How to Make Soil Blocks
There are a lot of advantages to using soil blocks for growing seedlings.
If you’re just starting out, and don’t have a soil blocker, and if you do have a stash of pots - by all means, start with what you have.
However, soil blocks are a good option to consider using at some point. The soil blocker is a small investment that will last many years, and the blocks are basically compressed soil mix, which means the seedlings are never growing in a pot - they only grow in that block of soil until they’re planted into the garden.
Advantages of soil blocks
No single-use plastics required (some types of plant pots are so flimsy, they hardly last more than a season)
Roots are naturally “air-pruned”, which means they grow outwards to the edges of the block in a bushy pattern, versus roots in pots, which tend to circle around and around the base, leading to “root-bound” plants, which is not ideal
Less transplant shock - which means the plants establish faster when they’re transplanted, because the roots receive less injury at transplant time, and continue to grow outwards in their bushy pattern instead of learning a new way to grow after circling around a pot for weeks
How to Make Soil Blocks
The biggest difference between making soil blocks versus filling pots with soil is that the soil must be VERY wet for making blocks. A lot wetter than you think it should be!
The first step is the same as for prepping soil mix to fill trays: in a waterproof tub (eg a rectangular plastic or enamel dishwashing tub) add some water to moisten the soil; let it sit for a few minutes while you do something else, then gently mix it.
Add any compost or soil amendments (eg, worm castings; you could use approximately 5% worm castings to soil mix, by volume); mix together
Add more water, and mix in
Add yet more water! At this stage, you’ll get a good feel for how much water is required. Here’s what to look for:
the soil should be very wet, like mud
there should be some water accumulating at the bottom of the tub
when you squeeze a handful of soil together, it should hold its shape when you let go
Wet your soil blocking mold, either by submerging the block portion of it into a dish of water, or holding it under running water for a few seconds. The idea is to wet the inside of the blocks, so that the soil won’t stick to the mold.
Shift the soil mix in the tub so that you have a “layer” in the centre that’s about 3-4 inches deep. This is assuming that you’re using the 2-inch soil blocker. If you’re using the mini blocker, make a layer about 1 - 1.5 inches deep. Push the rest of the soil mix to the edges of the container, and make sure the layer in the middle is very wet.
Hold the blocker so that it’s perfectly vertical, and press it straight down into the wet layer. Hold the blocker on the upper part of the frame (it works better with 2 hands), and avoid touching the handle. If the handle is depressed at all while filling the blocker, the resulting blocks will be shorter, since pressing on the handle will reduce the capacity of the mold.
While the blocker is fully pressed into the soil, push it away from you (~ a 45 degree angle), then lift it, and press it straight down again into another deep layer of soil. This will help to firmly pack the soil mix into every corner of the blocker.
Lift the blocker, and use an old stick or your hand to brush excess dirt from the bottom of the blocker. Use a firm but gentle motion, so that dirt is not pulled out of the mold. It’s important that the excess dirt is removed from the bottoms of the block, so that the blocks will sit flat in the tray.
Set the blocker on your tray, and push down on the spring-loaded handle; this will push the soil blocks out of the mold. You may need to gentle wiggle the mold until all the blocks release. This will be easier when your soil mix is very wet.
Keep the handle pressed all the way down, and slowly lift the blocker straight up (to make sure that all the blocks have slowly released) - and ta-da! There are your new soil blocks!
Repeat, to make your desired number of blocks.
When you set a row of blocks beside an existing row, you can snug the blocker right up tight against the previous row, to get as many blocks in one tray as possible. Setting them tight together like this will also help hold moisture in the blocks. Any blocks that are not sitting tight beside their neighbouring blocks will dry out significantly faster.
With some practice, you’ll be able to make a tray of soil blocks in 2 - 4 minutes!
If your soil blocks don’t release easily from the mold, the mix is likely too dry. Put the broken blocks back into your tub of soil mix, add more water, and make another set of blocks.
If your soil mix is too wet, that will be obvious too… you’ll see lots of water on the bottom of the tub, and lots of water coming out the mold when you try to make blocks. If there’s too much water in the mix, simply add a bit more dry soil mix… a little at a time.
If the blocks look ragged instead of with crisp edges, add a bit more water to your soil mix. You may need to add a small amount of water to your tub after making several sets of blocks.
Size of soil blocks to use
Mini Blocks: personally, I use this size almost exclusively for flower seeds that are: a) very tiny, and b) grow slowly, taking several weeks to reach a small seedling size.
For most vegetable crops, I feel this size of block is too small and creates too much extra work. They can be used, of course - but you MUST insert these mini blocks into a 2” block within 1.5 - 2 weeks of planting the seeds; they will need the larger soil volume and fertility of the large blocks very soon. Therefore, I question the efficiency and practicality of using the mini blocks.
The only time I would consider using mini blocks for vegetable seedlings is if I was using old seed with questionable germination. Then, it might make sense to plant a whole bunch of seeds and then keep the ones that germinate.
However, for a home garden scale, it almost always makes more sense to start with the bigger blocks.
1.5-Inch Soil Blocks: Great size for broccoli, cabbage, cauliflower, lettuce, kale, swiss chard, bok choy, onions (3-4 per block), spinach. All of these can also be planted in 2-inch soil blocks, if you have only one blocker.
2-Inch Soil Blocks: Great size for peppers, tomatoes, eggplant, cucumbers, zucchini, melons, and everything on the 1.5-inch blocker list.
4-Inch Soil Blocks: Good for tomatoes and peppers, especially if they are started extra-early (eg end of March / early April), and good for melons, cucumbers, zucchini and squash, especially if they’re started more than 2 weeks ahead of transplanting time.
Where to find Soil Blockers
If you’re going to get only one soil blocker, I would recommend the 2” blocker to start with. It’s the most versatile, and produces high quality seedlings of a good size.
There are multiple places to buy soil blockers; here are 2 Canadian sources:
Once you’re ready to plant seeds, here are some tips for successful seed-sowing indoors.