In response to national and local consumer demand, organic crop production in Florida has expanded during the last decade. Consequently, high tunnels have emerged as a protected production system increasingly used by Florida organic growers because of their utility for season extension of high-value vegetables. As organic production in high tunnel systems continues to expand, research is warranted to guide producers in terms of pest and nutrient management. One method which has proved effective against several soilborne diseases across a wide range of horticultural crops and environments and shows promise as a weed management strategy is anaerobic soil disinfestation (ASD). ASD is a preplant method based on creating anaerobic soil conditions by incorporating labile carbon sources, saturating the soil to field capacity, and covering the soil with a gas impermeable barrier. ASD promotes soil microbial shifts toward facultative and obligate anaerobes which produce short-chain organic acids and volatile compounds that are likely toxic to and inhibit weed seed germination. While previous studies have focused on disease management and the underlying mechanisms, limited field studies have investigated the effects of ASD treatment on soil and crop nutrient dynamics as well as ASD for managing weeds in high-tunnel systems.
It is expected that soil treatment by ASD may differ between high-tunnel and open-field systems given the differences in environmental conditions, which may lead to differential impacts of ASD on soil mineral nutrient availability, leaching potential, and gaseous N loss. Therefore, to avoid negative environmental impacts related to soil and crop nutrient dynamics, research is warranted to optimize ASD application techniques in open-field and high-tunnel systems. Furthermore, recommendations for ASD application are needed to address the use of single-season plastic alternatives. While plastic mulch is not strictly prohibited in organic agriculture, many organic producers are interested in reducing plastic use in their farming operations, both for environmental and economic reasons.
Based on inputs from organic growers, we propose the use of 6-mil silage tarp as an alternative to totally impermeable film during the ASD treatment period. Silage tarp does not require specialized machinery, is able to be recycled over multiple seasons and can be implemented in high tunnel systems. The main goal of this 2-year project is to examine the application rate of ASD amendments in organic high-tunnel and open-field systems for developing recommendations on ASD application in production of direct-seeded baby leafy greens in high-tunnel and open-field systems. Project findings will contribute to improving ASD as an environmentally-friendly method for weed and nutrient management and benefit both organic and conventional production systems.
With the long-term goal of developing cost-effective and environmental-friendly ASD application for improving baby leafy green production systems, the specific objectives of this project include:
Objective 1: Determine the level of soil anaerobiosis in ASD treated soil using totally impermeable film (TIF) and 6-mil silage tarp (ST) in high-tunnel and open-field systems.
Objective 2: Assess the ASD impact on soil and crop nutrients dynamics using TIF and ST in high-tunnel and open-field systems.
Objective 3: Assess the ASD impact on weed management using TIF and ST in high-tunnel and open-field systems.
Objective 4: Assess marketable yield and quality of baby leafy greens as affected by ASD treatments.
A preliminary Summer field trial was conducted in July – Aug. 2020 and a Fall trial was conducted in Dec. 2020 – Jan. 2021 at the University of Florida Plant Science Research and Education Unit in Citra, FL. The preliminary field trial was conducted following a split-plot design. Production system (high tunnel vs. open field) and soil treatment were assigned as whole-plot and subplot factors, respectively. No crops were included in this preliminary study as the main focus was on examining the feasibility of using silage tarp for ASD application. The Fall trial was conducted according to a split-split plot design, with production system (high tunnel vs. open field), ASD treatment, and post-ASD period spinach seeding date (1 day vs. 3 day after ASD treatment period) as whole-plot, subplot and sub-subplot factors, respectively. Spinach cultivar ‘Space’ (Johnny’s Selected Seeds, Winslow, Maine) was grown during the Fall trial. Using ChickMagic® (CM, 5%N-3%P2O5-2%K2O) at a rate of 6.8 Mg ha-1, and molasses (M) at a rate of 6.93 m3 ha-1, soil treatments consisted of Control0 (no tarp, no CM, no M), Controltif (TIF, no CM, no M), Controlst (ST, no CM, no M), Controlnt (no tarp, with CM, with M), ASDtif (TIF, with CM, with M), ASDst (ST, with CM, with M). After application of CM and M, soil was tilled to a depth of 15 cm with a rotary cultivator, and beds were covered with either a 6-mil silage tarp or a 1-mil black/white VaporSafe1 TIF (Raven Industries Inc., Sioux Falls, SD). Using overhead sprinkler irrigation, ASD plots were irrigated to saturate air-filled pore space in the top 10 cm of the bed for enhancing the development of anaerobic conditions. ASD treatment period were maintained for three weeks before tarp was removed.
Following the protocol outlined by Paudel et al. (2018), oxidation–reduction potential sensors (PT combination electrodes, Ag/AgCl reference, Sensorex, Garden Grove, CA) were installed to a depth of 15 cm in each plot to measure the redox potential, and thus, to evaluate the level of anaerobiosis achieved in the soil during the first 3 weeks after treatment application. Using a soil probe (1.75 cm internal diameter), three soil cores were taken randomly to a depth of 15 cm from each plot prior to ASD treatment application and at 1, 7, 14, and 21 days after treatment application (DATA). Baseline soil samples were analyzed to determine soil pH and mineral contents of N, P, K, Mg, Ca, S, B, Cu, Fe, Mn, Zn, Ni and Mo. Subsequent samples were analyzed for soil pH, NO3-N, and NH4-N. Soil temperature and volumetric water content were continuously monitored during ASD treatment period. Soil temperature/moisture sensors (CS655, Campbell Scientific, Logan, UT) were installed to a depth of 15 cm in each plot before initial irrigation. During the ASD treatment period, anion exchange membranes (AEMs; Ionics AR204-SZRA, Durpro, Candiac, QC, Canada) were used to measure soil NO3-N flux following procedures by Castro and Whalen (2016). Anion exchange membranes (2 cm × 10 cm) were placed in each sub-subplot, vertically oriented 0-10 cm below the soil surface, and they were retrieved and replaced when soil samples were taken. The AEM measurements also continued after seeding baby spinach until the final harvest.
Number of emerging weeds and percentage of weed coverage per plot were evaluated quadrat (0.25 m2) samples prior to ASD initiation and at 11 and 21 DATA. Weeds were classified into grasses, broadleaves, and sedges. Weed assessment was also performed each week during the baby spinach growing season. Total fresh weight of baby spinach per plot and biometric assessments were taken at harvest. Above-ground biomass was oven-dried at 65˚C until constant weight to measure dry matter content. Dried samples were ground and will be analyzed for nutrient contents of N, P, K, Mg, and Ca. Sampling quadrats (0.25 m2) were used to determine specific leaf area and specific leaf weight in each plot at harvest. Roots were also harvested from 0.25 m2 quadrats for measuring root dry biomass and nutrient contents.
Data were analyzed using a general linear mixed model in JMP (v.15). Block, days after ASD treatment application (DATA) were considered as random effects. Growing season (Summer, Fall), production system (open field, high tunnel), and soil treatment were considered as main effects for analysis of cumulative soil redox potential (Ceh, mV hr-1), number of nutsedge emerged per bed, percentage of weed coverage, NO3-N flux, and soil NO3-N concentration. Spinach was only grown during the Fall trial, so seeding date, production system, and soil treatment were considered as main effects.
A two-way interaction of production system (high tunnel vs. open field) and soil treatment (p<0.05) significantly affected cumulative soil redox potential (Ceh, mV hr-1). In general, ASDst and ASDtif treatments achieved similar Ceh levels in open-field and high-tunnel systems. When averaged across production systems, the main effect of soil treatment revealed highly significant differences between ASD soil treatments and the controls. Specifically, ASD soil treatments increased Ceh levels compared to Controlnt, Controlst and Controltif. These results suggest that the addition of CM and M alone do not promote soil anaerobic conditions in the absence of a gas impermeable barrier. On the other hand, the use of ST or TIF without added CM and M as labile carbon and N sources do not promote soil anaerobic conditions. Compared to previous studies by Shrestha et al. (2018) and Poret-Peterson et al. (2020) which reported Ceh levels ranging from 50000 mV hr-1 – 115000 mV hr-1 in growth chamber environments, levels of Ceh achieved in the current study were lower, ranging from 25558 mV hr-1 in ASDtif in open fields to 35223 mv hr-1 in ASDst in the open field. Ceh achieved in the current study are similar to those reported by Paudel et al. (2018) for open-field tomato production in Florida using 6.93 m3 ha−1 of molasses and 11 Mg ha−1 of composted poultry litter for the ASD treatment. The current study was conducted under field conditions without forming raised beds which might have allowed lateral movement of soil water, thereby limiting anaerobic soil conditions. In addition, to mimic farmer production practices for growing leafy greens, overhead irrigation was used prior to tarping. This represents a deviation from the ASD method applied using drip irrigation after tarping.
In Florida where many production fields consist of sandy soils with low levels of organic matter, it is recommended to supply N for microbial decomposition of labile carbon during the ASD treatment period. The N contained in the ASD organic amendment also contributes to the seasonal crop N requirements. To date, limited research has focused on ASD soil treatment in high tunnel systems. Specifically, information regarding N dynamics in high tunnels is lacking. In the current study, main effects of production system and soil treatment significantly impacted soil NO3-N flux, although their interaction was insignificant. Compared to the open field system (1.77 ug cm2 day-1), NO3-N flux was three-fold greater in high tunnels (6.77 ug cm2 day-1). Compared to Controlnt (3.57 ug cm2 day-1), greater soil NO3-N flux was observed under ASDtif (6.12 ug cm2day-1) and ASDst (6.53 ug cm2 day-1) treatments. A two-way interaction of production system and soil treatment significantly (p<0.05) affected soil NO3-N concentrations. In the high tunnel system, significantly greater levels of soil NO3-N were observed under ASDst (57.29 mg N kg-1 dry soil) compared to ASDst (23.83 mg N kg-1 dry soil) in the open field. A similar trend was observed for ASDtif, though the differences between high tunnel (28.36 mg N kg-1 dry soil) and open field (25.85 mg N kg-1 dry soil) systems were not significant. In general, all treatments tended to exhibit greater levels of soil NO3-N in high tunnels compared to their respective treatments in open fields. These findings suggest that use of recyclable ST for ASD soil treatments can minimize NO3-N leaching at levels comparable to traditional TIF. Furthermore, high tunnels may be suitable production systems for implementation of ASD soil treatment in terms of improving N availability and minimizing NO3-N losses.
Soil treatment significantly influenced the number of emerged nutsedge (p<0.001) and the percentage of weed coverage. The fewest number of emerged nutsedge per bed was observed in ASDst (<1) and Controlst (<1) treatments, which represented a thirteen-fold and nine-fold reduction compared to ASDtif (8.54), respectively. Similarly, the number of nutsedge emerged per bed in ASDst and Controlst treatments represented a thirty-fold and twenty-fold reduction compared to the uncovered Control0 (20.75) treatment. Percentage weed coverage was significantly increased in Control0 (22.4%) and Controlnt (18.5%) compared to ASDst (<1%), ASDtif (<1%), Controlst (<1%) and Controltif (<1%). These results are consistent with findings by Di Gioia et al. (2016) who reported weed coverage less than 1% in ASD soil treatments using a combination of 13.9 or 27.7 m3 ha-1 of molasses with 22 Mg ha-1 composted poultry litter at 9 and 22 days after tomato transplanting. Our findings suggest that ASD soil treatment using ST may confer benefits to growers in terms of weed management in addition to soil biological and chemical changes related to the ASD soil treatment; rather, ST may serve as a more restrictive physical barrier to nutsedge emergence compared to conventionally used TIF.
During the Fall trial, production system (p<0.001) and soil treatment (p<0.001) significantly impacted the yield of baby spinach, with a lack of a significant interaction effect. The effect of planting date (1 vs. 3 days after end of ASD treatment application period) was not found to significantly affect crop yield. However, baby spinach yield was more than two-fold greater in high tunnels in contrast to open fields. The greatest yields were observed in ASDst (and ASDtif treatments. Leaf area under ASDst and ASDtif was significantly greater (p<0.001) compared to the controls when averaged across production systems. However, leaf area did not differ significantly between ASDst and ASDtif treatments.
Educational & Outreach Activities
We plan to present our research findings at the American Society for Horticultural Science 2021 Annual Conference. In addition, we hope to prepare a research manuscript for publication in a peer-reviewed scientific journal.