In this episode of the Nutrient Management Podcast, we’re talking about variable rate technology. What are some reasons that farmers might want to use variable-rate fertilizer instead of applying a single rate? Is there a difference between variable rating, different nutrient types? What are the technologies that are used to prescribe variable-rate fertilizer and what is the theory behind them? If farmers apply fertilizer at a variable rate, how should they evaluate those outcomes?
Welcome to University of Minnesota Extension's Nutrient Management Podcast. Each month we bring you the latest research in nutrient management for crops and how you can incorporate the latest tips and best management practices to your farm.
University of Minnesota Nutrient Management Podcast Episode: “Variable rate technology”
January 2023
Written transcripts are generated using a combination of speech recognition software and human transcribers, and may contain errors. Please check the corresponding audio before referencing content in print.
(Music)
Jack Wilcox:
Welcome back to the University of Minnesota Extension's Nutrient Management Podcast. I'm your host, Jack Wilcox, communications generalist here at U of M Extension. In this episode, we're talking about variable-rate technology, and we have three members of Extension's Nutrient Management team here with us today. Can you each give us a quick introduction?
Brad Carlson:
I'm Brad Carlson. I'm an Extension educator. I work out of a regional office in Mankato.
Jeff Vetsch:
Hi, I'm Jeff Vetsch. I'm a researcher at the Southern Research and Outreach Center in Waseca.
Dan Kaiser:
My name's Daniel Kaiser. I'm a state nutrient management specialist. I'm specializing in soil fertility and crop nutrient recommendations. I'm located out of the St. Paul campus.
Jack Wilcox:
What are some reasons that farmers might want to use variable-rate fertilizer instead of applying a single rate?
Brad Carlson:
Well, I think one of the main things a lot of farmers think when they first start exploring this is that there's these spots in the field where they are not meeting their potential. I think while it's a very basic concept, I think it needs to be thought about purposefully when we're talking about variable-rate, is that there's really only two ways you're going to pay for this technology. One is to increase your yields and obviously increase your income. And in the absence of that, you need to reduce inputs or reduce the expenses that are out there. And so that's really ultimately what's going on out there. And so I think from a farmer's standpoint, you need to start thinking about, "Do I really have parts in the field that are not reaching their full potential?"
In a lot of cases when we're talking about fertilizer, most farmers tend to be rather conservative or maybe even, I guess dare I say, aggressive with fertilizer because they want to make sure that they're not leaving yield behind. And in a lot of cases there just simply aren't those spots in the field that are not reaching their potential due to fertility. And so it's only reasonable that we would not really expect to see yield increases in those areas.
And so from that standpoint, farmers need to be willing to take a reduction in fertilizer expenses as the way they're paying off variable-rate technology in a lot of cases. And if you don't feel comfortable with that, if you're not willing to do that, then in a lot of cases variable rating, your fertilizer's not going to pay for itself. And so I think that's really kind of the main question a lot of farmers need to ask themselves before engaging in variable-rate technology.
Jeff Vetsch:
Brad, I agree with what you're saying. The only thing I would add to that is thinking about the fields themselves. There are areas of the fields that we just know have lower potential. And it could be related to a lot of things. The great example is what I see in southeast Minnesota where I grew up, where we have this lust cap soil on top of bedrock. And in some areas of some fields it's really shallow and they just don't have the water holding capacity. Or maybe you have a coarse textured soil that comes in a glacial till environment. Identifying those zones of the field that have poor yield potential is another way that you could think about managing that variability. You're right, there's some areas that maybe aren't meeting their yield potential, but maybe they are. But there are others that just have less yield potential and probably require less fertilizer or maybe just different management of that fertilizer. Maybe it's a change in application timing of nitrogen.
Brad Carlson:
Well, I think the one thing that a lot of farmers know intuitively is that their best soils produce their best yields, that you can't just simply go to a field that historically doesn't yield as well and just pour more fertilizer on it and make it yield as good as your better soils doing. So from that standpoint, we need to acknowledge the fact that our yield maps are finding our better soils, which you always knew were more productive. That's not necessarily a factor, simply pouring on a lot more fertilizer to get those higher yields.
Dan Kaiser:
Well, and that's one of the things in the recommendation side we struggle with. I know that's one... You look at some of the research, what some of these commodity groups want. I mean, one of the things that comes up more often than not is looking at recommendations for some of these medium and low productivity areas. It's not an easy thing really to look at because that's somewhat dependent on the year too, because we can have areas that maybe in a normal year more low productivity, but in a wet year or excessively drier might be slightly higher productivity. So it's one of those things that it's all really when you start looking at a lot of this variable-rate, it's in these fertilization strategy a lot of times the risk mitigation question where it boils down to.
So it's one of the things I think on the recommendation side we struggle with because for instance, nitrogen, we have recommendations for what we call medium productivity soils. And to me, I don't know exactly where those numbers came from because if you look at it, honestly we could go and look at say a non irrigated sand versus an irrigated sand and say, "Okay, well we know that water is going to be the more limiting factor in this particular field that we may not want to put as much fertility to it and we could say the same thing maybe within different zones of the fields." But it's one of those questions that I guess I don't have a clear answer for, so we just have to give recommendations for what we consider to be more normal for our areas and go with that and then just give information to growers how should you make an adjustment based on specific factors.
And this question between just a single rate versus a variable-rate, I ran into that just recently just looking at some of the ground I'm helping manage down in Iowa and with some of my family's farm with lime where we've got a number of zones where you have essentially the same rate and just maybe one or two zones in the field that are recommending the slightly less. So it's kind of just a cost situation with a lot of this stuff just kind of figuring that out. And the variable-rates, since you are likely going to be paying more for your application cost, is it really worth it in the end with what you're trying to manage?
I know really I think one of the keys with a lot of the times with variable-rate for goals for some growers might be to try to get your fields evened out, particularly with your P and K level, is you might get to a point where you may not need to look at it. But certainly the nice thing about it right now is just to know what variability out there, particularly with questions we get with fertilizer prices being high, give them some options if you're looking at cutting some costs in there. And certainly that's really where I think variable-rate shines, is you've got some of those options maybe to look at some reductions in cost.
Brad Carlson:
I think the other area that's worth discussing is, and Jeff alluded to, poor producing soils, particularly shallow soils and areas with coarse textured soils. Now there are definitely environmental concerns in some places and the ability to be more prescriptive with both rates as well as application practices to have less impact on the environment is definitely something that I think we all know is out in our future. I mean, at the moment we don't really have any requirements to be managing in that way, but the variable-rate technologies and the application technologies are going to allow us to be more prescriptive with our inputs in the future.
From that standpoint, there definitely are some farmers who are out there with that mindset that that's what they're trying to accomplish. And so from an environmental standpoint, that's a good thing also. I think for the majority of farmers who are just still going from year to year trying to be profitable, I think part of what we need to be accomplishing from our side is getting some comfort level in this technology and the ability to do this in a prescriptive way and not hurt yield. And so that's a big part of where our efforts have been too over the last decade or so.
Jack Wilcox:
Is there a difference between variable rating, different nutrient types?
Dan Kaiser:
This is a kind of good question because a lot of times when we think of variable-rate, we know that we can use it for multiple nutrients, both N, P and K. But really in terms of differences between those two, it's really how you get to the end goal or the end point of your recommendation. And certainly with P and K, management with a soil test gives us more... Well, I wouldn't say really flexibility. I mean we have much greater breadth of data with P and K in terms of what the number you're getting back from that lab means.
And certainly, nitrogen, we're seeing a lot of interest in variable-rate nitrogen. It's been a bigger question for us on the research end, how do we get to that point where we can assign or assign a value in terms of how much nitrogen to supply, whether it's aerial imaging or soil test or some other factor you're really trying to use. And really a lot of the difference comes back to the nutrients themselves and how they're held in the soil. With P and K, we know a lot of times we consider them to be able to be banked in the soil where they can be stored. It's not a perfect storage solution, the soil is for P and K, although we know that if you over apply, your soil test will increase. But we know that there can be some loss parameters if you get to a certain point as well.
I'm not going to really dig into some of that, but it's one of those things I think really when it comes down to it, producer goals really in mind is where we tend to see a lot of difference. Particularly with P and K, you start looking at a lot of them where I get this era that people consider this advanced management to be this variable-rate based on yield maps and crop removal, which to me that's less advanced than anything else because you're just taking one factor times another. And it's the easiest solution to do it and it's really not the most advanced one out there.
But that's kind of the thing with it. P and K, we know we have that database behind it to tell us or of its risk assessment of what nutrients supply should be from the soil to that given crop and how much should need to be applied as more of a risk assessment tool. So that's I think the difference between them, is just essentially the way the nutrients behave in the soils and how we can utilize the information we have in terms of making variable-rate decision.
Brad Carlson:
One of the ways I like to frame this to farmers or ag professionals is that really variable-rate is just a series of rate decisions as you move across the field. It's no more complicated than determining a single rate for a field except you're doing it multiple times hopefully with different types of information as you move along that change what your decision will be as you go. And so it's important for producers to remember that those decisions need to be made appropriately. We know that we don't store nitrogen in the soil, so we'll use soil testing for P and K and pH decisions. And then to some extent there's been some people have wanted to use yield maps for crop removal. But getting back to what I talked about, that your best soils produce your best yields, that doesn't always correlate with what's happening from extraction of nutrients from the soil.
I think very particularly if you're using a yield map for producing a variable-rate nitrogen prescription, that's really a misuse of the technology. We've gone a long ways as far as discussing and convincing the agriculture as a whole that we don't simply base our nitrogen rate decision based on yield potential. That was an old technology that we went away from 20 years ago. Yet if you're taking a yield map and you're producing a variable-rate nitrogen map just simply based on the yield out in the field, you're actually doing that. You're going backwards 20 years to where we were in making nitrogen rate decisions. And we know that there's a lot of factors that go into what the optimum nitrogen rate is just simply beyond what the total yield is. And so from that standpoint, you also need to be think really critically about what it is that you're doing and how the whole thing works.
Dan Kaiser:
We've had some discussions internally too, that was when I first started here in Minnesota back around 2008, about particularly phosphorus, do we have higher yield potentials from high testing soils? Because that would be kind of more that advanced management approach of trying to push for higher soil tests to push for greater management. And what we've seen is that there's no more. If you take the same soil, a low versus high test, I mean the yield potential's still there if you fertilize accordingly. It just depends on where the plant's extracting its nutrients from, whether it be fertilizer from the soil. So that's kind of one of these things. I mean, again, I get this era that you need to really be pushing more and more and more fertilizer for higher yield potentials. And as Brad said, we look at a lot of the data. I look at particularly nitrogen. We look at tying our optimal rate to our yield and there's absolutely no connection between the two.
So if you look at it in terms of where you have high yield potentials, there's other things that go on, and certainly nutrients are part of that. But it isn't the only factor that impacts your overall yield potential. So that's one of the things I think that's kind of forgotten about because nutrient management fertilizer application is something that can be controlled. I mean we can actually control it, you can apply nutrients. Yet I think the understanding is, and it's what we've tried to do with a lot of our recommendations, is at least to provide some of the information behind what's that probability of response, particularly with P and K.
So if you're applying fertilizer at a given soil test value, what is that percentage chance that that fertilizer is actually increasing yield and on average and what would be my yield or my potential yield loss without fertilizer application? Some of that data for phosphorus is in the corn and soybean [inaudible 00:13:52] right now. So you can look at that and you just kind of look at it, because it's really what you look at variable-rate, it should be more of a risk assessment. It should be putting it where you need it. And where you don't need it, cutting back to save on costs and getting that information out to growers.
I think we've been a good job selling fertilizer with a lot of our research. Maybe if you go back 30, 40 years with Extension, it's one of the things I know my dad always talked about with some of his ground. It was low at one time and he saw the benefit to it, so he just continued to do the same thing over and over again for years. And it's one of those things now we have management options where we can look at being more prescriptive and I think that's one of the advantages particularly variable-rate offers us.
Jeff Vetsch:
To add to what you said, Dan, I think that we know there's nutrients that interact with one another. I mean nitrogen and sulfur is a great example, potassium and nitrogen. But if your sulfur or nitrogen is applied at well above limiting levels or well above the optimum levels, then that interaction goes away. I mean, just applying three times of what the amount of sulfur you need is not going to make your nitrogen more effective. So it's more when they're less than limiting that it's an issue and not when they're above critical values.
Jack Wilcox:
What are the technologies that are used to prescribe variable-rate fertilizer and what is the theory behind them?
Brad Carlson:
Well, we've talked about already the fact that some of the things we variable-rate are just simply soil test-based. And so that's pretty simple and straightforward. If you're looking at a P and K test, if you're looking at applying variable-rate lime based on your pH map, that's fairly simple. We know that sulfur application does not always correlate with soil test value and that may be a little more complicated, although there are soil factors that do play into that. And so using some soil information from that standpoint also may be useful if you've really got a good handle on your field.
It's really nitrogen that's kind of the wild wild west. There's all sorts of things that have been used. There's active and passive sensors that look at the health of the crop in season to make recommendations. We've used crop models extensively to try and predict what the needs are, sort of a math equation. What's out there versus what the demand is, therefore how much else is needed in order to get us to the end in line of optimum yield. And then there's also been the theory for using soil testing for nitrogen primarily in season. All of these technologies have shown some potential, but they've also all got some disadvantages.
And so I think the key is to understand how each of these work. I've had a lot of experience with the crop models in terms of at least looking at using the recommendations on those. And in a lot of cases I've been fairly impressed with those. However, what we've kind of found out is that they're kind of disappearing from the marketplace. They're not being used as much. And some of these seem to be doing a fairly accurate job of accounting for, particularly what the weather has been like and then making some fairly accurate predictions on how much nitrogen is necessary. These are just simply modeling crop growth and potential and taking some soil factors into account.
In a lot of cases long-term, these models were designed also to look at yield history. And so to some extent they're able to account for what I've already talked about a couple times, you're better producing soils, not by necessarily defining what makes them better producing, but just simply defining that they are better producing because of their track record and their history. Long term I still think this has a lot of potential. But in the short term, these products have really started to go backwards in the marketplace.
Some of the other technologies, I know we've all worked extensively with the pre-sidedress nitrate test. That is something that does have some potential as far as looking at the status of nitrogen in season. But there's some drawbacks to that, particularly if we've already applied a base rate of fertilizer. Whether you've accurately accounted for that with a soil test and how that plays into the status in season and therefore how much extra you need, some of the systems that have had manure on them may be a little bit better suited to that technology.
And then in addition to that, on corn situations particularly where there's a lot of residue, we know there can be immobilization in nitrogen and the soil nitrate test only picks up nitrate. So if that nitrogen gets tied up in some biological process, it's not finding it and accurately predicting what's out there. So there's been some downfalls in that. I know Dan and Jeff, you guys could maybe chime in on some of your experience with that. Dan, I know you're continuing to work with the pre-sidedress nitrate test. I know Fabian's also looking at this, and both of you also have experience with sensors.
Dan Kaiser:
I think one of the challenges we look at really with a lot of these applications is timing. And logistics I know does factor a lot into grower's decisions in terms of what they do. And that's one of the challenges I look at, particularly some of this in-season work is, can you get the information you need to help you make the decision then also be able to apply what you need at a given point in time to where it's not going to be too late? Because I know, Jeff, some of the data you've, you and Gyles, Randall collected, well that's been close to 20 years ago. I mean, some of that stuff in the early 2000s, I mean you can kind of see some instances where you really didn't want to go too late especially with some of the in-season applications.
So that's one of the challenges particularly when I look at some of the work we've done with aerial imaging, is first of all effectively see what you're going to find at the end of the season with the nitrogen rate you're applying. So if I take a nitrogen rate response curve, there's a deviation in color correlated positively to what I see in terms of a deviation and yield at the end of the season. And that's been the challenge particularly if we look at the minority of Minnesota, let's say central and southern Minnesota, is that with the amount of nitrate we have in the profile and some of the other factors that we have with the amount of fertilizers being applied, being able to see that, being able to detect the differences that we're going to be reflective of the yield differentials at the end of the season is extremely difficult.
I've seen it in the northwestern part of the state around, if you look at in the Red River Valley, you can get a yellowing earlier on that the crop can show that that seems to be better correlated to yield at the end of the season. But if you start seeing some of that severe yellowing, have you already lost yield? That's been one of the challenges with management in season, is we need something that gives us something like we see with P and K with the soil test where it gives us a risk assessment. And the PSNT, the pre-sidedress nitrate test hasn't been perfect because it's what I call more of a qualitative test. You get to a certain point. We know you're probably okay if you're under that point. You probably need to apply something, but can you quantify how much you need to apply? That becomes very difficult.
What I've been looking at has been the pre-plant nitrate test as a way to look at adjusting my N rate, because what we've seen in a lot of our studies is that with a split versus pre-plant application in several of the studies that took the same amount of nitrogen, whether we split it or put it all on pre-plant. So if I know it's going to take the same amount of nitrogen, if I have something that tells me essentially some sort of adjustment factor, then I could go with a split application, hopefully adjust within field.
The thing is not a perfect system because nitrogen is so complex. And that's one of the things when I look at a lot of these crop models, you struggle with all those components that go into the input and output sides of that nitrogen cycle. You can make a lot of assumptions based on them, but there's always errors associated with that assumptions. And as you add those errors up, it just the predictability becomes less and less. I don't know if that's something of what some of these companies have seen, what have you seen some of these come off the market more recently, but it's been a challenge. If we had that factor, it'd be less for us to do I guess here on the research side.
Jeff Vetsch:
Yeah. Dan, you mentioned the issues with some of these later season samplings. And that gets back to the sensor work that we did back in the early 2000s where we started using the active sensors and we looked at how late can we sense and how late can we detect differences and what's the best calibration of using that data to know that there is going to be a difference that's going to affect yield. One of the challenges that we had to deal with was if we were sensing out there in a corn after bean field and it had even a modest or a very low rate of N to start with and we were sensing at V6 or V7 or V8, which is the time that you could still probably get through the field maybe with conventional equipment, we couldn't see much for differences and we certainly couldn't calibrate them into kind an algorithm that would be effective at predicting the amount of end that would be needed at that point.
So what's the alternative? Well, you go later. You wait later and later and later. But then you have the challenge, as you mentioned Dan, that you can't recover some of that yield. Maybe that fertilizer end if it's supplied at V14 or later doesn't get in the plant or the plant has already been a deficient long enough that it hurts yield. Not only do we see that 15, 20 years ago, but our colleague was just talking at UCIN, who's our nutrient management precision ag person here at the U of M, he was talking about this at a meeting last week that some of their sites this last two years, that they've seen the same thing where they were out there putting applications on that were a little bit later than typical and that they didn't see that response or they couldn't recover that yield and it actually hurt yield a little bit.
So yeah, that's one of the challenges if you're going to use imagery or you're going to use sensors to make these in-season applications and variable-rate applications, is trying to fit those into timing. As you said, Dan, the logistics of doing that. How many acres can you do that on? How many days do you have to do that on? That's all a challenge and that's a big hurdle. And that's not going to go away. That hurdle's always going to be there.
Brad Carlson:
I think one of the other concepts that is worth thinking about is, and we've talked about this already, some of our poor producing soils or challenging soils, actually some of those sites may be much more amenable to predicting the total nitrogen rate. One of the things that we've had to a address in Minnesota is that some of the data has come in from other parts of the country where soils just simply don't have the yield potential we have. They don't have the soil organic matter. There may be other limiting factors.
And in a lot of cases, there is just a linear response to nitrogen based on crop potential that's very predictable, for instance if you're in central Missouri, that we don't see in Minnesota with high organic matter. And then also if you've got adequate drainage, it's really an unknown what the soil's going to supply. But in a lot of cases it's going to supply a lot. And so it is very difficult to know exactly what that is. However, some of our sandy soils, particularly in places like where we're using irrigation and so forth, some of those soils may actually have a lot higher potential to be using some of these technologies than I guess what we would consider our prime agricultural soils in Minnesota.
Jeff Vetsch:
Yeah, I agree with what you're saying, Brad. I was thinking the same thing that maybe you don't try to do every field. Maybe you target fields that have been problematic in the past and have a lot of variability. And maybe you do a combination of practices that may include variable-rate. But really maybe from a nitrogen standpoint, I got a field that's problematic in the past, maybe it's got poor drainage and I plan a sidedress application on that field and then use some kind of imagery or sensor data or something or other to decide do I put on a conventional rate that would meet my total rate, maybe I put on 40 pounds or maybe there's a certain area of that field that looks much worse than others that has yield potential that I could get back if I put on a higher rate. And it may be to make it simpler and not try to make it so complicated.
Dan Kaiser:
Well I think the other thing, Jeff, you could throw in there too is when you look at yield potential in fields is maybe looking at land tenure too. If you've got rented ground, you want to have areas where you want to cut back on. That might be a good spot to look at. Certainly variable-rates going to cost you more, but if you can save on at least fertilizer costs in some areas to cover that cost at least to try to maximize profitability, it might be something you want to do.
But I want to get back to another point here too. When we start looking at technology, I remember we started talking a lot about variable-rate nitrogen. I think the number one thing a lot of growers were talking about, or a lot of consultants were, we're looking at organic matter maps in fields as measuring the mineralization of nitrogen and a potential nitrogen available to the crop. But the more and more I think about that, you look at across the state of Minnesota's, why do we have this variation in organic matter within fields? I mean, certainly we know that erosion does come into play in hilltops, we're likely going to have less organic matter. You may have a more... But really what it boils down to when I look at a lot of these fields, it really boils down to drainage.
So if you look at nitrogen, you look at lower areas of the field, we're likely going to have higher organic matter. Well why is that? It's likely because that organic matter isn't breaking down as quickly because there's water logged and you just can't get the amount of mineralization in those areas and you likely see more denitrification. So I could probably make the argument that some of those areas that are higher organic matter may need more nitrogen than less nitrogen, and just simply you looking at mineralization as a factor of organic matter. While if I do it in the lab where I control my aeration of the soil, I don't have water log soils, I get a nice relationship, it probably doesn't work in the field.
So again, we're looking at a lot of these times, or I'll go back to the comment I had before where we look at this advanced management, where you're really to me oversimplifying a lot of things and it's really not what I would call advanced management when it comes to some of this stuff. I mean really to me, advanced management would be looking at each area of your field and making a decision on an area by area basis using a number of factors, not just some simple factors to try to make predictions on what that crop's going to need.
Jack Wilcox:
If farmers apply fertilizer at a variable-rate, how should they evaluate those outcomes?
Jeff Vetsch:
I think that it's been mentioned before. You got to think about the economics, it's not going to be free. But I think first you also think about what's their goal? So if their goal is, say they acquired a new piece of land and it's got a lot of variability, maybe they did grid sampling and they've identified that the P and K soil tests are all over the board and they're very high in one area and low in the another area, do they want to even that field out? Is that their goal? If that's the case, that might work quite well with variable-rate. It might be an opportunity over a period of a couple application series to make that field more uniform.
But then the other part of it is getting back to what Brad said earlier, is their yield out there that they think they're losing or missing and can they garner or get that yield back? Or is it the opposite? If there's areas of the field that have extraordinarily high soil tests and they could reduce inputs in those areas, those are really the only two ways that they're going to increase profits by doing this variable-rate application. And if you're not going to in increase profit, you're not going to pay for the cost of that method. So it comes down to economics, but it also is kind of their management style. If they see as making that field more uniform and P and K reducing risk and that's their risk avoidance method, maybe they don't care if it costs a little bit more. But it really comes down to either your increasing yields or you're reducing inputs if you're going to have an effect on economics.
Brad Carlson:
I think I guess getting to a technical standpoint for evaluating this stuff, if a farmer wants to really scientifically look at does this pay or am I getting a higher yield and so forth, you're going to just [inaudible 00:30:43] leave some strips behind in the field where you do a single rate across. Your variable-rate application's perfectly capable of applying a single rate on a strip across the field, identify where those strips are, have them randomized in the field to make them representative, and then just come back and do a yield estimate and make sure you incorporate all those costs into there, the cost of what it paid to do the technology whether that's grid soil sampling or whether you purchased some nitrogen recommendation, whether it's a software or other technology imaging, whatever that might be. Take the cost of the technology into effect as well as the cost of making that supplemental fertilizer application versus just what a single rate, what a cost you to put on across the field and then look at your yield.
And in a lot of cases, like we've already talked about, you're maybe not going to see a yield increase. And so then you need to really do some complex economics across those strips and see whether or not there was an economic advantage to using that variable-rate technology. At least I personally am not going to commit to Jeff and Dan, but I've been willing to help farmers with making those evaluations. I've looked at a lot of yield maps over the years and put them into my software and come up with some yield differences between variable-rate and flat rate and help do that analysis. And I still remain willing to do that because at this point the demand has not exceeded the amount of time I have available. That could be different in the future, but for now I'm willing to extend that offer.
The other area that I think also needs to be addressed in this is that we'll hear reports of reductions in fertilizer use based on our variable-rate technology. Sometimes those of us at the university will say, "Yeah, well the guy reduced his rate from some excessive rate down to what we were recommending all along. Is that actually a result when you could have just simply been following our recommendations?" I'm willing to take the W on that. I guess if a farmer was not comfortable for some reason or another and wanted to apply a higher rate and now some technologies made him comfortable with following recommendations and the end result was a reduction in fertilizer application and increased profits, it was real. It happened. In the long run though, I guess if that turns out to be a high percentage of what we're seeing for results in this, I guess maybe we'll need to step back and reevaluate.
Dan Kaiser:
Well, and I look at a lot of the work I do is with P and K. I mean obviously I've done a little bit of work with nitrogen. But overall, I mean the bulk of the work I do is looking at correlating soil test values to crop response. And you get beyond a certain point. I mean just say what we call the critical level, the probability that you're going to get yield increase to that fertilizer you're applying is maybe less than 1%. And so you're looking at essentially being in near maximum yield. So if you look at from the standpoint of variable-rate where I get to these more advanced systems where they're just taking yield map and then multiplying it by a factor in crop removal, I mean you're likely at a point at which essentially that fertilizer giving you absolutely nothing in terms of a yield increase. So it's more of an insurance policy than you're going to be at maximum yield potential on a year by year basis. So that's the thing you got to think about with this.
Again, I know. I mean when you start looking at, the portion of your yield is going to be attributed to your nutrients. But really what the soil test, if you look at breaking your categories down, I mean if you look at that low, very low, medium high, very high, that's really you look at that being the potential or equating to the potential that the soil will be able to supply all the given nutrients that that crop's going to need within a given year. So a very high class, there's a very high probability that you will need no fertilizer for that particular area. So it's one of the things to think about.
And again, you look at on the retailer side, I mean I see a crop removal push pretty hard in some of these areas. Again, I guess I can see it makes sense where we know how much we're taking off roughly applying that removal rate. And a lot of growers will ask me, "Is this economic?" And really I don't really see the economics for fertilization until you get beyond what we deem to be the critical level. So for Bray phosphorus, I mean you look at somewhere around 20 part per million of potassium. We're working on that right now. But if you're in a high clay soil, less than 200 part per million. If you're in kind of a medium, more of a silt loam to sand and you can likely survive on less in the soil because they're different in how they hold the potassium. So it's the thing to really think about is those probabilities. And if you're at such a low probability of response, I mean really looking at applying more fertilizer isn't going to increase yield if you're already near that critical level.
So really in terms of what you're looking at is more of a cost savings approach and trying to put it where it's needed and put it where it's not. I guess recently with the fertilizer prices, I've really been pushing a lot of growers to look at their potash levels or their potassium levels in their fields because I think there's some under fertilization going on there. The thing I hear about that is well it's expensive. And you look at it, there's I don't think really any cheap fertilizer sources really right now. So it's really trying to get what you're going to get the most benefit for in that particular area of the fields and it'll really reduce back on what's going to give you less of a benefit.
So it's really the thing. Looking at it, we've got a lot of information right now on some of these nutrients that we can supply to growers. If you really want to look at some of these numbers, you just have to... I have a lot of confidence, particularly on our phosphorous values, if you're making a decision based on some of that and reducing in some areas that you're likely not going to see much of a yield effect. I think some growers are starting to see that and really starting to work with more and more now that are cutting back on some of the areas, particularly in high testing areas and really avoiding some of those areas, try to put the money or their investment in area that's going to make a more profit on a yearly basis.
Jeff Vetsch:
Yeah. And getting back to that, to what something Brad said, Dan, in that regard is when you look at phosphorus and nitrogen, but we have environmental concerns with those nutrients as well. So if you back off on that phosphorus in that high soil testing area that's very high or well above the critical value, not only are you saving money by not applying the expensive fertilizer there, but you're also probably going to have a minimal or a positive environmental effect as well in that same area.
Jack Wilcox:
Are there any last words from the group?
Brad Carlson:
Well, the one thing I guess I'd like to say is remember that every year needs to be considered on its own merits. And so this really with evaluating anything that you are looking at on the farm, you need to think about what the previous year was like, growing conditions wise and other things that you experienced when you're evaluating the results. And so the same thing certainly applies to variable-rate technology. Like for instance, this last year was extraordinarily dry. If you were managing because you had some of these coarse textured soils in a field, the odds are those areas just burned up because there wasn't a lot of moisture. We ended up with some pretty good yields and most of our heavier textured soils. But those areas were not really a factor of how you manage your fertilizer. It was just simply too dry. Similarly, when we have a really wet year, you'll have some drown out spots where your drainage may not have been adequate and so forth.
I remember a project I was involved in many years ago when we were at the end of the year, we were looking at the yield map and I asked a guy, "Did the yield really go to zero here? What's happening?" He said, "Well my herbicide failed. And yeah, actually the giant ragweed got so tall, the yield really did go to zero in that spot." And to which my response was, "Well could then we really accurately say what happened in any of this field if the weed control was that bad?" And so you do need to use your own senses and filter your own knowledge as far as how you're evaluating this stuff.
Dan Kaiser:
I'm going to put a plug in here for soil testing. I mean, obviously it's what I do, but it's what we see happening right now, is people foregoing that and just going to the easy button route and just doing crop removal.
With potassium, we know right now there's not technically any environmental issues with that. Although over application we have seen some problems. Particularly with potash, it's a topic for another day, just talk about that. But phosphorus too, you get to a certain point where soluble phosphorus can be an issue, whether it's coming from runoff or even through leeching. I mean you get to a certain point. We know that we can see some phosphorus start to move through the tile drains that most cases or in all cases when we start getting those problems are well above what we consider to be the soil test that's agronomically justifiable in terms of where we see above and below or we see a crop response with it.
So it's one of the things, it's just nice to have that information and know where you're at and just foregoing that and just applying removal year after year. I mean one of the things I really want to say that removal does not equal maintenance. What we tend to see with crop removal rates is we tend to see the slow build over time in many of our soils. Particularly you get to that medium and high class that you think that, "Well I just put on what was removed." Well you don't really know how much was removed because you're not measuring the amount in that grain and that number can vary annually. So we look at our data where we've applied exact removal. Again, we see a slow build. So decoupling that maintenance and removal is really one of the things we have to do. And really the best thing to do is just have that soil test information because then at least I know the probability of what you're going to get for a crop response within those given areas.
So again, I mean I'd like to see that more and more. You don't necessarily need to be grid sampling, but just having a starting point from a field is kind of a good thing to do it. At least know where you're at roughly and help you make a decision for this or for future years.
Jeff Vetsch:
Yeah. The other thing I would add too is if you're using precision ag technologies, whether it's imagery or drones, sensors, any of those things and you identify an area of the field that you think has an issue, go out there and validate. You're going to have to still do some infield scouting to make sure that that issue or concern is what you think it is. Is it really a nitrogen deficiency or has it got poor stand or has it got poor weed control? All those things. Some of these tools can be effective at identifying where to look. And then take a soil sample, take a tissue sample of the plant or something like that if it's at the appropriate growth stage and try to do some other diagnostics to help confirm that it is what you think it is and don't just assume it's something and then I'm going to go out there and put more nitrogen on or more something or do something else and correct that problem. Because it may be related to some other issue.
Dan Kaiser:
Yeah, that's a good point, Jeff. Yellow, there's a lot of things that can cause the plant to turn yellow, so you've got to kind of understand that and not just assume it. That's one of the issues with remote sensing right now is you see yellow, but you need a little boots on the ground just to know what that issue might be.
Jack Wilcox:
That about does it here for this episode of the Nutrient Management Podcast. We'd like to thank the Agricultural Fertilizer Research and Education Council, AFREC, for supporting the podcast. Thanks for listening.
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