Final report for GS21-239
Project Information
Organic agriculture emphasizes the improvement of soil organic matter and stresses the foundation of maintaining healthy soil structure and diverse biological activity. With a fundamentally unique approach to soil fertility, the avoidance of many synthetic inputs leaves the organic grower much more dependent on soil biological processes that govern nutrient pools and transformations. Differences in fertility management and the challenges associated with navigating site- and season-specific variations in nutrient release and availability may result in different tissue nitrate contents in vegetable crops. Previous studies have indicated that nitrate content is often lower in organic foods than their conventional counterparts. Celery is known as a nitrate accumulator, and celery powder has been utilized as an alternative to traditional food preservatives. Regardless of the controversy associated with nitrates and human health and the restriction on maximum nitrate levels of certain leafy greens in Europe, the USDA National Organic Program currently places conventional celery powder on the National List for non-organically produced agricultural products allowed as ingredients in processed products labeled as Certified Organic. Concerns have recently been expressed questioning the use of conventional celery powder in the production of organic processed meats, although a lack of consistent, research-based recommendations for organic growers to meet the needs of the processing industry remains a significant barrier. To date, little research-based information is available regarding varietal differences in celery nitrate levels, and the impact of soil fertility management on nitrate accumulation in celery.
In addition to the inherent variation of tissue nitrate content between species and within organic and conventional counterparts, there are also other factors that can influence the accumulation of nitrate. Intraspecific variation among different cultivars is common and has been observed in species including celeriac and lettuce. Diurnal fluctuation in nitrate content has been observed in several species, likely due to the influence of light quality and intensity on the activity of nitrate reductase. Given the lack of information on nitrate accumulation dynamics in organically grown celery, this project focuses on the examination of cultivar, harvest time, and nutrient management factors affecting nitrate contents in celery under organic production. Findings from this project will help to improve our understanding of celery nitrate accumulation and will also provide meaningful information toward promoting timely soil nitrogen (N) availability, minimizing N losses, and enhancing crop productivity under organic management.
Objective 1: Assess commercially available celery cultivars in terms of their yield performance and capacity to accumulate tissue nitrate throughout the growing season under organic production.
Objective 2: Determine the effect of early morning and afternoon harvesting on the accumulation of tissue nitrate in organic celery.
Objective 3: Compare different rates of total seasonal N application using various preplant and liquid organic fertilizer types to quantify effects of fertilizer source and rate on crop yield, soil N availability, and tissue nitrate content.
Cooperators
- (Educator)
Research
Objective 1: Assess commercially available celery cultivars in terms of their yield performance and capacity to accumulate tissue nitrate throughout the growing season under organic production.
In Sep. 2021, a selection of 10 commercially available cultivars of celery (‘Balada’, ‘Conquistador’, ‘Command’, ‘Kelvin’, ‘Merengo’, ‘Pink Plume’, ‘Tall Utah’, ‘Tango’, ‘TZ 6200’, and ‘Victoria’) were seeded in a research greenhouse at the University of Florida campus in Gainesville, for producing transplants to be used in the cultivar evaluation trial under organic production at the UF/IFAS Plant Science Research and Education Unit in Citra, FL. Plants were supplied with 200 mg/L N using 2N-1.3P-0.8K Neptune’s Harvest fish and seaweed fertilizer (Gloucester, MA) via weekly fertigation, and eight weeks after emergence on 22 Nov. 2021, plants were transplanted into plastic-mulched raised beds in double rows (1.2 m between-bed spacing and 30 cm plant spacing) with drip irrigation. Nature Safe 10N-0.9P-6.6K granular organic fertilizer (Darling Ingredients Inc., Irving, TX) was applied at 35% (78.8 kg/ha N) of the total seasonal rate of 225 kg/ha N as a preplant application, and weekly fertigation using Aqua Power liquid fish fertilizer 5N-0.4P-0.8K and Big K 0N-0P-41.3K (JH Biotech Inc., Ventura, CA) supplied the remaining N along with potassium (K) needed throughout the season. No supplemental phosphorus (P) was targeted due to very high levels of soil P at baseline soil testing, and K was applied at a total rate of 148 kg/ha. The experiment was arranged in a split plot design with four replications, where cultivar and harvest dates on a 21-day interval served as the whole and subplot factors, respectively. Total aboveground fresh weight was determined at each harvest based on six celery plants from each experimental unit. The sampling unit was reduced to four plants as biomass increased during the season. Sub-samples from each experimental unit were divided into leaf and petiole tissues to identify differences in nitrogen accumulation across different tissue types. Leaf and petiole tissue samples were dried separately to a constant weight at 60 °C and ground to pass a 3 mm sieve, where total N content was determined at Waters Agricultural Laboratories (Camilla, GA). Nitrate was extracted by boiling 2 g fresh tissue in 10 mL deionized water for 30 min. Solution was filtered and analyzed for nitrate-N content using the nitration of salicylic acid microplate assay. Briefly, salicylic acid in concentrated sulfuric acid was reacted with nitrate-containing samples, and sodium hydroxide halted the reaction after 20 min. Samples were then analyzed at 410 nm and compared to a standard curve developed using KNO3 standards. Fresh tissue was centrifuged at 14,000 × g for 30 min at 4°C, and supernatant was filtered through cheesecloth before analysis of total soluble solids using a Brix refractometer. At the final harvest, relative chlorophyll content was measured from most recently mature leaves in each experimental unit using a SPAD-502Plus Chlorophyll Meter (Spectrum Technologies, Aurora, IL), and crown diameter was measured using a digital caliper at the cut surface immediately above the soil line. Plant height was assessed my measuring from the bed top to the highest leaflet tip. Leaf number was recorded by counting leaves longer than 12 cm. Celery biomass, nitrate-N content, and total N accumulation curves were developed using data across the production season.
Objective 2: Determine the effect of early morning and afternoon harvesting on the accumulation of tissue nitrate in organic celery.
In the same cultivar evaluation study described in Objective 1, celery cultivars were harvested in the morning (±30 min from sunrise) and in the afternoon (±30 min from solar noon) on each sampling date, following protocols outlined in Objective 1. Harvest time was added to the experimental design as a sub-subplot factor, so that interactions between cultivar and sampling time can be analyzed for each harvest date.
Objective 3: Compare different rates of total seasonal N application using various preplant and liquid organic fertilizer types to quantify effects of fertilizer source and rate on crop yield, soil N availability, and tissue nitrate content.
In Nov. 2021, a trial was initiated to compare various seasonal N application rates and different granular organic preplant fertilizer sources in organic celery production. Celery transplants (‘Tango’) were produced in a greenhouse as previously described, and subsequently transplanted into plastic-mulched beds with drip irrigation. Prior to transplanting, false beds were constructed, and preplant fertilizers were incorporated into the soil using a rototiller, per the experimental design. The experiment was structured in a split plot design, with the total seasonal N application rate as the whole plot factor and the preplant fertilizer source as the subplot factor. Total seasonal N application rates included 140 (N140), 224 (N224), and 308 (N308) kg/ha. Preplant fertilizer was applied at 35% of each total seasonal rate, with the remaining N applied using 5N-0.4P-0.8K Aqua Power liquid fish fertilizer through weekly in-season fertigation. A zero-N control (N0) and a preplant-only control (N78C) were included, matching the 35% preplant contribution of N224 (78 kg N/ha), but omitting any in-season fertigation. Additional 0N-0P-41.3K was applied as necessary to achieve a total K application rate of 148 kg ha-1 across treatments. The two types of preplant fertilizers included Nature Safe 10N-0.9P-6.6K and Everlizer 3N-1.3P-2.5K (Organic Growing Solutions, Live Oak, FL). Nature Safe 10N-0.9P-6.6K is a commercially formulated, granular organic fertilizer composed of processed feather-, meat-, bone-, and bloodmeal along with additional sulfate of potash. Everlizer 3N-1.3P-2.5K is a heat-processed poultry litter product. These preplant fertilizers can be considered contrasting in their N source and overall composition but are both popular products used by local organic growers in the North-Central Florida region. Plant samples were taken on 69 and 101 days after transplanting (DAT) and were weighed, dried, and processed as previously described to assess aboveground biomass and tissue nitrate content. Total N content was determined at Waters Agricultural Laboratories, Inc. (Camilla, GA) by dry combustion analysis.
In Feb. 2022, an expanded replication of the Fall 2021 fertilization study was initiated, aiming to compare different total seasonal N application rates as well as both preplant and liquid organic fertilizer sources. ‘Tango’ celery transplants were grown as described in Objective 1 and transplanted into similar plastic-mulched beds. The study was arranged as a split-split plot experiment, with liquid organic fertilizer as a fertigation source, total N application rate, and preplant granular fertilizer source serving as whole, sub, and sub-subplot factors, respectively. Liquid fertilizer sources were either Aqua Power Liquid Fish Fertilizer 5N-0.4P-0.8K or Allganic Nitrogen Plus Chilean Nitrate 15N-0P-1.7K (SQM Industrial, Santiago, Chile). The total seasonal application rates of N included 0 (N0), 84 (N84), 168 (N168), 252 (N252), and 336 (N336) kg/ ha. A preplant-only control was included (N88C) matching the preplant contribution of N252 as described below but omitting in-season fertigation. As in the previous experiment, either Nature Safe 10N-0.9P-6.6K or Everlizer 3N-1.3P-2.5K was applied as a preplant fertilizer source at 35% of each total seasonal N application rate, with the remaining N applied through in-season fertigation according to the experimental design. Plant samples were taken at 72 and 94 DAT, corresponding to a midseason and final season assessment. Yield, tissue nitrate content, and total N content were determined as previously described.
In both experiments focused on N application rates and organic fertilizer sources, soil NO3-N content and NO3-N fluxes were monitored on a weekly basis beginning immediately after preplant fertilization and continuing for 6 weeks after preplant fertilization (WAPF) in the trial initiated in Nov. 2021, and for 10 WAPF in the trial initiated in Feb. 2022. Soil NO3-N content was determined at the 0-30 cm depth through traditional soil testing using a soil probe, and soil NO3-N fluxes were monitored using anion exchange membranes.
Data Analysis
To analyze the data of the organic celery cultivar trial and fertilization management studies, a linear mixed model was used within the GLIMMIX procedure of SAS (Version 9.4, SAS Institute, Cary, NC). Multiple comparisons were conducted using Fisher’s Least Significant Difference test at P≤0.05.
Cultivar Trial
In the organic celery cultivar trial, cultivar did not significantly affect crown diameter or plant height at final harvest, though significant differences in relative chlorophyll content (expressed in SPAD value) were observed at final harvest across cultivars. ‘Pink Plume’ showed significantly higher SPAD values compared to all other cultivars, and ‘Tall Utah’ exhibited SPAD values significantly higher than ‘Tango’ or ‘Victoria’. At final harvest, celery yield was significantly affected by cultivar selection, where ‘Command’, ‘Merengo’, and ‘TZ6200’ exhibited impressive yield performance, significantly outperforming ‘Kelvin’, ‘Pink Plume’, ‘Tall Utah’, ‘Tango’, and ‘Victoria’. Celery yield was affected by the significant two-way interaction between cultivar and harvesting date, where the early harvest at 70 DAT did not show differences among cultivars. NO3-N content was not significantly affected by cultivar across harvesting dates, though NO3-N accumulation was affected by the significant two-way interaction between harvesting time and cultivar, where ‘Merengo’ and ‘Tango’ showed increased NO3-N accumulation in the afternoon harvests. NO3-N accumulation was also affected by the significant two-way interaction between harvesting date and harvesting time, where successive significant increases in NO3-N accumulation were observed from early, mid-, and late season harvests at 70, 91, and 112 DAT, and afternoon harvesting resulted in significantly higher NO3-N accumulation at 112 DAT. Soluble solids content was significantly higher in the afternoon at all harvesting dates, suggesting diurnal accumulation of photosynthates and NO3-N. Interestingly, nitrate reductase (NR) is a light-mediated enzyme, so more research should focus on monitoring cultivar-specific enzyme activity, diurnal fluctuations in nitrate content and accumulation, and dynamic NO3-N uptake and assimilation patterns in celery.
Fertilizer Source and Nitrogen Rate Studies
In the fertilization management and N rate study initiated in Nov. 2021, Nature Safe as a granular organic preplant fertilizer significantly increased soil NO3-N fluxes for the first 3 weeks after preplant fertilization (WAPF) across all fertilizer rates, compared to Everlizer. The soil NO3-N flux peaked at 3 WAPF, where Nature Safe resulted in a 2.5-fold increase in flux relative to Everlizer. Total seasonal N application rate also exhibited significant impacts on NO3-N fluxes from 3-6 WAPF, where N308 consistently maximized NO3-N fluxes. N308 exhibited significantly higher fluxes than N140 or N78C at 3-5 WAPF, although N140 and N78c were similar from 1-6 WAPF. In the follow-up study initiated in Feb. 2022, similar trends in soil NO3-N flux were observed, where Nature Safe resulted in increased early season flux. Under both Nature Safe and Everlizer, soil NO3-N flux peaked at 2 WAPF, though Nature Safe exhibited over a 150% increase in soil NO3-N flux compared to Everlizer. Monitoring soil NO3-N fluxes can provide insight into the mineralization patterns of organic fertilizers. Likewise, variations in cumulative soil NO3-N fluxes and peaks in soil NO3-N flux highlight the role of site- and season-specific factors in mediating N mineralization, facilitating or inhibiting NO3- leaching, and driving seasonal availability of N from organic fertilizers.
In the Nov. 2021 experiment, preplant fertilizer had a significant effect on organic celery yield at 69 DAT, with Everlizer significantly increasing biomass across all rates compared to Nature Safe. N application rate also had a significant effect on organic celery yield at the midseason harvest, and as N application rate increased, significant increases in yield were seen up to highest rate N308. At the final harvest on 101 DAT, Everlizer significantly improved yields at each fertilizer rate, with N78C< N140 < N224 < N308, while Nature Safe showed similar yields at N140 and N224, and N308 did not significantly increase yields compared with N224. At the final harvest in the follow-up study initiated in Feb. 2022, similar effects of organic fertilizer source and N application rate were observed, with the significant two-way interaction resulting in significant yield improvement between N252 and N306 under Everlizer, but not under Nature Safe. Significant differences in crop yield between Nature Safe and Everlizer were observed at the highest N application rate N336 and in the preplant-only control N88C, further supporting the trend that early season differences in N mineralization can impact late season crop yield.
At 69 DAT in the nutrient management trial initiated in Nov. 2021, N application rate significantly affected plant tissue NO3-N content and accumulation. While N78C increased tissue NO3-N content compared with N308, NO3-N accumulation was highest in N224, significantly higher than both N140 and N78C. N accumulation was also promoted under Everlizer. At the final harvest, N application rate significantly affected plant tissue NO3-N content and accumulation, where NO3-N content was maximized under the preplant-only control N78C, and NO3-N accumulation was maximized at the highest N application rate of N308. In the follow-up study initiated in Feb. 2022, Tissue NO3-N content on a fresh weight basis was significantly increased across rates and preplant fertilizers when using liquid fish fertilizer, exhibiting an over 50% increase relative to Chilean nitrate at 72 DAT.
In terms of plant total N accumulation, Everlizer organic fertilizer promoted N accumulation in the Nov. 2021 trial at 69 DAT, increasing total N by 20% compared to Nature Safe. Nitrogen application rate also significantly affected aboveground total N accumulation at 69 DAT, following the order of N78C < N140 < N224 < N308 on a per plant basis. At 101 DAT, there was a significant two-way interaction between N application rate and preplant granular organic fertilizer source. While N308 resulted in the highest total N accumulation across organic fertilizer sources, the difference between N308 and N224 was only significant under fertilization with Everlizer but not Nature Safe. Further, while N78C and N140 were similar under fertilization with Everlizer, N140 had significantly greater total N accumulation compared to N78C under Nature Safe, resulting in a reduction in total N of over 75% in N78C compared to N140. The interaction between fertilizer source and N application rate highlights the differences between the two preplant organic fertilizers in their N supply to a long-season crop like celery. In the follow-up study initiated in Feb. 2022, N application rate and source significantly impacted N accumulation at 72 DAT, in line with the Fall 2021 trial. In this follow-up study, Everlizer enhanced N accumulation compared to Nature Safe at all N application rates, and N336 maximized N accumulation under both organic fertilizers. Differences between the preplant granular organic fertilizers suggest enhanced N use efficiency with Everlizer, and continued N accumulation with increasing N application rates suggests that plants are able to effectively utilize and uptake the N from higher application rates. At the final harvest on 94 DAT, the significant two-way interaction demonstrated continued N accumulation from N252 to N336 under Everlizer, despite no significant differences beyond N252 with Nature Safe, closely in line with observations from the Fall trial. Overall, these soil and plant tissue data suggest differences in the decomposition and mineralization of organic fertilizers, indicating the need for further tailoring nutrient management to organic systems to enhance N use efficiency, increase crop yields, and minimize N losses.
Educational & Outreach Activities
Participation Summary:
Research findings were disseminated at the American Society for Horticultural Science 2022 and 2023 Annual Conferences, and the Florida State Horticultural Society 2022 Annual Conference.
Ray, Z.T. L. Zotarelli, and X. Zhao. 2023. Effects of Organic Fertilizer Source and Nitrogen Application Rate on Growth, Quality, and Tissue Nitrogen Dynamics in Organic Celery. American Society for Horticultural Science Annual Conference, Orlando, FL.
Ray, Z.T., L. Zotarelli, and X. Zhao. 2023. Evaluating Integrated Nitrogen Fertilization Programs for Organic Celery Production. American Society for Horticultural Science Annual Conference, Orlando, FL.
Ray, Z.T., L. Zotarelli, and X. Zhao. 2022. Comparing Two Types of Granular Organic Fertilizers in Organic Celery Production. Florida State Horticultural Society Annual Meeting, Sarasota, FL.
Ray, Z.T. and X. Zhao. 2022. Yield and Quality Attributes of Organic Celery as Affected by Cultivar Selection and Harvest Scheduling. American Society for Horticultural Science Annual Conference, Chicago, IL.
Research results from this project were also presented at the ‘Research Updates in Organic Vegetable Production Field Day’ in December 2022, as well as at the ‘Integrated Organic Vegetable Production Systems Field Day’ held in January 2024, both at the UF/IFAS Plant Science Research and Education Unit in Citra, FL. At both field day workshops, work related to soil mineral N dynamics using organic fertilizers at preplant and during the production season was discussed with farmers and agricultural professionals. Discussions were led in regard to the benefits of integrated approaches to nutrient management, through the application of organic fertilizers, composts, and the incorporation of cover crop residues. Field plots were also toured where attendees made observations of different N fertilization treatments, and we further discussed ongoing research objectives, preliminary findings, and future research needs.
Blog posts focused on organic celery production are in preparation for dissemination through UF/IFAS Extension, and EDIS (Electronic Data Information Source) Extension articles focused on organic celery cultivar selection and integrated nutrient management in organic crop production are in preparation. Two manuscripts for peer-reviewed journal publications are also being prepared for submission.
Project Outcomes
This project aimed to identify celery cultivars that are well-suited for organic production in the Southeast, and to optimize N application rates from different sources of granular and liquid organic fertilizers. By contributing to research focused on cultivar selection and integrated nutrient management under sandy soil conditions found throughout the Southeast, these research findings will enhance agricultural sustainability through improved soil health, plant productivity, and input use efficiency. Specifically, this project sheds light on strategies for reducing N losses and better synchronizing N supply with crop demand, a critical challenge for organic producers in the region and beyond. These insights into nutrient management can help to guide future recommendations that aim to optimize resource use while reducing environmental impacts.
Economically, identifying regionally adapted cultivars and fine-tuning N application rates can help organic growers to achieve improved yields and reduce input costs. Through the synchronization of nutrient supply with crop demand and the minimization of nutrient losses, farmers can improve the profitability of organic operations. Introducing affordable tools like AEMs for the dynamic monitoring of soil nitrate-N can also support precision nutrient management, further contributing toward economic sustainability by guiding informed decisions.
Environmentally, our emphasis on enhanced N use efficiency can help protect resources like groundwater by reducing the risk of nitrate leaching. By contributing to more responsible nutrient management, this research offers the potential to improve long-term metrics of soil health and preserve local agroecosystems. While this project is focused on organic celery, the insights gained from this project can inform updated recommendations and decision-making in other vegetable cropping systems, offering broader benefits across diverse organic and sustainable cropping systems.
Socially, this project can benefit both local farmers and the wider community. Growers can gain practical skills and knowledge to improve their operations by implementing monitoring tools and adopting an integrated approach to organic nutrient management. Meanwhile, consumers can benefit from the increased availability of locally grown, nutritious organic produce. By fostering more sustainable farming practices, our research findings contribute toward the overall health and resilience of southeastern farming communities, promoting a stronger local food system.
Through the research trials related to organic celery cultivar selection and organic nutrient management, my advisor and I greatly expanded our understanding and practical skills in the realm of sustainable agriculture. This unique project allowed us to further refine our expertise in integrated nutrient management, particularly as it relates to the optimization of nutrient availability and use efficiency in organic systems. As a graduate student, I also developed practical skills ranging from field experimental design and setup, data collection and analysis, and project management. A key theme that emerged was the critical role of organic fertilizer sources in N mineralization, highlighting important considerations for developing organic nutrient management recommendations. We also explored the use of affordable monitoring tools to provide solutions for growers that aim to monitor soil N status, such as the anion exchange membranes for dynamic monitoring of soil nitrate-N fluxes. Tools like these can offer great potential for precision nutrient management, further enhancing sustainability in organic farming.
As the graduate student leading the project, I was grateful to gain hands-on experience meeting with local growers, county Extension agents, and other stakeholders during Extension and outreach events. I also received two academic scholarships from the Florida Agrichemical Association where I was afforded additional opportunities to network with agricultural professionals and discuss the implications of this project. Research findings shed light on the importance of synchronizing nutrient supply with crop demand through integrated approaches, while also raising new questions. For example, the potential role of composts and cover crops in these integrated systems warrants further exploration, as does expanding the work to other vegetable crops to identify crop-specific responses to different organic fertilizers and amendments. These future areas of inquiry provide nuance to our findings and represent exciting opportunities for future exploration and collaboration toward the advancement of sustainable agricultural practices.
Thank you very much for the funding support! We hope SARE continues to support graduate studies focused on evaluating and developing sustainable agricultural and food systems including organic farming systems.