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Mike Howell (00:08):
The Dirt with me, Mike Howell, an eKonomics podcast where I present the down and dirty agronomic science to help grow crops and bottom lines. Inspired by eKonomics.com, farming’s go-to informational resource. I’m here to break down the latest crop nutrition research use and issues, helping farmers make better business decisions through actionable insights. Let’s dig in. Welcome back to The Dirt. This is Mike Howell, senior agronomist with Nutrien. We’re glad everybody’s joining us today, and we have a special guest with us today. We have Dr. Antonio Mallarino. He’s an agronomist with Iowa State University. Dr. Mallarino, thank you so much for joining us today. If you would, tell our listeners a little bit about yourself.
Dr. Antonio Mallarino (00:57):
It’s a pleasure to do this with you. I’m a professor of research and extension here in Iowa State University, and I started my career way south in Uruguay, which is between Argentina and Brazil. I was a professor there. And I started there being mainly focuses on research and teaching in fertilization of forages, but then when I work on my PhD and masters and so forth, I came for a postdoc here in Iowa and I worked with mainly corn and soybean. So especially since the early nineties, I have been working mostly on corn and soybean and until about one year ago, mostly focusing on phosphorus, potassium, lime, micronutrients, and then phosphorus in relation to water quality. Those have been my emphasis. Since early last January, I am the only extension nutrient management faculty in the department of agronomy, so I had to be dealing with nitrogen too, so I have been very busy lately.
Mike Howell (01:54):
I understand. It seems like a lot of our universities are getting into that situation. People are retiring and moving on and we don’t have as many professionals out there as we used to have.
Dr. Antonio Mallarino (02:03):
Yes, that’s the case.
Mike Howell (02:05):
Okay. Dr. Mallarino, let’s get right to the dirt and jump in and start talking about potassium today. We’re in the middle of a series. We have already talked about nitrogen and phosphorus and potassium is the next one of the big three. If you would, kind of tell us what’s the function of potassium inside a plant?
Dr. Antonio Mallarino (02:21):
Well, potassium is very important and is one of the three macronutrients together with nitrogen and phosphorus. That’s because of the amounts extracted from the soil and amounts needed. The main role of potassium is being a co-factor or mediating enzymatic relations that relates to carbohydrates, synthesis. And this includes starch or sugars, for example, like sugar beet, things like that, but also cell walls. So it is also important for strength stock and stem strength in plants and resistant to lodging and things like that.
(02:58)
The other major role is as a mediator or regulator of the water relations in the plant, tubular pressure and stomata function and things like that, cell growth. It is key for all those issues. And one interesting thing is that potassium is not as much critical for cell division early in early growth of crops, although of course if there is an efficiency it is, but that’s a major difference with nitrogen and phosphorus. So potassium sometimes if there is efficiency can be fixed by early season applications of potassium, but it is very important and we can see all the time when we see lodging in, say, wheat or cereals like that, or stalk breakage in corn, and then of course for production of sugars.
Mike Howell (03:44):
Right. It’s a vital part of our plant structure and helps to regulate a lot of things in the plant as you mentioned.
Dr. Antonio Mallarino (03:50):
One thing that I forgot to say is there is no organic potassium. So even though it does all the things in the plant, it does this as a cation. And this is important because people tend to forget about that because when they are considering, for example, nutrient availability in animal manure or crop residues, that for nitrogen and fossils, we have all these issues with organic forms that may or may not be available and all that stuff. See, potassium is not part of any organic compound, so that’s one reason that the recycling of potassium with crops and the availability from animal manure essentially a hundred percent and the same as fertilizers. So that should be an important consideration too.
Mike Howell (04:31):
Well, Dr. Mallarino, we’ve talked about nitrogen and phosphorus earlier and how they can move in the soil and move in the plants. Talk a little bit about potassium. Is it mobile in the soil and is it mobile once it gets into the plant?
Dr. Antonio Mallarino (04:42):
Well, in the major fertility books is classified as an immobile nutrient, mostly together with phosphorus. In reality, it is a little bit more mobile than phosphorus, but still it’s not too mobile, and for example, in sandy soils, it can be leached with excess rainfall very easily. But in soils that are long or finer texture, seldom this is a problem. Now because it is only a bit more mobile than phosphorus, we have all decisions related to the uptake. Where is water in the soil, where are the roots? So it can be taken up, and even though you see it moves a bit more than phosphorus, in practice, sometimes it’s the issue of the placement concerning to where the roots are and the soil moisture is more important than fossils because the amount of potassium that needs to be absorbed is much, much higher than phosphorus. So this is an important issue to be considered.
Mike Howell (05:37):
And I’m glad you mentioned that it can be leached in the sandy soils. That’s something we experience in my area down here on the Gulf Coast. We’ll get these big rains and leach it out of these sandy soils, and we actually have growers that have to put out split applications of potassium just because they’re leaching so much potassium. I’m glad you mentioned that for us today.
Dr. Antonio Mallarino (05:56):
Yes, that’s not a problem here in Iowa where I work, but yeah, there are many, many areas even here in the corn belt that are kind of sandy, so that’s an issue. Yes.
Mike Howell (06:05):
Okay. What forms of potassium can be available to be taken up by the plant?
Dr. Antonio Mallarino (06:09):
Well, that’s one of the things that potassium is easy. The availability for crops or predictability is not easy, but the uptake is easy. It’s just potassium, you see? The cation, free cation in the soil, so most of the fertilizers that are applied, the salts that are transforming into potassium, that of course is first in the soil solution, then can be retained in the soil and so forth. As I said, the potassium recycled from crop residues is very important and it’s just the cation. Yep, so that’s the key. So there has to be sufficient supply of the ion in the soil.
(06:43)
Now, when the potassium is applied to the soil, it is retained by the soil, which retained may not mean unavailable for plants, but sometimes could be. The key is that the soil has to have this capacity of supply potassium ions to the soil solution as the plants begin their uptake. It’s not as much as the amount in the solution, but that the soil needs to be able to react to absorption to supply this potassium to the soil solution, because otherwise we may have tremendous amount of potassium in the soil but the plants cannot get it.
Mike Howell (07:18):
So if you don’t mind, talk a little bit about some of the factors that could tie up that potassium or make it unavailable to the plant.
Dr. Antonio Mallarino (07:24):
Okay. This is the key issue because it’s complicated and also affects the issue of the soil testing, you see, and testing for availability. We have essentially three major pools without going into the test of chemistry or mineralogy, which is a potassium solution. Then there is this potassium that is somehow retained by the soil, so part of it is called exchangeable potassium. The exchangeable potassium is weakly return, and that pool is in a very fast equilibrium with the potassium in the solution. That means that we apply potassium, so that moves very fast to this exchangeable, but still is easily available form. And the other way too, as the plants begin to reduce the concentration, the solution, then some of these potassium exchangeable goes to the solution.
(08:15)
So that’s easy and can be measured. Well, it has been measured for many years, and that’s the basis of most soil tests that are in use in the world, and they are trying to estimate this exchangeable potassium in soils. But then we have a fraction that it is retained by the soil, but it’s not so easy in equilibrium with other more available pool. Which people have been called for many years, soil chemist, non exchangeable potassium, and I always thought that that’s is wrong. It’s not a good name because initially we thought that actually is not exchangeable, and then of course not available, but we are seeing more and more that it can go to the solution. So I like to call it slowly exchangeable potassium.
(08:58)
And then we have of course, the potassium in minerals that it may be available over a thousand of years or things like that. The main issue with potassium is that equilibrium or those reactions between the exchangeable and the slowly available pool. That’s what has been complicated in our lives for hundreds of years, and even now, you see after 30 years of working on potassium, me and others still feel very frustrated because there are some factors that is kind of easy that affect the equilibrium. For example, very fine textures. If we have very fine texture, then there is a tendency to be more potassium retained, and that’s slowly available fraction. Then we have the mineralogy. Okay, there are some clay types that it’s not much of a problem, but others, when you get that potassium in the layers of the minerals, then they just close up and it becomes like fixed quote, see, more available.
(09:52)
But the problem is, of course, it’s expensive, but soil mineralogy doesn’t change much, at least over a few years, can be measured. So in theory, one could say, well, we could know that we have different soil taps, different regions. We can classify soils depending on the texture and the clay mineralogy, and then go for it, and maybe we need different calibration of soil test for different mineralogizes. But it’s not so easy because these factors are very, very, very affected by other things that cannot be predicted well.
(10:23)
So one is moisture. Moisture relations in the field. We are seeing that mainly the type of alternating with saturated or dry conditions in the soil, in the field, that affects incredibly in an important way what is really available, not on these processes. The other thing is how the soil sample is treated. Research here in Iowa, in the United States in the sixties shows that when in some soils when we take the soil sample and then it’s dried in the lab in order to do the testing, that changes the dynamics of the potassium there, and then the soil test based on that dry sample is not as good. So we have been working with this moist test, things like that.
(11:04)
Then the other thing that also complicates, and I know I’m talking too much now, but I said it was complex, is the recycling, you see, in relation to rainfall. If we have, say for example, after you harvest corn, that much of the potassium is in the store, it’s not in the grain, and then if it rains, you may have a hundred, 150 pounds of potassium or kilo per acre or per hectare that is available. It’s the same as if it were fertilizer. But if it is dry, then that doesn’t recycle. So all these things make this evaluation of availability complicated, and they answer of your questions complicated.
(11:41)
Of course, we agronomists always say it depends, but this is one of those cases that most people accept this for nitrogen because nitrogen availability is complicated because of the mineralization, the weather, all the stuff. Well, potassium can be as complicated as for nitrogen. Well, we are lucky with potassium that potassium is not an environmental hazard for water quality, you see. If there is leaching, there is losses of potassium, that’s an economic loss you see for the farmer and for a resource, but not for water quality. But yes, it is important, and we are realizing more and more, not only here, but research here in Iowa, Brazil and other parts of the world, the consideration of this interaction between the rainfall and the rainfall patterns when it is not the total rainfall is when it happens in relation to the potential recycling of the potassium and then when the new crop can be taken up or not.
(12:35)
So here in Iowa, for example, we have the issue, since we don’t have many sandy soils or things like that, the most important issue is that if we take samples, for example, immediately after the crop. So here we essentially plant summer crops. There is no winter crops here in the north central region, but most people want to take soil samples, soil testing in the fall. So if they take those samples when it has been a dry fall and there is much potassium in those residues, then that soil sample, that soil testing will underestimate the availability for the next crop. But, okay, so then we say, well, so in order to assess that, you need to wait on sample in the spring just before the crop. But the issue we have here in Iowa, like this spring, which is very wet, people don’t want to do that because see, they don’t have time actually to take this whole sample, get the results, and then go and apply fertilizer when they can. The most important thing is plant.
(13:29)
So these are the most important issues with the recycling here. Now, in terms of potential leaching in sandy soil, that’s important too. So for example, it’s common that corn may need 180, 200 pounds of K per acre or kilos per acre. So if you have a sand solar, you apply all that before planting. Then if it rains, half of that is gone. So that’s where you mentioned before, split applications is a good practice.
Mike Howell (13:54):
Okay, and Dr. Mallarino, you mentioned that we had several different sources of potassium fertilizer. Go through some of the most common fertilizer sources, and if there’s any difference in those sources, go through those as well, if you don’t mind.
Dr. Antonio Mallarino (14:06):
Well, I like to say that potash is king. Potash, which is potassium chloride is by far the most common fertilizer, and it’s applied granulated, dry, and it is the most important fertilizer source all over the world, no doubt. Now, there are other potassium sources. For example, I have been doing, before the COVID-19 pandemic, I was doing all kind of international work in Argentina, in Uruguay and South America. They have all kind of mixtures, dry mixtures, where they… Even part is potassium chloride, but also in mixture with others. So it’s like nitrogen or phosphorus. Here in the United States is mostly applied as potassium.
(14:45)
Now, here in the United States, we have also, it’s very important liquids, but mainly for starters, you see, liquid fertilizers are more expensive. They’re very good sources but are more expensive. So we have all kind of starter liquid fertilizer here that they use other forms of potassium, some have potassium chloride, but those have what is called a high salt index. So they need to be careful if they apply in furrow. The same thing, for example, in potassium thiosulfate, which is commonly used, but it has to be applied in very small amounts when applied in furrow. But we have other sources like potassium nitrate, potassium acetate, so all these forms. We do have a few crops that they don’t like too much chloride, like tobacco, for example. So in tobacco, potassium chloride is a no-no. So they have to use potassium sulphate or things like that. But most of the extensive crop, that’s not problem, and in fact, chloride is needed. It’s a micronutrient that is needed. So there is a variety of sources, but as I said, potash is king. Okay.
Mike Howell (15:46):
Dr. Mallarino, you mentioned that you’ve been doing this type of work for over 30 years now. Give us a little bit of information about some of your most recent research on potash and what you’re finding out.
Dr. Antonio Mallarino (15:56):
Well, it’s my research, but also some other colleagues here, and also for example, in Uruguay and in Argentina, we are working to that. But the main issue is that soil testing for potassium is far more uncertain than for phosphorus. I mean, we have some of those soils in some areas that are nice. You see that they have this type of clays that they don’t fix much potash, so we don’t have some of those issues, but in most part of the world are like Iowa, that we have different mineralogies. So the issue of classifying soils according to mineralogy or texture and then having a different calibration for the common soil test that we use, you see.
(16:35)
In theory is nice, but often that’s not work. You see, because we have all these different interaction, as I mentioned. So one of the things that I have been working for years is on a moist test, you see. When we sample soil and we dry the sample, usually at about 35, 40 degrees centigrade, when we have this type of clays like smectite, vermiculite, things like that, drying, the soil samples really screw up. The amount extracted does not represent really what will be available in the soil.
(17:07)
One of the things that we have been working is on a field moist test. So we bring the sample and then do not dry. We determine moisture, of course, but then we analyze the sample without drying. So in many soils, but especially soils that are kind of fine texture and also that have these cycles of being dry to too wet or saturated, this is a much better test because in those soils, it tests better. So we introduced that test in 2013 here in Iowa, and I know that recently been done in other states and countries. In soils that are well drained normally it’s not that better.
(17:43)
The other thing that we have been working, not just here but in Indiana and other places for long, is okay, it seems obvious at this time that this testing of exchangeable potassium in the soil is not enough. Very often that test doesn’t work well because we have this slowly available potassium that actually is available even for an annual crop, and we are not measuring it. Now, the problem is that there are methods very old 50, 60, 70 years old to measure the total non exchangeable potassium. But the key is not that total. The key is what is the most reactive fraction of that slowly exchangeable potassium, and we are struggling with that. I can tell you that I’m frustrated. We don’t have a solution. We do have one. It’s called the tetraphenyl boron test, and instead of having a long time of digestion, we digested for about 10 minutes, and that measures what is the most reactive, slowly exchangeable potassium.
(18:41)
So my grandiose plan was because this test is expensive here, for example, it may be $20, $25, so nobody was using a routine test situation. We say, okay, what if we study this and we classify soils according, not in relation only to mineralogy, only to texture, but not the total slowly exchangeable K but the partially structural K. So then by classifying different soils doing that, then maybe we can improve the interpretation for the cheap low cost soil test. But we are struggling. We really are struggling. So that’s one thing that we continue working with this test.
(19:19)
And the third thing is that cause of all the complications that I mentioned before, the recycling and availability, the ideology of the site over time, we are finding, for example here in Iowa that the best timing for soil testing for potassium is not in the fall, but it’s actually after we plant the crop, you see. Because in that point, late spring, early summer, all the things that could happen in the soil since the previous crop already happened. But the problem is, of course, you need to know how to apply potassium before planting. So we are working on these tests, say for example in June, especially in fields that people are maintaining, for example, in soybean, that normally many farmers here plant corn, soybean rotations, and they apply all the protection for corn. So in the soybean, we could be testing at that time, maybe we can get a better estimate of the protection needs for the next fall. And at the same time, as I said earlier, people very commonly side dress nitrogen, you see. For corn, for example, they apply, they can split the dose, but not as much for side dress potassium.
(20:24)
Of course, phospho doesn’t make sense because phospho is needed early. But in the case of potassium, I think that this test and then in season application of potassium, especially of liquid fertilizer, I think it could be potentially a good thing. So that’s one of the last things I have been doing in potassium, looking at liquid potassium side dress for corn. That, of course could work if we get some rains in the summer. I mean, if we have some drought, then it’s not very useful. But those are the three major issues that have given me strong headaches over the last 10, 12 years.
Mike Howell (20:56):
I understand the headaches, but at least you have something to keep working on and keep you going for another 20 or 30 years. We sure appreciate all you’ve done for the fertility world in the last 30 years and want to see you around for a long time to come. Dr. Mallarino, we’ve talked about a lot of things about potassium today. Is there anything that we’ve missed is something else that you feel we need to touch on this morning?
Dr. Antonio Mallarino (21:15):
Yes, we missed the issue of the placement. I refer to it in general because of all these situations of the mobility of potassium, things like that. In some soils, in some climates where the potassium is supplying, the soil is important. So that’s something that actually it was the first research that I did in Iowa as a professor in the early nineties was to look at deep placement of potassium, not just banding with the planter. So there are some situations and some tillage systems where we need to put that potassium deeper than just in the surface of the soil, not deeper than maybe four or five, six inches, but because that would complicate soil sampling and testing. So that’s another thing. It’s not for everybody, but for example, some people here are doing strip tillage instead of no-till. Then they are applying potassium and phosphorus deep. In some conditions, it pay in dozens. So that’s the other issue is the people in different regions, and as I said, different climates and soils may consider.
Mike Howell (22:16):
Okay, Dr. Mallarino, we sure appreciate you taking the time to be with us today. Listeners, thanks again for joining us today, and thanks again to Dr. Mallarino for his insights on potassium and potassium fertilization. I want to remind everyone, please visit our website at www.nutrien-ekonomics.com for more information on potassium or any other subject that we are covering here on The Dirt. Please join us again next week as we come to you live from Tampa, Florida at the World Sulphur Symposium. We’ll be talking everything related to sulphur next week. Until then, this has been Mike Howell with The Dirt.
