UMN Extension Nutrient Management Podcast

University of Minnesota Nutrient Management Podcast Episode: “Phosphorus fertilizer guidelines: Four Midwest experts talk profit, yield, soil tests & more"
August 2024

Written transcripts are generated using a combination of speech recognition software and human transcribers, and may contain errors. Please check the corresponding audio before referencing content in print.

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Jack Wilcox
Welcome back to University of Minnesota Extension's Nutrient Management podcast. I'm Jack Wilcox in the communication side of things here at Nutrient Management. Today we're going to talk about state phosphorus guidelines throughout the midwest. What they are now, and what makes them so important for growers. We have four experts on phosphorus guidelines here with us today. Can you each please introduce yourselves?

Dan Kaiser:
This is Daniel Kaiser. I'm with the University of Minnesota Department of Soil, Water and Climate. I've been working on phosphorus guidelines in Minnesota for the better part of the last 17 years. Here, I'm located on the St. Paul Campus.

Antonio Mallarino:
Sure. I am Antonio Mallarino. I am at Iowa State, and I have been here for about 35 years with research and extension department and focusing essentially in more things about fertility except nitrogen.

John Jones:
I'm John Jones. I'm a soil fertility extension specialist with the University of Illinois Urbana-Champaign. I've worked on phosphorus research related to agronomic crop production in Wisconsin for the last three years and now have recently moved to Illinois and will be doing very similar work in the State of Illinois as well.

Dorivar Ruiz Diaz:
I'm Dorival Ruiz Diaz, Soil Fertility Specialist at Kansas State University, and I've also been working on fertilizer recommendations specifically including phosphorus for the last 16 or 17 years here in Kansas.

Jack Wilcox:
Could everyone describe their current state guidelines and perhaps some history for context? Dan, why don't we begin with you In Minnesota here?

Dan Kaiser:
Minnesota is interesting just because we're kind of at a crossroads here. If you look at our soils, we've got the western part of the state. A lot of the stuff we've got soils similar to the Great Plains versus you go to the east and the south where we're more similar to the central corn belt. And the guidelines though, if you look at Minnesota, have been more conservative than many of the other states around us, and the focus has been on what we call the sufficiency approach, which essentially is only applying the fertilizer that's needed to maintain maximum yield. And there's been a lot of criticism on that just mainly because of many growers looking at focusing on something more that we'd see at some of the other states, more of a maintenance style strategy. So it's one of the things that in starting at Minnesota, I've been working towards at least providing some grower flexibility.

And we have changed some of our guidelines as we've gone along. And I think a lot of the really emphasis is you start getting closer to the Iowa border, we've got a fair number of consultants or growers that are working across the border. So you're looking at both sets of recommendations. And I think that's the emphasis really of this podcast is talking a little bit more about this because we see these state borders, we see soils that extend across state lines, and there's always a lot of questions on that.

And if you look at the guidelines, the main challenge for some of us has been trying to figure out where some of those guidelines have come from. For me being that my predecessor had a pretty strong relationship in Nebraska, it's where we brought a lot of the Western, the sufficiency approach into Minnesota with the recommendations. So that's where we're sitting now, although I've been working with a number of things here in the state just to look at gathering more trial information and looking at regionally just more interest in looking at some of what we are doing potentially regionally for comparing some of our guidelines across state lines.

Antonio Mallarino:
Since about 2002, we have interpretations for the Bray one, for the Olsen, for the Mehlich-3 colorimetric, and for the Mehlich-3 ICP. Those are the interpretations that we have. And the Mehlich-3 are the big higher numbers.

So we essentially tell the people very clearly that they need to input that, not the yield goal, the prevailing yield in order to estimate the removal, but, of course, we have some defaults. But every five, six, seven years we update those defaults because some labs tell us that many farmers, they don't put the yield, so they need that to put in there. Then we have the high that we define as having about 5% probability response or less. We recommend only starting phosphorus in some condition, and then we have very high that we recommend nothing.
So this is the key. So essentially we say, well, in the very low there is a list of about 80% probability response for the low about 55, the optimum about 25, five for the high and essentially less than one or less for the very high.

John Jones:
The history behind Wisconsin phosphorus recommendations for both grain crops and forage crops is not too dissimilar to most other states where a rich history of field research had been done to support those predictions of a crop response to key fertilization. And that was indicated by soil test phosphorus. The current status of recommendations within Wisconsin, there's really two delineations that make up the interpretation of a soil test, and that is the soil group or the specific type of soil that the farmer will be working and sending a soil sample in. Those are usually broken down into loamy and then sandy and organic pairings essentially. So really two categories, but then also there's crop demand levels, and those demand levels are really based off of the amount of removal that would come off of that specific field with the harvested portion from the crop.

Dorivar Ruiz Diaz:
In the case of Kansas, for us thinking a little bit or general guidelines and what is the history behind that, the guideline for us is really have two different types of recommendations, if you will. One is a strictly sufficiency approach, and we do have an alternative, which is a build and maintain. So essentially that provides flexibility for the producer to basically use either one. And so, again, in some ways, it may be similar to what Dan mentioned earlier, the context of why we have these is because in Kansas, if you think about the gradient going from east to west, the eastern side of the state with a little bit more like the corn belt, but very quickly you get central and west is really the Great Plains where limiting factor tends to be more precipitation. And that's really our main limitation. So a lot of our farmers, especially dry land farmers out west, they typically like to build that soil test a little bit more and at least take care of that fertility.

And then hopefully when you get that rain, they're going to have good yields. And so they don't want to have that as a limiting factor. So I would say that's a little bit of the context of why we move to this approach of providing both options to farmers. There are also obviously the issues of tight budgets, costs of fertilizer and grain prices like again today where unfortunately the grain prices are going down and starting to get some of that question. And in that situation, a sufficiency approach is usually a better option for that farmer where we're really not trying to build up or putting additional phosphorus fertilizer, we really just want to maximize or optimize yield for that particular crop. And, again, our recommendations provide that flexibility. And so that's a little bit of our approach. I would say both use quite a bit, and it really depends on where you are in the state and the philosophy that each farmer has, and that's a little bit of the context.

In terms of what is the soil test that we use, historically it has been Bray, but over the years and with more data, of course, we find that Bray has some limitations, especially in our high pH soils. And so I would say we move quite a bit more to Mehlich-3. In recent years, that's been really more of the default that we tried to use. And Olsen as well, which is one that's very commonly used in the western part of the state. With the use of ICP as well, we are working to develop interpretations similar to what Iowa has essentially provide the flexibility for the producer and having all the different soil test methods; Mehlich-3 colorimetric, ICP, as well as Olsen and Bray for the farmer to use those different interpretations. So that's the direction we are moving in terms of the soil test methods that we have.

Jack Wilcox:
While we're discussing regional usage, are today's state guidelines satisfactory?

Dan Kaiser:
Well, one of the questions I think is something, I mean you look at all of us talking about the recommendations, you look at the differences across state lines in terms of interpretation, it becomes a little bit confusing, and I think Antonio hit the nail on the head with some of what he was talking about is being more straightforward in terms of what we mean with our guidelines. I think all of our states have five ranges that we use for separating out the different sufficiency levels or the ability of the soil to supply phosphorus. But the interpretations and some of the history behind those interpretations differ from state to state, and it does become an issue when you start looking at across or larger areas. Combining data sets, it becomes a problem because for one, looking at Minnesota, we recommend the Bray and the Olsen like many of the other states, but a lot of labs are starting to use the Mehlich-3 tests.

And with phosphorus, the thing to remember is we can't use the interpretations from one test to the other always essentially looking at different interpretations for different tests. And that becomes a problem with if you're looking at more of a regional type approach to recommendations. So then it comes to us in terms of looking at what are some of the fundamental things we can look at for our soil tests that gives us some of the key information that we might need for making decisions to look at comparing things from state to state. I think that a couple of really key pieces now, and it's one thing that we've been providing more, has been the probability and magnitude of response for the given soil test class. It can give an idea to growers on what they might be looking at for a return on investment, but it also, I think, helps if we start looking at comparing across states because that information is a bit more, I think, comparable versus some of the interpretations.

I think that's what a lot of people want. In terms of looking at the data, they want the ability to make the decision themselves but have some of the key pieces of information out there to help them make those decisions. I mean, I hope they're taking soil tests. That's something that we see some growers go to more maintenance style approach, but then forget about soil testing, which that is hard when you get the questions. When we start seeing these imbalances with high fertilizer prices and low commodity prices, is those questions on how do I make ends meet in these situations? And really soil test is kind of the key thing. So that's a good starting point. And then just looking at what information is out there to help you interpret things to make the best decision economically.

I was at a meeting recently, and there were two growers there, and it's interesting, you had an old grower or an old farmer that'd been farming for a number of years that says inputs makes yield, so you wouldn't cut back on any inputs. Then you got the young guys sitting there saying the commodity prices are bad, and I can't really afford anything, so how do I make this work? And that's the thing is really I think the growers when it comes to their goals it's really key when it comes to providing information to them and just making sure we have the information that can answer some of their questions.

Jack Wilcox:
On the subject of profits, yields, and cash flow. When finances are tight, do farmers forgo taking a soil test? Is that the first thing on the chopping block? Or do farmers prioritize soil tests even though financially lean situations?

Dan Kaiser:
I could throw that out to the group, but the thing about soil testing, if you look at price changes, I mean, yeah, it tends to increase over time, but it isn't as volatile as some of the commodity and the fertilizer prices. So to me, it makes sense. Something where even if you're not taking some very intense grids to at least have some information to go by is I think really important on that end. So I can let that up to the group too if anybody else wants to touch on that one.

Dorivar Ruiz Diaz:
Yes, I completely agree with that. And this is something I try to emphasize, and I hear this often. Some people say budget's pretty tight, so I'm not going to soil sample. Well, that's the wrong place to cut on expenses. The value of having that soil test information is much more when you have tight budgets because you may have opportunities to save money on fertilizer when in other cases maybe you have opportunities to capture a little bit of yield and make profit. So this is very important and I think something definitely to emphasize with the conditions we have this year.

Antonio Mallarino:
Yeah, I agree with what Dan and Dorivar say. We have about a similar situation in Iowa. There are many, a large area of manure soils with phosphorus is sky-high. It's not so high as in some states of the east that they have these problems, but it is very high. So, of course, all of us have our different philosophies for approaching extension and outreach. But I have been trying for the last 20 years to show in every single of my articles or talks the economic returns from fertilization. So they see that if you have low testing soils, you have to be sure that you have the right amount of phosphorus there. Don't reduce, don't cut. Even people that are renting, as long as they're sure that we have that field one more year, they should apply, don't cut. You will get your money back. There is a very high probability of economic response.

Now, you are in the high or very high, as long as you have a good soil test plan or soil test sampling approach that you can trust it, you should not apply one atom of phosphorus in very high testing soils other than maybe started in some conditions. So this is our current struggle and actually has been for many years. We have not been doing surveys random, but the last kind of random survey that we did about 12 years ago over 33 counties was about two-thirds of the Iowa fields test optimal high or very high. We have some spots, of course, in every field. That's why I am this maniac in this north central region that I encourage grid sampling and variable rate application. We have some spots in southern Iowa where we do have some soils that are low, and they just don't apply. But the main issue is the people need to understand that they're not making much money as they could in those high testing soils.

John Jones:
Well, I think a lot of the contemporary issues that were brought up are ubiquitous in Wisconsin as well. As Antonio said, manure is the monkey on everyone's back in terms of needing to make sure that we don't excessively build up P. I know some of the more recent research I've done in the last few years in Wisconsin related to the state guidelines has confirmed that the philosophy works pretty well. Adjustments were needed to be made to interpretation ranges, but there are some big cropping system changes that occur around the state that I think are changing sometimes the way we have to think about some of interpretations and really just the boots on the ground agronomics of it. Our crop rotations are getting shorter. There's much less crop rotations in Wisconsin, for example, with alfalfa in the rotations. The dairy sector is getting more aggregated and less spatially distributed across the state.

And so that means there's fewer farms receiving manure, but certainly the same amount of cows in the state. And so there's a manure distribution challenge that's always going to occur there, and it's something that we pick up in some of the county level soil test summaries around the state as well. So those are really important things that I think have come up that are related to the recommendations and challenges we face. I think another challenge is simply just trying to have these goals and either reducing high soil test phosphorus fields to meet water quality standards or water quality goals as well. That's something that's been really challenging because honestly with the variability in soils and crop removal, the rate at which we can draw down some of these high testing soils is very difficult to predict. And so I know that's something that there's ongoing regional work on.

That's a big challenge. And then also P is really ubiquitous in fertilizer plans around the state, whether it's starter, fall applied MAP and DAP. There's P usually in a fertilization or fertilizer program that we have to account for. And there's just a lot of different combinations of when it's applied, what the application methods are that we have to account for, not just for the agronomic but for the water quality purposes as well. And so those are some challenges that maybe are a little more Wisconsin focused, but relate really well to what Dan, Antonio and Dorivar just summarized.

Antonio Mallarino:
We don't have anything against animal producers. We understand that animal production is extremely important for the economy of our states, and we understand that it's not easy to distribute manure, so it really is applied where the phosphorus is low. So we put ourselves in the boots of the farmers of the animal producer, and that's why all of us, the four of us, for a long time have been working in phosphorus and water quality issues. We have been working with the state agencies. We have been working with national groups, all of us, for example, the SERA17 Group, which is a phosphorus environmental. There's Anifa, so this committee. I just want to make clear that we do understand that farmers sometimes they cannot just take the manure a hundred miles, but that's why the four of us had been really working, trying to help develop phosphorus index, for example, so they could use that.

If the risk of phosphorus loss is not too high, then they can still apply manure for the nitrogen value because we need to use the nitrogen in manure. So I just want to say these things to clarify. And actually when EPA, as I said, I am the oldest folk here, when in the early nineties, EPA wanted to put a threshold where soils tests be where farmers could not apply more manure, which everybody, all the soil fertility people in the United States were against because each state should have a hundred thresholds. Because depending on the soil properties, the climate, the rainfall, the production system, all the things, then we have what is the maximum that's sustainable from the point of view of water quality is different.

So I just want to say that the way we were talking about high-testing soils in phosphorus is that some of those farmers that they have high-testing soils and they keep applying fertilizer. That's when that could benefit everybody. They could benefit their profits, they could benefit water quality, and that's why continuing research in soil test calibration, fertilizer recommendations, formal applications, we have not talked anything about formal application, methods of application, things like that. So all those things are important in my view.

Jack Wilcox:
So we're talking about phosphorus guidelines almost at a very granular level, granular by soil type, granular by location. Having said all of that, is it building up to where state phosphorus guidelines might need to be revised in the future? Or what would that process be? Or would you be surprised if they needed adjustment?

Dan Kaiser:
Well, I think there's a big push now for some regional work or even national work. This fertilizer, FRST, Fertilizer Response Support Tool some people may have heard of that's looking at a national database to try to look at curating some response data that can be queried by individuals if they're looking at more regional response trials. I mean, there are some issues specifically related to what I mentioned before with the soil tests being different, extracting different amounts of phosphorus. For something like that to work, we'd all have to have some commonality in terms of what we do. So the same sampling depth, this is the same again, methodology for soil tests. So that's I think one of the major challenges. I think in the north central region, we're pretty close. I mean, the only ones save I think is Wisconsin's potassium, although I think they're moving maybe from the Bray, I don't know.

But I think that's one of the main challenges. And with AI and with some of the modeling we can do right now, I mean, certainly we can handle larger data sets. The thing though I want people to know is that you're not going to find data that's in every single county across every single state. I think that's the challenge because when it comes to looking at something more regionalized or something where we need large data sets, you need a lot of data to do that. And with phosphorus, that's one of the things I do see, at least for us in Minnesota, is that if you look at things like the probability that you're going to get a response at a given soil test, it doesn't necessarily change all that much given some of the differences in soil properties. So these are the things that I think need to be looked at in the future on a larger scale just to see, because I think there's a lot we can do by consolidating some of the data in the region.

I think that's generally what we're looking for, but we're still looking at, I think, in most cases states having their own recommendations. But, I mean, at least being able to be more upfront with growers in terms of terminology and things that we commonly use in some of our recommendations, I think it's really going to be key on that in terms of interpreting some of this. So I think that's really the thing moving forward is we're looking at more regional work, which I think is good, more focus on phosphorus because that hasn't always necessarily been the case, and that's why the four of us are on here because we're the four in the region that have a lot of work on phosphorus, but isn't the case across some of the states. And so that's the main thing is we just need to keep pushing forward, getting some trials in place, but also not forgetting the fact that there's a lot of data out there that could be potentially used.

So there's a fair amount we can do, and I think there's some things that I think people are going to see moving forward, but I just don't think that we're going to have a response trial in every county, in every state. It just doesn't happen. I mean, you're looking at drawing data. You're going to need to draw data from a wider scale still, even if you're looking at some more localized recommendations with some of the things that are in place right now, particularly with something like the first tool that is just its infancy in terms of release.

Dorivar Ruiz Diaz:
Just to follow up on that, and I agree, I think there are great opportunities for more regional approach, but I think the key here is really looking at regional and look at common soil types and conditions and crops, but also without ignoring the difference that we have in soil types and soil test methods that we need to use. I think those are the key aspects. Again, we mentioned earlier, for example, the Bray is sometimes very common in Illinois and Indiana and other places. It works well, but we may have some issues as we go west in higher pH soils and that type of aspects that we need to keep in mind. So there's still going to be ideally, in my opinion, some level of specificity for what works best in that specific soil type and condition. So that's something that we need to keep in mind.

Obviously, leveraging more data that we have out there, also, that's an opportunity. For us in Kansas on the question of the guidelines today, I feel like we do have also some challenges. Specifically one of them is looking at some of the soil test methods. We have commercial labs that are using soil test methods that maybe has not been necessarily calibrated with data in the field, and that's something that we need to be looking at. I think there are potentially some methods that maybe we need to develop more data, and that's okay. That's one thing that we are always trying to look and pay attention to that working with our colleagues in commercial labs. I like something that Dan mentioned earlier in terms of looking a little bit more in the context of probability and magnitude of response, which is something that we need to do more of that as well in our case and tie that to the economics.

We've done a very good job in nitrogen to do the economic optimum nitrogen rate and so on but very little for phosphorus. Except for Antonio, he's done a lot of that aspect. But, again, this is something that we need to continue to do that in our case. In the case of people using the build and maintain approach, for example, a few years ago with a high price of fertilizer, they were able to cut back on phosphorus fertilizer and use some of that extra phosphorus that's there, which is something that's also an opportunity for those using manure for example, right? There's an opportunity there, and it goes back to what we discussed earlier, using soil tests. Fertilizer prices are high or grain prices are low. Hey, we can cut back on duration, and that's okay as long as we have that information. And so those are some of the key aspects that for us, I think, it's still important to continue to develop and definitely some challenges there and great opportunities as well, especially working with colleagues in the region.

Antonio Mallarino:
Dorivar, those are key issues that we've done too. Now, the other thing that we need to understand and farmers and consultants and scientists needs to understand that soil fertility and soil test calibration and soil chemistry, those may be exact sciences, but nutrient management at a practical level is not. There is much of economic concepts, risk, risk management, things like that. So we can all agree in coming with an equation how to calculate the critical level whatever for the brain, blah, blah, blah, and really that's not very important. We can show that. We have been showing that for 50 years. Farmer consultants companies know that. The key issue is, what do we do? So how we can provide interpretations and suggestions and guidelines so that the consultants and the farmers can say, "Okay, that's great, but this is my philosophy. This is what I would like to do." And I remember when in Kansas, Dale Laken many years ago, he was the one that first introduced this dual recommendations approach, the strict sufficiency level and the build up and maintenance.

I defended it. You have no idea. Everybody wanted to shot us, our colleagues in soil fertility. I wanted to do the same thing in Iowa, and I could not convince my peers. No, what you are doing is to essentially let the farmer do whatever they want. Well, come on. They will do whatever they want anyway unless there is regulation that all of us oppose as much as possible. But I believe then that in addition to all these things that we talked about, and I agree a hundred percent. A couple of years ago, we started a project with Dan and Doriver to really build on that, but also include this idea, how do we explain what our recommendations are, how people can interpret in different ways, and to show that they are known for us? When my friend Paul Fickson started talking about the 4Rs years ago, I said, "Come on Paul, there are no 4Rs. There are a hundred Rs because that depends on the situation, the farmer and the philosophy of management and the attitude toward risk."

So I believe that that is something that we can do in our interpretation of recommendation if we could. It's not easy. If we could somehow make clear, listen, this is our philosophy for the recommendation, but look at these points, one, two, three, four and then you can adapt this to your particular philosophy and concerning risk of investment of fertilizer or manure application. Now, this is very difficult, but I think that we need to start talking about it, and we are trying. Actually, Dan and others, we have started inviting each other to our stage to speak, and so we need to start talking about things. But it's not easy. It's not easy.

Jack Wilcox:
Is there a piece of guidance or advice pertaining to phosphorus guidelines in your state that you might want to offer growers?

Dan Kaiser:
Well, I think one of the things I just go with is the fact that if we look at phosphorus, I mean, even with the differences, we have a across state lines, it still, when it comes to soil testing, to me, one of the easiest ones to assess with the soil test versus potassium. So that's one of the things. Looking at costs, to me, it's one of the ones, if you have the soil test value, you can pretty easily make a decision on whether you need it or not. So that's one of the things I've been really preaching to a lot of growers as they start talking about economics is that phosphorus data is pretty solid compared to some of the other nutrients we're looking at.

Antonio Mallarino:
Yeah, I agree. Now, one thing that I believe has been useful in our state until now at least, is that we had been in very good communication between the university, the researchers. They applied fertility researchers an extension with the state agencies in charge of nutrient management like NRCS or DNR. And even companies, and, for example, the Agribusiness Association of Iowa, even though we have our differences, there have been talks about it. And all of this group had been participants of what we call the Nutrient Reduction Strategy. The name should be Nutrient Export Reduction Strategy. So we have started working with that about a few years ago, and I believe that that's very positive because I believe that we are going to get not just the farmers, but the consultants, the companies, the agribusiness and the public to understand the complex issues around nutrient management. You see, all these problems. We need to understand each other. We need to understand these different views so that way we can collaborate, and we can get to something good for everybody.

John Jones:
Well, I think with one of the biggest challenges in Wisconsin is working with soil variability across given fields. And working in the state, there's no better place for intensive soil sampling and zone or grain management to work because of the changes in even soil order, so parent material and soil texture within one field. And so really trying to capture that variability generally has benefits, not just agronomically and for nutrient loss, but also for your pocketbook. I think that's really important. And, finally, when a lot of the guidelines around manure application are based on how fast you can reduce high testing soils, one of the important things is to make sure that you're optimizing, whether it's corn silage, corn, soybean grain or alfalfa, or even an alternative forage, maximizing and optimizing those yields to remove phosphorus even faster from high testing soils. And so you can have just as effective, essentially freedom to operate, so to speak.

We've seen interactions between potassium nutrition and phosphorus removal and corn silage, and that if you don't have your potassium nutrition, at least at a sufficient level, your ability to not just your silage yield, but your ability to remove phosphorus and potentially draw down your soil test level will be reduced. And so it's a complex system where a lot of these nutrients interact with each other. And so I think that's really important is to not just focus on that because at the end of the day, most likely your fertilizer program has multiple nutrients in it. You're looking at your soil test data where you're trying to capture what's going on across your whole field, look at the whole picture, and then probably your phosphorus management might be a little more simple.

Dorivar Ruiz Diaz:
Yes, in some ways I agree with what others already say. I really want to emphasize in cases like Kansas, for example, we do have much diverse cropping systems, and so keep that in mind how the different crops differ in terms of phosphorus response and needs. We need to be specific to that. I think there are great opportunities to improve, not just agronomics, but also water quality as we think more about placement, and can we do a better job there? Oftentimes, a lot of the benefits tend to be more on the water quality, which it is something that we need to keep an eye on.

And the other thing too that John just mentioned, I think, is something that we do see in Kansas quite a bit is, what else is a limiting factor? I mean, we talk about phosphorus today, but what about pH? What about other nutrients and making sure we maximize the use of that phosphorus we apply? That's something that we tend to see. Oftentimes, we think a lot about some of the nutrients, but what about some of the basics such as pH, for example? So things to think about.

Jack Wilcox:
Before we go, if you have a question or comment for one of our guests or a topic you'd like addressed in a future episode, please email us at nutmgmt@umn.edu. Thank you. We look forward to hearing from you.

That about does it for this episode of the Nutrient Management Podcast. We'd like to thank the Agricultural Fertilizer Research and Education Council or AFREC for supporting the podcast. Thanks for listening.

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