University of Minnesota Nutrient Management Podcast Episode “Potassium management”
May 2021
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)
Paul McDivitt:
Welcome back to University of Minnesota Extension's Nutrient Management Podcast. I'm your host, Paul McDivitt, communications specialist here at U of M Extension. Today, on the podcast, we're talking about potassium. We have three members of Extension's nutrient management team. Can you each give us a quick introduction?
Dan Kaiser:
This is Daniel Kaiser. I'm a nutrient management specialist with the U of M with the Department of Soil, Water and Climate and University of Minnesota Extension. One of the key areas of focus I've been doing over the last few years has been on potassium management related to revisions of the fertilizer guidelines.
Jeff Vetsch:
This is Jeff Vetsch, and I'm a nutrient management researcher here at the Southern Research and Outreach Center in Waseca. I'm generally a nitrogen guy, but since Dan's been here, we've been working together on potassium on several different projects across the state, and me focusing on the southern half of the state.
Leanna Leverich:
Hi, everyone. I'm Leanna Leverich, and I'm a PhD candidate in the Department of Soil, Water and Climate. I work with Dan on potassium studies, particularly working on optimizing those potassium recommendations.
Paul McDivitt:
Great. Starting off, can you each give us a quick update on the research you've been working on related to potassium management?
Jeff Vetsch:
Yeah, Paul. So back in 2012, with funding from the Minnesota Corn Research & Promotion Council, I started three sites where we did some long-term studies and we designed it after the Alps or the phosphorus project, where we created larger plots that were fixed and developed a range in soil test with the idea of coming back into those plots at a later date and putting a different rates of P or of K on so that we could look at the correlation calibration side. It was complimenting some of the work that Dan and I had been doing, or Dan had been doing on farmer's fields, where we were just doing side-by-side comparisons, looking at the critical value or where did we see response starting.
Jeff Vetsch:
We picked three sites, one at Waseca, and that was a site that we had been mining potassium out of for several years. We hadn't applied K there in that site. We had a site at Rochester on a silt loam or a loess soil, which was different than the parent material at Waseca. And then we picked one site at Becker. Of course, the coarse-textured soils are very different. It's been interesting because we've identified the differences, especially coming from the coarse-textured soil. I'll probably let Dan expand on that a little bit. And then in 2015 or '16, AFREC funded that project, and we continued with the correlation side looking at different rates. And then the last couple of years, we've been looking at comparisons of band versus broadcast.
Leanna Leverich:
So a little bit about my research and what I'm working on with Dan is looking at the different clay mineralogy types across the state, as we'll get into a little bit more, the basic or underlying mineralogy in fields tends to really affect how potassium reacts and how it's available and how it flows in each soil. So in addition to looking at mineralogy, we're also doing some studies looking at CEC [Cation Exchange Capacity] and pH and how do we consider those chemical properties for optimizing potassium recommendations. And then the last piece of our work is looking at weathering and as we see different cycles, freeze–thaw cycles in soils, does it affect how potassium is being released? So that's the major areas that we're looking at to try and nail down potassium recommendations for different areas of the state based on those physical and chemical properties.
Dan Kaiser:
So, as Jeff said, I mean, I've had studies here in Minnesota, I think, dating back to around 2010 with potassium. The first studies we were looking at were side-by-side yes/no strips in farmer fields, where we essentially had none versus a high rate, which was either 150 or 250 units K2O. The idea of that, as Jeff said, was to look at the critical level. We're looking at essentially the point at which in fields for corn and soybean do we not see a response to K a soil tests. We do not see response to K beyond that. A couple other things I was working at that point in time, we were starting to look at the moist K test because I came out of Iowa. That's where I did my master's, my PhD and that was the group I was working in, that was a big thing that we were looking at. We were looking at some different soil test options. So that's where I started that.
Dan Kaiser:
And then I had a secondary study where we looked at... Actually, it was over 10 years. It started in 2009 and went to, I believe, 2018, where we're looking at timing of potassium in a corn-soybean rotation. So we had blocks within fields of both corn and beans, either all the K applied ahead of the corn or all ahead of the beans, or a split, where two-thirds was applied ahead of the corn or a third ahead of the beans. So that's where I started. Following that, then working with Jeff, we were working with a number of things, as Jeff mentioned, with looking at that sufficiency versus build approach. That was those blocks, similar to the long-term phosphorus trials. One thing I started to see in some of our trials were some yield reductions and particularly in the soybean year. So then I started looking at some of the data that was out there, looking at chloride.
Dan Kaiser:
My latest work really has been looking at obviously the stuff that Leanna was talking about, but then also looking at different sources of potassium and then looking at management options to try to limit some issues that we've been seeing with chloride. Now, I'm going to talk about, I think, this is a little bit later. I don't want to hit this all right now, but we're seeing just some small yield reductions in beans. I mean, they have a lot of times weren't significantly different if you looked at the with and without, but one, maybe one and a half bushels and they're consistently there. That was one of the things that through funding through the Minnesota Soybean Research & Promotion Council was able to start looking at more closely and looking at the contribution of chloride to some of those yield reductions and looking at timing of application and within the rotation to try to manage some of that.
Dan Kaiser:
So that's been the big thing because the package and what I've really been trying to do is finetune our guidelines, looking at providing some better guidelines that are more soil-specific, because that's one thing that you see about Minnesota is that we have a wide diversity in soils across the state. And management, looking at a lot of our data, it's been interesting that there's some situations where we thought we didn't need K where we needed it and there are some situations where we thought we did and we didn't. So we were trying to really hone in on that a little bit more moving forward and looking at soil-specific factors to get a better handle on what's going on.
Paul McDivitt:
What key factors impact potassium availability in soils?
Leanna Leverich:
Well, potassium is mainly mediated. It's availability is mediated by what's in the mineral fraction of the soil. So apart from fertilizer, our potassium balance on the soil is derived from what we have there mineralogy-wise. Different types of clay directly impact this. We'll talk a little bit about three major types, kaolinite, mica and smectite. This is something that we're looking to refine in our work. But basically, kaolinite doesn't have a huge effect. It's not really interacting all that much apart from K binding on the outside of it. Micas, they can create this space where we have fixed potassium there. So, there's two layers of clay and the potassium can get caught up in those layers. And then smectite also has two layers, but it allows for the potassium to move in and out of that. And so, as we work towards mapping our clay mineralogy, we can keep track with those three different types of clay.
Leanna Leverich:
So, mineral matters when we talk about potassium availability. To go right along with that is CEC. CEC is influenced by those mineral types. When we think about CEC, we can also think about your dominant type of soil, whether you have sandy soils or clay-dominated soils. So where you have sandy soils, you might have a lower CEC, which may mean that you have a lower ability to hold on to K. So, those are our major areas, CEC, the clay type, and then also pH. So when you have changes in pH, you can also influence the different binding sites that are available. So we call that a pH-dependent charge. Different pH levels can also affect when potassium might be available.
Dan Kaiser:
So, you can visualize clay as a series of plates, so if you had a bunch of stacked dinner plates on top of each other in the soil. Depending on the clay species, when those clays hydrate, we see the spaces between those individual clay particles expand, particularly with smectites. And what that does, if you look at those plates is that there's pockets. They aren't necessarily flat. There's individual pockets inside of those that the size are about perfect for a hydrated potassium ion to fit in it. So that's when we talk about fixation, what happens as those are hydrated and those layers are apart, you'll have potassium that will move in and out of those layers. As they dry, they can collapse. With smectite, that's where we get a fixation. Fixation isn't necessarily indicative of that potassium is gone. It's just temporarily made unavailable until those clay species will hydrate again and that potassium can be exchanged without it.
Dan Kaiser:
In Minnesota, we tend to have our soils are what we would call mixed. They contain smectite, which is that hydrating clay. It also contains illite, which is a non-expanding clay. It has lower cation exchange capacity, and it tends to have potassium in between the clay layer, those layers that are held tightly. That's in actual, it's a weatherable source of potassium. Where I see a lot of illite in the state are soils in the Southeast, so the silt loam soils, and those are soils that we don't always see a strong yield response to from potassium, which could be explained potentially by some of that illite or that mica weathering over time. So that's been the important thing looking at this, is when we start looking at clay species is trying to get a handle on how this might be affecting our potassium availability. So that's a lot of what Leanna is really trying to do.
Dan Kaiser:
With the sands, since they don't have a lot of clay, the organic matter provides the most of the cation exchange capacity, and that's what Leanna was talking about as a pH-dependent charge. So, that was one of the things that I wanted to further look at and see, because in some of the stuff that, Jeff, what we've been seeing with some of our responses, particularly some of that build and sufficiency work at Becker is that we didn't always see a strong response to potassium. We saw very low K soil tests, but seeing a very low rate of potassium being more sufficient for a given crop, and that was the surprising thing. Then we started looking at, "Well, maybe we need to be looking at a different set of recommendations based on what soils we have."
Paul McDivitt:
Currently, Minnesota suggests banding K is more efficient than broadcasts application of K. What are your thoughts on the band versus broadcast debate?
Jeff Vetsch:
Yeah. So that data probably goes back to the John Lamb, George Rehm days, and also our neighbors to the South that Dan mentioned. He worked with Antonio Mallarino's group and they published some papers back in the late '90s, looking at band versus broadcast applications. We generally not all the time, but showed a consistent advantage for band applications of potassium in or near the row or below the row. It works really well in systems like strip-till or no-till if they could be placed in a starter band or they could be placed in a deep band. The effectiveness, I think, comes from just the fact that we think about how K is taken up more as long term, kind of like nitrogen as biomass accumulates in the corn crop and that's where that advantage comes from.
Jeff Vetsch:
So the study that we initiated back in 2019, we took a couple of our sites that we had a long-term studies at the Waseca and the Rochester site. We had a wide range in soil tests K in these medium size plots. So we split them and we did half with band and half with broadcast, and 2020 was the first growing season of doing that. We saw some mixed results. What we found at Rochester is we got a significant yield advantage to band over broadcast across a range of soil tests Ks and across a couple of different rates of application. At Waseca, we had a small trend towards a positive, but it was not statistically significant. We're going to do this study again in 2021 and hopefully in 2022, to verify these recommendations. Interestingly enough, our current recommendations do not have any change in band versus broadcast or adjustments for soybeans. It's only for corn.
Dan Kaiser:
This is a big debate, I know my former advisor down in Iowa. I know he always gives me grief because the equipment manufacturers really love our recommendations here in Minnesota because of the band efficiency. There's been a lot of questions on that because a lot of it hasn't really been able to be duplicated. I mean, I think a lot of the work, Jeff mentioned Doctors George Rehm and John Lamb, I think a lot of the band recommendations we have actually come from work that George did in Nebraska before he started here in Minnesota. So, replicating it is really been the key in trying to figure out whether or not there's some actual truth to some this. We don't see a wide range in grower's banding. I mean, I know there's been pockets of growers based around certain areas that have more consistent with banding. I think it's going to be more common in situations like strip-till.
Dan Kaiser:
And then historically, ridge-till I think was the main thing, and that was, I think, the big thing that some of that Mallarino data, they were looking at ridge-till versus chisel plow tillage or no-till to look at differences between those systems in response to banded versus broadcast fertilizer. So that's one of the things I've struggled with being here is whether or not we can justify having those guidelines. There's a couple of things though about banding that you need to remember that when it comes to efficiency, that really the greatest efficiency always is with very low soil test levels. So if you look at our rate recommendations where they change, our greatest change in those recommendations is going to be for the very low and low testing class. The reason for that is it's essentially the ability of the soil to fix tie up or just render some of that potassium unavailable that you're applying for the crop you're applying it to.
Dan Kaiser:
When you start getting towards the medium and high testing classes, that efficiency tends to go away. So, that's one of the things to think about with your banding is that if you are reducing the rates, you have to be somewhat careful, particularly with situations if you're looking at a soil tests changing over time that we do tend to see. I mean, the tendency, particularly with soybean and the rotation for the soil test to drop, just because soybean removes a fair amount of potassium. So that's one of the things to watch out for. If you are banding, I mean, obviously there's some benefits. Jeff, I mean, looking at some of the historical Minnesota data, I've looked at some of George's data, but generally, what we've seen is a greater benefit when it comes to banning potassium for reduced tillage systems. A lot of that thought behind that was just how potassium is taken up in the soil, it takes moisture. As the soils dry, if you have most of your potassium in that upper surface, that the crop can have a difficult time to take up potassium.
Dan Kaiser:
The corn crop in general, the stuff we've measured had been close to 200, 200 plus pounds per acre uptake of K2O. So the plant does take up a substantial amount. It just doesn't remove a lot of it. And that's one of the things to remember that corn is, if you look at corn versus soybean, soybean crop is going to remove more K in the grain versus corn. So typically, where we've seen instances where soil test decline, it have been situations where a soybean have been grown more heavily in the rotation. So that's one of the things I see a lot right now with growers, putting more emphasis on applications to beans, and that's one of the things that we've been looking at more closely, just because we know that beans put a higher demand on it, but I'm not anticipating any changes. Jeff, the study we're doing is just going to be corn next or this upcoming year. Correct?
Jeff Vetsch:
Correct.
Dan Kaiser:
So we're not going to be looking at soybean at all in terms of that band application?
Jeff Vetsch:
Correct. One of the things that I'd add to what you said, Dan, is I think you hit it a couple of good points. One is when we think about no-till, strip-till systems is we're recycling all this K. As Dan said, the amount of K taken off in the grain in corn is not tremendous. There is a significantly more taken off in the soybean seed, but we're recycling a lot of K through the biomass. Now, in corn tillage system, that's a different story. We're going to take most of that away. But in no-till, when we recycle all that K through the biomass, then it leaches back out into the surface soil and it's going to just probably stay in the top inch or two, because it's an immobile nutrient. So if we're not mixing that soil with tillage, we're creating a lot of stratification. That's, I think, where the advantage of the K banding comes in these reduced tilt systems, which we may not see that advantage in a more conventional or conservation till system where we're taking a chisel plow or field cultivator out through there regularly.
Jeff Vetsch:
The other point that you mentioned that it bring up, and Leanna might be interested in weighing in on this, is thinking of these low and very low soil test levels and how the mineralogy would react or interact with that in a band situation. So my first thought is this could be a bigger issue in South Central Minnesota, where our clay mineralogy is to more likely to fix or tie up K than it would be in Southeast Minnesota, where the clay mineralogy would be less likely to tie up K. So if we rented a piece of ground, a farm or a field that we'd never farmed before, and we found out it had really, really low soil tests K and we were going to band apply, that may interact with the kind of clay mineralogy that it has and how effective that that band might be. Now, whether we can prove this in our study, Dan, I don't think we will be able to, but it's something to think about.
Dan Kaiser:
Yeah. Some of the results you had, Jeff, I mean, looking at both the sites you had at Rochester, that's a silt loam, so that would be more, what I would say, would be higher illite content versus Waseca that's a clay loam loam soil, that would be more dominated probably by smectite. I mean, the results you were seeing, you were seeing benefit of banding at both locations. Were you not?
Jeff Vetsch:
Well, primarily, significantly at Rochester with a trend towards the benefit at Waseca. But Dan, you were there the spring and saw the amazing growth differences we had at Waseca and the severity of the K deficiency in the plants early on. So, it's really surprising that there wasn't a bigger yield difference.
Dan Kaiser:
And that's one of the things, too, I saw this year quite a bit. We had some areas that were relatively dry early on. I mean, it was amazing how much more K efficiency I saw this year than I've seen in some years past, particularly for soils that I would kind of call marginal. We had some sites... One of Leanna's project was at Rosemount and it was just a clear stair-step in terms of increasing in height across the different rates of K. You could see the checks were really strong potassium deficiency, the firing on the edges of loess, in the lower loess was there within those particular sites. What's interesting is we had two locations that a lot of our work has been on farm where we've been using commercial-grade equipment. I had two locations though that were hand-applied.
Dan Kaiser:
Both of the sites responded, made a site closer to Le Sueur. That site, not as large of a yield increase, but the yield increase was to about a similar rate. It was around 80 to, I think, 90 pounds of K20 of that site tested, then closer to 160, 170 part per million, where the Rosemount site was closer to 100 to 120 on that. But the Le Sueur site didn't show the deficiencies, but there was a small yield increase. So that's the thing to weigh on this and that's one of the issues with potassium, as well as phosphorus is the economics becomes a little trickier, because well, this year we had a decent response, the rates we applied, we don't always get that. That's one of the things with nitrogen. The advantage we have, we can put an economic model to it, make it a lot easier to figure out economically optimum rates, when with potassium, it becomes more of a problem.
Dan Kaiser:
So, that's where we're really trying to track down with historical data and with current data to try to fill in some gaps with some of this newer research to see where we can really put a probability of response to the individual soil test classifications and hopefully split things out. I know Leanna has done a little bit of work looking at some of the initial results from that, presented some of that data in the fall of 2020. I don't know, Leanna, what you found for some of the sites. It did look like a few some things were segregating a little bit among some of the sites for potentially the optimal or the critical level of potassium for the individual sites based on some of the chemistry.
Leanna Leverich:
Yeah. To follow up there, Dan, yeah, there is. I think that we are going to be able to see a little bit of a difference here in response, and maybe some of that is driven by mineralogy. More to come on that hopefully in the future here though.
Paul McDivitt:
So there have been some questions over the winter related to potash application on soybean. What are some key takeaways from your research? Should farmers consider alternative forms of potassium fertilizer?
Dan Kaiser:
Well, I put out a article through Minnesota Crop News. I think it was in January of 2021. I'm talking about some of the data we collected in 2020, where we put on a very rate of chloride. What I was really trying to do is trying to induce a problem to try to be able to separate out with whether or not it was indeed the chloride or the potassium fraction of potash. It contains about equal quantities of both. I mean, if we think of potash fertilizer, we think of it as 0-0-60 or 60% K20, even though the fertilizer doesn't contain any K20 in it at all. It's just the way we sell fertilizer on that. If you look at K versus chloride, it's roughly 50/50 of the two. So you can put a significant amount of chloride on the field with potassium.
Dan Kaiser:
So I put that article out and I didn't realize how much legs that thing would have in terms of how many questions I would get with it. There is some concern. Looking at the data, some of the sites... I think, Jeff, one of the studies we had in that, yes/no strips that was down by Grand Meadow was the first time I'd really started to see some of these small yield reductions. I think that actually site actually came back significant where we put on zero versus 200 units K20, where it was just a small one bushel yield reduction at that given site and no positive response to K for soybeans. So, I started seeing it back then. We saw it in some dryer years in the Western part of Minnesota more clearly, where I could see linear yield decreases to increasing rates of potash beyond roughly around 100 pounds of 0-0-60 per acre. So, that's really what kind of necessitated it.
Dan Kaiser:
So key takeaways out of some of this current work, one is if you need it, you need it. And that's one of the things that I don't want to put it out there, that there's a lot of concern where you can't apply potash at all to soybeans. If it's low, the soil test says that you need to apply some, the data still shows that it's economical to apply. It's really the key takeaway from a lot of this work is if it doesn't need to be applied, if you're looking at a removal-based system, that really what I'm stressing is to not split the application up. If you're going to apply a two-year removal, it's better off doing that ahead of the previous crop, because if you look up potassium sales in this state, it's 98% of our sales are potash or 0-0-60, and that's the most common source.
Dan Kaiser:
There isn't really another alternative source out there that's cheap. I mean, if you look at potassium sulfate, it's not widely available. Sophomag has a lower concentration of K, so it doesn't necessarily make itself as a good source. It's a better source, I think, of magnesium or sulfur. And then we also tested a product called polyhalite, which was 14% K2O, which again, that's more of a sulfur source because it was 23% sulfur. So we're a little limited in what we can do. So, I'm not saying it can't be applied. I just think we need to start looking at where it's being applied in the rotation.
Dan Kaiser:
I've been seeing more and more growers trying to push yields apply every year. With potassium, I'm just not so sure that's a good idea. If you're going to do it, I would say keep your rates to no more than 100 pounds of 0-0-60. It's one of the studies we still need to do. I need to look at fall, spring application different rates and just see what that overall tolerance is. But based on some of the long-term data, you could really see the yield was flat up to about 60 units K20 or 100 pounds of 0-0-60, and that's really where the yields started to decline.
Dan Kaiser:
So, I really don't want to cause a panic here because the thing is this issue has been happening since soybeans have been grown in the state and potash is being applied, so it isn't new. This isn't anything that just started happening. It just with some of the dry years we've had, we've been able to notice it more on some of the research, and that's what I've been really trying to put into place research, where we can define that overall risk potential. So I think it's going to be a wait and see. Minnesota Crop News is where we've been posting a lot of this. I think I'm going to be posting some time late spring, maybe early summer of a follow-up to some of that data. We'll just talk a little bit more about what we've been finding from single long-term trials. Because again, they've been very small yield reductions, but they're still there and it's still a yield reduction so it's still something to probably consider and maybe look at changing some of the practices a little bit to fit so we don't have that great of a risk.
Jeff Vetsch:
Dan, I was going to throw something out there that maybe you've talked about before and when you present this information and I've missed it or forgot it is, are there any other crops that people grow in Minnesota that would be sensitive to chloride?
Dan Kaiser:
So, I got some questions on edible beans. I don't know because we haven't done a lot of edible bean research. I know sugar beet can have some issues with quality, particularly with higher application rates. So you got to be careful with that because the excess potassium can be picked up and can impact quality. The only other data I've seen has been with corn. NDSU, Dave Franzen, their recommendations are suggesting no more than 200 pounds of potash applied ahead of corn, because they're seeing actually what we call a quadratic response, where the yield will increase to a point and it won't plateau and stay flat. It'll start to decline at that particular point. I have not seen that in many of our trials. I have seen some yield reductions in corn at one location. It's at Becker in 2011. That was just an incremental yield decrease for all the rates of K we applied. I've never been able to replicate that though in corn.
Dan Kaiser:
So the two I'd be more concerned about beans is number one, potentially sugar beet, and that's more on the potassium side. Just watch the rates. There's really no reason. It doesn't seem like they respond as much in particularly in some of our heavier soils that are higher in K. So, those are the ones I'd be worried about right now. The only other one that does tend to come up and that's mostly a K issue are forages. We know that's one thing up potassium has taken up in excess quantity, that excess K uptake in forages can be an issue, particularly for dry cows. So that's just one of the things to watch out.
Dan Kaiser:
If you're looking at application of high rates to forages, you may want to split that up, because it's one thing we see more commonly is if plants, if it has K available to it, it will take it up no matter whether it needs it. We see almost a linear increase in potassium uptake with increasing rates applied even without an increase in yield. So the forage, particularly where corn, we see a substantial amount of K that it has to take up. So, just be careful with that. But chloride, I mean, looking at it, I think the main one I'm worried about with chloride is beans. The rest of the stuff would be more probably the K issue, just watching it with some of the other crops in terms of how they accumulate potassium.
Paul McDivitt:
Any last words from the group on potassium management?
Jeff Vetsch:
I think there's this one thing I'd mentioned that K can, in corn, be an important nutrient for plant health. We see it sometimes helping out in stay green, stock strength, shank strength, and that can be advantageous for growers that farm a lot of acres. They may not get to that last field to combine to harvest until late November, early December and they want that crop to still stand. We certainly see that that's a place where potassium can provide some other benefits that aren't necessarily yield-oriented or directed to just towards the yield.
Dan Kaiser:
Yeah. That's one of the other things, too, that I would say, just keeping up-to-date in terms of what's going on with the guidelines. That's one of the things with Leanna's project that I'm interested in seeing, particularly in the sands, this availability issue, because one thing I really would kind of like to know is are all sands the same? We know that looking at some of her data, as we've modified pH, we've seen that a change in CEC or cation exchange capacity with some sands. There's one of the samples that inexplicably, I don't know, we probably need to test to see if it is truly a sandy soil or if it has something else in it that was relatively flat. And that's kind of the thing, is our recommendations would suggest a lot of potassium being applied to sandy soils. The issue with sandy soils though, having a lower cation exchange capacities, they won't hold it. So leaching is an issue.
Dan Kaiser:
So that's one of the things that I've started looking at, maybe dialing back our recommendations on some of our sandy soils, because if it's leaching out of the profile, it really doesn't make any sense to apply high rates, particularly if our data doesn't support it. So I would just say keep on looking at the data. We'll keep updating things through Minnesota Crop News. So it's one thing to pay attention to as we go along here. We're getting to a point at which we should have most of the data collected for the mapping for the illite, smectite here within the next year, I'm hoping. And then we'll start getting some of the data put together and looking at critical levels and those types of things after that. So just keep in touch with what's going on through Minnesota Crop News.
Paul McDivitt:
All right. That about does it for the podcast this week. We'd like to thank the Agricultural Fertilizer Research and Education Council, AFREC, for supporting the podcast. Thanks for listening.
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