How to Make Soil Blocks? | Firm Seed-Starting Cubes

The trick behind soil blocking is getting the mix wet enough. Iowa State University Extension describes the right consistency as resembling wet concrete, peanut butter, or brownie batter — when you squeeze it, it holds together without dripping water. Understanding how to make soil blocks starts with that moisture level, because a mix that’s too dry simply won’t hold its shape.

Beyond the mix, the method comes down to firm packing, a clean release, and watering from below. This guide walks through the full process and the mistakes that cause crumbling blocks.

Choosing and Moistening Your Mix

Start with a quality seed-starting mix, not garden soil. The mix needs to hold together when compressed, so it should be thoroughly moistened before you begin.

A workable starting ratio is 3 parts mix to 1 part warm water, but the exact amount depends on your specific mix. Add water gradually and test the consistency as you go. When it’s right, a squeezed handful holds its shape firmly without water dripping out.

If the mix seems dry, add more water rather than trying to work with it as-is. A wetter-than-expected mix holds together far better than one that’s too dry.

What’s the Correct Packing Technique for Soil Blocks?

Press the blocker firmly into the moist mix several times, twisting slightly as you go. This ensures each chamber fills completely with no air pockets.

Once all chambers are full, scrape the excess mix off the bottom of the blocker with a straight edge. You want a level surface so each block sits flat on your tray.

Place the blocker on a sturdy, flat tray or table surface. Press down with steady, even pressure, then use the release mechanism while lifting the tool straight up. Lifting straight up is critical — pulling at an angle disturbs the blocks and can cause them to crumble or lose their defined edges.

Each finished block should have a small indentation on top for your seeds. If germination is a concern, sow more than one seed per block and thin later.

Common Mistakes That Ruin Soil Blocks

• Using a too-dry mix: Blocks won’t hold their shape. Aim for the wet-concrete consistency described above.
• Poor packing: Loose fill causes crumbling after release. Pack the soil tightly into every chamber.
• Disturbing blocks after release: Always lift the blocker straight up so edges stay crisp and defined.
• Top-watering: Pouring water over the blocks can break them apart. Always water from the bottom.

Label your trays immediately after forming. Once the blocks are side by side, they all look alike — and you won’t remember which is which by morning.

Watering and Seed Coverage Tips

Water soil blocks from the bottom by adding water to the tray and letting the blocks wick it up. This keeps the cubes intact and encourages roots to grow downward.

Seed coverage varies by crop. Some seeds need light coverage with mix, while others should remain exposed on the surface. Check the seed packet for the specific crop you’re planting.

The method works with a manual soil blocker and a flat tray or table setup — no special equipment beyond that. If you’re looking for the right starting medium, the best soil blocking mixes for firm cubes are tested and compared in our roundup.

Iowa State University Extension’s guide, “Starting Seeds in Soil Blocks,” covers this method in depth and is a reliable reference for getting the technique right.