Organic spinach is among the top organic vegetables sold in the United States, and growth of spinach production tops the list. Climatic conditions have limited spinach production in the southeastern U.S., where high humidity and warm temperatures negatively affect spinach growth and quality. Florida’s climate also includes drastic variations in temperatures during the spinach growing season. High tunnel system has the potential to mitigate many of these environmental extremes, yet there is a lack of research-based information on organic spinach production under Florida conditions. Additionally, compost application may help overcome spinach germination difficulties, which is a challenge to growers particularly in warm climates. Given the growing interest in spinach production in Florida, the main objective of this project was to assess the effects of compost and high tunnel system on organic spinach production to develop techniques for improving crop establishment and yield performance in sandy soils.
Results from on-station and on-farm trials demonstrated that compost additions affected soil physical, chemical, and biological properties, and compost application method may be important for realizing these benefits and enhancing crop performance. The pilot study on anaerobic soil disinfestation (ASD) method also showed promise for managing weeds in high tunnel systems prior to direct-seeded spinach. Irrigation, fertilization, and other management practices may be impacted by compost application method and future research is warranted to optimize the outcome and consider the cost-benefit analysis based on production scales.
The goal of this project was to identify production techniques that promote sustainable spinach production in Florida. The specific objective was to assess the effects of compost and high tunnel system on organic spinach production for improving crop establishment and yield performance in sandy soils. By visiting different growers, the project was also aimed to identify the current spinach production practices of organic farmers and challenges of organic spinach production in Florida to understand the range of issues and most common difficulties faced.
An organic spinach (cv. Reflect) production trial was conducted at the University of Florida Plant Science Research and Education Unit (PSREU) in Citra, FL during Dec. 2017 ‒ Feb. 2018. A split-split plot design with three replications was used, with the production system (high tunnel and open field) as the whole plot factor, compost application method (tilled and furrowed) as the subplot factor, and compost type (plant based and cow manure based) and rate (0, 10, and 20 tons per acre) combination as the sub-subplot factor. Soil samples were taken at planting to explore treatment effects on physical, chemical, and biological soil quality indicators. Spinach germination was assessed via the Canopeo App (www.canopeoapp.com), which calculates the coverage of green canopy in a picture frame. In Spring 2018, another high tunnel trial was developed to assess compost treatments in organic baby spinach production in another high tunnel location at PSREU. A split-plot design was used, with the seeder type (Earthway and Jang) as the whole plot factor and compost application method (no compost control, tilled, and furrowed) as the subplot factor (applied at 20 tons per acre). Spinach seed germination rate and crop yield were evaluated.
In addition to the on-station experiments, three on-farm high tunnel trials were carried out at a commercial high tunnel farm in North Florida. In Jan. 2019, a small, organic observational trial was conducted in which compost was applied to 6-ft sections on top of raised beds at 1-inch and 2-inch depths. A soil sanitizing product was used on two beds within the same tunnel. Multiple microbial products were also used in adjacent, organically certified high tunnels (as well as in multiple plots throughout the production season). Spinach seeds were drilled directly into the compost, as well as into the unamended control, sanitized, and microbially amended treatments. Spinach crop stand was assessed using the Canopeo App. In mid-April 2019, two large-scale, replicated trials were initiated at the high tunnel commercial farm to further examine the compost application effects. The first was on a conventional section of the farm in which the soil was very coarse and drained rapidly, and in which, over two years, a profitable spinach crop has yet to be grown. In this trial the purpose was therefore to examine how treatments would affect nutrient and water holding capacity, in addition to overall crop performance. Original treatments consisted of compost added to 1-inch depth on bed tops, as well as 1-inch of compost added and tilled, and an unamended control. Additionally, there was a polymer and polyacrylamide product used as a fourth treatment, which claims to improve water holding capacity of the soil. However, due to the compost quality issue, the trial had to be adjusted to include the control, a polymer and polyacrylamide product, 0.5-inch tilled compost (tilled 4 inches deep), and 1-inch tilled compost (tilled 4 inches deep). Soil moisture content, plant available water, saturated hydraulic conductivity, and bulk density were measured, as well as crop response to different soil management treatments. This conventional production trial was included as part of this project in order to understand soil moisture dynamics of the compost treatments at the field level. For the organic trial, the compost product received was fully composted, yet it again was not the retail product used in the observational trial and was too coarse to drill spinach seeds into. The drill machine was not able to roll on top of the non-tilled compost, as the roller would instead remove the compost layer from the bed. Adjusting the moisture level of the compost and/or underlying bed could eliminate the issue, yet an alternative method was chosen, in which a portion of the compost was applied, followed by seed dribbled onto the top of the compost, followed by the remainder of the compost. This seeding technique provided minimal error (regarding seed rates) while assuring the untilled compost treatment would be intact and valid for comparison. The treatments of the organic trial consisted of the soil sanitizer used in the observational trial, a no treatment control, and compost spread at a 1-inch depth. Half of the compost was tilled in and beds were reformed, while half remained on top of the raised beds. Each treatment was 5 ft wide by 100 ft long, with 3 replications (in 3 separate tunnels).
During the final year of this project, a pilot study was conducted at PSREU to investigate the impact of anaerobic soil disinfestation (ASD) as a pre-plant biological soil treatment method to manage weeds in high tunnel systems for organic baby spinach production. Molasses and a processed chicken manure based organic fertilizer were applied in the 3-week ASD treatment with planting beds covered with either totally impermeable film (TIF) or silage tarp. Redox potential was measured to monitor anaerobic conditions during the soil treatment period. ‘Space’ spinach was seeded at 1 day vs. 3 days after the ASD treatment. Weed emergence and coverage were assessed at the end of ASD application period, and baby spinach yield was measured at harvest.
In the 2017 winter trial conducted at PSREU, results showed lower soil bulk density, higher soil CEC and nutrient concentrations, and higher total carbon and organic matter content in furrowed treatments than tilled treatments, while no significant differences were found between tilled treatments and the no compost control. In tilled treatments, a slight increase in plant available water and decrease in soil bulk density were observed at the 20 tons per acre compost rate compared with the unamended control, while no difference was found at the compost rate of 10 tons per acre compared with the control. Interestingly, saturated hydraulic conductivity of soil was 65% greater in the 10-ton tilled treatment and 318% greater in the 20-ton tilled treatment compared with the unamended control. Untilled compost treatments at 20 tons per acre, however, increased plant available water by 600%, decreased soil bulk density by 70%, and dramatically increased saturated hydraulic conductivity in the top 2 inches (the compost horizon). Significant spinach seed germination increases were found with increasing amount of compost additions in the furrowed treatment, while the tilled treatment was not significantly greater than the control. Additionally, germination was greater in the cow manure-based compost than the plant-based compost, most likely due to the poorer quality (larger particle size) of plant-based compost received. Harvest units were established by thinning plants to 3-inch spacing in order to eliminate germination effects and assess plant growth rates on a per plant basis. Spinach was harvested 50 days after seeding. Fresh weight revealed no significant differences in growth rates between compost treatments, while high tunnel yields were 174% greater than the open field, due to an increase in both leaf number per plant and leaf size.
The 2018 high tunnel spring trial showed that the seeder type varied significantly, with the Earthway seeder dispensing more seeds per ft than the Jang seeder. The variance in the Earthway seeder was 33.0% while the variance in the Jang seeder was 6.6%. This was to be expected as the Jang seeder is advertised as a precision seeder. With the Earthway seeder, tilled and furrowed treatments increased germination by 104% and 123%, respectively, compared to the control. With the Jang seeder, tilled and furrowed treatments increased germination by 65% and 113%, respectively. Because yield was significantly influenced by treatment effect on germination, harvest units were selected to minimize areas with poor germination and represent uniform growth. Yield data on a per linear ft basis indicated there was no significant difference between tilled and furrow treatments, while both averaged 117% and 97% greater than the control with the Earthway and Jang seeders, respectively.
In the small, organic observational trial conducted at the commercial high tunnel farm, the averaged compost treatments showed significant increases in crop stand compared to the other treatments. The compost treatments had consistent results, while all other treatments had very high variability. In fact, after two and a half cycles of continuous plantings (alternating spinach and arugula in each location, while planting in seven different blocks per cycle and one block per week), the microbial treatments did not improve the crop stand compared with the unamended control. The performance was so poor that half of the 16 plantings have been terminated prior to harvest. In the large-scale conventional high tunnel spinach trial, compost additions significantly reduced soil pH, increased soil CEC and concentrations of K, Mg, Ca, Cu, total N, nitrate-N, and total carbon as well as soil organic matter content and EC. Interestingly, the 1-inch tilled compost treatment was only significantly greater than the 0.5-inch tilled compost treatment in soil K and Ca concentrations. Yield data were not collected due to the high May temperatures’ reduction of plant health. Soil moisture data indicated non-tilled compost has the potential to reduce the amount of water percolating through the soil, and irrigation management may therefore change with this technique. Additionally, less water was removed from the soil during daily cycles. This reduction in the magnitude of daily soil moisture oscillation, if managed correctly, may benefit plant health. In the organic high tunnel spinach trial carried out at the commercial farm, soil chemical analyses at planting (sampled from the top 4 inches) indicated the non-tilled compost significantly increased K (178%) and Ca (36%) concentrations over the control, while tilled compost only significantly increased the level of soil K (156%) compared with the control. In all the soil chemical analyses conducted, the non-tilled and tilled compost treatments were not significantly different. The lack of widespread soil chemical differences compared with previous trials was likely due to the native soil being relatively high in organic matter compared with previous trials (1.5% vs. less than 1%), while a smaller compost horizon was added to the soil (1 inch vs. 2 inches in previous years). This smaller compost horizon was used because it approached the maximum applicable rate that would permit the treatment to be economically viable. Canopeo data captured the relative plant population at two weeks after planting, indicating treatment effects were trending at P=0.07. The lack of significant difference was primarily due to the variability in control treatments across different replications. While the compost particle size was not ideal for this study and may have also minimized the non-tilled treatment benefits, this trial demonstrated the potential for compost to be used as a soil substrate medium to isolate seeds and seedlings from soil pathogens in organic spinach production. Greater benefits would have also been expected if the trial occurred during the peak growing season, as the high May temperatures reduced the health of the spinach and no yield results (including in concurrent conventional plots) were obtained.
Findings from the ASD study demonstrated that the ASD treatment using 6-mil recyclable silage tarp and the ASD treatment using TIF achieved comparable levels of anaerobic conditions. The number of emerged nutsedge per bed during the 3-week ASD treatment period was the lowest in the ASD treatment with silage tarp, suggesting that the silage tarp may serve as a more restrictive physical barrier to nutsedge emergence compared to conventionally used TIF. The ASD treatments were also effective in reducing the percentage weed coverage in the planting bed. These results indicated that ASD soil treatment using silage tarp may offer additional benefits to growers in terms of weed management for direct-seeded crops. The two ASD treatments also produced similar yields of organic baby spinach in high tunnels, while the seeding date did not show a significant impact on crop yield.
Educational & Outreach Activities
The graduate student visited six spinach growers from north, central, and south Florida and corresponded with several others. Interestingly, there are numerous conventional spinach growers who are only limited by the subtropical heat of Florida’s summers. For organic growers, spinach cultivation is so difficult that the majority do not even attempt to grow it. Poor germination and lack of crop stand are the common reasons organic growers keep spinach as a periphery crop. This was confirmed through an internship experience, in which the graduate student worked part-time at a high tunnel, baby leafy greens farm in North Florida. This enabled the graduate student to understand large-scale baby spinach production systems, detect some of the constraints in both conventional and organic spinach production in Florida sandy soils, and identify potential applications for the research conducted in this project. The principal constraint in organic spinach production on sandy soils in Florida is the presence of soil-borne seedling diseases that cause damping off, limiting crop stand. Most times the incidence was so great which has caused complete crop failures.
The graduate student presented some of the research results at the Southern Region American Society for Horticultural Science 2018 Annual Conference. The student is also planning to work on a research or extension publication to summarize the project findings.
Soil pathogens are the most limiting factor for spinach production in the southeastern U.S., affecting crop stand and therefore yield. High soil temperatures potentially exacerbate these effects. The on-station and on-farm studies conducted in this project demonstrated that compost additions could affect soil physical, chemical, and biological properties, and compost application method (as well as particle size) may be important for realizing these benefits. More specifically, compost has the potential to be used as a soil substrate to isolate seeds from soil-borne pathogens in the early stages of growth when they are most vulnerable, which can increase crops stand and yield potential. Irrigation, fertilization, and other management practices may be affected by compost application method and future research is necessary to maximize benefits.
Compost cost may be a key limiting factor, with the cost of shipping surpassing 30% of the material cost in some locations. Therefore, a cost-benefit analysis needs to be considered by producers and researchers to determine suitability for use. Our studies also indicated that the traditional, tilled application method may result in limited impacts of compost on soil physical, chemical, and biological properties in sandy soils and thus compromising the gains for direct-seeded crops. This project demonstrated that modified application methods of compost have the potential to maximize economic and environmental benefits to growers, yet also highlighted the challenges faced by growers for using compost to improve organic baby spinach production.
This project transformed the graduate student’s outlook on sustainable agriculture from one focused on the environmental aspects of growers’ production systems to include economics in the equation. Sustainable agriculture must identify practices that improve soil health and the environment and provide a return for the investment. While compost additions have widely been advocated for to improve soil health, this study demonstrated traditional application methods have little to no measurable effects on most soil physical, chemical, and biological properties at rates that are affordable to growers in sandy soils under Florida conditions. Alternative application methods have the potential to maximize economic and soil health benefits in certain production systems.
While the lack of compost additions resulted in complete crop losses in organic commercial baby spinach production system in North Florida, the cost of compost appears to limit the suitability of this method, regardless of the increase in production or if commercial yields are achieved (which most likely would have been the case if the large-scale trials were conducted during the ideal growing season). Results from this study indicate that alternative application methods have the potential to provide economic benefits in other cropping systems, and future research is needed to identify which cropping systems are most suitable. This research should target growing systems in which banding compost at affordable rates would provide compost depths between 1.0 and 2.0 inches for each row of crop. Additionally, it is crucial that all future research in sustainable agriculture quantifies the economic impact for the methods, techniques, or inputs being assessed. A failure to do so may result in practices being recommended to growers that are not economically feasible.
Results from this project indicate that alternative compost application methods have the potential to provide economic and environmental benefits that are unable to be attained with standard application methods. Future research is needed to identify which cropping systems are most suitable to provide an appropriate economic return on the investment when compost is considered an essential organic amendment to improve crop productivity and quality. This research should target growing systems in which banding compost at affordable rates would provide compost depths between 1.0 and 2.0 inches for each row of crop. Additionally, it is crucial that all future research in sustainable agriculture quantifies the economic impact for the methods, techniques, or inputs being assessed. A failure to do so may result in practices being recommended to growers that are not economically feasible.