Strategies to enhance organic vegetable production systems: Soil blocking, cover crop mixtures, and poultry integration
Date
2024-08
Authors
Carey, Anne Maureen
Major Professor
Advisor
Nair, Ajay
McDaniel, Marshall D
Slack, Suzanne
Thoms, Adam
Wiedenhoeft, Mary
Committee Member
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Abstract
Organic agriculture applies many principles of agroecology, including a focus on increasing diversity, enhancing biological relationships or synergies, and working with the local environment. Certified organic producers must use cover crops, crop rotations, and manure and compost applications to meet their needs, but continue to rely on organically-approved fertilizers to meet high nutrient demanding vegetable crop needs. Additionally, the complexity of building a biologically based organic system is a challenge for many growers. The overarching goal of my research was to investigate strategies for enhancing sustainability and diversity in organic vegetable production systems, including an alternative transplant production method, cover crop multi-species mixtures, and crop-livestock integration.
Healthy and vigorous transplants are a key to successful vegetable production. Many organic growers are interested in reducing plastic waste and reducing their carbon footprint by using local products. The soil block method is an alternative approach to the plastic flats most often used, creating free-standing blocks which can be directly transplanted into a field. Additionally, many growers purchase commercially available certified organic growing media, but the multitude of products available from all over the country makes selection a challenge. In Chapter 2, I compared the growth and performance of cucumber and bell peppers transplants in plastic flats and in soil blocks, with the use of four commercially available growing media. Cucumbers grown in flats had a significantly greater dry weight than those grown in soil blocks, by 20% in 2022 and by 38% in 2023. In contrast, pepper transplants grown with the soil block method had between 50% and 130% greater dry weight in the final sampling in 2022. Seedling emergence was delayed in soil blocks due to a higher bulk density and lower medium temperature, but peppers were able to overcome this delay with a longer growing period than cucumbers. Although there are claims of improved root development in soil block grown transplants, differences in root surface area were related to shoot performance, and no advantage in soil blocks was found. I recommend two locally produced products, Purple Cow Organics (‘Seed Starter Mix’, Middleton, WI) and Beautiful Land Products (‘Soil Blocking Mix’, Tipton, IA) to growers in the Upper Midwest, while Cowsmo (‘Green Potting Soil’, Cochrane, WI) stunted plant growth.
Many researchers have established the benefits cover crops bring to an agro-ecosystem, known to improve soil health and support cash crop yields. The practice of growing three or more cover crop functional groups in a mixture is gaining popularity for the opportunity to combine the individual benefits of cover crop species, creating a multifunctional mixture. In Chapter 3, I analyzed the performance of four summer cover crop species in monocultures, two three-way mixtures, and one four-way mixture grown prior to a fall cabbage crop, seeking the provisioning of weed suppression, nitrogen contribution, enhanced cabbage yield, and support of soil biology. Results revealed that mixtures performed similarly to monocultures, able to provide comparable weed suppression, cabbage yield, and fertility contributions as the best monoculture. Additionally, soil microbial biomass was increased in plots with mixtures in one year. Future research should expand on questions of appropriate seeding rates in mixtures to avoid interspecific competition while maintaining multifunctionality.
Developing a system which combines cover crops and livestock production could further enhance diversity and benefits to organic farmers. Poultry are an ideal livestock for integration in a vegetable system, with potential to cycle unharvested plant material back as nutrient-rich manure. In Chapter 4, I compared two poultry-integrated rotations, vegetable-poultry-cover crop (V-P-CC) and vegetable-cover crop-poultry (V-CC-P), to a no poultry control, vegetable-vegetable-cover crop (Control). We evaluated vegetable yield, cover crop performance, soil nutrients, soil microbial biomass, earthworm abundance, and field saturated hydraulic conductivity. Our results show poultry integration (V-P-CC and V-CC-P) increased soil inorganic N concentrations by 71% to 110% compared to Control in the spring, supporting greater leaf lettuce yields and overwintered cover crop biomass. In V-CC-P, two cover crops were grown, and greater improvements to soil physical and biological properties than poultry integration alone were found, including higher field saturated hydraulic conductivity and earthworm abundance after two years. Our study confirms the improvements in soil conditions and potential reduction in fertilizer use by integrating poultry in an organic vegetable production system, but highlights the risk of an excessive accumulation of nutrients and a need to further explore the use of cover crops in poultry-vegetable integrated systems.
By introducing livestock to vegetable fields, unique and potentially pathogenic microorganisms are introduced, altering the soil microbiome. Using 16S rRNA sequencing, we analyzed the soil bacteria following two years of poultry integration, exploring changes to diversity, relative abundance of bacteria phyla, commonness of soil bacteria genera, and the presence of human pathogens. Our results show an increase in Shannon and Inverse Simpson diversity in soil bacteria following integration and a high degree of commonness (78%) of soil bacteria genera present in poultry-integrated and no-poultry treatments. This may be indicative of a limited impact of poultry integration in soils already under organic management. Importantly, Salmonella and E. coli were not present in any samples.
In summary, this research explored strategies for increasing sustainability of transplant production with the soil block method and locally produced commercially available growing media and the benefits of expanding diversity with cover crop mixtures and poultry integration. The research in this dissertation offers practical, science-based findings that will support organic vegetable producers as they seek to maintain organic certification, realize the biological synergies espoused by organic agriculture founders, and meet production goals.
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dissertation