Quantifying and Understanding Factors Affecting Tissue Nitrate Accumulation in Organic Celery

Progress report for GS21-239

Project Type: Graduate Student
Funds awarded in 2021: $16,497.00
Projected End Date: 08/31/2024
Grant Recipient: University of Florida
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Dr. Xin Zhao
University of Florida
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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 lower in organic foods than their conventional counterparts in many cases. 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 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 improve our understanding of celery nitrate accumulation in relation to soil nitrogen (N) availability and crop productivity.



Project Objectives:

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.


Materials and methods:

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 Nov. 2021, 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-1 N using 2N-1.3P-0.8K Neptune’s Harvest fish and seaweed fertilizer (Gloucester, MA) via weekly fertigation, and eight weeks after emergence (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-1 N) of the total seasonal rate of 225 kg ha-1 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-1 based on UF/IFAS recommendations. 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 nitrate 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. Nitrate was extracted by boiling 0.5 g dried tissue in 10 mL deionized water for 30 minutes. 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.


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 after sunrise) and in the afternoon (30 min after 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, another trial was initiated aiming to compare different seasonal N application rates and contrasting granular organic preplant fertilizer sources in organic celery production. Celery (‘Tango’) transplants were produced as previously described and transplanted into similar plastic-mulched beds with drip irrigation. First, false beds were formed and preplant fertilizers were incorporated according to the experimental design using a rototiller. The experiment was arranged in a split-plot design, with total seasonal N application rate and preplant fertilizer source serving as whole and subplot factors, respectively. Total seasonal N application rates included 140 (N140), 224 (N224), and 308 (N308) kg ha-1. 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 preplant contribution of N224 (78 kg N ha-1) 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 granular organic fertilizer commercially formulated and composed of processed feather-, meat-, bone-, and bloodmeal along with sulfate of potash. Everlizer 3N-1.3P-2.5K is a heat-processed poultry litter product. These preplant fertilizers are 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 also 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 onto similar plastic-mulched beds. The experiment was arranged as a split-split plot experiment, with liquid organic fertilizer as a fertigation source, total N application rate, and granular preplant 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-1. 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 the total seasonal N application rate, with the remaining N applied through in-season fertigation according to the experimental design.

Preplant fertilizers were incorporated using a rotary tiller as described in the previous experiment, and soil NO3-N content and fluxes were monitored on a weekly basis beginning immediately after preplant fertilization and continuing for 10 weeks after preplant fertilization. In addition to monitoring soil N dynamics, biometric parameters including relative chlorophyll content, crown diameter, plant height, and leaf number were assessed at 72 and 94 DAT, corresponding to midseason and final harvest plant samples where total yield, soluble solids content, and total N and nitrate contents were determined.


Data Analysis:

To analyze the data of the organic celery cultivar trial, a generalized linear mixed model was used within the GLIMMIX procedure of SAS (Version 9.4, SAS Institute, Cary, NC) for response variables of fresh weight, dry matter content, leaf and petiole (stalk) nitrate content, relative chlorophyll content, crown diameter, plant height, and leaf number. Multiple comparisons were conducted using Fisher’s Least Significant Difference test at P≤0.05. Additional analyses are underway for plant tissue total N and SSC. A similar model was used to analyze data from the fertilization management studies, with additional response variables including soil NO3-N content and fluxes.

Research results and discussion:

Crop yield and aboveground dry tissue NO3-N content were analyzed separately in leaves and petioles from the organic celery cultivar trial since the completion of the 2022 annual report. At 112 DAT, aboveground dry tissue NO3-N content was significantly affected by harvesting time, although there was no significant effect of cultivar. NO3-N content was higher in the afternoon in both leaf and petiole tissues by over 50% and 25%, respectively, which deserve to be validated through the analysis of additional samples from this trial. Crop yield was not affected by cultivar at 112 DAT, although significant differences among cultivars were observed at an earlier harvest at 70 DAT. ‘TZ6200’ exhibited impressive yield performance on this sampling date, significantly higher than ‘Conquistador’, ‘Tango’, ‘Balada’, ‘Tall Utah’, and ‘Pink Plume’. ‘Pink Plume’ was the lowest performer at 70 DAT, with yields significantly reduced compared with ‘TZ6200’, ‘Merengo’, ‘Victoria’, and ‘Kelvin’.

In addition to soil NO3-N content, soil NO3-N flux, and crop yield data reported for the Nov. 2021 fertilization study in the 2022 annual report, tissue N parameters including NO3-N content on a dry weight basis and NO3-N and total N accumulation have been analyzed at midseason (69 DAT)  and final harvests (101 DAT). At 69 DAT, N application rate significantly affected tissue NO3-N content and accumulation. While N78C significantly increased tissue NO3-N content compared with N308, NO3-N accumulation was highest in N224, significantly higher than N140 and N78C. In terms of plant total N accumulation, Everlizer organic fertilizer promoted N accumulation 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 granular preplant 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 expanded, Spring 2022 fertilizer study, Nature Safe organic fertilizer significantly increased soil NO3-N flux relative to Everlizer organic fertilizer for the first two weeks after preplant fertilization (WAPF), consistent with findings from the Fall 2021 experiment. At 5 WAPF, there was a significant two-way interaction between liquid and preplant fertigation sources, with Nature Safe exhibiting significantly higher fluxes than Everlizer, but only when using liquid fish fertilizer. This suggests liquid fertigation sources might have an effect on mineralization of granular fertilizer and can impact the efficacy of an integrated nutrient management program. In terms of soil NO3-N content, there were no significant differences between liquid fish fertilizer and Chilean nitrate across the 8-week monitoring period. Nature Safe exhibited higher soil NO3-N content for the first WAPF, although Nature Safe and Everlizer resulted in similar NO3-N content in later weeks.

At 72 DAT, the interaction between preplant fertilizer source and N application rate had a significant effect on celery fresh biomass. With Everlizer, N336 resulted in higher yields compared with all other rates, although Nature Safe did not show significant yield improvement beyond N252. Everlizer resulted in higher yields vs. Nature Safe among all rates of N application. Tissue nitrate 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. At 94 DAT, a significant two-way interaction between preplant fertilizer source and total N application rate was observed, with Everlizer increasing fresh weight vs. Nature Safe under higher N application rates (N252 and N336) and within the preplant-only control N88. Under Everlizer, celery accumulated significantly more biomass at N336 compared with all other N application rates, although with Nature Safe, no significant yield improvements were seen beyond N252. Analyses for determining plant tissue nitrate content at 94 DAT along with data analysis of other biometric attributes are currently underway.

Participation Summary

Educational & Outreach Activities

1 Tours
2 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

14 Farmers participated
11 Ag professionals participated
Education/outreach description:

Research from this project was presented at the ‘Research Updates in Organic Vegetable Production Field Day’ at the UF/IFAS Plant Science Research and Education Unit in Citra, FL in December 2022. Work related to soil mineral N dynamics using organic fertilizers at preplant and during the production season was discussed with farmers and agricultural professionals, and field plots were also toured where we further discussed research objectives and preliminary findings.

We plan to present at the American Society for Horticultural Science Annual Conference in August 2023 with findings from this research project. We also plan to develop extension materials related to nutrient management and N application sources for organic systems and cultivar selection for organic celery production. Two manuscripts for peer-reviewed journal publications are being prepared for submission.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.