Soil Blocking: A Better Way to Start Seedlings
Air-pruned roots (left) compared with root-bound roots (right).
What is Soil Blocking?
Soil blocking is a clever way to start seeds in cubes of pressed soil mix instead of plastic pots or cell trays, and it has several benefits:
- Stronger seedlings: When roots reach the edge of a soil block and meet air, they stop growing; this is called air pruning. In a plastic pot, by contrast, roots can circle the inside wall and become root-bound. Besides preventing that, air pruning gives roots more oxygen, which can make seedlings more vigorous.
- Less transplant shock: Air-pruned seedlings start growing again quickly after transplanting and suffer less shock than plants from plastic pots.
- Less waste: Soil blocks are an eco-friendly way to start seeds, cutting the waste and cost of plastic pots and cell trays, and you don't have to find room to store stacks of plastic.
Choosing A Soil Blocker
Soil blockers come in several sizes. Which one you need depends on what you're sowing and how much.
The 12-cell handheld blocker is designed to be easier on fingers and hands.
Soil Blockers
These blockers are sized to fill a 1020 tray completely and evenly with no wasted space, and they're built for better comfort and efficiency.
- 10-cell handheld blocker: makes two rows of five 1 7/8" x 1 3/4" x 2" blocks, with dibbles. Five presses fill a 1020 tray with 50 blocks. Its shape is easier on fingers and hands.
- 12-cell handheld blocker: makes twelve 1 1/2" x 1 1/2" x 2"-deep blocks, with dibbles. Six presses fill a 1020 tray with 72 blocks. Like the 10-cell model, it's designed to be easy on hands.
- 32-cell handheld blocker: makes 32 blocks 1" x 1 1⁄8" x 1 1⁄2", with dibbles. Four presses fill a 1020 tray with 128 blocks. Like the 10- and 12-cell models, it's easy on fingers and hands.
- For commercial growers there are the 72-cell commercial blocker and the 50-cell commercial blocker, both great for soil blocking at scale. Each fills a whole 1020 tray with blocks in one press.
Additional Soil Blocking Options
Other soil blockers we carry:
- 20-cell handheld blocker (mini blocker): our smallest blocker makes twenty ¾" cubes with dibbles. It's great for very small seeds and for crops that like bottom heat, since the tiny cube lets warmth from the heat mat reach the seed. Move them up into 2" blocks when the seedlings are ready.
- 5-cell handheld blocker: makes five 1 ½" square blocks for starting all kinds of garden seeds.
- 4-cell handheld blocker: makes four 2" cubes, good for larger seeds or for potting up mini blocks from the 20-cell blocker.
- 1-cell handheld blocker: the largest, making 4" cubes for the biggest seeds or for potting up 2" blocks from the 4-cell blocker. It forms a 2" x 2" x 2" recess to hold a 4-cell block.
- Stand-Up Soil Blockers suit growers who like the posture and leverage of a standing tool. We carry 6-, 12-, 20- and 35-cell stand-up models.
The 72-cell commercial blocker is built for soil blocking on a commercial scale.
What About Dibbles?
Dibbles press small holes into blocks for sowing or potting up.
Dibble holes in soil blocks.
These blockers have built-in dibbles:
- 20-cell handheld blocker (mini blocker)
- 1-cell handheld blocker
- 10-cell handheld blocker
- 12-cell handheld blocker
- 32-cell handheld blocker
- 50-cell commercial blocker
- 72-cell commercial blocker
We also carry snap-in dibbles, including:
- Round dibbles: snap them into the blocker to press a seed hole into every block.
- Square potting-up dibbles: use them to make room in a block for a smaller block being potted up.
Note: check the product details to be sure a snap-in dibble fits your blocker.
Soil Block Recipe
Good soil blocks start with the right mix, with the correct balance of peat, compost, soil and sand or perlite. Blocking mix needs extra fiber to hold the block together and keep it moist.
The mix also has to be at the right moisture level. As a rule of thumb, it should hold together when you squeeze a handful, with no water dripping out. Too dry and blocks may fall apart; too wet and the mix may stick to the blocker or slide out of the form.
Mixes we suggest for soil blocks:
- 512 Mix and Germination Mix: both are suitable for organic growing (check with your certifier) and make professional-quality blocks straight from the bag.
- DIY blocking mix based on Eliot Coleman's recipe: to make your own blocking mix, we suggest the recipe from Eliot Coleman's published work.
DIY Soil Blocking Recipe
From The New Organic Grower, by Eliot Coleman
From The New Organic Grower, by Eliot Coleman
Measure bulk ingredients with a standard 10-quart bucket and the smaller ingredients with a standard 1-cup measure. This makes about 2 bushels.
- 3 buckets brown peat (ordinary peat moss).
- ½ cup lime. Mix well.
- 2 buckets coarse sand or perlite.
- 3 cups base fertilizer (equal parts blood meal, colloidal phosphate and greensand). Mix well.
- 1 bucket garden soil.
- 2 buckets well-rotted compost. Mix everything well.
Wet the mix thoroughly with about 1 part lukewarm water to 3 parts mix.
How To Make Soil Blocks, Step-by-Step
A few handheld blockers and the blocks they make.
- Mix soil and water to the right consistency (see above).
- Use the blocker on any flat surface or our soil blocking and potting tray. Pile the mix 1 ¼–1 ½" higher than the blocker is tall. Press the blocker down until it hits the surface, then give it a quarter turn so it sits firmly on the surface.
- Scrape extra mix off the form; a skimming trowel helps.
- Move the blocker to where the seedlings will grow, such as a wooden or plastic tray.
- What you do next depends on the blocker:
- 12-cell handheld blocker: line the plunger up with the cells and press lightly. Keep slight pressure on the plunger while you gently lift the grid off the blocks.
- Other handheld and stand-up models: push the handle down as you lift the blocker to release the blocks onto the tray.
- Rinse the blocker in warm water so the next batch is less likely to stick.
- Repeat.
Full instructions for handheld and stand-up models:
- 10-cell handheld blocker instructions • PDF tech sheet
- 12-cell handheld blocker instructions • PDF tech sheet
- 32-cell handheld blocker instructions • PDF tech sheet
- Soil block maker instructions • PDF tech sheet
Note: commercial blockers work a little differently; see the 50-cell commercial blocker manual and the 72-cell commercial blocker manual.
Watering Soil Blocks
Water soil blocks from below so they're less likely to crumble. Leakproof lightweight deep trays work well for bottom watering.
With soil blocks, potting on means starting seeds in small blocks and moving seedlings into larger blocks as they grow.
Potting Up (a.k.a. Bumping Up) with Soil Blocks
Once roots poke out the bottom of the blocks, it's time either to plant out or to move up to the next block size.
- Move ¾" mini blocks up into 2" blocks.
- Move 2" blocks up into 4" blocks.
