Evaluating the soil block technique for organic vegetable transplant production

Progress report for GNC21-322

Project Type: Graduate Student
Funds awarded in 2021: $14,926.00
Projected End Date: 09/30/2023
Grant Recipient: Iowa State University
Region: North Central
State: Iowa
Graduate Student:
Faculty Advisor:
Dr. Ajay Nair
Department of Horticulture, Iowa State University
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Project Information


Evaluating the soil block technique for organic vegetable transplant production

Proper transplant production is an important part of a successful vegetable operation, ensuring better field establishment, increased yields, and higher profitability for farmers. Many organic vegetable farmers in the North Central region employ the soil block method for transplant production, following anecdotal evidence that this alternative technique improves root development of seedlings through air pruning and an increase in root volume, but there is limited scientific research evaluating these claims. Furthermore, much research has been conducted on specific recipes and components of organic media for vegetable transplant production, but less is known about commercially available products which are commonly used by organic farmers. This project aims to address these issues by scientifically evaluating the growth parameters and root system architecture of organic vegetable transplants grown with the soil block technique, in plastic flats, and with five commercially available organic media. The experiment will be conducted at the Iowa State University Horticulture Greenhouse over the course of seven weeks with organic tomato seedlings. On three occasions data will be collected on plant height, stem diameter, and chlorophyll leaf content of seedlings in each treatment. On each occasion four representative seedlings will be destructively sampled to determine root to shoot ratio. Final samples will be analyzed with the WinRhizoTM software for root system architecture characteristics.

As a result of this project and the output activities farmers will increase knowledge and awareness of the soil block method, successful organic transplant production practices, and regionally available organic media options. Farmers will also build skills in transplant production and with the soil block technique. By employing the new knowledge and skills, farmers will improve their transplant production, improving yields and farm profitability. Farmers will also have scientific knowledge to make decisions when purchasing organic media, including a consideration of regionally produced organic media. A targeted outreach effort focused on beginning farmers will enhance their relationship to extension services, offering guidance for the future and opportunities for research collaboration. These outputs will be evaluated through follow up interviews with workshop participants.

Publication of extension articles, a journal article, and a thesis chapter will increase scientific understanding of the soil block method and organic media options, spurring more research for organic methods. The success of this will be evaluated by the number of downloads and citations of the articles.

Project Objectives:

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.


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Materials and methods:

The experiment was conducted in the Iowa State University Department of Horticulture greenhouse. A total of five different media were used in plastic flats and soil blocks in a split block randomized complete block design with four replications. The five different media evaluated were: Purple Cow Organics ‘Seed Starter Mix’ (Purple), Cowsmo ‘Green Potting Soil’ (Cowsmo), Beautiful Land Products ‘Soil Blocking Mix’ (BLP), Vermont Compost Company ‘Fort Vee’ (Vermont), and a lab mix (Lab). The lab mix was comprised of 50% peat, 25% compost, 12.5% perlite, and 12.5% vermiculite by volume. This formulation is based on general growing medium mix recommendations. All media are specifically stated to be used with the soil block method. Three regionally produced, certified organic growing media, from Purple Cow Organics (Middleton, WI), Beautiful Land Products (Tipton, IA), and Cowsmo (Cochrane, WI), were compared against the nationally recognized Vermont Compost Company product (Montpelier, VT).

Bell pepper ('OG Yankee Bell', Johnny's Selected Seeds) and cucumber ('OG Marketmore 76', Johnny's Selected Seeds) transplants were grown in 50-cell flats cut in half to create 25 cells for each treatment (method x growing medium) with four replications. The soil blocks were made with the Stand-Up 12 soil blocker tool purchased though Johnny’s Selected Seeds.  Flat size and block size were chosen to ensure the most equal volume possible, taking into consideration the compression of the medium with the blocking tool. Plastic flat cells had a volume of 8.3 cm2 and soil blocks had a volume of 5.33 cm2.

Cucumber and peppers were seeded into each cell or block on 17 January 2022. All transplants were monitored daily and watered to ensure appropriate moisture. Pepper transplants were fertilizer with 150ppm Aqua Power™ 5-1-1, an organic fish emulsion fertilizer, on 24 February 2022 (38 days after seeding [DAS]) and 3 March 2022 (45 DAS) to prevent nutrient deficiency. Cucumber transplants were not fertilized.

To evaluate the growth parameters and root system architecture of transplants with each method and growing medium combination, cucumbers were destructively sampled on 8 February 2022 (22 DAS). Five cucumber plants were selected from the center of the plastic flat or soil block tray (n=200), leaving the exterior plants as guard plants, and the plant height, stem diameter, and the SPAD value were recorded. SPAD is an indirect, unitless measurement of the chlorophyll content of a leaf, and a good indicator of 'leaf greenness' and overall plant health. The roots were thoroughly and carefully washed to remove all remaining growing medium. Plants were then placed in an air-forced oven at 67˚C for seven days to remove all water content. Upon removal, the whole plant weight was be recorded, roots were carefully removed, and the shoot weight was recorded. This data was used to determine the root to shoot ratio and plant dry weight.

Pepper transplants were destructively sampled after five weeks (36 DAS), six weeks (43 DAS), and seven weeks (50 DAS) from seeding. At each sampling, three peppers plants were selected from the center of the plastic flat or soil block tray, leaving the exterior plants as guard plants (n=120 at each sampling). The same growth parameter measurements were collected (plant height, stem diameter, and SPAD). The roots were washed and plants were dried and weighed as per the cucumbers. Dry cucumber transplants and pepper transplants from the final sampling were ground and sent to Ward Laboratories Inc. (Kearney, NE) for plant tissue analysis.

One transplant was reserved from the cucumbers and from the final sampling of peppers from each treatment/replication and was analyzed with the WinRhizoTM software to determine root system architecture characteristics, such as root surface area and root volume. This data was used to evaluate the differences between growing medium and method on root development of the seedlings.

The growing media was further analyzed by performing the pour-thru extraction method (Torres, et al., 2010). Once per week for the duration of the study leachate from the plastic flats with pepper transplants was collected as per the method and data on leachate pH and electrical conductivity (EC) was collected. The pour-thru method was performed only on plastic flats growing peppers, due to an inability to sample only selected data cells in soil blocks.

Research results and discussion:


Seedling emergence data was collected for cucumbers 10 days after seeding (10 DAS) and for peppers 15 DAS by counting the number of emerged seedlings in each cell and block. Plant count data was collected at time of destructive sampling of cucumbers, 22 DAS, and at first destructive sampling of peppers, 36 DAS, by counting the number of plants in each cell or block. 

Emergence in cucumber and peppers transplants grown in soil blocks was significantly reduced as compared to transplants grown in plastic flats (Table 1). For cucumbers, averaged across all growing media and replications, 24 out of 25 seeds had emerged in plastic flats and 18 out of 25 had emerged in soil blocks (p-value=0.0441). For peppers, averaged across all growing media and replications, 23 out of 25 seeds had emerged in plastic flats and 20 out of 25 had emerged in soil blocks (p-value=0.0253).

The plant count was significantly reduced in peppers grown in soil blocks, 24 out of 25 in plastic flats and 22 out of 25 in soil blocks (p-value= 0.0162). Although not statistically significant (p-value= 0.0559) the plant count was higher for cucumbers grown in plastic flats, 24 out of 25, than in soil blocks, 19 out of 25. 

Growing media had a significant impact on cucumber and pepper emergence. Cucumbers grown with 'Purple' and 'BLP' had higher emergence, 23 and 22 out of 25, respectively, than cucumbers grown with 'Cowsmo', 'Lab', and 'Vermont', all 20 out of 25 (p-value= 0.0342). The trend was not similar in pepper emergence by media. Peppers grown with 'Cowsmo', 'Vermont', and 'BLP' had higher emergence, 23, 23 and 22 out of 25, respectively, than peppers grown with 'Purple' and 'Lab', 20 and 19 out of 25, respectively (p-value= 0.0103). 

The plant count was significantly higher in peppers grown in 'Cowsmo', 'Vermont', and 'Purple', 24 out of 25 for all, than peppers grown in 'BLP', 22 out of 25, and 'Lab', 20 out of 25 (p-value= 0.0019). Although no significant differences were found in plant count among the growing media for cucumbers (p-value= 0.1486), cucumbers grown with 'Purple' had the highest plant count.


Table 1. Cucumber and pepper transplant emergence and plant count. All numbers reported out of 25 cells or blocks.

  Cucumber Emergence Cucumber Plant Count  Pepper Emergence Pepper Plant Count 
Flat 24 24 23 24
Soil Block 18 19 20 22
P-value 0.0441 0.0559 0.0253 0.0162
BLP 22 22 22 22
Cowsmo 20 21 23 24
Lab 20 21 19 20
Purple 23 23 20 24
Vermont 20 22 23 24
P-value 0.0342 0.1486 0.0103 0.0019

The significant reduction in pepper and cucumber emergence in soil blocks may be due to differences in bulk density and growing media temperature. A high bulk density reduces the pore space and oxygen in the growing media. Soil blocks are made by compressing wet growing media into the soil blocker tool. This action may increase the bulk density to a point that limits the ability of a germinated seedling to emerge above the growing media surface. Temperature plays a significant part in seed germination. Many vegetable species have temperature dependent seeds, requiring a certain temperature before germinating and emerging. The cells of plastic flats are made of black plastic, which surrounds five sides of the cube. On the other hand, soil blocks are surrounded by other blocks or air. There is less opportunity for the growing media to dry out and increase in temperature in soil blocks. This reduction in temperature may lead to a decrease in seedling emergence.

In the second year of this study, I will collect data on bulk density of the soil blocks and plastic flat cells for each growing media. Additionally, I will collect data on the temperature of the growing media during time of emergence and before the plant count. This will help to elucidate the reasons for the differences found. 

The differences found between the growing media may be due to the bulk density, influenced by the aggregate size of the growing media, as well as other physicochemical properties. A larger aggregate size reduces the bulk density of a growing media, as the larger particles are not held as tightly together as smaller particles may be. The pH and electrical conductivity (EC) are two important characteristics of growing media which influence seedling emergence and plant growth. A high EC, as is often found in growing media made with a high proportion of compost, can negatively impact germination and plant growth. Table 2 shows the pH, as reported by AgSource Laboratories (Lincoln, NE), and EC data collected for each of the growing media using the pour-thru method.  An EC of 4 or higher is generally considered to be detrimental to plant growth. The EC decreases over time as leaching of soluble salts occurs through continued irrigation and as plants uptake available nutrients.


Table 2. pH and electrical conductivity (EC) of each growing media.

Media pH

EC (mS/cm), Week 1

EC (mS/cm), Week 2 EC (mS/cm), Week 3 EC (mS/cm), Week 4 EC (mS/cm), Week 5 EC (mS/cm), Week 6
BLP 5.8 6.3 4.3 2.8 2.2 1.4 1.0
Cowsmo 7.2 4.8 3.4 2.8 2.2 1.9 1.4
Lab 6.9 6.5 4.6 2.9 2.2 1.9 1.2
Purple 5.5 5.9 2.9 1.6 1.5 1.2 1.0
Vermont 5.4 7.8 6.7 5.2 3.9 2.7 1.2




Table 3. Cucumber transplant growth parameters at destructive sampling. 22 DAS. 

  Dry Weight (g) Plant Height (mm) Stem Diameter (mm) SPAD
Flat 0.22a 61.49a 3.81a 40.50
Soil Block 0.18b 53.95b 3.55b 38.97
P-value 0.0474 0.0446 0.0347 0.0753
BLP 0.21a 68.26ab 4.04a 39.68b
Cowsmo 0.08b 27.41d 2.07b 42.06a
Lab 0.22a 62.70bc 3.96a 36.29c
Purple 0.25a 71.23a 4.27a 40.93ab
Vermont 0.22a 58.95c 4.06a 39.73b
P-value <.0001 <.0001 <.0001 0.0003


Table 4. Pepper transplant growth parameters at destructive sampling. Week 5= 36 DAS, Week 6=43 DAS, Week 7= 50 DAS.

  Dry Weight (g)
Plant Height (mm) Stem Diameter (mm) SPAD
  Week 5 Week 6 Week 7 Week 5 Week 6 Week 7 Week 5 Week 6 Week 7 Week 5 Week 6 Week 7
Flat 0.13 0.20b 0.32b 49.16 66.54b 82.16b 2.14b 2.52b 3.01 28.39b 25.51b 27.99
Soil Block 0.19 0.40a 0.84a 64.76 110.56a 159.76a 2.62a 3.36a 4.06 39.42a 37.64a 51.34
P-value 0.1133 0.0048 <.0001 0.0645 0.0019 <.0001 0.045 0.0033 0.0748 0.0015 0.0004 0.0783
BLP 0.24a 0.45a 0.64b 71.69ab 110.31b 150.76b 2.88a 3.56ab 5.11b 32.038b 34.54b 32.43
Cowsmo 0.02c 0.02c 0.02c 16.71d 18.15d 21.95d 1.06a 1d 1.05c N/A N/A N/A
Lab 0.14b 0.27b 0.55b 53.83c 81.29c 102.21c 2.36c 3.03c 3.36b 28.28c 31.3c 50.03
Purple 0.19ab 0.31b 0.74b 64.75b 106.2b 145.45b 2.65b 3.25bc 3.63ab 30.29bc 30.5c 37.84
Vermont 0.23a 0.46a 0.93a 77.83a 126.79a 184.4a 2.95d 3.85a 4.51ab 35.69a 39.28a 38.38
P-value <.0001 <.0001 <.0001 <.0001 <.0001 <.0001 <.0001 <.0001 0.0012 0.003 <.0001 0.5985


Contrasting significant differences were found among cucumbers and peppers grown in soil block and plastic flats. Cucumber transplants performed better in plastic flats, with significantly higher dry weight, plant height, and stem diameter, than those grown in soil blocks. On the other hand, pepper transplants performed better in soil blocks across all weeks for which data was collected. Peppers grown in soil blocks had significantly higher dry weight in weeks five, six, and seven. Soil block grown peppers also had significantly larger plant height, stem diameter, and SPAD reading in two out of the three samplings. In the one week that was not statistically significant, soil block grown pepper transplants continued to have a numerically higher plant height, stem diameter, and SPAD reading. 

The growing media treatments show cucumber and pepper transplants grown in Cowsmo to have a reduced dry weight, plant height, and stem diameter. Due to the small size of the leaves of the pepper transplants grown in Cowsmo, no SPAD reading could be recorded. This again displays the decreased development and growth of transplants grown with this media. Pepper and cucumber transplants grown in BLP, Purple, and Vermont performed similarly. Although, in some weeks pepper plants grown in Vermont had significantly higher growth parameter measurements than all the other growing media, such as the plant height in weeks six and seven and the dry weight in week seven.

Similar results were found in the cucumber transplants. Plants grown with BLP, Purple, and Vermont had a similar dry weight and stem diameter. Purple and BLP grown cucumber transplants had a greater plant height than plants grown with the other growing media.




Table 5. Cucumber transplant root volume and root surface area.

  Root Volume (cm2) Root Surface Area (cm3) Root to Shoot Ratio
Flat 0.26a 39.87a 0.24
Soil Block 0.10b 14.71b 0.26
P-value 0.0069 0.0008 0.7908
BLP 0.20ab 31.32a 0.18b
Cowsmo 0.12c 18.38b 0.46a
Lab 0.20ab 27.61ab 0.22b
Purple 0.22a 34.06a 0.17b
Vermont 0.16bc 25.09ab 0.20b
P-value 0.0449 0.0188 0.0004


Table 6. Pepper transplant root volume and surface area. Root:shoot ratio from Week 7 sampling.

  Root Volume (cm2) Root Surface Area (cm3) Root to Shoot Ratio
Flat 0.51b 62.91b 0.34a
Soil Block 0.80a 86.73a 0.18b
P-value 0.0316 0.077 <.0001
BLP 0.61c 73.67b 0.22bc
Cowsmo 0.03d 3.91c 0.38a
Lab 0.61c 69.07b 0.26b
Purple 1.18a 121.96a 0.26b
Vermont 0.86b 105.49a 0.19c
P-value <.0001 <.0001 <.0001


Similar to the growth parameter measurements, cucumber and pepper transplant root development differed in soil blocks and plastic flats. Cucumber transplants had significantly greater root volume and root surface area in plastic flats. On the other hand, pepper transplants had significantly greater root volume and surface area in soil blocks.

In terms of growing media comparisons, cucumber and pepper transplants grown in Cowsmo had a significantly lower root volume and root surface area than transplants grown with the other media. Cucumber transplants grown with BLP, Purple, and Vermont had similar root volume and root surface area. Pepper transplants grown with Purple had the greatest root volume, followed by Vermont. Purple and Vermont also produced pepper transplants with the largest root surface area.

A higher root to shoot ratio is generally considered an indication of transplant quality. As the proportion of root development increases as compared to shoot development, the transplant will be able to access more water and nutrient resources within the media and, once transplanted, withstand transplant shock better. No significant differences were found in cucumber transplant root to shoot ratio when comparing the two methods. Pepper transplants had significantly higher root to shoot ratio in plastic flats as compared to soil blocks. 

When comparing the different growing media, Cowsmo cucumber and peppers transplants had significantly higher root to shoot ratio as compared to the other growing media. Although, this result may be misleading due to the overall small size of Cowsmo grown transplants. The relationship between the transplant root to shoot ratio and growth parameter metrics needs to be explored further. Additional details and analysis will follow in the final report.



The differences found between the cucumber and pepper transplant growth parameter performance may be due to the duration of transplant growing time between the two vegetable species. Cucumbers only grew for three weeks, while peppers grew for seven weeks. If data collected in 2023 confirms differences in growing media temperature and bulk density, this may help explain the differences found. A cooler growing media could decrease the rate of plant growth, but soil blocks may provide an growing advantage once the temperature difference is overcome. As soil block grown transplant roots are not restricted by the walls of the plastic cell, the plants, over time, may be able to grow at an exponential, increased rate. Further research and investigation is needed to confirm these thoughts. With more data collected in 2023, more conclusions can by made.

Two regionally produced growing media, BLP and Purple, produced cucumber and pepper transplants of similar quality as Vermont. Growers in the North Central region can be confident when using these locally produced products. On the other hand, concerns arose about transplants grown with Cowsmo. With significantly reduced growth and root root development in both plastic flats and soil blocks, cucumber and pepper transplants performed poorly when grown in Cowsmo. Further investigation into the chemical concentrations and physicochemical parameters is needed to determine what may have caused these results. With an additional round of the experiment and data collection in 2023, more thoroughly conclusions can be made.

Participation Summary

Educational & Outreach Activities

5 Consultations
3 Tours
3 Webinars / talks / presentations
2 Other educational activities: Videos posted to YouTube and lab website

Participation Summary:

30 Farmers participated
275 Ag professionals participated
Education/outreach description:

I gave an oral presentation of the research project and first year results at the American Society of Horticultural Sciences Annual Conference on August 2, 2022. 

The research was also presented at the Graduate Program in Sustainable Agriculture Research Symposium on May 4, 2022. This also included a graduate student competition, where I place 2nd.

I presented in a session at the Great Plains Growers Conference on January 13, 2023 to an audience of mostly farmers and growers. This was a full, 45 minute session where I gave background on the topic and presented first year findings. 

Additionally, I have given three tours of the project to interested researchers and student recruiters.

Two videos have been made about different aspects of the project, available to the public through YouTube. The first discusses and shows how to make soil blocks and the second discusses the pour thru method for understanding growing media qualitites.

This coming spring, in April 2023, I will participate in a full day workshop for Master Gardener's of Iowa. I will give an introduction to transplant production methods and guide a hands-on portion on the making of soil blocks to an audience of gardeners and growers. As part of the workshop, I will be developing a factsheet and informational handout to be made available to participants and posted online for the public.

I will also be presenting the research in the form of a poster at the International Society of Horticultural Sciences Annual Conference in June 2023 in Montreal, Canada and at the American Society of Horticultural Sciences Annual Conference in August 2023 in Orlando, Florida.

I will be writing an extension publication covering the soil block method and findings concerning the methods and growing media from the project, to be completed in spring 2023. Finally, I am working on a journal article covering the results of this project, to (hopefully) be published in HortScience in 2023. 

Project Outcomes

30 Farmers reporting change in knowledge, attitudes, skills and/or awareness
2 New working collaborations
Project outcomes:

As we have only completed one year of the project so far, our outcomes are still in progress. As we are able to continue presenting and disseminating the results of our work, we will have a greater opportunity to affect agricultural sustainability in our area. 

Using the soil block method for transplant production decreases plastic use, providing economic and environmental benefits for farmers. Using a long-lasting, metal tool instead of plastic flats, growers are able to reduce their costs and do not need to rely on purchasing and transporting new plastic flats every few years. 

With the skills and knowledge to produce healthy, vigorous vegetable transplants, growers are also able to increase their economic returns. High quality transplants leads to high quality crops, which sell for higher prices. Growers who participate in my talks, workshops, and poster presentations can learn about what makes a quality growing media as well as other, general considerations for growing good transplants. This knowledge can be used to improve overall transplant production, regardless of plastic flat or soil block use.

Additionally, having the confidence that a regionally produced growing media is of similar (if not greater) quality as a product from the Northeast, North Central growers can increase their economic, environmental, and social sustainability by purchasing these products. When goods are purchases from a local company, the money is often recycled within the local economy. These local dollars build social sustainability, growing communities and building resiliency. Shipping costs and emissions are also reduced and the products become less expensive, providing economic and environmental advantage.

Knowledge Gained:

After the first year of the project, my awareness, skills, and attitude about sustainable agriculture have increased. I was aware of the soil block method for transplant production previously, but did not consider the all of the advantages such a technique could afford small-scale growers looking to increase their sustainability. Making soil blocks with a reusable tool instead of the flimsy plastic trays most common on farms opened my eyes to other ways farmers can reduce plastic use. I am now more aware of all the ways plastic is commonplace and depended on by small-scale (oftentimes organic) growers. Some may think it is impossible to grow efficiently without this plastic (such as in plastic mulch), but by showing that transplants can be successfully grown with significantly less plastic, I am now more open to the other changes in behavior that would be needed to reduce plastic on the farm. Additionally, I have develop my skill of making soil blocks and experienced the nuances that come with growing transplants in this way, such as their irrigation needs. 

My advisor has experienced similar changes in awareness and skill. Having the opportunity to discuss transplant production with many growers and others throughout this project has given him (and myself) a greater understanding of the challenges regional growers have in producing quality transplants. We have become aware of networks which grown transplants for each other and buying clubs for purchasing growing media in bulk quantities, amongst other things. Coming from an academic environment, one may have a limited view of what is necessary for 'proper' transplant production, relying on expensive greenhouses, inline fertigation, and expensive lighting. Yet, most growers in our region do not have access to these amenities. In our increasing awareness of the practices used by growers in our area, we are better able to tailor extension materials and develop research projects which benefit them and increase their sustainability.

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.