On-farm Evaluation of an Innovative Anaerobic Soil Disinfestation Practice for Improving Organic Carrot Production in North Florida

Project Overview

Project Type: On-Farm Research
Funds awarded in 2020: $19,995.00
Projected End Date: 03/31/2023
Grant Recipient: University of Florida
Region: Southern
State: Florida
Principal Investigator:
Dr. Xin Zhao
University of Florida


  • Vegetables: carrots


  • Crop Production: nutrient management, organic fertilizers
  • Education and Training: on-farm/ranch research
  • Pest Management: biofumigation, mulches - general
  • Production Systems: organic agriculture, transitioning to organic
  • Soil Management: soil quality/health

    Proposal abstract:

    The ASD method used in fruit and vegetable production involves addition of a labile carbon source to stimulate microbial activity and community shift in the soil. In Florida, feed-grade blackstrap molasses is typically used as an organic carbon source together with composted poultry litter as an organic amendment to help with water holding and microbial promotion (Guo et al., 2017). In addition, the amended field is irrigated to saturation point to fill soil pore space and reduce oxygen levels. The field is then covered with a totally impermeable polyethylene (TIF) film to limit gas exchange and promote anaerobic conditions. This creates a shift in microbial community to facultative and obligate anaerobes. Previous studies indicate shifts in microbial communities, reduced oxygen levels, production of volatile fatty acids and organic compounds, release of metal ions, decreased pH and increased temperature under the tarp contribute to weed and soilborne disease control. In Florida, previous studies have shown 3 weeks of anaerobic conditions are sufficient to promote the shift in microbial communities. Before planting, the TIF is either removed or planting holes are made directly into the tarp to allow oxygen to diffuse into the soil and stimulate the degradation of by-products from anaerobic decomposition.


    Research conducted in Florida and Tennessee have focused on raised-bed high value crops including tomato and strawberry (Lamers et al., 2014). Previous studies have reported significant suppression of bacterial and fungal pathogens by ASD. Shrestha et al. (2018) found significant reductions in germination percentage of yellow nutsedge using soil amendments with low carbon-to-nitrogen ratios. Comparing ASD treatments with conventional soil fumigation for tomato production in Florida sandy soils, Gioia et al. (2016) and Guo et al. (2017) reported positive impacts on nematode suppression while increasing tomato yield. Paudel et al. (2018) also demonstrated suppressive effects on weed coverage for ASD treatments in open-field tomato trials in Florida. Our preliminary studies indicated significantly higher yields for organic okra in ASD treatments compared to the grower’s practice (Paudel, unpublished). As weed management presents a major challenge in organic carrot production in Florida, on-farm assessment of the innovative ASD method would contribute substantially to developing integrated management practices for organic vegetable cropping systems.


    This project is a site-specific evaluation of ASD as an option for managing weeds and soilborne disease in organic carrot production. To address cost concerns from the grower, we are proposing to use locally sourced labile carbon amendments. Additionally, we are proposing the use of re-usable 6-mil silage tarp as suggested by the grower. It will allow the grower to reduce waste from using single-season plastic covers. Furthermore, in collaboration with Cody, we will use a commercially available carrot cultivar recommended by the grower in this study. 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 with respect to its potential for improving long-term sustainability of high-value vegetable production systems.

    Project objectives from proposal:

    Field experiments will be conducted to evaluate ASD treatments involving different combinations of the use of feed-grade black strap molasses (Terra Feed, LLC, Plant City, FL), an OMRI-approved chicken manure-based organic fertilizer Chick Magic 5-3-2, and on-farm compost, and. The Chick Magic organic fertilizer is currently used at Siembra Farm, and the grower will supply the on-farm compost that is produce on site at Siembra Fram. The grower typically direct seeds carrots after irrigating an area to stimulate weed seed germination then covering the area for a certain period of time to kill any emerged weeds. In this project, the proposed ASD treatments will be compared to the grower’s standard practices at Siembra Farm using a combination of molasses, chicken manure-based organic fertilizer, and on-farm compost. For each of the ASD treatments tested, amendments will be applied with each plot occupying an area of 6 ft × 50 ft. Amendments will be incorporated into the soil using a rototiller and 2 acre-inches of irrigation water will be applied to saturate the soil prior to tarping. The 6-mil silage tarp will be securely tucked around the edges to decrease gas exchange beneath the tarp. Carrots will be directly seeded after three weeks of ASD treatment when the tarp can be removed.


    The objectives of this study are to: 1) develop ASD treatments based on farmer-recommended inputs that fit within the site-specific farming system; 2) determine the effectiveness of ASD for controlling weeds and soilborne diseases in the production of organic, direct-seeded carrots; 3) assess marketable yield and carrot quality as affected by ASD treatments.


    This on-farm research project will be conducted at the Siembra Farm located in Gainesville, FL.  Development of this project is largely driven by the grower’s interest and his active participation in brainstorming. Cody at Siembra Farm suggested using the carrot cultivar ‘Shin Kuroda’ for the on-farm research project, as this cultivar has received positive feedback from CSA customers of Siembra Farm. ‘Shin Kuroda’ is an open-pollinated cultivar that matures in 75 days. The seeds will be purchased from Fedco Seeds (Clinton, ME).  


    Experimental design: There will be 4 treatments of ASD application with different rates of feed-grade blackstrap molasses, Chick Magic organic fertilizer, and composted farmyard waste. Two treatments based on the grower practice will be included as controls. Carrots will be direct seeded during the Fall production seasons of 2020 and 2021. The ASD treatments include: 1). ASD with Chick Magic organic fertilizer at the rate of 2.3 tons/acre and black strap molasses at 741 gal/acre, without on-farm compost; 2). ASD with Chick Magic organic fertilizer at the application rate commonly used by the grower and black strap molasses at 741 gal/acre, without on-farm compost; 3). ASD with Chick Magic organic fertilizer at the rate of 2.3 tons/acre and black strap molasses at 741 gal/acre, with on-farm compost applied at 12 tons/acre; and 4) ASD with Chick Magic organic fertilizer at the application rate commonly used by the grower and black strap molasses at 741 gal/acre, with on-farm compost applied at 12 tons/acre. The two grower practice controls include the standard method used at Siembra Farm for preparing the field for direct seeding carrots with the two application rates of the Chick Magic organic fertilizer as in the ASD treatments. All the ASD treatments and the grower practice controls will be covered by silage tarp for 3 weeks before carrot seeding.


    All the ASD treatments and the grower practice controls will be arranged in a randomized complete block design, with four replications. The standard seeding rate of 630,000 seeds/acre will be used following the spacing and other in-season management practices used by the grower.   


    Disease resistance assessment: In general, the disease resistance of different treatments will be evaluated during the production season and at final harvest. Both disease incidence and severity will be assessed as appropriate. Plant samples will be sent to the University of Florida Plant Diagnostic Center for disease identification and confirmation. A 0-5 rating scale (0 = no symptoms and 5 = plants dead) will be used to evaluate the disease severity. For assessment of root-knot nematodes, plants will be dug up and root galling will be evaluated based on a 0-10 scale (0 = no galls and 10 = plant and roots dead) developed by Zeck (1971). Procedures for disease assessment will be fine-tuned as the experiment progresses with disease identification confirmed.


    Weed species and density: ASD treatments and grower practice controls will be assessed for number of emerged weeds, percentage of weed coverage, and weed density count by species (e.g., broadleaf, grass, sedge) during the production season and at final harvest. Data will be collected from fixed quadrat areas once per week.


    Soil parameters: The level of anaerobicity achieved in the ASD treatments and the grower practice controls will be monitored using oxidation-reduction probes (ORP). ORPs and temperature probes will be inserted into the soil at 6 inch depth at the time that the tarp is secured. Soil samples for nutrient analysis will be collected from each replicated plot prior to treatment set-up, immediately after treatment completion, at mid-season, and at harvest in order to monitor changes in soil pH and nutrients. Soil temperature and moisture will also be monitored throughout the production season. In addition, soil respiration will be measured at the completion of soil treatments before carrot seeding and during the production season.


    Carrot yield and quality: Carrot root weight and number (with marketable and cull separated) will be measured at final harvest to determine total and marketable yields. Carrot soluble solids and dry matter contents will be measured, and representative plant tissue samples from each treatment will be taken at mid-season and final harvest for mineral nutrient analysis.


    Data analysis: Data will be transformed if necessary to ensure normality. Analysis of variance will be performed using the GLIMMIX procedure of SAS. Multiple comparisons between treatments will be conducted using Tukey’s test at the 5% level of significance. Cost and benefit analysis will also be conducted to compare economic feasibility among different treatments.

    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.