How to Soil Block Without A Blocker: 7 Seed-Starting Hacks

Seed-starting gives plants a jumpstart on strong roots and healthy growth in a protected environment. With soil blocking, there’s no need for cell packs to start seeds. Individual soil cubes become the foundation for their development and are easy to transplant into the garden. The blocks are both the “pot” and the growing media where roots develop. A simple blocking tool filled with a specialized mix forms individual blocks for seeding. 

Soil blocking is a streamlined technique that uses minimal resources. It takes a bit of practice to finesse, especially in forming solid blocks for seeding. The goal is to create dense plugs of moistened soil, free of air pockets, that hold together to support seedling growth.

The initial investment includes a commercial soil-blocking tool and soil materials. A soil blocker can last for decades, or you can make your own using household materials to try it this season. Seed-starting is accessible using any containers that hold potting mix and have drainage holes. Soil blocking can be, too.

What Is Soil Blocking?

Soil blocking relies on a specialized potting mix, purchased or homemade, formed into condensed cubes using a mold. Each block holds one to two seeds. As they germinate, seedlings develop as they do in other seed-starting methods, rooting in their plastic-free containers before moving either to four-inch pots or straight into the garden bed.

Seedlings in soil blocks develop robust roots with the increased oxygen available. Roots naturally air-prune as they reach the block edges, filling out the volume of the block and holding the dirt in place until transplanting. With the roots full and holding, they’re ready to quickly establish at planting. There’s also minimal disturbance during transplanting, free of manipulating the tender roots out of cell packs.

Growing space is precious, and blocks are efficient space-savers, whether sowing in bulk or a small amount. Without cell pack spacing, more “pots” occupy a tray.

Basic Supplies

A big advantage of soil blocking is its flexibility in materials. Get creative with supplies you have on hand, or invest in a few to use season after season.

Materials to start include a blocking tool, a potting mix formulated to form the blocks, and bottom trays. The trays hold the blocks, and a flat one is best for even development and bottom watering. The base tray can be a nursery bottom tray, a baking sheet, or even a takeout container. Anything flat with a lip to hold water will work.

Gather a large bowl or pan to mix the potting media with water. The mix needs to be wet but firm (not drippy) to fill the mold and hold the form. After seed-sowing, a misting bottle, watering can, or pitcher to fill the base of the tray will keep the blocks moist.

Commercial blocking tools come in varying sizes. For home use, two-inch blocks accommodate most seeds. As you experiment with your tool and technique, you may want to create a small one, too, for bulk seeding tiny seeds or those with long germination times. Half-inch cubes work well for these seed types (think lavender, oregano, thyme, milkweed, rudbeckia, and more).

Alternative Blockers

A number of “at-home” items can become molds for soil blocks, and you’ll probably think of more to add to this list as a resourceful gardener. The premise is to create a two-inch (or so) uniform block or round for even root development.

In the first two ideas, a cylinder becomes the mold and a dowel (or any item to press the dirt cake out of the mold) is instrumental.

Can and Dowel

A soda can or canned good (soup, beans, tomato sauce, tuna, cat food) becomes a handy mold to pack with soil. To use a soda can, cut off the top and bottom, saving the center section for the form (be careful – those cans are sharp!). Then, cut the center portion lengthwise for sizing.

Use a two-inch or so dowel or other rod as an implement to press out the block. Once you have this tool at hand, size the open piece of can so it slips easily around the dowel. Use electrical tape to seal the can’ seam and edges. Now, you’ve got an aluminum form to pack with moistened soil. Use the dowel to tamp the dirt evenly, and then press the round out onto a tray.

To use a soup or other can, leave it at full size. Fill it with two inches of soil, and use the dowel as in the soda can method.

PVC Tube

To reuse extra PVC or irrigation couplings, use the dowel method and a small PVC section. A two-inch diameter PVC tube is a handy mold, and any workable length is fine. Hand scoop a couple of inches of moistened potting mix into the tube and use a rod to tamp it in place. Then, use the rod to pop it out onto the bottom tray. 

A convenient way to use PVC is to press the pipe directly into the potting mix to fill it a couple of inches. Then, complete it with the dowel.

Ice Cube Tray

Ice trays become seed starters by forming cubes of moistened soil. A silicone tray works best for easier extraction. To use an ice cube tray to make soil blocks, fill each cube densely with the mix. Use a spreader or flat tool for compaction and to fill the corners.

Let the cubes dry for an hour or so before pressing them onto the bottom tray. Test this timing and technique as you go – trial and error is part of soil blocking. Use a butter knife, if helpful, to pry out the cubes. If the mix sticks to the trays, a light sprinkling of sand before adding the potting mix helps.

Cookie Cutter

They may look more like brownies, but a soil cookie works well, too. Use a uniform cookie cutter to carve out a piece of compressed mix from a pan or tray. Press it out from the top, using the indention from your finger as the seed reservoir. Like the ice cube method, this takes experimentation, but it makes a fun activity with kids to begin the spring garden.

Related Materials

At the heart of it, seed-starting takes finesse but needn’t be expensive. Get a headstart on your spring or summer favorites with any vessel that supports the potting mix and provides drainage. Like soil blocks, tubes from paper towels or toilet paper filled with media are viable seed starters.

These can also be block molds as an alternative to aluminum cans. Egg cartons, too, are accessible mini pots.

Paper Tubes

To use cardboard tubes as seeding pots, cut them into smaller pieces. Fold the base, line them up in a tray, and fill them with seed starting mix. Sow one to two seeds in each “pot.” When ready to plant, open the bottom and place them in the ground as biodegradable planters to minimize root disturbance. Or, cut them away or slip the plug out of the sleeve.

Egg Carton

As with paper tubes, egg cartons become upcycled seed starters. Punch a hole in the bottom for drainage and fill each with a seeding mix. Place the cartons on a tray, and keep each segment evenly moist.

At planting time, clip off each pod for direct planting (biodegradable cartons) or pop them out of plastic ones.

Make Your Potting Mix

Making your own potting mix requires an initial investment, but it goes a long way, and you can use your own compost and added minerals. You can also buy ready-to-go blocking mixes to make things easier.

Blocking requires a base that takes shaping well, and a peat or coir component is essential. Coir is somewhat more sustainable than peat (as ancient peat bogs provide loads of carbon sequestration and take generations to recover after harvesting). So, to lessen peat usage, coconut coir is useful. Gardeners are working on eliminating coir from the blocking system, too, to rely on more renewable resources.

There are great organic recipes for soil-blocking potting mix. These need approximately two parts water for the best consistency of being wet and sticky but not drippy. Here are two simple ones to experiment with: