Starting vegetable and flower seeds in soil blocks is an efficient, low‑waste method that promotes healthy root development.
Soil blocking is a seed‑starting method in which seeds are sown in compressed soil cubes rather than in plastic trays or pots. A handheld soil‑block maker compresses a moistened soil mix into firm blocks that hold together through their own structure and moisture.
Advantages and Disadvantages | Materials Needed | How to Create & Sow Soil Blocks | Care | More Information
Advantages and Disadvantages
Soil blocking offers several key benefits for starting seeds. Because roots naturally stop growing at the edge of the block, seedlings develop compact, non‑circling root systems that transplant with minimal shock. The method also does not use plastic containers, reducing the amount of plastic used and eventually thrown away. ti is highly flexible, allowing growers to make exactly the number of blocks needed, and adapts easily to both small home gardens and larger production.
Despite these strengths, soil blocking has a few limitations. It requires purchasing a soil‑blocking tool, and beginners may find that blocks fall apart until they master the right mix and moisture level. The exposed sides dry out faster than traditional pots, so blocks need more frequent watering, and the watering method is different and takes some trial and error to perfect. Very small seeds can also be challenging unless a micro‑blocker is used.
Materials Needed
Soil Blocker
Soil blocks are made using a specialized tool that forms and releases one or more blocks, often with a small dimple in the top for sowing seed. Different sizes are available to grow various crops and accommodate different growth stages. A typical soil blocker creates four blocks and features a spring-loaded handle to eject them from the mold.
Standard handheld soil blockers typically make 1½‑ to 2‑inch blocks and are the most versatile option for most vegetable and flower seedlings. Larger 3‑ to 4‑inch blockers are used when seedlings need more time before transplanting or when potting up from smaller blocks.
Micro‑blockers produce tiny blocks and are ideal for germinating very small or slow‑to‑sprout seeds before transferring them into larger blocks. Special adapters can be used on the larger sizes to create an opening that perfectly fits the micro-block.
Soil Mix
The right soil mix is essential for success using soil blocking. The mixture must hold together but not be so compressed as to reduce pore space for air and root growth.
The best place to start is to buy a pre-made mix intended for use in soil blocks. These mixes contain the right balance of different components to be successful. With more experience, custom soil mixes can be created that perfectly suit the seedling's needs.
A classic soil block mix is:
- 3 parts peat moss or coco coir
- 2 parts well‑finished screened compost
- 1 part coarse sand or perlite
- 1 tablespoon lime per gallon of peat‑based mix (omit if using coir)
A simple soil mix utilizing easy-to-find ingredients is:
- 2 parts peat moss or coco coir
- 1 part compost
- 1 part vermiculite or perlite
There are many different formulations that can be used for soil blocks. These recipes can be used as a starting point, and you can make adjustments to create a mix that suits your needs.
Adding Fertilizer
Fertilizer can be added to the soil mix if desired. About 1/2 cup of a low-analysis all-purpose fertilizer (like a 5-5-5) for every cubic foot of mix works well. (1 cubic foot of soil is equal to about 1.5 five-gallon buckets) A fertilizer mix of equal parts blood meal, rock phosphate, and greensand can also be used, adding 1.75 cups per cubic foot of soil.
Trays
Trays for soil blocking need to provide support while allowing good drainage and bottom watering. For many gardeners, reducing plastic waste is a goal, so trays are reused or repurposed. Good trays include shallow, rigid cafeteria-style trays or sturdy boot trays because they are stable, flat, don't restrict airflow around the sides, and hold a small amount of water to facilitate bottom watering. Other gardeners like mesh-bottom trays, which provide good airflow and prevent overwatering. These porous bottom trays are submerged in another tray full of water for a short period to facilitate bottom watering.
The tray you choose is largely a matter of personal preference, as long as it is sturdy, facilitates bottom watering, and provides good drainage and airflow.
How to Create and Sow Soil Blocks
Prepare the Mix
To make soil blocks successfully, begin by preparing the mix. The soil mixture should be thoroughly moistened before blocking. A mixture of 3 parts mix and one part warm water is a good place to start. Aim for a consistency similar to wet concrete, peanut butter, or brownie batter. The soil mixture will be wetter than you would anticipate for planting. When you squeeze a handful, it should firmly hold together without dripping water.
Getting to the right moisture level is essential for success. When the mixture has the right moisture level, it will easily compress into the soil blocker and hold together when ejected. Mixtures that are too dry will crumble apart, and those that are too wet will stick to the soil blocker.
Load the Blocker
Once the mix is ready, load the soil blocker. Place the mix in a large flat tray, container, or table. Press the tool firmly into the mix several times, twisting slightly as you push to ensure each chamber fills completely. It’s important to pack the soil tightly so the blocks hold their shape. After the chambers are filled, scrape excess mix off the bottom using the edge of your container or a flat surface. See the video below for a tutorial.
Form the Blocks
After loading, form the blocks by placing the soil blocker on a sturdy, flat tray surface. Press down with steady pressure, then use the blocker’s release mechanism to leave the blocks in neat rows. Lift the tool straight up so the blocks aren’t disturbed. They should stand on their own with well-defined edges and a small indentation in the top for seeds.
Plant Seeds
Plant seeds by dropping one seed into each depression. If the germination rate is low, sow more than one in each block. Follow the germination requirements for each crop. Some seeds need to be lightly covered, while others should remain exposed to light. If covering is necessary, gently place a pinch of moist mix or fine vermiculite over the seed.
Label
Finally, label your trays right away. Soil blocks all look alike once formed, so clear labels help you keep track of varieties, planting dates, and any special care instructions. Because trays often hold multiple rows, include markers at both ends or create a simple map for accuracy.
How to Care for Soil Blocks
Watering
Use a bottom-watering system to water soil blocks, rather than pouring water over the top, which can cause the blocks to break apart. When using an impermeable tray (such as a cafeteria tray) add water to the tray and allow the soil blocks to wick up the water. If using a permeable tray (like a web flat tray) submerge the tray in a shallow container of water and allow the blocks to absorb the water.
Bottom watering has the advantage of keeping the surface dryer and reducing the risk of fungal issues. However, the blocks cannot sit in water long-term or they become oversaturated. Soil blocks should feel evenly moist but never soggy. If they begin to look shiny, slump, or lose their shape, they are too wet. Regularly check the tray and allow excess water to evaporate or pour it off to maintain the ideal moisture balance.
Light
As with seedlings grown using containers, once seeds germinate, provide strong, consistent light to ensure healthy, compact growth. A bright, sunny window may be sufficient, but seedlings often perform best under grow lights. Maintain 12–16 hours of light per day to prevent stretching or leggy growth. Good light at this early stage encourages sturdy stems and well-developed leaves, laying the foundation for successful transplanting later on. Learn more in this article: Growing Indoor Plants Under Supplemental Lights.
Airflow
Adequate airflow is essential for keeping seedlings healthy in soil blocks. Good ventilation helps reduce humidity around the plants, preventing damping-off disease and other fungal problems that thrive in stagnant air. If a plastic dome or other cover was used to increase humidity for germination, remove it as soon as the young plants emerge.
A small fan set on low to gently circulate air in the growing area works well and can also help strengthen seedling stems. Just ensure the airflow is indirect and not strong enough to dry out the blocks too quickly.
Spacing
As seedlings grow, it is important to maintain a space between the blocks. Slightly separating the soil blocks allows the edges to dry between waterings and encourages air pruning, a natural process where roots stop at the block’s edge rather than circling. This results in compact, fibrous roots that transplant more effectively and establish more quickly in the garden.
Spacing is also important to prevent roots from growing into neighboring blocks, making transplanting much more difficult.
Transplanting
When seedlings are ready for the garden or a larger container, transplant the entire soil block without disturbing the roots. One of the biggest advantages of soil blocks is that the plants experience virtually no transplant shock, since the entire root system remains intact. As with all seedlings, transplant shock is reduced when they are properly hardened off. Learn more in this article: Selecting, Hardening, and Planting Bedding Plants.
To transplant, simply dig a hole the size of the block, set it in place, and firm the soil around it. The block will quickly integrate with the surrounding soil, allowing the plant to resume growing with minimal stress.
More Information
- Guide to Starting Seed Indoors
- How to Successfully Start Seed Indoors
- Containers for Starting Seeds
- Supplies for Starting Seeds Indoors
- Germination Requirements for Annuals and Vegetables
- Sources for Annual and Vegetable Seeds
- How to Store Seeds and Test Germination Rates
Photo credits: 1: andreaobzerova AdobeStock; 2: Petra Richli AdobeStock; 3: andreaobzerova AdobeStock; 4: eurobanks AdobeStock; 5: Petra Richli AdobeStock; 6: Anna Hoychuk AdobeStock; 7: andreaobzerova AdobeStock