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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, news, and issues, helping farmers make better business decisions through actionable insights. Let’s dig in.
(00:39):
Well, hello again, everyone, welcome back to The Dirt. Glad you’re tuning in this week. Today, we’re going to be doing a live episode from the 18th International Symposium on Soil and Plant Analysis. To help us do this today, I’m happy to say that we’ve got Bryan Hopkins back with us. Bryan has been on the show in the past, and it’s an honor to have him back with us. Bryan, thanks for doing this, and if you will, remind everybody who you are and what you do.
Bryan G. Hopkins, Ph.D. (01:02):
Yeah. I’m Bryan Hopkins, I’m a professor at Brigham Young University, I’m also the director of the Soil Science Society of America’s North American Proficiency Testing program.
Mike Howell (01:12):
Bryan, what got my attention today, I was looking through the program, and the title of your presentation was Tailoring Nutrition for the #1 Vegetable: Potato’s Unique Fertilizer Recommendations. Now, I am a big fan of potatoes. Growing up, if my mom didn’t have potatoes on the table for supper in some form, I refused to eat. You can look at me and tell, I didn’t miss very many meals, so potatoes are still a very important part of my diet, I like them just about any way you can fix them. Talk a little bit about potatoes and why they’re such a unique crop.
Bryan G. Hopkins, Ph.D. (01:42):
I’m similar, and my girth would reflect that as well. I love potatoes, and my mom fed me plenty when I was a kid. Potatoes are the number one vegetable crop in the world and in the United States, they’re number one in acreage as well as number one in value. So, they’re an important crop, we need to pay attention to them in terms of that footprint, and being able to do it right.
Mike Howell (02:05):
Dr. Hopkins, you talked a little bit about the Green Revolution and how most crops have increased their yields since that time, but potatoes have increased a lot more than other crops. Talk a little bit about why that happened and why these yields are continuing to go up.
Bryan G. Hopkins, Ph.D. (02:18):
The Green Revolution was fueled by all sorts of things, innovations in machinery and fertilizer. The availability of fertilizer was a huge reason, soil testing is part of it, but other things too, a wide variety of things to give us these steady increases that we’ve had since about World War II. Potatoes is kind of head and shoulders above the others in terms of its trajectory. Our yields have increased dramatically as we’ve learned how to grow this crop. It’s a really unique crop in that it’s extremely sensitive. I like to call it a wimp. It’s just really susceptible to about every problem you can imagine, more so than most other crops. And so, because of that, back in the day, they just didn’t know those things, they just treated it like everything else, and as a result, they just didn’t have very good yields.
(03:09):
Well, we’ve figured a lot of those things out. Growers have done a lot of things, including my family as part of that, my great-grandfather’s one of the very first potato farmers in Idaho, and they’ve just, through trial and error, figured a lot of things out. There’s been a lot of research that’s been done. And a big part of it, a huge part of it, is figuring out the nutrition of potato. It’s really unique compared to most other species, and so it needs to be fertilized right, and if you don’t, you’re going to be out of business.
Mike Howell (03:37):
You kind of led me into what I wanted to talk about today and that’s the fertility. But before we get into that, you mentioned several facts about potato production, things like pH and drainage, can you talk a little bit about the needs of a potato crop before we get into the fertility aspects?
Bryan G. Hopkins, Ph.D. (03:51):
You bet. One of the big myths out there is that the potatoes are acid-loving. If you do a search on the internet, you’ll find that all over the place. Less than it used to be, but it kind of drives me crazy. Most of the potatoes grown in the United States are not grown in acid soils. In fact, in the Pacific Northwest, the vast majority of those are just the opposite, in alkaline soil. So, they’ll tolerate acid soils, but they’re not acid-loving.
(04:16):
They are a root crop, and as a result, they cannot have wet feet, that’s what the old-timers would say to you. You just can’t have wet feet. And so it’s important to have good drainage, well-drained soils. There are some exceptions to that, there are some fields around that I’ve seen people grow potatoes in that I just thought, “Man, this is a bad idea.” But somehow they do it. So, some varieties are less susceptible to that than others. Most of the russet types, the ones that are the famous Idaho potatoes, the russet potatoes, they especially are susceptible to having wet feet, and so we need well-drained soils. Idaho is the number one producer of potatoes in the United States for a reason. We have well-drained, volcanic, sandy soils, and they are really conducive to growing potato.
(05:03):
Another reason why we do so well in Idaho and Washington, Oregon is less humidity, and less disease as a result of that. Again, potato is really kind of a wimp in terms of struggling with diseases, and it’s just susceptible to about everything that comes along. The Irish potato famine is an important historical event, and that was just a disease that killed millions of people because it killed their potato crop, and we still face those kinds of challenges today.
Mike Howell (05:30):
Well, let’s talk a little bit about the nutrition. I’m familiar with the S-shaped curve that many of our crops utilize, they don’t need as much in the beginning of the season, and then as that crop gets developing through the year, it needs more and more nutrients up until the point of maturity, and then obviously it’s not going to need as much. You pointed that out with potatoes, but you showed that potatoes have a more pronounced curve. Talk a little bit about that S-shaped curve, and why potato’s curve is more pronounced.
Bryan G. Hopkins, Ph.D. (05:56):
I’ve done a lot of research in my life on all kinds of crops, and it’s not uncommon, especially in a cold spring, cold soils, where they’re not mineralizing organic matter, so you’re not getting much nutrient from that source, and so we apply a starter fertilizer, and we often get a response on corn and wheat and other crops. Well, I started doing fertilizer trials and looking at starters, and I just wouldn’t get responses like I would in most other crops, and I figured out what others maybe knew, and I just didn’t figure it out until after they did, but figured out that the potato has its own built-in starter fertilizer. We plant potatoes as seed pieces instead of true seeds, and those seed pieces are just cut up potato tubers, they’re about two and a half ounce ideal size, which is important, but that thing is packed with nutrients.
(06:46):
We planted about 20 hundredweight per acre, so that’s about 2,000 pounds per acre we’re putting out there of seed, and that seeds’ got nutrients in it. So, for the first few weeks of growth, that potato’s got its own built-in supply. You can see that in your cellar, or in your closet, your pantry, when you have potatoes that start to sprout, shoot, those things will grow, and they will not be nutrient deficient for a long time. So, that’s a little bit of a unique thing for potato.
(07:13):
The other thing is is the potato grows really slowly. It takes them 15 to 20 days to emerge out of the soil, and then even after that, they’re kind of slow growing initially. They just don’t really need much nutrient, they don’t have much of a root system, they’re kind of self-sufficient. Now, when they take off though, that S-shaped curve really kicks in, even more so than other crops, because now all of a sudden I am just taking up a ton of nutrients.
(07:40):
One of the challenges with potato is that most cultivars have very poor root systems. They’re shallow, they don’t have many root hairs, and so as a result they’re just not efficient at utilizing nutrients, and that’s a challenge. So, we’ve got to have a pretty big supply of nutrients sitting there available for uptake while they’re trying to build their factory, it’s really critical.
(08:02):
One of the most important things for potato is to close the rows early. We want to close the rows, and nutrition is a big part of that. Closing that space between the rows is going to give me better heat management inside the potato hill. If I get sunlight shining down on there on a hot summer day, it’s going to potentially heat that tuber up and cause problems, all kinds of problems, that give me poor tuber quality. And if I can avoid that by closing the rows earlier, even five days earlier we’ve shown often makes a big impact on total yield, but more importantly, the quality of the crop. And so, that’s really a key.
(08:43):
And then it’s going to continue to need nutrients to sustain that canopy, although it’s going to start tapering off, and we want it to taper off. You’re going to start forming tubers, call it tuber initiation, we create these tubers. Well, then they’re beginning to bulk. If I have too much nitrogen especially, I am going to have a problem with tuber bulking. I’m just going to be creating a lot of vines and no tubers. And so that’s a problem. I don’t want too much nitrogen. I got to have enough, because if I don’t have enough, then my crop dies early and I lose a bunch of yield. You’re really walking a fine tightrope of being able to have enough nitrogen, but not too much. Similar for the other nutrients, although it’s not as critical, we just need to make sure we have enough there. We don’t worry so much about excess. It is possible to get excess, especially phosphorus, causing problems with micronutrients, but nitrogen is really the most important one to try to dial in.
Mike Howell (09:38):
So, you mentioned the importance of nitrogen and how important it is to make sure that it’s there when the plant needs it, and not have too much. Talk a little bit about how you manage the nitrogen in a potato crop.
Bryan G. Hopkins, Ph.D. (09:48):
Well, you have to have a steady supply throughout the season, including even into the end of the season. I’ve got to be able to do something to do that. Now, again, we’re often growing on sandy, low organic matter soils that don’t have a lot of nutrient-holding capacity. Typically, like most growers over the years, have fertigated. So, we might put on 25 to 40% of our nitrogen pre-plant, or even better, apply it at emergence, and then we’ll fertigate by injecting nitrogen into the irrigation system to give me a nice, steady supply of nitrogen. And we’ll just pump it every week. That’s a pretty good system; it’s also very labor-intensive and costly. You’re typically applying urea ammonium nitrate as the liquid form, which is usually more expensive than just straight urea, for example. It has a cost to it, it’s definitely beneficial.
(10:39):
Alternatively, we use various slow-release enhanced efficiency fertilizers. I’ve used polymer-coated ureas, I’ve used nitrification and urease inhibitors, and some others potentially, those are kind of the main ones that we know are very effective, to give that nice, steady release throughout the growing season. It’s really a key.
(10:59):
And then being able to monitor the crop through petiole analysis, to be able to dial that in is important. We want to kind of estimate it at the beginning, get on what we think we’re going to need early, but then we’re going to spoon-feed it based on that analysis.
Mike Howell (11:15):
Dr. Hopkins, we spend a lot of time talking about 4R nutrient management. We’ve kind of talked a little bit about time already, we’ve got that one covered, but one thing that I really took out of your presentation was the rates of some of these nutrients that are needed on potatoes. Talk a little bit about the rates, especially in P and K.
Bryan G. Hopkins, Ph.D. (11:32):
Because potato has high demand uptake, but also because of that poor rooting efficiency, we find that we need almost double the rates of nutrients than we have on other crops. For example, nitrogen, it’s not uncommon for us on a high-yielding crop, like let’s say I have a yield potential of 700 sacks to the acre, where a sack is a hundredweight. So, let’s say my yield goal is 700 hundredweight to the acre. I’m going to be putting on 350-ish pounds of nitrogen per acre.
(12:07):
I may have to adjust that upward though, if I had previous crop is a grain, which is common, to have wheat, for example, as a previous crop. If I have a lot of straw residue, and more and more because of the soil health initiatives, we have more and more growers putting more straw back into the soil. Well, that means I’m going to have to put on a little bit more nitrogen, so I might bump that nitrogen rate up by 30 pounds of nitrogen per acre to account for the grain. I might need to bump that up on sandy soils; our data shows that we need higher rates because of leaching losses, the leaching potential in the sandy soil. Bottom line is, again, we might be applying 400 pounds of nitrogen per acre.
(12:48):
It’s even more pronounced for phosphorus and potassium, especially in Western soils that are calcareous. In these calcareous soils, we have data that shows we ought to bump up the phosphorus rate by about 10 pounds of P205, or phosphate, per acre, for every percent of lime in the soil. That’s really unique, we don’t really have that recommendation for any other crop, but we have data that shows that we ought to be doing that. Not uncommon to have 10, 12% limestone in the soil; that means more phosphorus that’s going to be applied. So, we can see phosphorus rates at 400 pounds per acre. That’s about double what we see on most other crops.
(13:31):
And then potassium rates are just astronomical. At very low soil test levels, we can see 600 pounds of potash applied per acre. Now, most growers have got their soils built up to a point they’re not applying that much, but I’ve seen some rates that I just was sure, in our research, like, “Oh, we’re just going to kill these potatoes with so much potassium.” But we didn’t, we saw tremendous yield responses. That’s not just University of Idaho data, that’s Oregon State data, Washington State data, as well as grower experiences. So, we’re needing pretty high nutrient rates on these potatoes.
(14:07):
I think it’s wise to do soil testing, I think the soil test correlations are there, and that we shouldn’t be applying it if I’ve already got levels that are super high. I work with a lot of growers who’ve already built their soils up to levels that they just don’t need that much. Low soil tests, we need some pretty high rates.
Mike Howell (14:26):
So, we’ve talked about time and rates, let’s cover the other two Rs. What about placement and sources? Is there a benefit to putting fertilizer out in a band, or are there some sources that we need to make sure we use, or some sources we need to stay away from?
Bryan G. Hopkins, Ph.D. (14:40):
Placement is interesting. We do see some response typically to a concentrated band that’s placed, even though I made the argument previously that we don’t really need a starter. However, later in the season we actually see some benefit to having a concentrated band. And we typically are placing that about three inches to the side of the seed piece. And I’ve done some work, and I can go three inches above the seed piece, three inches below, always to the side, three inches to the side, but I kind of have some range there in terms of where I exactly put that. But typically we want it off to the side, that’s where the roots are going to be more concentrated. So, that’s good, we like that. Otherwise, most of the broadcast fertilizer, when we do hilling, so we plant in hills, and then usually at emergence we’re going to go through and hill again, and that’s going to help throw, kind of concentrate that fertilizer up into the hill, so that’s important.
(15:33):
Fertigation actually, it’s not as efficient as soil-applied fertilizer, we just get less uptake. But it’s surprisingly good. Potato, once the rows close, you start getting a lot of surface-feeding roots, and they’re pretty efficient at taking up those nutrients that get injected into the irrigation system. We actually have some pretty good data showing good uptake, even with phosphorus, which is kind of the one that’s the biggest problem in terms of placement. But if I have a petiole analysis that says I’ve got low phosphorus, then I can go ahead and inject that in my irrigation system, and have some confidence that it’s going to actually get in there.
(16:10):
We get some folks doing foliars. I don’t really see too much of a benefit to foliars versus injections, in most cases. Sometimes we do that, especially if I’m partnering it with a fungicide, for example, then foliar applications can be a good placement as well.
(16:26):
Sources can make a big difference. As I mentioned, I think these enhanced efficiency fertilizers, I’ve had some really good luck with polymer-coated ureas, like ESN. I’ve had really good luck with some of these inhibitors. They don’t last quite as long as the polymer-coated ureas, but we do see some good sources there in terms of nitrogen. If we can get a nice steady release, then yeah, I’m all for it instead of doing the injections, I think it’s great.
(16:51):
Sources on phosphorus, there are some that are very important. Done a lot of work with humic acid. Actually, combining that with phosphorus and micronutrients, we’ve found that it enhances the solubility, at least in the alkaline, calcareous soils that we work with out West. I don’t know about the data on the more acidic soils, that could be, but there’s definitely some opportunities there to have some source differences. We’ve tested some other products, like some copolymers that have shown enhanced availability, and I can cut my P rate down by half in some cases, with some of these enhanced efficiency fertilizers that have a track record, and have got some research behind them.
Mike Howell (17:33):
Dr. Hopkins, one last thing that you mentioned, you talked about some nutrient disease interactions. I know that’s prevalent in some crops, but I didn’t realize they had it in potatoes. Talk a little bit about these interactions.
Bryan G. Hopkins, Ph.D. (17:44):
I could talk for a month about that, and there’s so much we don’t even know. But potatoes, as I mentioned, are very susceptible to diseases. Nitrogen in particular has a lot of interactions, like early die complex. For example, if I am a little lean on the nitrogen, I’m much more likely to be susceptible to that early die complex. Sometimes if I have too much nitrogen, I can create a very humid, vine-heavy canopy that is more susceptible to white mold and other types of diseases that thrive in those humid, wet environments, and so that can be a factor too. There’s a lot of data on phosphorus, for example, having phosphorus deficiencies, creating some increased disease pressure. I think it’s really important to do that. Some cultivars are more susceptible than others, that’s one of the things that we’ve learned, is that some of these cultivars are less susceptible, and as a result they seem to actually need less nutrients, and so that’s an interesting thing.
(18:45):
I could tell 100 stories, but I’ll just pick one. I did a project where I had this very large grower, second-largest potato farm in the United States, actually, AgriNorthwest in Washington State. They were just swearing that we have to pump phosphorus through our irrigation system, even though our soil tests are so high. And I was just like, “No.” So, we agreed to do this little research trial, and I was convinced that I was right. Well, I was not right, I was wrong. We actually found that at least on Russet Burbank, not on the other cultivars by the way, but on Russet Burbank, we were getting a response late in the season to pumping on some phosphorus. And what we learned eventually is that the potatoes were getting this vascular disease, and it was preventing nutrients from being translocated from the roots up into the leaves late season. And by putting on that little bit of phosphorus late, we got a nice response. So, I’m a big believer in soil testing, but sometimes we have this interaction happening that causes me to have to do something a little different.
Mike Howell (19:45):
One other thing that you talked about was petiole samples and how important those were. Now, I’m familiar with petiole samples, we do that in cotton pretty regular down in the South, but we do it maybe once or twice during the growing season. You talked about weekly petiole samples; talk about the importance of that.
Bryan G. Hopkins, Ph.D. (20:01):
I would say the average potato grower, at least in the Western United States, samples their petioles weekly. And the petiole is the tissue that connects the leaves to the main stem. And so we go out there and pull about 40 petioles, pull the leaves off immediately, and get those shipped into the lab. Timing’s important. But that’s really the pipeline of the nutrients getting to the leaves. So, it’s a great snapshot, right at that moment in time, of what nutrition that I have. And we’ve just learned that we have to monitor those petioles in order to avoid excess nitrogen or too little nitrogen. The other nutrients as well, they’re not quite as critical or quite as well studied, but nitrogen in particular really needs to be studied. And it’s interesting, sometimes we’d be going along and everything seems fine, and then the petiole nitrate values will just crash.
(20:52):
And so, we’ve learned that we really do need those weekly, and we can’t just do once a season. Like wheat, for example, which is often grown in rotation with potato, we sample it once. We sample it flag leaf, it’s pretty good, that’s kind of a once and done. But with potato, we really do need to have those weekly analyses. And we don’t just look at the nitrate. Some growers only look at that, but it’s important, like potassium, for example, those potato petioles can have like 10% potassium, and that’s what they need early. And it’ll drop over time, but it needs to start high, and it needs to follow that pattern that we’ve learned, or otherwise we’re in trouble. We become deficient too early in the season. We monitor the other nutrients as well, including the micronutrients with that, and if we see something that’s a problem, we’ll adjust, we’ll try to pump some in, and then maybe even adjust what we do the next time we plant potatoes in that field, and say…
(21:47):
I had one field for example, where I was working, and soil tests actually looked pretty good. So we didn’t really put on a heavy rate of anything, other than nitrogen, but the petioles just really crashed too early. And it kept happening more than one year, and so we said, “Okay, this is a pattern that is unique to this soil.” So, the petioles kind of helped us calibrate our soil test for that field, to say, “Most of the other fields do fine at this soil test level, but this particular field doesn’t. We’re going to have to bump up that so that we don’t have to inject the phosphorus and the micronutrients late in the season.” Which saved us money in the long run, plus, we got good yields.
Mike Howell (22:26):
Well, Dr. Hopkins, I grew up scouting cotton and managing cotton fields, and I knew that was really labor-intensive, still is, still do a lot of that today. Did not realize that potatoes were nearly as labor-intensive, probably more so than we ever thought about in a cotton crop. But we really appreciate you taking time to visit with us today. Before we go, do you have any closing comments, or anything else we need to mention today?
Bryan G. Hopkins, Ph.D. (22:47):
I wouldn’t trade you cotton for potatoes because I don’t think I can handle that humidity where you are doing that work. They’re both labor-intensive, but man, I don’t know how you walk around those fields that are so humid. I did my PhD at Kansas State in Manhattan, and about killed me, I was very glad to get back to the West, in our low humidity environment.
(23:06):
Anyway, I think it’s important that we do things right, that we don’t just try to do the same thing on each crop. We need to understand root architecture and morphology, we need to understand the life cycles of these different species. Potato has been studied probably more than most of the other vegetables, and so we’ve got the advantage of having some data behind us, but we really do need to customize our fertilization based on various species, and understanding these different crops. And it really is a complicated world, and there’s a lot we don’t know still. There’s still plenty to learn, especially with some of these minor crops.
Mike Howell (23:42):
Dr. Hopkins, thanks for joining us today, I know I’ve got a lot out of this, and I know our listeners will as well. Listeners, we appreciate you tuning in, and as always, if you’ll stick around for just a couple of moments, we’ll be right back with segment two.
(23:55):
Farming isn’t farming without questions, and now there’s a place to go for answers. At eKonomics, an entire team of agronomists is waiting and ready to help, for free. No question is too big or too small. Visit nutrien-ekonomics, with a K, .com, and submit your question with the Ask an Agronomist feature.
(24:18):
Well, listeners, welcome back for segment two. As you know, this is a part of the show where we ask one of our agronomists a question of the week. Today we’ve got Dr. Karl Wyant, director of agronomy, back in the studio with us. Karl, welcome back to The Dirt.
Dr. Karl Wyant (24:30):
Hi, thank you for having me.
Mike Howell (24:31):
Karl, today we wanted to talk to you a little bit about soil testing. We know soil testing is important, but tell us a little bit about the importance of soil testing, when someone should do a soil sample, and how often they need to take these soil samples.
Dr. Karl Wyant (24:43):
Great question, Mike. Think about a soil test, that document you get back from the lab, think of about almost as a bank statement for your soil. What’s in the savings account, what’s in the checking account, and what really do you have to work with during that growing season? Or maybe where do you need to grow your savings account to into the future? It’s a very useful set of tests to get done, put it all in one place so that you remember where to go look for it later. But the soil tests give you a snapshot of the nutrients in the field, they can give you a snapshot of what the pH of the field is, it can give you a snapshot of some of your other parameters, like soil organic matter, and soil texture, the clay, silt and sand percentages in your soil. Depending on where you send a soil sample to, you get a lot of information back, but for your dollar spent it’s a great investment.
(25:30):
The question about when to soil sample, and I always say this, if you haven’t sampled your fields in a while, now is the time to start thinking about it. There is some debate in the agronomy world of spring sampling versus fall sampling, and I think for those that do sampling every other year or every three to four years, that’s a good debate to get into. But for the listener on the call here, if you haven’t soil sampled in quite some time, five years down the road maybe or more, it’s just time to go out, grab some soil samples, see what’s in the bank account, and you can use that information to make some really informed crop nutrition decisions that are based on your soil, your fields, and then build your fertility program that’s almost custom-made for your operation.
Mike Howell (26:12):
Oh, you referenced that soil bank account several times. In my personal bank account, I’ll get a notice on my cell phone when my account’s running low. Unfortunately, our soil’s not that advanced yet. Any way we can get an update on what’s in our soil checking account?
Dr. Karl Wyant (26:26):
That’s funny. Well, you can wait for the crop to tell you that you’re low in the savings account, with diminished yields, deficiency symptoms, like purple in corn because you don’t have enough phosphorus, or yellow corn because you don’t have enough sulfur or nitrogen. You don’t really want to wait for that kind of text message, because you’ve already lost yield and you’ve already lost profitability for that acre. So, taking that proactive approach and getting the soil sample done can really… Until we figure out some sort of text messaging, send your thing in the soil, your best bet is just to try to be as proactive as possible and get that information and plan ahead.
Mike Howell (27:01):
Karl, thanks again for joining us today, we really appreciate it. Get a lot out of these Ask the Agronomy questions.
(27:06):
Listeners, thanks for tuning in this week, and as always, if you need more information on anything we’ve talked about today, you can visit our website, that’s nutrien-ekonomics with a K, .com. Until next time, this has been Mike Howell with The Dirt.
(27:22):
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