When HLB arrived in Florida, it changed the world for citrus growers, and a lot of other things had to change with it. One of the biggest changes has been how growers manage their fertilizer programs.
The nutrient recommendations that had been developed by the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) before HLB became outdated because trees no longer take up fertilizer the same way after infection.
LESSONS LEARNED SO FAR
Davie Kadyampakeni, UF/IFAS associate professor of citrus water and nutrient management, has been leading research efforts to update fertilizer programs for HLB-affected citrus. He is conducting a six-year study to develop new recommendations.
Photo by Frank Giles
“The goal of this research is to improve our understanding of how integrated plant nutrition using fertilizer blends containing a little bit of every nutrient in a balanced fashion along with some chemicals (for controlling fungal and bacterial diseases and soil pathogens) would mitigate tree decline against HLB,” Kadyampakeni says.
It has been a learning process each year as the research has progressed. One of the biggest takeaways is that it takes time for HLB-affected trees to respond to good nutrition programs.
“First, we learned that plant response to improved nutrition takes time, in our case between 24 to 36 months,” he says. “Second, we noted that combined use of fertilizer blends and crop protection chemicals restored root density and mass close to pre-HLB levels that would, in the long-term, help HLB trees with improved nutrient and water acquisition, and eventually improved fruit yields. Finally, we also found improved juice quality, especially from the third to the sixth year of the project with Brix reportedly around or greater than 10.5.”
ALL NUTRIENTS MATTER
The research has been attempting to analyze the role and importance of each nutrient in promoting tree growth and health. The research has helped confirm Liebig’s barrel theory: The barrel can only be filled as high as its shortest stave. Similarly, plant growth will be limited by the nutrient or input in shortest supply.
Photo courtesy of UF/IFAS
“We are now realizing with HLB that every nutrient is critical. Growers cannot take the luxury of omitting a nutrient in their program unless it is in excess from the periodic leaf tests,” Kadyampakeni says. “We are also learning that HLB trees need more frequent fertilization for conventional granular fertilizers. Applying three times a year for controlled-release fertilizers or 20 to 30 times a year for liquid fertigation will promote nutrient uptake and minimize leaching on Florida sandy soils. All essential nutrients are critical, including nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, manganese, boron, zinc, iron and molybdenum.”
Given the ravages of HLB, many growers are running on budgets that have them looking for ways to cut back and save money. But Kadyampakeni suggests growers should not dial back their nutrient programs if at all possible.
“Rather than cutting back, I would explore using alternative but cheaper sources of fertilizers that are available (compost, etc.). Also, we have been learning about legume cover crops that fix nitrogen in the soil and can build nitrogen in the soil over time. This helps growers cut back on nitrogen fertilizer needs in the long-term,” he adds. “And some cover crops sequester micronutrients such as zinc and manganese in seasonal crops, but this is yet to be well documented in citrus.”
FERTILIZER AND TRUNK INJECTION
With a large percentage of commercial citrus growers in Florida now applying oxytetracycline (OTC) trunk-injection therapy, Kadyampakeni has rolled that into his observations as the nutrient research continues. He says there is more to learn about OTC and nutrient interactions.
“This is a tricky question because we have not had an opportunity to test variable treatments with OTC and assess the interaction,” he says. “My speculation is that growers need to keep their nutrition program at its optimal level to get the best results with OTC. Doctors typically recommend taking medication with a meal. The meal in this case is provided through a solid fertilization program.”
NUTRIENTS AND BMPs
The citrus nutrient research is being conducted in concurrence with plans to update the state’s best management practices (BMPs) manual for citrus, which is administered by the Florida Department of Agriculture and Consumer Services (FDACS). The updated manual will use UF/IFAS recommendations as guidance for the program which regulates nutrient and water management in citrus and other crops.
“The ongoing BMP study is addressing challenges related to nitrogen and phosphorus. We are conducting trials on different rates of nitrogen and phosphorus at several locations to account for variability in space and time,” Kadyampakeni says. “So far, we have not yet established trends in production and canopy size and need more time for research. We also have yet to finalize water quality and economic analysis to see the impacts on the environment and profitability of each nutrient application rate. We have done this work for two years but would like to do more study for an additional three years to factor in variability due to extreme weather such as hurricanes and freezes, and of course, HLB impacts.”
So, when will all this work show up in new BMPs? J.P. Fraites, assistant director of the FDACS Office of Agricultural Water Policy, addressed that question during a recent meeting at the Mid-Florida Research and Education Center in Apopka. In short, it is a work in progress.
“Those research reports are currently being collected for all the crops where updates are being made, and we don’t have a final report for those rates (in citrus and other crops),” Fraites said. “The way that the manual and the rules are developed, we don’t include the specific rate in the manual. We provide guidance on using the highest rate. So, when the new rates have been determined and published (by UF/IFAS), we will update the manuals to fit the specific results of those reports.”
COLLABORATION IS KEY
Kadyampakeni says grower involvement in his nutrient management research has been invaluable. Working in commercial groves in a real-world environment is the only way to confirm research recommendations.
“I want to thank our grower collaborators and other sponsors, especially Bayer and YARA for our six-year study and FDACS for supporting our citrus BMP research,” he says.
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