Season 3, Episode 8 | Rethinking Critical Values for Grapevine Nutrition

00:00 Patty Skinkis

This is the HiRes Vineyard Nutrition Podcast Series, devoted to helping the grape and wine industry understand more about how to monitor and manage vineyard health through grapevine nutrition research. I am your host, Dr. Patty Skinkis, Professor and Viticulture Extension Specialist at Oregon State University.

00:21 Patty Skinkis

Vine nutrition is important to all vineyards, whether they are wine, juice, table, or raisin grapes. Most of our podcast episodes thus far have focused on wine grapes, as much of our team in the Hi-Res Vineyard Nutrition Project is focusing on wine grape production throughout the US. However, I want to turn our focus during this episode to nutrient management in table and raisin grapes-- and to do so, I invited Dr. Matthew Fidelibus to join us.

Matt is Viticulture Cooperative Extension Specialist at the University of California Davis, but he's located at the UC Kearney Agricultural Center located in Parlier, California--basically in the heart of the San Joaquin Valley and the heart of the table and raisin production areas.

Matt is co-PI on the Hi-Res Vineyard Nutrition Project and was first interviewed for this podcast back in season one. If you want to go back to that episode, you can listen in. He talked a lot about tissue sampling and the history of it and how it's really become part of what we do in viticulture based on work that both he and his predecessor had done on grapevine nutrition. His research during his career has been improving the foundational knowledge on how to best monitor vine nutrition for table and raisin grapes amongst wine grapes as well. He's joining us today to give us an update on his project advancements over the past two years for the High Resolution Vineyard Nutrition Project.

01:49 Matthew Fidelibus

Thanks for inviting me, Patty.

01:50 Patty Skinkis

Glad to have you back and to tell us more about your work. I know you do really great research and you've been busy for the past few years. Can you tell us about the research you are conducting?

02:03 Matthew Fidelibus

Our research has basically been on raisin and table grapes. In raisin grapes, what we decided to do, because when we started this project, we had just happened to develop a new vineyard with a new type of raisin grape. Raisins have been made in more or less the same way for like all of eternity, by just harvesting ripe grapes and putting them somewhere to dry. There's been some development in the so-called “dry-on-vine,” where they cut the canes that bear ripe fruit when their fruit are ripe and allow them to dry on the vine, and then they harvest them and there's some labor savings there. But still, even with that type of production, it's still a person causing the fruit to dry either by picking the grapes off or cutting the canes.

But in recent years, there's been a new a type of raisin grape that's been developed, which dries on its own after ripening, so there's no reason to separate the fruit from the vine. It just starts drying on its own. And that really has the potential to revolutionize raisin production because it could allow complete mechanization of pruning and all of the raisin-making processes, which there's no other variety that's enabled us to do that before. But the problem with it is it's kind of a black box. In the past, growers have been able to have a lot of control over how ripe the grapes get and how soon they start drying.

That's important because the level of ripeness really dictates the quality, so the more sugar that the berries accumulate before they start drying, the better the quality. The other thing that growers don't have much control over is the weather. If the fruit don't start drying soon enough, then they're in danger of never being able to dry, because in the fall we start to get into shorter days and cooler days and more rain and so on.

And this natural DOV (dry on the vine) varieties are kind of like a black box. You don't really understand how they work, so we don't really know how to control it. As sort of a first attempt at this, we decided to set up a rootstock and trellis experiment to try to see how those things affected fruit quality and pre-harvest fruit drop, which is another unusual trait of this variety. The fruit actually fall off before they're harvested, which is obviously bad.

We had this opportunity to introduce a nutrition component to it when we started this work with a specialty crops research grant. I figured that nitrogen would certainly be an important nutrient to look at because these drying and pre-harvest fruit drop behaviors are, you can think of them, they're probably a form of senescence, and I've learned from previous experience that nitrogen can have an impact on how quickly fruit mature or don't, and the ultimate senescence basically of the leaves and also the fruit. So, we tested various rates of nitrogen on these new DOV varieties to see how they affected yield and quality. That was one of the main goals in raisins.

05:20 Matt Fidelibus

A secondary goal in this project-- and all of our projects--was to look at relationships between petiole and leaf nutrient content. Partly we did that because growers in my area, some of them prefer to test petioles and others prefer to test blades, and I wanted to have some data to show how those two tissues compare to each other. Another major part of this project, and by project now, I'm referring to the larger grants that we're working on, was to develop new tools for assessing the vine nutrient status and a lot of teams are working on remote sensing.

In remote sensing, they're going to be imaging the leaves for the most part, not petioles. I wanted to have a better understanding of how petiole nutrient content compared to blades, because if anyone is ever going to adopt these remote sensing estimates of nutrient content, they're going to have to have some understanding of how that relates to petiole tests. So, we were doing some work with that as well and then ,of course, reporting all of this data or sharing it with a remote sensing team so that they can make use of it in developing their models.

Then in table grapes, what we were mainly interested in there was developing nutrient budgets. So, we have been collecting tissues at all different stages of development, determining what the biomass is and what the nutrient content is. Then you can estimate the quantity of any of those nutrients by just multiplying the dry weight per vine of any particular organ times its content percent of these nutrients, basically. That gives you an estimate of how much of the minerals there are in the different organs at different times. What we can do with those--for one thing, we can determine what the peak amount of any of those nutrients is. Then we have at least a floor for replacing the amount of nutrients and a replacement program that you might develop for table grapes.

07:20 Patty Skinkis

I have a question for the nutrient budget work. Are you doing that work in a potted system or is this in a field-based system where you're digging up vines?

07:30 Matthew Fidelibus

That's a good question. We have done, as part of this project, some work with potted vines. The potted vine work--we harvested the roots and quantify the nutrients. The main interest, from my perspective, was using the leaves for remote sensing technologies that we were testing. I think that, especially in my region, trying to compare potted vine studies with the situation in the ground is... I don't have a lot of confidence in it because it's such a different system. The soils in my area are very deep, the vines are very big, have a lot of fruit, the canopy is very big, and I just feel like it's a very different situation from being grown in a pot. We did do some potted vine studies, and we did measure roots and so on. We did dig up some whole vines in the ground here at Kearney [research center] with a backhoe as part of this project, although I wasn't planning on discussing too much about it. It's a big job. I mean, it literally takes a full-size backhoe in several days to dig up a relatively small amount of vines and then sift through all that soil, trying to find all the roots. At some point, you just have to call it and allocate a certain amount of time for the operation because it's otherwise almost impossible to get every scrap of root out of there. The large pieces that contribute the most to the biomass, they're actually fairly easy to collect. Once the backhoe takes a scoop with the trunk base, you get almost all of the large structural roots come along with that one first scoop, but then after that there's just a myriad of smaller roots, just amazing, all throughout the whole soil profile.

9:17 Patty Skinkis

I ask that question because oftentimes in research we have these plots of data and nutrient budget work, in particular. Until you're a researcher who's done this work, you have no idea how much work goes into that and why there's so few studies. I get asked that all the time: “Well, why aren't there studies in this?” …because it's a hugely expensive operation to be able to do digging up plants and those sorts of things. I didn't want to take you off the rails too much on your thought process but wanted to have you comment on that a little bit.

9:51.05 Matthew Fidelibus

Luckily, one of my colleagues here at the station, he's now retired, Larry Williams, he's done a lot of digging, so I don't have to. That's kind of the way that I think of it. He has dug up quite a few vines and, looking at his data, it's pretty common that, from year to year, there's not a huge increase in mature vines in the nutrient content of the trunk and roots. I think it's an order of maybe 5% or 10% increase in mature vines year over year. Now, over time, that makes a huge difference because if you had a 5% to 10% increase every year in your retirement fund, you'd have a huge difference from now to 20 years from now, right? But from year to year, it's not a big increase, so 90% or so of the of the mineral nutrients that are accumulated over the course of the season, they go into the annual growth, which is the leaves, the stems, and the fruit. You can capture about 90% of how much nutrients the vines need in an annual budget, at least in mature vines, by just looking at those tissues.

Most of our data in mature vines are just simply from leaves, stems, and fruits. We didn't really look at the trunk and the roots. We did on occasion, but not regularly. The reason why, partly, it’s very expensive and very difficult, and other people have done it.

In mature vines, it's not a huge contribution from year to year, but in young vines, that's probably not true because the difference between a young vine, say in the first three years or so of its growth compared to an old vine, the biomass in young vines increases substantially from year to year in the trunk and the roots. Whereas the large vines don't have a huge increase in biomass of trunk and roots from year to year. Those young vines, there's probably a lot going on in the trunk and the roots that you need to be aware of, as compared to the older vines, when you're planning a nutrition program.

11:59 Patty Skinkis

You were on the track of talking about your table grape studies and the nutrient budgets.

12:02 Matthew Fidelibus

It kind of goes back to what you what we were just talking about. When you're trying to think of a budget for the vines. The information that you need is what the biomass of the organ is, let's say the leaves or fruit, at the time that you're taking the data, and then the other piece of information you need to know is what is the content of the of the nutrient, like for example, nitrogen. We usually measure or express nutrient content on a dry weight basis. You could, for example, take all the leaves or all the fruit off the vine and figure out what the dry weight is by putting them in an oven and weighing them a few days later. Then you could measure the nutrient content. Let's say it was 2%, so you would multiply the dry weight times 2% and then you would figure out how much nitrogen was in the fruit.

That's why from year to year, you don't really see a huge increase in either nitrogen or potassium content of the trunk or roots in mature vines because they don't increase in biomass very much. But in a young vine, the trunk and the roots increase in biomass a lot. That's why they needed so much more nutrition for their trunk and roots, in terms of the total seasonal accumulation than mature vines do. It relates to what you were just asking.

13:25 Matt Fidelibus

But in table grapes, the studies were mainly collecting NNK budgets mostly just from the annual growth because we were mostly working in mature vines, although we did have a young vine study in here as well. And then we're also looking at relationships between mineral nutrients and yield and quality, because it's always important to the growers that they understand that. We were trying to look at critical values there as well. We are, again, looking at relationships between petiole and leaf nutrient content.

And then also sending all of our data over to the engineering team so that they can make use of it in their and the development of their models for remote sensing of mineral nutrients. One thing that we realized in the course of our studies there and working with the engineering team is that leaf age really makes a big difference in terms of their ability to determine mineral nutrient content accurately. So initially we were just collecting all the leaves, say on a shoot and not differentiating between primary leaves and lateral leaves. But in my region, the lateral leaves eventually become very important in terms of their contribution to the canopy size. Later, we started collecting lateral leaves separately from primary leaves and reporting that data to them as well so they can develop, use that information in their models basically.

14:44 Patty Skinkis

Is it using of a relative percentage of amount of primary leaves versus secondary leaves (or lateral leaves) and then also leaf age going into some of their models?

14:56 Matthew Fidelibus

Yeah. One of the differences between lateral leaves versus primary leaves, in terms of their population, is that lateral leaves are just inherently younger than primary leaves because they form after primary leaves have already started to form for the most part since they're coming out of that leaf axil on a primary shoot. The lateral leaves are on the lateral shoots. What we found is that young leaves, in general, have high nitrogen, maybe the highest nitrogen, but they don't necessarily have the highest chlorophyll content, especially very young leaves, as a group. As the leaves age, they start to accumulate a lot of biomass that is not necessarily rich in mineral nutrients, mainly carbohydrates. Their dry weight tends to increase as they age, which is kind of the same with extension specialists. I think at least speaking for myself, I've accumulated some dry mass over time. The leaves are getting heavier basically, but they're not necessarily becoming more--they're becoming less rich actually in mineral nutrients because the weight that they're gaining is mainly carbohydrates. The nutrient content actually diminishes as the leaves get older. And then as they get really old, they start losing their chlorophyll content too. The old leaves tend to have low chlorophyll and low nitrogen content, but the young leaves tend to have low chlorophyll but high nitrogen content. The leaves that are kind of in the middle there, in between those two extremes, there's usually a good relationship between chlorophyll and nutrient content or nitrogen.

16:33 Patty Skinkis

That's a lot of your findings from working with the engineering team. I know that they were scanning a lot of those leaves. Ali Pourreza talked about some of his work on a few other podcast episodes we had. Would you say that the work is going towards more sensing of the canopy or more into models? After you feel like you've sensed canopies enough to know that if you have certain timing, you could just say we're at this point and maybe do less sensing, so actually putting out a sensor and more integrating models that calculate based on where the plant is at a certain phenological stage.

17:15 Matthew Fidelibus

I don't want to speak for the engineering team, but one of the ideas that we had was to collect very fine-scale data in the canopies as we started to realize these differences: these effects of leaf age on nutrient content and chlorophyll. That's why we were separating the shoots into lateral versus primary leaves, for example, and reporting those results to their team and also reporting the biomass in the leaf area that's in these different components of the canopy. That way they could develop models. They're actually, I'm not sure if it's in one of your podcasts, but I know that he's [Alireza Pourreza] been working with some really interesting 3D models of the canopy where you can assign things like nutrient content, for example, or reflectance data and alter one or the other and see how the predicted values of the response variable change. One of our goals as viticulturists was to provide the engineers with as much information as they can make good use of. It was a rare opportunity to collect a lot of data and share it with an interdisciplinary team to try to get the most out of this information that we can.

18:27 Patty Skinkis

What do you think at this point are the biggest findings from the vine nutrition status or plant physiology level from your work?

18:36 Matthew Fidelibus

One of the things that was a real eye opener to us, because we happened to be doing a nutrition study within our vineyard that was originally designed to test rootstocks, we ended up overlaying nutritional treatments over a rootstock trial, basically, which we hadn't done before, even though it's kind of an obvious thing to do. We hadn't done it. But when we did and we looked at the main effects of rootstock and fertilizer and even trellis, we found that all of these factors affected vine nutrition, which was surprising. It was surprising and not surprising. I mean, I had worked with enough rootstock trials in the past where everything was fertilized the same way to realize that rootstocks had a repeatable and strong effect on vine nutrition. I guess the reason why it's concerning to me is that it makes it difficult to develop a recipe that growers can use, regardless of what rootstock or even what trellis that they have, for nutrition. We might say, well, if you produce three and a half or four tons of raisins per acre, with this variety, you can probably expect to need about 40 pounds of nitrogen to replace the nitrogen that was in the fruit. You could say that, but that doesn't necessarily mean they need to apply 40 pounds of nitrogen. It depends on what rootstock they have. It might depend on what trellis they have. It might even depend on how they irrigate the vines.

19:58 Matt Fidelibus

These were things that are good information to have, but they end up requiring a nuanced explanation to growers. It's not as simple as just a one line, apply 40 pounds. We could say that they require 40 pounds, but how they get that is going to depend on the situation in your vineyard. That was one result that I thought was interesting.

The other thing I have been thinking a lot is this concept of critical values. And critical values sound like really critical, right? But trying to figure out what are they critical for has been something that's been kind of difficult to answer very clearly. We have found in some vineyards, particularly vineyards that started off in the trial as being deficient in some nutrient, like nitrogen, that when you fertilize those vines, there is a response in some variables. One of the most common responses we see is in berry size. In table grapes, you want to maximize berry size. We found that between 3 and 3.5% total nitrogen in the blades of bloom seems to be about when several varieties of table grape achieved their maximum berry size. That's kind of a high level of nitrogen and blades of bloom. Not every vineyard might have that. I'm not sure if this applies to every variety or not, but we found it in a few.

Critical values is something that we've been thinking about. I've also been thinking about, as I've been doing research on what does it mean to have a critical value for a plant. I came across an interesting article by Patrick Brown, who is a plant scientist at UC Davis. He wrote an opinion piece or a literature review on the concept of beneficial nutrients rather than essential ones. He pointed out that sometimes a level of a nutrient might be beneficial, but it's not essential.

When we think of essential elements, I was trained as a biologist, not really as a horticulturalist. When I first heard of essential nutrients in the context of grape growing, I was thinking of it as I was exposed to in plant science where this nutrient, at this level is required for the plant to complete its life cycle, basically. What does that mean in table grapes? And table grape growers have been telling me, you've given us a lot of information about nitrogen, a lot of information about potassium, we're pretty, clear and what we need in terms of how much nitrogen, potassium and so on. What they're more interested in, or some are interested in, is some of these other nutrients that we don't understand as well and how they contribute to product quality.

For example, calcium, that might be something that I work on in the future. And I think calcium in the context that they're interested in is more of like a beneficial nutrient. So of course, calcium has an essential role in plant physiology, and it's needed for the plant to grow normally. But in berries, we found that the calcium content of berries doesn't really change very much between veraison and harvest. The reason why is because after veraison, grapes tend to import most of the most of the metabolites and the water that they need through the phloem rather if than through the xylem. And calcium is not very phloem-mobile. So, most of the calcium that ends up into the berry through the vascular system is probably imported to the vine before or soon after veraison, and not in between veraison and ripening. Growers have found that if they can increase the calcium level of the fruit, that it has a longer shelf life, or maybe it's more firm. The way to do that would have to be a foliar application. That's not probably essential for the life of the berry or the life of the plant, that extra calcium application, but it could be very beneficial to the product quality. This concept of critical value, we might get away from using that term and it might be more that we're thinking of what are the beneficial amounts of nutrients in different organs rather than the critical amounts.

24:14 Patty Skinkis

Yeah, absolutely. I know we started having that conversation here in Oregon, where we have low crop yields, and our critical values were not really that critical. We turned to calling them sufficiency levels. Even then, that's after me working with Paul Shriner over the years that we call them sufficiency thresholds. If you were above it, you're okay. Even that, we kind of were still unclear on--maybe I was more unclear on it and he's more solid on what this would be. We got away from the critical value. You're using that term. Certainly, I know that from a grower standpoint, I think a lot of times they're looking at what is that value that shows up on their tissue sample that they just sent to a lab. Now they've got a threshold that's either reached or not, and there's a chance there to impact those as well.

25:07 Matthew Fidelibus

Right, I agree with that usage of sufficiency versus critical. At the same time, I think most growers, at least in my region, they have a sufficient amount of most nutrients. It's rare that you see a real deficiency, though we have seen deficiencies, even of important and common nutrients like nitrogen. We've seen it occasionally, but it's uncommon. So that's the low end, right, the sufficiency--you have to have at least this much or you start to see problems because you don't have enough.

What about adding a nutrient that is already biologically sufficient, like calcium? If we could provide even more to the fruit, for example, after it would normally be accumulating it, like after veraison, and we would see an improvement there. Maybe a measurable effect on firmness or something like that. What's the definition of that? That's “beneficial,” I guess. It's a beneficial amount of calcium, it's on the high end. Sufficiency or critical value, those are mostly concerned with the low end; make sure you're not deficient, right? But there could be a boost that's important economically, but not biologically. That's an area that I think needs a little bit more attention, at least in my region.

26:29 Patty Skinkis

What have you learned so far in the work that you're doing that is unexpected or is surprising to you? Or maybe you just have so much experience that nothing surprises you at this point.

26:42 Matthew Fidelibus

I’m trying to think, what was surprising? I mean, it was sort of surprising to me when I started doing biomass calculations, and the quantity of all these various nutrients in the different organs, just how much calcium there was in the leaves. I guess I just never really thought about it. I mean, I've looked at plenty of nutrient reports and you can look at the percent calcium, percent nitrogen and so on. And the calcium percentage is high or relatively high usually compared to those other nutrients. It shouldn't have been surprising to me what that translates to and the quantity of that nutrient. But still when I did the math, I was kind of surprised how much calcium there is in the leaves.

I have a tortoise and I'm always trying to figure out how to increase the calcium content in his diet. And I was thinking, God, a lot of these leaves actually already have a lot of calcium? I wasn't expecting how much calcium that they have. But the fruit, that's another matter.

27:32 Patty Skinkis

You've fed him some grape leaves?

27:34 Matthew Fidelibus

Yeah, he likes grape leaves. He only likes young grape leaves. He doesn't like old grape leaves. I don't know if they accumulate some non-palatable flavor or what. Actually, in this area where I live, grape leaves are another grape product that's actually harvested commercially. People collect leaves, in the spring for dolmas, basically for a product that is a leaf that they treat and can usually. Then you can use it for Mediterranean foods, often it's stuffed. I guess even people like the young ripe leaves, but not the older ones.

28:15 Patty Skinkis

Interesting. What are the next steps or future directions of your work? You already talked a little bit about this beneficial versus essential nutrients and calcium, but are there other areas that you think are worthy of consideration for vine nutrition research?

28:33 Matthew Fidelibus

Something that I need to do more research on to educate myself more, I think, as much as anyone else, is understanding the role of soil fixation in potassium availability. We know how much potassium that the vines accumulate over the course of a season, but then making a recommendation on that is a little bit more difficult for me to understand. Certain soils in our area, soils of certain origins, tend to trap a lot of potassium in the soil. There could be more work looking at the type of potassium fertilizer that you're applying, how you apply it, and the kind of soil type you're on, and maybe how you irrigate as well, to figure out what you really need to apply to satisfy those needs. We were applying big differences.

This year--I didn't really talk about it yet, but I don't want to talk about it too much because I don't have too many results yet--we did a rate study in this raisin vineyard where we were previously doing a nitrogen study. We did a potassium rate study, and we saw clear effects of the rate of potassium on the amount of potassium in the fruit--in the raisins. We were applying, let's say, 75 or even 150 pounds of potassium to these vines. The amount of potassium that increased in the raisins was really modest by comparison. At the zero rate, we had about 13 pounds of potassium per ton of raisins. In the 75 pound of K (potassium) per acre rate, we had about 15 pounds of potassium in a ton of raisins. And then when we applied 150 pounds of potassium per acre, we ended up with about 17 pounds of potassium in a ton of fruit. Our return on investment, in terms of pounds of potassium in the raisins is not very good. But, I don't have the data for the leaves and the stems yet, so I'm not sure what the total recovery efficiency was of that application. Just judging by the raisins, it doesn't seem like we were able to improve the potassium content of the raisins very much with big differences of potassium [fertilizer].

There's some certain nutrients where a replacement theory works very well, like nitrogen, for example. We found in most of our studies that if you replace the amount of nitrogen that you took out, in the fruit, as long as the vines had a good amount of nitrogen already and they're growing well, you could probably just replace the nitrogen in the fruit. And that works for a couple of years at least and doesn't seem to have any effect on the yield or the quality. But for something like potassium, I'm not as confident about because there's so many other factors that affect uptake of the potassium by the by the plant. So, I'm still thinking about like what does our potassium budget data mean in terms of a potassium application plan.

31:39 Patty Skinkis

That's really interesting because there were some Oregon studies on potassium and they weren't very clear either. We knew that foliar potassium didn't do much, but soil applied potassium really did not have much impact at all. It had some, but not much. And if you stopped, it really went back to deficient. Deficiency was not improved. It was constant adding of potassium; it wasn't a correction and then you're okay. I agree, there's probably a lot more work we could do on potassium to understand that better.

As you wrap up the project, I know you're involved in a lot of the outputs from the project. And certainly you being an extension specialist, I'm looking forward to working with you and generating some of the results and generating useful outcomes for the growers. I look forward to hearing more about your work.

To wrap up this episode, I have a fun question for you, although I thought the tortoise information was pretty neat. I'll add another fun question: Where is the most interesting place you've ever traveled to and why?

32:51 Matthew Fidelibus

Just for pleasure, I went to Tonga, actually for my 50^th birthday. My wife and I went snorkeling with humpback whales. That was pretty interesting. It was kind of terrifying at the same time but interesting.

33:09 Patty Skinkis

How close were you to these whales?

33:13 Matthew Fidelibus

Very, very close in some cases, not necessarily by design, but it turns out that humpback whale calves are very curious. They kind of act like a dog with zoomies and they will swim you know right up close to you in a very fast and kind of unpredictable way. I nearly got hit by one of their pectoral fins.

33:37 Patty Skinkis

You survived to tell about it. That's pretty cool. Thank you so much, Matt, for your updates. We look forward to hearing more about your work.

If you’re interested in learning more about Matt's project or the High Resolution Vineyard Nutrition Project, we have a website (www.highresolutionvineyardnutrition.com). And we're also on social media. There will be additional links with this episode that you can check out about Matt and about his work and the project.

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This podcast is funded through the National Institute of Food and Agriculture’s (NIFA) Specialty Crop Research Initiative Coordinated Agricultural Projects (CAP) grant. Project Award Number: 2020-51181-32159.

Audio mixed by John Adams.