How to Use a Soil Block Maker | Better Seed Starts

Soil blocking replaces plastic pots with compressed cubes of growing medium, and using one correctly comes down to two things: the right moisture level and enough pressure. Get those right, and the blocks hold together for weeks while seedlings develop strong, air-pruned roots. This guide walks through the full process, along with the mistakes that cause blocks to crumble.

Getting The Mix To The Right Consistency

The single biggest factor in whether soil blocks hold their shape is moisture. The mix needs to be thoroughly moistened but not dripping wet. Multiple sources describe the target consistency as like peanut butter, brownie batter, or a wrung-out sponge. If you squeeze a handful, it should hold together; if water runs out, it’s too wet.

Iowa State University suggests starting with about 3 parts mix to 1 part warm water, then adjusting from there. Warm water incorporates more easily than cold. Work the water through the mix thoroughly—dry pockets will cause blocks to fall apart later.

Use a seed-starting mix or fine compost, not garden soil. Garden soil is too dense and heavy for blocks to hold their structure.

Packing And Pressing The Blocker

Fill a flat tray or work surface with a mound of the moistened mix. Johnny’s Selected Seeds, which sells the 72-cell Soil Blocker, recommends making that mound about 1 1/4 to 1 1/2 inches deeper than the height of the blocker itself. That extra depth ensures every chamber fills completely.

Press the blocker straight down into the mound until it hits the flat surface beneath. Push firmly so each cavity packs tight, then give the blocker a quarter-turn twist. This twisting motion helps fill any remaining air pockets. Lift the blocker straight up; the blocks should come with it.

Fiskars’ instructions for its Soil Block Maker add one extra step: partially press the plunger while the blocker is still full of mix to remove air pockets, then fully press to eject. This two-stage press makes a denser, more stable block.

Ejecting And Sowing Seeds

Set the loaded blocker onto your growing tray—a standard 1020 tray works well—and push down on the handle while lifting the blocker away. The blocks release in neat rows. Avoid sliding or twisting the blocker as you lift, since that can disturb the blocks you’ve already placed.

Each block has a small indentation on top designed for seeds. Green Gardener’s guidance is one seed per block for most varieties, or a few tiny seeds for very small ones. Check your seed packet for exact planting depth and whether the seed needs covering at all. Some seeds germinate better with light and should sit uncovered in the indentation.

Rinse the blocker in warm water before making the next set. This keeps the plunger mechanism moving smoothly and prevents buildup in the chambers.

Watering Your Soil Blocks The Right Way

Water from below, never from above. Add water to the tray rather than spraying the tops of the blocks. Top watering breaks down the block structure over time and can dislodge seeds. Bottom watering lets the blocks wick up moisture evenly while staying intact.

Check the blocks daily, especially in warm or dry conditions. When the tops begin to look dry, add more water to the tray. Don’t let blocks dry out completely—once they shrink and pull away from the tray, they’re hard to rehydrate successfully.

Common mistakes that cause crumbled blocks include underpacking the chambers, using mix that’s too dry, and skipping the twist. If your blocks fall apart, the fix is almost always more moisture or more pressure, not a different brand of blocker.

For a rundown of the most reliable models and features worth paying for, see our roundup of the best soil block makers on the market today.

FAQs

Why do my soil blocks fall apart?

Your mix is likely too dry or underpacked. The mix should feel like a wrung-out sponge and hold together when squeezed. Press the blocker firmly into a mound of mix that’s deeper than the blocker, twist a quarter-turn, and compress fully before ejecting.

Can I use regular potting soil in a soil blocker?

Seed-starting mix or fine compost works best. Standard potting soil often contains large bark chunks and other coarse material that prevents blocks from compacting tightly enough to hold their shape. Stick with finely textured mixes without large particles.

How long do soil blocks last before seedlings need transplanting?

Most seedlings can stay in soil blocks for 3 to 4 weeks before roots fill the block and growth slows. Because the blocks air-prune roots at the edges, you can leave plants longer than you could in pots without root circling. Transplant when roots are visibly filling the block.