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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.
(00:38)
Well, hello again everyone, and welcome back to The Dirt. Today we’re going to continue our discussion of plant nutrients and we’ll be talking about phosphorus today. Phosphorus is one of our 17 essential nutrients, and today to help us go through this important nutrient we have Dr. Cristie Preston, Senior Agronomist with Nutrien. Cristie, welcome to the program.
Dr. Cristie Preston (00:57):
Thanks for having me, Mike.
Mike Howell (00:59):
Cristie, before we begin, could you tell our listeners a little bit about yourself?
Dr. Cristie Preston (01:02):
Of course. As you said, my name is Dr. Cristie Preston, and I’m a Senior Agronomist with Nutrien. I have a PhD from Kansas State University and have been with Nutrien serving as an agronomist in the Midwest region for about the past five years. And while I was at Kansas State, I actually had the opportunity to conduct many years of trials with phosphorus, looking at different placement and tillage interactions within several of the cropping systems within Kansas so whether that was corn and soybeans, or wheat, soybeans, and corn.
Mike Howell (01:39):
Okay. Thank you, Dr. Preston. I appreciate you giving us that background. So let’s get to The Dirt and start talking about phosphorus. Why is phosphorus so important in the plant and what’s the function of phosphorus in a plant?
Dr. Cristie Preston (01:51):
So phosphorus is present in the cell membranes of all of the cells within the plant. It’s present in sugars in RNA, DNA, but really the big one that we pay the most attention to is phosphorus and how it’s used in energy transfer and respiration.
Mike Howell (02:09):
We’ve talked in the past about some different nutrients and found out that they’re available in different forms to the plant. What forms of phosphorus can be available to a plant?
Dr. Cristie Preston (02:18):
So phosphorus has to be in the soluble form to be plant available, and the majority of the time, those are the orthophosphates, H2PO4- and HPO4-2. Now, depending on what the soil pH is, you’ll have a shift between those two different forms, but as long as your pH is within an acceptable range for agronomic crop production, then you shouldn’t have any issues with phosphorus availability.
Mike Howell (02:44):
Dr. Preston, we talked a couple of weeks ago about nitrogen and how mobile it is in the soil. Is phosphorus going to be mobile in the soil like nitrogen or does it stay where we put it?
Dr. Cristie Preston (02:54):
So phosphorus is actually immobile within the soil, and that’s due to chemical formulations of insoluble complexes. If your pH is too low, then phosphorus can form complexes with iron and aluminum, and if your pH is too high, it can form complexes with calcium so that’s why we really stress to farmers that your number one goal is to adjust your pH because you could have enough phosphorus in your soils, but it could be chemically unavailable if your pH isn’t fixed. And usually that pH range is kind of between six and seven. Granted, different crops do sometimes require different phs, but in general, phosphorus availability is highest within that six, six and a half to seven range.
Mike Howell (03:43):
Dr. Preston, that kind of goes back to some previous episodes we did about pH and how different nutrients are required at different pH levels and how when we get above and below that we can have interactions and they’re not available to the plants so that goes right along with what we were talking about a few weeks ago. Dr. Preston, what are some common fertilizer sources of phosphorus? I know there’s a lot of different fertilizers out there. How can we get phosphorus in fertilizer?
Dr. Cristie Preston (04:07):
The most available forms are the orthophosphates, the HPO4-, which is your MAP or your monoammonium phosphates or your TSP, your triple super phosphates or the HPO4-2, which is DAP. Those are the most commonly used fertilizer sources whenever we’re talking about phosphorus. So MAP1152O and DAP1846O. And a lot of times it’s more regionally based, what form is most readily available as to which form the producer will use. But of course DAP does have more nitrogen in it as opposed to phosphorus concentration.
Mike Howell (04:48):
Are there any other concerns that a grower should be looking at when deciding which phosphorus source to use? Does one perform better than the other, or is it just a matter of what other nutrients may be in there?
Dr. Cristie Preston (04:58):
A lot of times it’s more just what other nutrients are also available in the fertilizer. When you’re looking at MAP and DAP, you can have some localized differences in pH with the application, but in general, the availability is relatively the same.
Mike Howell (05:16):
And Dr. Preston, we talked a little bit about phosphorus availability or sometimes we see deficiency symptoms in the field. Can you walk us through what we may see in the field as far as deficiency symptoms with the plant?
Dr. Cristie Preston (05:28):
Phosphorus deficiency is most noted for the purpling of the leaves. Now, we talked earlier about phosphorus not being mobile within the soil, but since it’s not mobile within the soil, that doesn’t necessarily mean that it’s not mobile within the plant. Phosphorus is mobile within the plant, and so we’ll begin to see phosphorus deficiency symptoms in those lower leaves because the plant can transfer phosphorus to the newer growing portions. If you think about it, the leaves that are on top are the ones that are doing the most photosynthesis, so we see phosphorus deficiency in crops that are planted in cooler soil conditions. So say planting corn in April, march in some areas, but April or early May will potentially see the purpling of the plant because the plant’s not actively taking up phosphorus.
(06:19)
The phosphorus doesn’t move with water like nitrogen does. It’s more of a root interception or diffusion. And so we’re not seeing that high amounts of uptake in the early spring, and actually you can see phosphorus deficiency in some crop rotations because say a previous crop like sugar beets before corn, sugar beets don’t form those relationships with the mycorrhizae, and since they don’t, they’re not staying in the soil and staying high in population so when you come back in and plant a crop like corn, that does rely on that mycorrhizae relationship you’re not necessarily going to see high amounts of phosphorus uptake until you reestablish that mycorrhizae relationship.
Mike Howell (07:05):
That purpling on the lower leaves is definite telltale sign, and I did a lot of my early work in the Mississippi Delta, still spend a lot of time there, and they do a lot of precision land levelling and grading those fields to a perfect slope so that they can irrigate with furrow irrigation, and that was something that we saw quite common in those precision-levelled fields. You’d move that topsoil around and it would move that phosphorus around and your soil sample would say you were good on your phosphorus, but you put the corn seed out early, especially with some cool conditions, and it was pretty common to see those purple leaves indicating that phosphorus deficiency on those precision-levelled fields and it tended to take a couple of years for those fields to come back around and readjust. So, Dr. Preston, if we find some deficiency symptoms and we do some tissue analysis and confirm that we do indeed have a phosphorus deficiency in that plant, when is it too late to apply phosphorus to a crop?
Dr. Cristie Preston (07:57):
That’s a great question. Several research studies have shown that over half of the phosphorus has taken up past the VT or R1 stage specifically in corn. And sorry, I do keep coming back to corn as an example because I’ve kind of got corn on the mind with planning right now. But if over half is being taken up after VT to R1, it’s essential to have enough phosphorus out there. But at the same time, phosphorus is really essential in the early growth stages so making sure that you’re not limiting your yield due to decreased availability in the early growth stages, phosphorus really needs to be out there either the fall before, at planning or at least before say, V4, V6 to make sure that crop gets a good start. And a lot of times that’s a great reason to apply a starter fertilizer is to make sure that you get that uniform emergence because that’s key for making sure that the crop gets out of the ground and it gets a good start because as soon as you put the seed in the ground, the yield potential decreases.
Mike Howell (09:01):
That’s right. Great point. Dr. Preston. So, Dr. Preston, you mentioned earlier that you did a lot of work with phosphorus working on your PhD. Any interesting results you want to share with us from your work at Kansas State?
Dr. Cristie Preston (09:13):
We actually found several key points whenever we did the research at Kansas State. We were looking at the long-term effects of the same fertilizer placements year after year. And, for example, in a corn soybean rotation, we looked at starter applications, with broadcast starter applications with deep banding, and then we even took it one step further to look at how phosphorus was stratified within the soil profile with those different management practices.
(09:43)
For the most part, our research results did follow what we’ve known since Barbour conducted research, gosh, that was what, 50 years ago, that the more soil that you fertilize, the higher yields you’re going to have. And another key that we picked up from it was specifically in a corn soybean rotation, was to see if the phosphorus supplied before corn would be enough for the corn soybean rotation. And at the highest amounts of phosphorus that we applied, we did see that additional phosphorus was needed to be added to the soybean crop because phosphorus was no longer the limiting factor for, say, the corn production. It may have been water in this instance, but there wasn’t enough phosphorus there to maximize the yield for corn and soybeans in a given two-year rotation.
(10:34)
And so I believe that’s one thing that farmers could take away from it was, historically, I save money. I do one application and I’m good for two years. Well, you really need to pay attention to what your corn yields are, to make sure look at removal rates, make sure that yes, I should have enough left in the soil since phosphorus is immobile, but a lot of times we’re not seeing enough left behind for the soybean plant.
Mike Howell (11:00):
Okay, great information. And that goes back to our very first podcast when we talked about soil sampling and then looked at interpreting those results and it all ties back together. We’ve got to make sure that those nutrients are available for that plant when they need it. So, Dr. Preston, we’ve covered a lot of information today. We’ve talked about the function of phosphorus and what it does, different forms of fertilizer that we can use to apply phosphorus to fields, and looked at some deficiency symptoms and some research results. Is there anything else that you think we need to mention before we wrap this up?
Dr. Cristie Preston (11:30):
I did mention starter applications and uniform emergence specifically in corn, but you really need to look at the entire rotation and make sure that you give the crop the best chance it’s got from the very beginning. Since phosphorus chemistry can be a little tricky, simply using a starter application can really make a big difference in making sure the crop gets out of the ground, it’s not deficient early on. Because the cool thing about corn and phosphorus is eventually that corn crop, the roots are going to go deep enough in the soil, they’re going to potentially run into more organic matter or higher amounts of phosphorus, and they’re going to potentially grow out of that phosphorus deficiency. But definitely making sure that it’s not deficient early in the seedling stages will make sure that you can maximize your yield.
Mike Howell (12:22):
Dr. Preston, we really appreciate you being here with us today and going through all this information about phosphorus.
(12:28)
Listeners, I want to thank you for joining as well. And as always, please pass this information along to your friends and neighbors. If you’re enjoying our podcast as much as we are, please give us a rating and put some comments in there so that we can get better feedback and adapt this program better to suit your needs. I want to invite everyone to tune in next week when we visit with Dr. Eric Snodgrass with Nutrien Ag Solutions. Dr. Snodgrass is going to talk about some meteorological events and things that are going on and help us get a better handle on what we can expect in terms of weather for the remainder of this growing season. Thanks a lot for joining. This has been Mike Howell with The Dirt.
