The Soil Block Mix

The mix is the whole trick, and the target is easy to remember: mashed potatoes. Take an organic seed-starting mix — we like Dairy Doo Seed Starter 101 — sift it, and work in warm water until it's smooth, dense, and fully saturated, with no dry pockets. That's the mix; everything else on this page is refinement. If you want to build your own mix from individual ingredients for larger batches, the fuller recipe below covers that too.

The quick-start mix

1. Start with 4 parts of an organic seed-starting mix.
2. Run your hands through it — or the Swift Scraper's seeder plate doubles as a sifter — to remove chunks and debris.
3. Add warm water slowly, a little at a time, stirring thoroughly as you go. A potato masher works better than a spoon or trowel for breaking water into a thick mix evenly.
4. Keep adding water until it's fully absorbed and the texture is smooth and dense — like mashed potatoes, with no dry patches left.

Err on the side of too wet rather than too dry: you can press out extra moisture while forming blocks, but you can't effectively rehydrate a block after it's pressed and set.

What ingredients go into a build-your-own mix?

• Peat moss or coconut coir — the base
• Sifted compost — nutrients and structure
• Perlite or vermiculite — drainage and aeration
• A small amount of garden soil or clay — helps the block hold its shape

How wet should the mix be?

Wetter than feels natural — water is what binds a block, not the dry ingredients. The standard squeeze test: grab a clump and squeeze; a drop or two of water should run out between your fingers, and the clump should still hold its shape after. A properly moist mix darkens to the color of chocolate cake batter as you work the water in — a dry mix stays patchy and pale by comparison. A rough starting ratio is about 1 part water to 3 parts dry mix, adjusted by feel.

Is there a more precise recipe for larger batches?

Market gardener Eliot Coleman's recipe, published in The New Organic Grower, is the most widely cited starting point for growers mixing in bulk: roughly 3 parts fine peat to 2 parts coarse sand or perlite to 1 part garden soil and 2 parts screened compost, with a small amount of lime (peat is acidic) and a balanced organic fertilizer blended in before adding soil and compost. Coleman mixes the lime into the peat first since it's the most acidic ingredient, then works in the sand or perlite, then the fertilizer, and finally the soil and compost — so everything distributes evenly. Treat the ratio as a starting point and adjust based on how well your own blocks hold together.

Do all soil-blocking mixes use perlite?

No — this is one of the more debated points among experienced growers. Some market flower growers favor a denser, more binding mix and skip perlite and vermiculite entirely, relying on compost and a longer soak to get the right texture; others (including Coleman's classic recipe) build drainage in with sand or perlite from the start. Both approaches produce sound blocks — the real test is whether your own blocks hold their shape through ejection and watering, not which camp's recipe you start from.

Do I need to treat my water if I'm using compost or worm castings?

If your mix relies on living biology — compost, worm castings, or any "living soil" approach — water it with dechlorinated water rather than straight tap water. Chlorine is there to kill microorganisms, so watering a biologically active mix with chlorinated tap water works against the reason you added that ingredient in the first place. Rainwater works well, or you can dechlorinate tap water by letting it sit out uncovered overnight, or treating it with a few drops of vitamin C or a humic acid product.