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The Role of Micro- and Macro-Nutrients in Rehabilitating HLB Infected Trees

Dr. Davie KadyampakeniDr. Davie Kadyampakeni, an assistant professor studying citrus water and nutrient management at the UF/IFAS Citrus Research and Education Center, was the speaker during the seventh session of the 2020 BMP Summit. Dr. Kadyampakeni discussed the role of micro- and macro-nutrients in rehabilitating HLB-affected trees.

Nutrients play a major role in improving plant health. Liebig’s Law of the Minimum states that the scarcest resource will restrict plant growth, thus it is called the limiting factor. These factors can include soil, light, carbon dioxide, temperature, and nutrients. All nutrients must be available in the correct supply for the plant to flourish.

These are the main roles of important nutrients in plant health:

  • Nitrogen: vegetative growth, metabolism, flowering, fruit yield.
  • Potassium: physiological functions, fruit formation, enhancement of fruit size, flavor, color, and juice quality.
  • Phosphorus: Photosynthesis, synthesis and breakdown of carbohydrates, and the transfer of energy within the plant. Phosphorus is involved in nutrient uptake and translocation.
  • Calcium: involved in cell division and cell elongation, an important constituent of cell walls and plays a major role in cell membrane integrity. It is an important element for root development and functioning. Root growth is severely restricted in Ca-deficient plants.
  • Magnesium: involved in photosynthesis, plays an important role as an activator of several enzymes.
  • Sulfur: Essential constituent of many proteins, vitamins, and some plant hormones.
  • Manganese: involved in the production of amino acids and proteins, an activator of several enzymes, plays an essential role in respiration and nitrogen metabolism, plays a role in photosynthesis and in the formation of chlorophyll.
  • Iron: Catalyzes the production of chlorophyll, involved in some respiratory and photosynthetic enzyme systems.
  • Zinc: involved in several enzyme systems that regulate various metabolic activities within plants.
  • Molybdenum: Assists in the formation of plant proteins, helps starch, amino acid, and vitamin formation, acts as a catalyst that aids the conversion of gaseous N to usable forms by nitrogen-fixing microorganisms.
  • Boron: Plays a key role in plant physiological and biochemical processes. Needed in smaller supply, but very critical.
  • Copper: Fruit formation and vegetative growth, plant defense against bacterial infection.
Root distribution for drip-irrigated citrus trees. Photo credit: Dr. Arnold Schumann  Version:0.9 StartHtml: 105 EndHtml: 986 StartFragment: 334 EndFragment: 954

Figure 1. Root distribution for drip-irrigated citrus trees. Photo credit: Dr. Arnold Schumann
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The 4R’s of citrus nutrient management are right source, right rate, right time, and right place. Source refers to choosing the right type of fertilizer to suit soil properties and ensure a balanced supply of nutrients. Depending on the situation, commercial fertilizer, livestock manure, or crop residue can be used. Right rate refers to applying nutrients according to plant demand and soil composition. The amount of fertilizer used depends on the results of a soil test and an economic analysis to calculate the costs and benefits of the fertilizer. Right time means applying fertilizer when it is most useful to plants and least likely to be lost due to leaching. A leaf and soil test can be conducted to assess how much fertilizer is needed, and then the fertilizer should be applied in between rains to avoid runoff. Right place involves applying fertilizer where it can best be utilized by the plants. Citrus crop rooting patterns show that 70% of the roots are in the top 12 inches of soil, so the fertilizer does not need to be applied deeper than this. Soil and tissue sample analyses will show where nutrients should be applied, then apply using fertigation, a spreader or broadcast, or with a variable rate application.

Florida’s unique soil requires special management to ensure the proper soil moisture and nutrient concentration. Although soils vary by region of the state, they are typically sandy and have a low nutrient and water holding capacity. Growers want to keep nutrients in root zones and prevent leaching, so it is important to note how the soil type in various areas affects nutrient management.

The results of several research studies have shown how macronutrients influence the performance of HLB-affected citrus trees. For potassium (K), research has shown that potassium is best utilized by the trees when it is present in the upper six inches of soil, and that higher concentrations of the nutrient are present in irrigated/fertigated areas as compared to non-irrigated zones. Measurements showed that potassium accumulates the most in branches, twigs, and trunk than other parts. Conventional Microsprinkler Practice (CMP) showed the greatest K accumulation, followed by Drip Open Hydroponic Systems (DOHS) and Microsprinkler Open Hydroponic Systems (MOHS).

In the same study, nitrogen concentration in the different parts of the trees showed uptake patterns. In the HLB-affected citrus trees, nitrogen concentration was found to be 36% in leaves, 26% in branches, 20% in roots, and 19% in fruits. The study found that overall, there was 45% greater nitrogen accumulation with DOHS and MOHS than with CMP.  Phosphorus accumulation remained at approximately the same value for all three fertigation methods.

The effect of calcium and magnesium on the yield of HLB-affected trees was also examined. The trees with the greatest yield received 150 pounds of nitrogen per acre and 40 pounds of magnesium per acre. The right balance of nutrient application, with lower calcium and higher magnesium, has been shown to produce greater yields. Application of the correct amount of calcium and magnesium also caused an improved leaf area index and improved canopy density for Hamlin oranges on two rootstocks. Leaf concentrations of magnesium in the Cleopatra rootstock had a higher concentration than the swingle rootstock. The results also varied greatly by season. The effect of calcium on leaf concentrations was similar across all treatments and seasons.

Several micronutrients also had therapeutic effects on HLB-affected trees. Regular foliar fertilization is desirable for all nutrients depending on leaf analysis. Soil boron availability is greater in irrigated zones closest to the trees. It is available mostly between 15 and 30 cm from  the tree, and applying it closer to the root zone can improve uptake efficiency. Boron may leach easily, so it is important to apply it when the plant can utilize it. For longer retention, boron should be applied to the soil. Foliar application is ideal for correcting short-term boron deficiency but soil application is preferred. To keep the leaf nutrient content of boron between 150-250 ppm, use between residual boron (no application) to twice the recommended amount.

Zinc and manganese are held strongly in the rooting zone, with availability depending on soil moisture conditions and pH. Both manganese and zinc concentrations in the leaves were greatest with a foliar application of twice the UF/IFAS recommendations. Manganese fertilization needs to be adjusted by season because the leaf nutrient content varies greatly between months.

When managing nutrients for HLB-affected trees, the critical 4Rs should be followed: right rate, right source, right time, and right place. These are particularly important in Florida because citrus producing regions primarily have sandy soils with low nutrient holding capacity. Optimal and balanced nutrient management is important for high nutrient use efficiency and minimizing leaching. Good nutrient management is necessary for promoting tree performance under HLB conditions.

Written by Natasha Roberts, CLUE Communications Intern

Q&A

Q: Clearly, you’ve done a lot of work with micronutrients and nutrients in general for HLB citrus. Compared to today where we have HLB, to previously, did we know as much about micronutrients or did that become important because of HLB?

A: When we had trees that did not have HLB, we did not see the need for tracking micronutrients because at that time the micronutrients could be applied once a year at a very low rate and we would get good yields. But what we learned is that, due to the changes of the physiology of the plant, most of those micronutrients tend to open up parts of the vessels in the plant, like phloem and xylem, which are closed due to citrus greening; they help mobility of other nutrients in the plant. And then at the same time, disturb the bacteria that causes citrus greening. So it seems that if you are using standard rates and you just put a little bump up in the amount micronutrients, up to about 3x the current recommendations, we tend to get a better response of the trees; better root growth, better yield, and better nutrient use by  the tree. That’s what we have learned.

Q: Which nitrogen source would be suitable for ridge soils?

A: That’s a tough one. I would say to be on the safe side, because ridge soils are extremely well drained, we normally recommend using controlled release or slow release fertilizer sources during the rainy months. The other months I would say a liquid fertilizer would be good, but make sure you target applications every two weeks, and put them in smaller durations – rather than using fertigation for 2 hours, use it for 30 minutes to 1 hour; that is quite optimal. This helps with uptake rather than triggering more leaching of the nutrients. So, I would emphasize on ridge soils using mostly liquid fertilizers, slow release fertilizers, and control release fertilizers – those would be what I would call ideal. If you want to use other fertilizer sources, then I would suggest not putting heavy amounts at one time – you can start at applications of maybe 4x a year if you are using conventional fertilizer sources. But for now, with HLB, we recommend using fertilizer sources that enable spoon-feeding nutrients into the root zone. For example, if you are using liquid fertilizers then applying every 2 weeks would be okay. If you are using controlled release fertilizers those can last for about 6-9 months, which can keep the fertilizer in the root zone, and the plant can use it as needed.

Q: If Florida soils have good calcium concentration (they’re high in calcium), why is the addition of calcium still showing good effects on citrus trees? I’m assuming irrigation water is probably somewhat, if not moderately hard water with high calcium magnesium content – so we have all this calcium in the soil and water, why is the addition of more calcium having an effect?

A: That’s a good question. Actually it’s a puzzle that my colleagues and I are working on and are struggling with at the moment. What we have learned is that we have a little calcium in the water, but what happens when we apply to the soil, most of it is tied; it is not available to the plant. So sometimes we have it there in the soil, but it’s not taken up. For example, the plots where we are doing the studies in Immokalee and Lake Alfred, we had calcium levels at 100 ppm  or greater– that’s unheard of! Usually we recommend the calcium levels to be about 50-60ppm, but we are getting thousands of ppm. And then we did the leaf tests and then we realized that we had low levels of calcium in the leaves. So, what we noted is that we might have calcium in the soil but it’s not available to the plant. So it’s important to still supplement. The only good news is that if you add calcium it is not known to be toxic to the plant. If we were actually having some toxicity to the plant, we would have backed off, but we do not have that problem. And what we are anticipating is that, if you encounter pH problems – for example if the pH goes higher, like close to 8 – we can go ahead and acidify the soil. So once we inject acid into the soil, that calcium that was bound can be released back into the solution. So that’s how it works, but we have been backing off from acidifying for now because pH is not a problem.

Q: How many growers use the variable rate nutrient application based on GIS?

A: So far we’ve haven’t seen many growers use the variable rate. But most of them are using fertigation, though it’s not a typical variable rate, but more or less it goes towards that precision nutrient management application method. But, I haven’t seen many growers use it yet because it requires you to have a tractor mounted with a GIS, and you want to get a better instrument, and you need proper training. There is one grower using it near LaBelle – but not many are using it at the moment. We hope to move towards that in the next few years, and help growers to use those standard tools.

Q: That’s kind of what I thought. That’s much more intense in terms of equipment, training, and learning for the growers, and it’s expensive in money and time too. A lot of growers are using the seeing eye, which is not exactly precision based on GIS mapping of the field, but it is a precision type application, right?

A: Yes.

Q: So you said most growers are fertigating, so if that’s the case then they have to come back if they want to apply any micronutrient foliar, and they have to come do that in a separate operation as opposed to fertigation, correct? And is it common for them to do that?

A: Yes. Most of them, when they apply insecticides, they just include the micronutrients in small doses within the product so they do not have to do it 2 or 3 times, they do it at the same time. So for example, they do have micronutrients such as boron, manganese, zinc, and copper that get applied with insecticide applications. So, you’re right. The only thing is that you have to make sure when using fertigation is you don’t have issues with clogging, because we have both chemistries going on and have problems with them. So we encourage growers to have separate fertigation lines for the micronutrients and macronutrients, and then applied over several days to make sure you don’t have issues. For example, calcium can have a reaction with phosphorous, and manganese can have a reaction with iron. So we encourage using different times and separate tanks for fertigation to make sure that does not happen.

Q: Good point – to avoid the clogging. And if I understood you correctly, some of the micronutrients have a different effect, whether they’re applied by fertilization to the soil versus foliar application.

A: Yes, we have learned that for some of our rich soils where we have sandy coatings with iron and aluminum, they tend to be tied to the soil. They’ll be there but then they don’t want to let go into the plant. So we encourage adding some foliar fertilization to make sure they don’t end up having the fertilizer in the soil and then it’s not able to be taken up by the plant. We did not know this because there were times when we would apply the fertilizer but we didn’t see a response in the plant – we were wondering what was going on. So we did some lab studies to just look at what stays in the soil and what goes into solution, and that’s where we found that some of those nutrients like manganese and zinc could be tied up in the soil. And we want to do that study now across most of the sites, because we did that with the Entisols, but we want to do that on other soils now and document that quite thoroughly to see if that is a common pattern other than on the Ridge.

Q: Which fertilization method is recommend for citrus greening, controlled release or fertigation?

A: I would recommend both. Because at this point you want to make sure that the grower is not losing nutrients, but is still within BMPs – and does not exceed BMP limits in the amount you apply. So if you are fertigating when it’s raining – between June to September – most of the nutrients will be washed out, which is a violation. So we highly encourage growers to use fertigation during months of the year that are drier because the tree will need irrigation anyway. So you can put the nutrients with the irrigation water via fertigation. Once the rainy months start, you can apply the controlled release fertilizer (CRF). If you use a 6-month CRF product it will stay in the soil from June all the way to November, and then you can start fertigating again. That way you are limiting nutrient leaching without compromising nutrient availability in the root zone.

Q: One of the topics we’ve talked about in this series earlier, not with respect to citrus per se, but there’s some interest in, and I know some growers are using, is compost. So, trying to add a compost material or some kind of organic material to change the characteristics of the soil; increase soil water holding capacity, maybe change the cation exchange capacity (CEC) so it’s more favorable – what do you see out there in the industry, is there interest in this from citrus growers?

A: Oh yeah, that’s a good question! Actually, I ended up missing that here because I said, “let me just stick with the nutrients”. But that is one part we are encouraging growers right now. We are encouraging growers to add compost or manure to make sure they elevate the amount of organic matter they have. We have found that in most of our sandy soils that grow citrus the organic matter is 3% or below; mostly 0.5%, 1%, or 1.5%, which is just so low you can’t do anything with it. We are finding a way to get over that and be able to produce citrus and other crops. So we are encouraging our growers to apply the organic matter to grow cover crops; apply manure (horse, chicken, or cow) to make sure that they can provide the organic matter the soil needs, and improve the soils ability to retain nutrients and water.

Q: Related to that, how many growers are actually able to do that? I can see there’s some pockets in the state where you can get horse manure, such as Marion County, and poultry manure is all going to be up in the Panhandle. So if I’m a citrus grower in Hendry County, the cost of trucking that would probably be prohibitive. So, how many growers are actually doing this?

A: I don’t have a number, but there are about 3 big ones in Polk County with about over 2000 acres like Peace River Packing. The McKenna Brothers do composting and have over 5000 acres of citrus, and the Duda’s have over 2000 acres of citrus, and are using compost. Trees Orange Groves and Packing have close to over 1000 acres of citrus and are applying compost and horse manure too. Most are applying compost or horse manure twice a year. They don’t have clear guidelines yet because we haven’t provided recommendations. We’ve just been encouraging them to do it. We are hoping to come up with recommendations in the next couple of years for the right rates to apply, but for now they are applying 2-4 tons of manure or compost/acre/year.

Q: If I understood correctly, you had more root growth in the two hydroponic systems and greater nitrogen uptake. I was wondering if you could elaborate on why you think that happened.

A: We concluded this because at that time we used the conventional practices of applying nitrogen once a month via fertigation. Then, with the two other hydroponic systems the micro sprinkler was injecting nitrogen every week. Then the drip open hydroponic system was injecting 5 pulses of it. What we think was the catch was that the micro and drip open hydroponic systems were providing the nutrients right in the root zone, at the right time, and as needed – all the time. With the conventional micro sprinkler practice we applied it just once a month. We wondered if we were applying too much and might get some leaching. If we did that, it was for a short window, so it was not an issue where you would see big flats of leaching for any of the systems – they were kind of similar. We suspect that the two open hydroponic systems resulted in better nitrogen uptake because it kept the water within the irrigated zone, and anything outside of the irrigated zone didn’t get much water. This resulted in better root growth. That might have resulted in greater uptake compared to the conventional practices that were irrigating and fertigating uniformly around the tree, so the root mass and density was about 1/8 of what you observe with an open hydroponic system. That was very interesting because at that time the trees were just becoming HLB positive – they were still in that stage where they were transitioning from getting recently infected with HLB to now becoming HLB positive. So they were not declining on site and they were still at that optimum level.

Q: What are the future trends for research in citrus nutrition?

A: We want to solve every problem now. But we cannot solve everything in one day. What we are happy to move forward with now is we’ll be looking at different ways of applying fertilizer. So we’ll not only work with foliar, soil, fertigation, and controlled release fertilizer applications, but we will also use trunk injections to see if we optimize the amount of nutrients in the tree. We would avoid soil and foliar fertilization applications and put everything in the trunk. Also, moving ahead we’d like to compare irrigation rates. We are suspecting HLB-affected trees will use less water amounts than the amount of water we put in because trees that are affected by HLB have less root mass. So we want to make sure we put the right amount of water in the root zone and see what happens. And we might have to determine what we call the crop coefficients; what we use to determine how much water is required by the tree. We feel like the coefficients for HLB-affected trees might be lower than what we conventionally use in citrus. There have been studies that have shown what’s appropriate for citrus, but we think that because the trees affected by HLB have defoliation and limited roots, this might affect the citrus crop coefficients. Lastly, we may look at the balance of the ratios of nutrients in the soil, the tissue, and the roots – what’s happening in the roots, what’s happening in the tree, what’s happening in the leaves, and what’s happening in the soil. And when we find that sweet spot, we’ll be able to provide the right suite of recommendations to the grower. Some of those studies are currently underway, but a few of those still need to get funding for, so we are still putting in some proposals to make sure we can have long-term studies. In citrus you can’t do a one season study, you need to have 3-4 years of studies to come up with guidelines. We have guidelines that have recently been produced from a few studies we just completed, but we are hoping that if we do another set of studies for the next 5 years, we can come up with more robust recommendations for HLB-affected trees.