Soil blocks produce some of the best transplants from seed starting I’ve ever grown. This is my hands down favorite way to start tomatoes, peppers and my earliest cold hardy crops.
The method uses a metal ‘press’ that you fill with an oversaturated (think cement slop for the right consistency) seed starting mix. You get this mix packed into the ‘press’ and push a lever to pop out the cutest soil cubes you ever saw. And if the mix is done right, they stick together great, even though it seems like they’re defying the laws of gravity.
This has proven a great way to get your seedlings off to a solid start and use less plastic. It was brought over to America in the 1970’s by Eliot Coleman, the OG Organic Gardener.
Soil Blocking Benefits
There are a few stacked bonuses to starting seeds this way. One is that it uses less plastic overall. It also creates a good environment for a seedling’s roots to ‘air prune’. This basically means the root hairs stop trying to grow out and instead bulk up closer to the plant. This avoids any circling of roots in a pot so no worry about root gridling. The air pruning process primes the plants for less transplant shock as well. The soil blocks also have great germination rates. They’re almost impossible to overwater as the blocks shed excess water right away.
The standard for seed starting trays is the “1020” which is roughly 10” X 20”. I mostly use a solid 1020 tray with a mesh 1020 nestled inside this for my soil blocks. But, I also have an assortment of older pots that I use to plant into, always with a solid 1020 tray underneath. I absolutely swear by Bootstrap Farmer’s trays. They cost more up front but will literally never need to be replaced- so much stronger then the box store trays!
You can also start soil blocks on any tray with a lip, and I’ve got some old school lunch room trays I’ve repurposed for this.
Getting the Seed Starting Mix Right
Start with a sterile seed starting mix. Do not use regular garden soil. There are quite a few good brands out there, I’ll use Promix if I’m buying a pre bagged blend. But when I do my main seed starting, I mix up my own, to be able to control the exact contents (yes, I’m a control freak) and to save money!
I buy compressed bricks of organic coco coir, like these, to make up about half of this mix. You’ll get roughly 2.5–3 gallons from one compressed brick after soaking in 1+ gallon of water and letting it expand for a few hours.
I have used peat in my soil blocks for years, knowing that it is a hotly debated medium for seed starting I’d been looking for alternatives in addition to the coconut coir. I trialed wool pellets from Kestrel Farms Pellet Co. for the first time in 2024 for my soil blocks. I am loving this medium so far. It is holding together like the peat and holding water as well too. I will update on my trial once the results are in!
There are lots of opinions and ways to create a good seed starting mix. Here’s what I found works well for me when starting soil blocks. If you’re making a seed starting mix for starting seeds in pots instead of soil blocks, you can skip the peat moss altogether.
🌿Soil Blocking Mix🌿
3 parts coconut coir
3 parts peat moss (*or 2 parts wool pellets)
2 parts compost or 1-2 parts worm castings
2 parts perlite
1/4 cup blood meal
< 1/4 cup bone meal
< 1/4 cup Azomite (aka rock dust)
SOAK COCONUT COIR FOR AT 2+ HOURS PRIOR TO MIXING
Here’s a link to my Amazon seed starting page if you’re interested in seeing all the products together. But remember, reusing any existing trays, pots and food grade plastic is an option too!
The soil blockers cost a little more than trays and pots but can make endless soil blocks for you – and all your garden friends. This is something I love demonstrating at schools too- it really is like playing with mud!
Enjoy starting seeds however you choose to go about it- and marvel at nature’s will to grow!
Dig In,
Michelle
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