Evaluating the Soil Block Method and Growing Media in Organic Vegetable Transplant Production

Project Director: Anne Carey, Iowa State University

Project Overview

Vegetable transplants are commonly utilized within U.S. vegetable production systems to ensure higher rates of field establishment and to secure greater yields. The ‘soil block method’ is one method for starting/transplanting vegetables in which seeds are sown directly into freestanding cubes of compacted soil, rather than into plastic ‘flats’ or starter trays. In recent years, the soil block method has received significant attention, owing to concerns about root malformations (i.e. spiraling, circling) when vegetables are started in confined flats, as well as environmental concerns about plastic waste.

This study, conducted at Iowa State University, evaluated organic cucumber and pepper performance (emergence, plant count, plant growth measurements, and root system measurements), both when (1) started in soil blocks or plastic flats, and (2) when grown in several commercial, regionally-produced growing media.

Farmer Takeaways

Transplant method (soil blocks vs. plastic flats) had a larger impact on cucumber and pepper performance than the type of growing medium used.
- Cucumbers tended to perform better when started in plastic flats, and peppers tended to perform better when started in soil blocks.
Growing medium had a relatively smaller impact on cucumber and pepper performance.
- Three of the commercial, regionally-available growing media evaluated (Beautiful Land Products’ Soil Blocking Mix, Purple Cow Organics’ Seed Starter Mix, and Vermont Compost Company’s Fort Vee) are suitable for growing certified organic vegetable transplants in both soil blocks and plastic flats.
- Cowsmo Inc.’s Green Potting Soil should be re-evaluated, as its performance was consistently poorer across treatments.
Additional, longer-term research is needed to better understand the interactions between growing medium, transplant method, time, and crop type.

Project Objectives and Approach

Compare growth parameters and root system development of organic bell pepper and cucumber transplants grown with the soil block method and in plastic flats

A two-year study was conducted at an Iowa State University research greenhouse. The whole-plot factor for the experiment was transplant method (soil blocks vs. plastic flats), and the sub-plot factor for the experiment was growing medium (five growing media, detailed below).
In mid-January of both study years, organic cucumbers and bell peppers were seeded into replicate soil blocks and plastic flats containing the five types of growing media.
Data was collected on seedling emergence (10 and 15 days after seeding (DAS) for cucumbers and peppers, respectively) and plant count (22 and 36 DAS for cucumbers and peppers, respectively), and plants from each treatment were measured for plant height, stem diameter, whole plant dry weight, and root surface area.

Evaluate the suitability of four commercially available, certified organic growing media and one in-house growing medium for vegetable transplant production in both soil blocks and plastic flats

The five commercial, certified organic growing media evaluated in this study were: (1) Purple Cow Organics’ Seed Starter Mix (‘Purple’); (2) Cowsmo, Inc.’s Green Potting Soil (‘Cowsmo’); (3) Beautiful Land Products’ Soil Blocking Mix (‘BLP’); (4) Vermont Compost Company’s Fort Vee (‘Vermont’), and (5) an in-house growing medium (‘Lab’) composed of 50% peat, 25% compost, 12.5% peat, and 12.5% vermiculite by volume.
Prior to seeding in both years, samples of each growing medium were sent for external analysis (nutrient analysis, bulk density, and water-holding capacity).

Key Findings

Cucumber and pepper emergence rates may be impacted by transplant method, with higher rates observed for plants started in plastic flats than in soil blocks.

In Year 1 of the study, cucumbers seeded in three of the five growing media experienced between 42 and 46% higher emergence rates and higher plant counts (22 DAS) when started in plastic flats rather than in soil blocks, suggesting that there may be a transplant method x growing medium interaction for cucumber emergence and plant count.
- In Year 2, however, there was no significant difference in cucumber emergence nor plant count between treatments. Longer-term studies are needed to further explore/confirm these interactions.
In Year 1 of the study, peppers seeded in two of the five growing media experienced between 26 and 45% higher emergence rates when started in plastic flats than in soil blocks.
- In Year 2, pepper emergence across all growing media was 33% higher in plastic flats than soil blocks. Pepper plant count (36 DAS) was similar among all treatments for both years.
- The researchers postulate that some of the observed differences in pepper emergence between transplant methods may be attributed to a temperature/compaction-induced delay in pepper emergence within the soil blocks, as temperatures were lower and bulk density was higher for soil blocks than for plastic flats.

Transplant method may have a larger impact on dry weight, plant height, and stem diameter than growing medium, with cucumbers tending to perform better when started in plastic flats, and peppers performing better when started in soil blocks.

Cucumber dry weight was greater in plastic flats than soil blocks by 20% and 38% in Years 1 and 2, respectively. Growing medium did not appear to have a significant impact on cucumber dry weight, with the exception of Cowsmo, which consistently yielded cucumbers with a significantly smaller dry weight.
- Pepper dry weight was between 50 and 130% greater in soil blocks than in plastic flats for all growing media during Year 1 except Cowsmo, which yielded significantly lighter peppers. No transplant method effect was observed for Year 2.
Cucumbers started in plastic flats were 13% taller across all growing media than those started in soil blocks during Year 1. During Year 2, cucumbers seeded in two of the five growing media were taller in plastic flats than soil blocks.
- Peppers started in soil blocks were between 47 and 74% taller across all growing media than those started in plastic flats during Year 1. During Year 2, peppers seeded in two of the five growing media were taller in soil blocks than plastic flats.
Cucumber stem diameters were 7% larger across all growing media when started in plastic flats than in soil blocks during Year 1. During Year 2, stem diameters for two of the five growing media were significantly larger in flats than in soil blocks.
- Pepper stem diameters were 26% larger across all growing media when started in soil blocks than in plastic flats during Year 1. During Year 2, stem diameters for three of the five growing media were significantly larger in soil blocks than in flats.
Cucumber root surface area was 92% and 118% larger in plastic flats than in soil blocks during Years 1 and 2, respectively.

Resources

Carey, A., A. Nair, and A. Thoms. 2024. Evaluating the Soil Block Method and Growing Media in Organic Vegetable Transplant Production. HortScience 59:542–551. https://doi.org/10.21273/HORTSCI17566-23.

Location

Iowa

Collaborators

Ajay Nair, Iowa State University
Adam Thoms, Iowa State University

Region

Midwest

Topic

Soil Health, Crop Nutrient Management, Cropping Systems

Year Published

2024