Soil Blocks for Seed Starting
Growing plants from seed is one of the most rewarding parts of gardening.
You can save money, grow a bigger variety of plants, and witness the miracle of seeds turning into plants. In this skill share video, I’d like to show you soil blocking. This approach uses a special tool to create small blocks of soil for growing seedlings.
This is our favorite approach for starting LOTS of seeds indoors without having to use a bunch of plastic.
Making soil blocks is a lot of fun too. It can be an enjoyable activity for even young members of the family (as you’ll see in the video). What kid doesn’t like to play in the mud!
Quick note – if you’re a beginning gardener, I suggest you wait to do indoor seed starting and focus on growing outside for the first season or two. You’ll direct sow in your beds or pickup transplants from a local grower. This technique can be used in a future season.
Soil blocks are created with handy tools called soil blockers.
(links to where we got our soil blockers at bottom of page)
This one makes ¾” small blocks – 20 at a time!
And this one makes 4, 2” blocks.
These have a small dimple in the top, but you can also switch out for attachments (square dibble accessory usually sold separately) that make a square hole. The small blocks fit right in the bigger blocks! So you can start small and move up.
I use the small blocker most often and will tell you why a bit later.
First, what are the benefits of using soil blocks for starting seeds?
Benefit #1, as I already mentioned, is Less Plastic.
I should probably say less single-use plastic. You might still use plastic to hold your soil blocks, but we are talking about sturdy, long-lasting reusable plastic like cafeteria trays, which are a great soil blocking sheet.
Benefit #2: Healthier Seedlings – One of the biggest problems with seedlings in containers is that they become root bound. As a plant’s roots spread and travel in search of nutrients and water, they hit the container wall and start to circle. Have you seen this in container plants before – a dense web of roots. When transplanted into your garden, root bound plants tend to have more transplant shot, take longer to get established, and often aren’t as robust.
Benefit #3: Easier to Transplant – Soil blocks get tucked right into the soil. No massaging containers to try to get them out or cutting open peat pots or keeping track of empty plastic cells.
Benefit #4: Grow more in less space. This is my top reason for soil blocking. We have a small indoor light setup, and I like to be able to start lots of seeds! The soil blocks use space efficiently since they line up nicely side by side. With the small blocker, you can fit hundreds of seedlings onto a single shelf.
We do mainly use the small blocker. We’re able to do this by timing our seed-starting so that we plant the seedlings into the garden after just 2-4 weeks inside.
So in early spring, we’re starting hardy crops like collards, kale, and spinach. If the weather is iffy, we can add a floating row cover to help smooth the transition. In late spring and summer, I’m starting our warm season plants.
For plants that need longer inside, like tomatoes or peppers, I do use the larger blocks or recycled plastic pots.
Benefit #5: Easier to store between seasons. Less plastic to organize and put away.
There are a few drawbacks to consider for soil blocking.
- Initial Cost – upfront investment in soil blockers – (pays off in long run though)
- Watering more challenging – bottom water in trays or gentle spray if overhead.
- Has a little bit of a learning curve (getting mix right consistency, watering, and handling)
Let’s see if we can help with that learning curve!
Here are the steps to soil block making:
Step 1. Prepare your growing area.
If it’s early in the season, you’ll need a simple indoor light setup.
When it warms up outside, I like to move our little seed nursery to a folding table on our porch. You could do the same in a protected area close to your house with bright morning and mid-day light or dappled shade. Just not in the open where they get rained on (the blocks will fall apart!) or in a super hot spot (the blocks will dry out).
Step 2. Gather supplies.
- Soil blocker – Small (3/4”) is our favorite, and we’ll plant transplants out after 2-4 weeks. For larger seeds like pumpkins, medium (2”) works well. Large (4″) might be needed for extended time inside.
- Trays to hold blocks – reused styrofoam trays, plastic cafeteria tray, old sheet pan. (Some growers with greenhouses use corrugated plastic or net trays…but since water drains out, those don’t work in our house!)
- You’ll also need seeds, plant markers or Sharpie, and toothpicks – will show you a trick for handling small seeds toward end.
Next, supplies for making the mix itself
- A tub or bin with a flat bottom
- Water source
- Growing mix – we use a recipe that has coconut coir, worm castings, rock phosphate and green sand. Detailed recipe below.
- Sifter with fairly large holes to break up clumps and keep out long fibers. This one is a wooden frame with ¼” hardware cloth stapled to it. Kids sand sifter works too.
Step 3. Make your soil blocking mix.
If you’re using coconut coir, it usually comes in a compressed block – you’ll add water to expand it. Follow the directions so you don’t have excess water. Drier materials are easier to mix.
Sift your ingredients into your tub or bin. Mix well
Step 4. Add water.
You’re looking for the consistency of thick brownies. Wetter than you might expect.
You can make a test block to see if you’ve got it right. If the blocks are crumbly on the corners, you need more water. If they slip out as you pick it up, it’s too wet. At that point, you can add more dry material or tilt the bin to let some water move out.
Step 5. Make soil blocks!
Pile the mix up to the depth of your blocker and press it in a few times to fill the cells. Twist and pick it up.
Scrape off the bottom on the edge of the tub.
Place it on the tray, press the spring lightly for a moment, and then lift the blocker up.
Repeat!
If you have leftover mix, you can cover if you’ll use within a few days. Or let it dry out to use later.
Step 6. Plant your seeds.
For small seeds, you can use a toothpick to pick them up. Moisten the tip and you’ll be able to pick up one or two seeds at a time. Press into the soil block.
You could also use a sharpened dowel or stick for this trick.
For small seeds, you can just press them firmly into the surface. No need to cover. You just want them nestled in so they can absorb moisture.
With larger seeds, press them a little deeper into the block and you might want to cover with a sprinkling of the mix or some vermiculite.
Step 7. Water daily.
Water gently on the side and let the blocks absorb the moisture for a few minutes. If there’s any standing water remaining, pour it off.
If you have plants in the blocks for more than a few weeks, add in a water-soluble fertilizer, like fish emulsion, once a week.
Step 8. Plant into the garden.
Once they’re ready to transplant, you’ll treat these seedlings like any other, clearing some space in your mulch and tucking them into the soil. Press gently and water in well.
I like to use a butter knife to open up a small space for the seedling and drop them in.
If the roots have grown between blocks, you just separate them gently.
Recipes:
In this video, we adapted the recipe from The Gardener’s Workshop.
For 20 cups (makes approximately 500 mini ¾” blocks or 36 of the 2” blocks)
16 cups sifted coconut coir
4 cups sifted worm castings
¼ cup greensand
¼ cup rock phosphate powder
Mix ingredients dry. Mix approximately 3 parts dry mixture to 1 part water.
I first learned about soil-blocking from Eliot Coleman in his book The New Organic Grower. We have used his recipe with success too.
A standard 10‐quart bucket is the unit of measurement for the bulk ingredients. A standard 1 cup measure is used for the supplementary ingredients. The following recipe makes approximately 2 bushels of mix.
- 3 buckets brown peat (standard peat moss).
- ½ cup lime. Mix ingredients together thoroughly.
- 2 buckets coarse sand or perlite.
- 3 cups base fertilizer (equal parts blood meal, colloidal phosphate, and greensand). Mix thoroughly.
- 1 bucket garden soil.
- 2 buckets well‐decomposed compost. Mix all ingredients thoroughly.
Where to get a soil blocker:
I have seen soil blockers (and accessories) in several online places like The Garden Supply Company and Amazon, but the one’s we have and recommend are from Johnny’s Selected Seeds. We have had ours for many years and they are quality (and that’s who Eliot Coleman partners with).
Your Turn
Have you tried soil blocking before? What questions do you have about this technique? Let us know in the comments below!