Final report for OS20-135
Project Information
This on-farm research project is a response to growers’ interest in seeking innovative solutions to overcome weed and disease management challenges in organic carrot production systems. Anaerobic soil disinfestation (ASD) as a non-chemical tool for controlling soilborne diseases and weeds also has great potential to enhance soil fertility and biological activity through the addition of organic amendments. However, the cost of implementing ASD treatments in commercial production is often perceived as an economic barrier for the wide-spread adoption among growers. To address cost concerns from the grower, we used locally sourced labile carbon amendments and re-usable 6-mil silage tarp for ASD application, which helps reduce waste from using single-season plastic covers. By teaching the grower how to implement ASD treatment utilizing locally available resources and re-usable tarp, we expect to establish a partnership with local growers for technology and knowledge transfer which allows for a practical assessment of ASD for improving high-value vegetable production systems. Results from a series of on-farm trials conducted at Siembra Farm in Gainesville, FL demonstrated promising effects of ASD application on managing weeds and enhancing carrot yields while maintaining or improving produce quality. The ASD treatment using Nature Safe organic fertilizer (10%N-2%P2O5-8%K2O) at a rate of 175 lb N/acre + black strap molasses at a rate of 741 gal/acre + on-farm compost at a rate of 7.5 bu/100 ft2 (ASD1) significantly reduced the number of weeds in the carrot field compared to grower’s practice using organic fertilizer applied at a rate of 130 lb N/acre + on-farm compost at a rate of 7.5 bu/100 ft2 (Control) and the other two ASD treatments, i.e., organic fertilizer at 130 lb N/acre + molasses at 741 gal/acre + compost at 7.5 bu/100 ft2 (ASD2) and organic fertilizer at 175 lb N/acre + molasses at 741 gal/acre (ASD3). Among the three ASD treatments, ASD3 led to significantly more marketable carrots than ASD1 and ASD2 by 33% and 34%, respectively. ASD3 also showed a numerical increase in marketable number of carrots relative to Control by 20%. Carrot quality assessment revealed significantly higher soluble solids content and total phenolic content in ASD3 vs. Control by 13% and 20%, respectively. The disease pressure including the nematode infestation level remained low throughout the carrot trials, with no ASD treatment effect observed. We demonstrated ASD treatment setup and presented research findings at the on-farm field day upon completion of the field trials. While more studies are needed to further improve on-farm ASD application and its effectiveness, this on-farm research project successfully introduced ASD as a biological management tool for enhancing organic vegetable production to growers and Extension agents.
The specific objectives of this project include:
Objective 1: Develop ASD treatments based on farmer-recommended inputs that fit within the site-specific farming system.
Objective 2: Determine the effectiveness of ASD for controlling weeds and soilborne diseases in the production of organic, direct-seeded carrots.
Objective 3: Assess marketable yield and carrot quality as affected by ASD treatments.
Cooperators
- - Producer
- (Researcher)
Research
Field experiment
Three field trials were conducted between September 2021 – May 2022 at Siembra Farm in Gainesville, FL. In each trial, carrot (Daucus carota cv. Shin Kuroda) was seeded using a Jang JP-1 push seeder (Johnny’s Selected Seeds, Winslow, ME) following the grower’s practice. Seeds were planted at the spacing of 2.5 in between plants with planting rows spaced at 11 in. At the time of field preparation, soil was tilled to a depth of 6 in below the soil line in each experimental plot. Experiments were arranged as a randomized complete block design with three replications. Four soil treatments were included: 1) Grower’s practice (Control): Nature Safe organic fertilizer 10-2-8 (Nature Safe Fertilizers, Cold Spring, KY) applied at a nitrogen (N) rate of 130 lb/acre, with on-farm compost applied at 7.5 bu/100 ft2; 2) ASD1: ASD with Nature Safe organic fertilizer 10-2-8 applied at 175 lb N/acre and blackstrap molasses (Agricultural Carbon Source; Terra Feed, LLC, Plant City, FL) at 741 gal/acre, with on-farm compost applied at 7.5 bu/100 ft2; 3) ASD2: ASD with Nature Safe organic fertilizer 10-2-8 applied at 130 lb N/acre and blackstrap molasses at 741 gal/acre, with on-farm compost applied at 7.5 bu/100 ft2; and 4) ASD3: ASD with Nature Safe organic fertilizer 10-2-8 applied at 175 lbs N/acre and blackstrap molasses at 741 gal/acre, without on-farm compost.
Raised beds (16 ft long, 3.6 ft wide, 0.5 ft high) were formed in each block with each bed randomly assigned to a soil treatment. A 1:1 (v:v) water dilution of molasses was used to set up the ASD treatment. The water and molasses mixture was applied to the top of the bed and tilled at the soil depth of 6 in with a rotary cultivator to evenly amend the soil following the grower’s practice. All ASD plots received an initial irrigation event using a high-angle overhead Xcel-Wobbler sprinkler irrigation (Senninger, Lubbock, TX) with an output of 2.9 gpm to saturate soil within the 0-2 in soil profile. Irrigation was run for 2.4 hr based on the total volume of water required and the output of irrigation system. Following the grower’s practice, the Control plots also received the initial irrigation event. At the end of the irrigation event, a 6-mil silage tarp (Farm Plastic Supply, Addison, IL) was used to cover all plots. The ASD treatment period lasted 21 days in each trial. Soil treatments were initiated on September 22, 2021, December 16, 2021, and February 7, 2022 for Trials 1, 2, and 3, respectively. The carrot crop was directly seeded on October 14, 2021, January 7, 2022, and March 1, 2022 for the three field trials.
Soil and environmental conditions
Soil oxidation–reduction potential sensors (Sensorex, Garden Grove, CA) were installed vertically in each plot to a depth of 0-6 in to measure the redox potential and evaluate the level of anaerobic conditions achieved in the soil during the ASD treatment period. Sensors recorded measurements every 15 min, with hourly average values reported using an automatic data logging system (CR-1000X with AM 16/32 multiplexers; Campbell Scientific, Logan, UT). Raw soil redox values were used to determine the cumulative number of hours soils remained below a critical redox level (CEh) according to Rabenhorst and Castenson (2005) as follows, in order to estimate soil anaerobic conditions achieved by the soil treatment:
CEh = 595 mV − (60 mV × soil pH)
Soil pH and soil nutrients
In each trial, soil samples were taken to a depth of 0-6 in, using a 0.75 in diameter probe (Oakfield Apparatus, Oakfield, WI) during the soil treatment period. Six soil cores from each experimental unit were collected and pooled into one composite sample at baseline (day 0, prior to treatment application), 1, 7, 14, and 21 days after treatment (DAT). Soil samples were analyzed for soil pH and levels of soil NO3-N and NH4-N by Waters Agricultural Laboratories, Inc. (Camilla, GA).
Weed and nematode assessment
The number of weeds in each plot was counted at regular intervals depending on weed pressure. After counting, weeds were removed by hand. In Trial 1, weeds were counted only twice near harvest because of the low weed pressure. Weed assessment was conducted weekly during January 14 – March 25 and March 4 – April 22, 2022 for Trial 2 and Trial 3, respectively. Root-knot nematode infestation was also assessed at harvest during each trial. Ten representative carrot roots from each plot were used to evaluate root galling based on a 0-10 scale (0 = no galls and 10 = plant and roots dead). Soil samples were also taken in Trial 3 for analyzing nematode population density (Waters Agricultural Laboratories, Inc.).
Carrot yield components and quality attributes
Carrots were harvested on January 25, April 21, and May 26, 2022 for Trials 1, 2, and 3, respectively. At harvest, an area of 7.9 ft × 3.6 ft in each plot was designated for determination of carrot yield components. Within the harvest unit, all carrot taproots were harvested and evaluated to determine marketable number and weight as well as unmarketable number and weight. Carrot roots with length≥3.9 in were considered marketable if no pest or rodent damage was evident. Six representative marketable carrot taproots from each plot were chosen randomly for determination of average root length and diameter. They were also used for measuring carrot quality attributes including soluble solids content (SSC), titratable acidity (TA), ascorbic acid content, total carotenoid and β-carotene contents, total phenolic content, and total antioxidant capacity. In addition, dried carrot taproot samples were sent to Waters Agricultural Laboratories for analyzing the contents of N, phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), sulfur (S), boron (B), zinc (Zn), manganese (Mn), iron (Fe), and copper (Cu).
Statistical analysis
Data were analyzed using the GLIMMIX procedure in SAS (version 9.4; SAS Institute, Cary, NC). Sampling date and trial were also considered in data analysis with the interaction between soil treatment and sampling date or trial included. Fisher’s least significant difference (LSD) test (P ≤ 0.05) was used to conduct multiple comparisons of different measurements among treatments.
The average daily air temperatures were 23.7 oC (74.7 oF), 16.8 oC (62.2 oF), and 15.3 oC (59.5 oF) during the 3-week ASD treatment period for Trials 1, 2 and 3, respectively. During the crop season (from carrot direct seeding to final harvest), the average daily air temperatures were 15.7 oC (60.3 oF), 15.9 oC (60.6 oF), and 20.7 oC (69.3 oF) for Trials 1, 2 and 3, respectively (Florida Automated Weather Network: https://fawn.ifas.ufl.edu/).
In contrast to ASD treatments with molasses application, the soil treatment following grower’s practice (Control) without molasses achieved the lowest levels of soil anaerobic conditions in all three trials, although the differences were not statistically significant (P = 0.71, 0.45, and 0.28 in Trials 1, 2, and 3, respectively). The ASD treatment without on-farm compost (ASD3) led to the highest levels of soil anaerobicity in Trials 2 and 3. Cumulative anaerobicity during the ASD treatment period may be determined by the sources and application rates of organic amendments, initial water applied, soil temperatures, and soil types (Guo et al., 2018). Moreover, cumulative anaerobicity is likely impacted by amendment quality such as lignin, hemicellulose, and soluble carbohydrate contents, and adaptation of the soil microbial community to a given amendment input (Butler et al., 2012). Compared to previous studies, which reported accumulated Eh levels under anaerobic soil conditions ranging from 73315 mV‧hr to 118515 mV‧hr for ASD treatments in open field tomato production systems in Florida (Guo et al., 2017), the levels of Eh in the current study were considerably lower, ranging from 16285 mV hr to 22960 mV hr in Trial 1, 14565 mV‧hr to 37766 mV‧hr in Trial 2, and 19658 mV‧hr to 54643 mV‧hr in Trial 3. One possible reason for this result could be different organic inputs (Nature Safe organic fertilizer and on-farm compost) applied in the current study. In addition, overhead irrigation instead of drip irrigation was used prior to tarping as requested by the grower, which may have limited the development of anaerobic soil conditions. The cumulative soil anaerobic level was higher in Trial 3, followed by Trial 2 and Trial 1. The difference between trials might be due to the soil temperature variations with production seasons. Environmental conditions, such as soil temperature and precipitation were reported to affect soil anaerobic levels (Butler et al., 2014).
Soil pH during the ASD treatment period differed significantly among sampling dates for the three trials. In general, soil pH decreased after one day of ASD application, and maintained at a lower level for several days, and tended to recover, reaching a level similar to the original value at 0 DAT. This result corresponds with previous ASD studies that have reported a decrease of soil pH during the first several days of ASD treatment application followed by a return of soil pH to the initial level during the following weeks (Vecchia et al., 2020). The initial decrease in soil pH may be explained by the production and accumulation of organic acids during anaerobic degradation of the added carbon source (Momma et al., 2006; Rosskopf et al., 2015). A significant ASD treatment effect on soil pH was detected in Trial 3, with ASD2 showing a higher soil pH compared with ASD1 and ASD3. ASD2 also exhibited the highest numeric levels of soil pH in Trial 1 and Trial 2. The higher soil pH of ASD2 may be associated with the lower amount of organic fertilizer applied (130 lb N/acre compared with 175 lb N/acre in ASD1 and ASD3), as the organic fertilizer used in this study has a pH around 6.0. The type of organic inputs used can influence soil pH under ASD treatments (Di Gioia et al., 2017).
Soil NH4-N and NO3-N levels during the ASD treatment period also showed significant differences among sampling dates in the three trials. Similar decreasing trends of soil NH4-N were observed in Trials 1 and 2 over the 21-day ASD treatment period. While no soil treatment effect was observed in the first two trials, a significant interaction between DAT and soil treatment was detected in Trial 3. Soil NH4-N levels of the three ASD treatments at 21 DAT were significantly higher than other sampling dates. At 0 DAT, ASD3 demonstrated a significantly lower level of soil NH4-N than Control, while at 1 DAT and 21 DAT, ASD2 showed an increase of soil NH4-N relative to Control. Soil NO3-N levels exhibited similar trends for all three trials, with highest levels at 14 DAT. Overall, soil NO3-N remained at a higher level than soil NH4-N during the ASD treatment period, which might be related to the relatively low level of soil anaerobic conditions achieved by ASD treatments.
Root-knot nematode infestation was assessed at harvest for the three trials. Overall, very few root galls were observed and soil root-knot nematode population density remained low, indicating a rather low level of root-knot nematode infestation in the field. With respect to weed assessment, soil treatments did not differ significantly in weed count in Trial 1, although the number of weeds tended to be lower in the ASD1 plots relative to other treatments during the carrot production season. For Trials 2 and 3, significant soil treatment effects were detected, with ASD1 showing significantly lower numbers of weeds compared with ASD2 and ASD3. Spiny sowthistle, American pokeweed, nutsedge, and common lambsquarters were among the major weed species detected.
Data from all three trials were analyzed together for carrot yield components and quality attributes. No significant interaction effects were found between trial and soil treatment in terms of carrot yield and taproot length and diameter. Significant soil treatment effect was detected in marketable carrot number. ASD3 increased the marketable number of carrots by 33%, 34%, and 21% compared with ASD1, ASD2, and Control, respectively. Marketable and unmarketable yields as well as carrot taproot diameter and length differed significantly among trials. A significantly lower marketable yield of carrot (both number and weight) was found in Trial 3 (planted on March 1, 2022) compared with Trial 1 (planted on October 14, 2021), which may be due to the relatively hot and humid weather condition during the growing season in Trial 3. Although the total number of carrots was significantly lower in Trial 3 compared with Trials 1 and 2, total weight yield was similar across the three trials. Carrots harvested from Trial 1 were significantly thicker and shorter than the harvests from Trials 2 and 3.
Regardless of the production season, no significant soil treatment effects were observed in mineral nutrient contents of carrot taproots. In contrast, soil treatment exhibited significant influence on SSC and total phenolic content of carrot taproots. Compared with Control, ASD3 significantly increased SSC and total phenolic content of carrots by 13% and 20%, respectively. No significant difference between the ASD treatments was detected. The growing season impact outweighed the soil treatment effect for all the carrot quality attributes measured. The three trials differed significantly in terms of carrot mineral nutrient level, SSC, TA, SSC/TA ratio, total phenolics, total antioxidant capacity, and contents of ascorbic acid, β-carotene, and total carotenoids. Of the three trials, Trial 3 showed highest levels of N, P, K, Mg, Ca, S, B, Zn, Mn, Fe, and Cu in carrots, while Trial 2 demonstrated higher SSC, SSC/TA ratio, total phenolic content, and total antioxidant capacity than the other two trials. In addition, Trials 2 and 3 exhibited greater levels of TA, ascorbic acid, β-carotene, and total carotenoids in comparison with Trial 1. Overall, ASD treatment did not show any adverse impact on carrot quality attributes. The variations among trials indicate pronounced effects of seasonal environmental conditions on nutrient uptake and partitioning as well as antioxidant phytochemical production.
Educational & Outreach Activities
Participation Summary:
An on-farm field day was successfully conducted to disseminate research findings in collaboration with the grower and a local county extension agent. Handouts were prepared to present key results. The ASD technique was demonstrated in the grower’s field, and integrated management practices for organic vegetable production were also discussed extensively at this field day. In addition, the use of ASD for direct seeded crops was included as a topic in another organic vegetable production research field day held at the University of Florida Plant Science Research and Education Unit. Growers, extension agents, researchers, educators, and other agricultural professionals participated in the field days. Information from this on-farm project was also used to develop ASD case studies as part of a field lab in an organic and sustainable crop production course taught at the University of Florida. We are currently working with a local county extension agent to draft an online blog for project outreach. We are also preparing a research manuscript for submission to a peer-reviewed journal.
Learning Outcomes
Integrated practices for soilborne disease and weed management in organic vegetable production
Anaerobic soil disinfestation
Project Outcomes
This on-farm research project introduced ASD as a biological soil management tool for managing soilborne diseases and weeds in organic carrot production. The ASD application was adapted to the grower’s farming system, taking into consideration the use of 6-mil silage tarp by the grower for weed control prior to seeding carrots and their existing crop nutrient management program. Although the soil anaerobic conditions did not reach a desirable level as in plastic-mulched raised beds with drip irrigation and the low level of soilborne diseases in the field trials made the disease management assessment challenging, this pilot work demonstrate the feasibility of ASD implementation in growing direct seeded crops for improving plant health and productivity. In particular, the ASD approach was shown to enhance weed suppression compared to the silage tarp alone practice, without any adverse impact on produce quality. As an alternative to single season plastic mulch, the 6-mil silage tarp could be innovatively used for ASD application with overhead irrigation by small growers and limited resource farmers.
The on-farm trials also offered an excellent opportunity to engage the grower in developing more sustainable production systems to address site-specific challenges and seek cost-effective solutions. Locally sourced black strap molasses was the only additional input brought to the farm for setting up the ASD treatment, while the organic fertilizer, on-farm compost, and silage tarp needed for ASD assessment were already used regularly by the grower. When we conducted the on-farm field day to discuss the research findings, the use of cover crop residues as a potential carbon source for ASD was brought up by the grower. We plan to evaluate cover crop-based ASD application and other ASD approaches that best incorporate on-farm resources in future studies to further improve the integration of ASD into diverse farming systems toward advancing the long-term environmental, social, and economic sustainability.
Thank you so much for supporting this on-farm research project! Despite the many challenges we encountered during the project period including the COVID pandemic, the lessons learned and new partnerships built will help us improve our future work.