Evaluating the soil block technique for organic vegetable transplant production

View the project final report

Commodities

• Vegetables: cucurbits, peppers

Practices

• Crop Production: greenhouses, transplants
• Education and Training: demonstration, extension, technical assistance, workshop
• Production Systems: organic agriculture
• Soil Management: soil analysis, soil chemistry, soil physics

Organic vegetable growers are interested in utilizing the ‘soil block’ method for transplant production as an alternative to plastic flats. The soil block method compresses growing media into a freestanding block in contrast to the cells of a plastic flat. Anecdotal evidence of soil block grown transplants with increased vigor and root development exists, but limited research has been conducted to evaluate these claims. Furthermore, identifying commercial growing medium for certified organic transplant production is needed. The objective of this study was to compare growth parameters and root development of cucumber (Cucumis sativus) and pepper (Capsicum annum) transplants grown in soil blocks and plastic flats, in combination with four commercially available certified organic media (Beautiful Land Products ‘Soil Blocking Mix’, Purple Cow Organics ‘Seed Starter Mix’, Cowsmo ‘Green Potting Soil’, and Vermont Compost Company ‘Fort Vee’). A volume-based 50% peat, 25% compost, 12.5% perlite, and 12.5% vermiculite growing medium was also evaluated. A split-plot randomized complete block design with four replications was used with growing method as the whole plot factor and medium as the subplot factor. ‘Marketmore 76’ cucumbers and ‘Yankee Bell’ peppers were seeded in 50-cell flats and soil blocks made with Johnny’s Selected Seeds Stand-up 12 Soil Blocker. Data were collected on growth parameters by destructively sampling cucumbers three weeks after seeding, and peppers five, six, and seven weeks after seeding. Root development was evaluated using WinRHIZO at the last sampling. Cucumber and pepper transplants performed differently in soil blocks and flats. 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. Cucumber and pepper transplants grown with Cowsmo ‘Green Potting Soil’ performed poorly, with an up to 144% lower dry weight and up to 167% lower root surface area than transplants grown with the other media. Root development correlated with shoot development, without a specific advantage in soil blocks, although differences in root system architecture should be investigated. The evaluated Beautiful Land Products, Purple Cow Organics, and Vermont Compost Company media can all be considered suitable for growing certified organic vegetable transplants in both soil blocks and flats.

 Our findings are relevant for certified organic vegetable transplant producers in the Midwestern U.S. looking to use regionally produced growing media, potentially limiting shipping costs and carbon footprint. Very limited research is available regarding the soil block method, even as it maintains popularity amongst small and medium-sized growers. More information and continued research on soil blocks is of great value, establishing optimal bulk density parameters and improving understanding of the impact this method has on vegetable transplant performance.

Demonstration workshops will be held at the Iowa State Horticulture Greenhouses and during on-farm extension visits. The developed curriculum will increase farmer’s knowledge of successful organic vegetable transplant production methods, the availability of good quality organic media, and increase awareness of the soil block technique as an alternative technique. In the workshop farmers will improve their organic transplant production skills, including appropriate fertilization, watering, and compaction of cells. This will lead to the action outcome of farmers adopting more successful production practices for transplants. These learning and action outcomes will be evaluated through follow up interviews with workshop attendees.

Outreach specifically targeted at beginning farmers will improve extension education for this historically under-served group, allowing for future on-farm problem solving and collaboration. This project will also spur further research on organic transplant production as more questions arise from collaboration with farmers and the results of the experiment. The project results will be printed in extension publications, published as an article in a scientific journal, and be a chapter in the graduate student’s thesis. These publications will lead to citations from other researchers and a continued increase in knowledge, awareness, and skill development of good organic transplant production practices. A recorded video of the soil block technique will also be made available online to increase learning opportunities for those not able to attend the in-person demonstration workshop, again increasing skill building and knowledge of organic growers.