. Right? Is the fertilizer still there or not? Not really, how this test was calibrated?

Mitch Baum:

No.

Dan Andersen:

And then as we move forward, I know the N FACTS tool gets updated. I'm assuming you'll keep updating, sort of the guidance from here to understand how year to year variability plays into our what's happening in recommendations?

Mitch Baum:

exactly both N FACT and this late spring nitrate decision support tools are living tools. So each year we will add more information that we can into it. For N FACT, that's going to be hundreds of yield response trials per year for this late spring nitrate tool. It's probably going to be around 30 to 40 per year.

Dan Andersen:

But that's still a big addition to the data set every year to sort of understand what was happening. And then where can people access the tool and see and learn more? What should I be Googling to find it? What website am I looking at?

Mitch Baum:

Yes. So this tool is housed on the Forecast and Assessment of Cropping Systems, web page hosted by Iowa State University. That web page itself is a host of decision support tools that help characterize weather, how it relates to historical averages, actually, soil mineralization rates per week, soil water values per week, and also other things as well, and now also this late spring nitrate tool. So if you go to that website, all the tools are on tabs across the top. You just go to the one that says late spring nitrate test, and it will take you right to our interactive web page.

Dan Andersen:

Perfect. And personally, I really love that mineralization tool as well. It was interesting to see last year when we started trend above normal and continue there. And I'd followed it really closely so far this spring, and I thought, Oh, we're off to a great start, and now we're back to average, given that we've had a few cold weeks here, but it's still nice to sort of see that trend. Okay, any last takeaways we have from you? Just encourage people to sample. And even if you don't use the results this year, think about what it would have suggested.

Mitch Baum:

Yeah, that that was, is my main takeaway. Soil sampling, knowing what you have in the soil is the best tool you have to help understand what your nitrogen demand is going to be at the end, by the end of the year. And that's it's really powerful tool to try to estimate optimum nitrogen rates.

Dan Andersen:

Perfect. From a guy who looks at nitrogen trials all the time and tons of data, I think it's great to hear that knowledge is still power. You need that information to help make better decisions. So once again, Mitch, thanks for joining us. Thanks for the great conversation. I really appreciated learning more about the new tool and how I can maybe get some better guidance from the late spring nitrate test.

Mitch Baum:

Yep, as always, happy to be back, and hopefully we'll have something else to talk about in the future.

Dan Andersen:

Sounds perfect. We'll make sure we have you back.

Mitch Baum:

All right, thanks, Dan.

Dan Andersen:

Thank you for joining this installment of Talkin' Crap. Be sure to take a look at the show notes on our website for links and materials mentioned in the episode. For more information, or to get in touch, go to our website, www.extension.ia.state.edu/immag/. If you found what you heard today useful, or it made you think, we hope you subscribe to the show on your podcast app of choice. Signing off from a job that sometimes smells but never stinks, keep on talking crap.