Best Soil Blockers: Sizes, Nesting Systems, and Top Picks
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A soil blocker is a simple hand tool that presses moist growing mix into firm, free-standing cubes. You sow seeds directly into the blocks, and the seedlings grow without any pot at all. When the roots reach the edge of a block, they stop at the air gap instead of circling, which growers call air pruning. The result is a seedling that transplants with little shock and no plastic cell tray to wash, store, or throw away.
Soil blocking has been used by European market gardeners for decades and was popularized in North America largely through Eliot Coleman's writing. This guide covers the main blocker sizes, how the nesting system works, and which tools are worth buying.
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Blocker Sizes and What They Are For
| Block size | Typical tool | Best use |
|---|---|---|
| 3/4 inch | Micro 20 (makes 20 blocks per press) | Germinating tiny seeds and lots of transplants in minimal space; potted up into larger blocks |
| 2 inch | Mini 4 (four blocks), Multi 6 or Multi 12 stand-up models | The workhorse size for most vegetable, herb, and flower seedlings |
| 4 inch | Maxi-style blocker | Growing on larger transplants like tomatoes, peppers, squash, and cucumbers before planting out |
These are the standard sizes in the Ladbrooke system, which is the benchmark most other blockers copy. According to Ladbrooke retailers, the Micro 20 produces twenty 3/4-inch blocks per stroke, and the Mini 4 produces four 2-inch blocks.
Block-in-Block: The Nesting System
The clever part of soil blocking is potting up without disturbing roots. Fit cube inserts into a larger blocker, and each block it makes has a square hole in the top sized for the smaller block. You drop the small block into the hole, and the roots grow straight into the fresh mix.
- Sow in 3/4-inch micro blocks for germination.
- Move them into 2-inch blocks made with cube inserts once true leaves appear.
- For big transplants, move 2-inch blocks into 4-inch blocks the same way.
This saves an enormous amount of space in the germination stage. A single tray of micro blocks can start a whole season's worth of lettuce, onions, or brassicas under one heat mat.
The Best Soil Blockers
Ladbrooke Mini 4 (2-inch)
Best overallMade in England from zinc-coated steel, the Mini 4 is the size most growers start with because the 2-inch block suits nearly every common seedling. It comes with seed pins that form a dimple in each block. Cube inserts and dowel pins are sold separately for nesting and larger seeds.
Ladbrooke Micro 20 (3/4-inch)
The Micro 20 makes twenty 3/4-inch blocks per stroke in a footprint of roughly 3 by 4 inches. It is designed to feed the Mini 4 and other 2-inch blockers fitted with cube inserts. Ideal for lettuce, brassicas, onions, and herbs.
Ladbrooke Stand-Up Multi Blockers (2-inch)
The long-handled stand-up models make multiple 2-inch blocks per press while you stand at a bench or over a bin of mix. They trade cost and weight for much faster production. Worth it once you are starting hundreds of seedlings a season.
4-Inch Maxi-Style Blocker
A 4-inch blocker makes large blocks for tomatoes, peppers, squash, and cucumbers. With a 2-inch cube insert, it accepts blocks from the Mini 4. Growers who sell or trade transplants find these especially useful.
Budget Generic Soil Blockers
Many unbranded blockers sold online copy the 2-inch and 4-inch formats in steel or plastic. Quality varies widely: some eject blocks cleanly, others stick or bend. They are a reasonable way to try the method, but read recent reviews and expect to upgrade if you stick with it.
What You Need Besides the Blocker
- Solid-bottom 1020 trays (no drainage holes) to hold blocks and water from below.
- A blocking mix. Regular potting mix often crumbles. Blocking mixes are usually heavier on peat or coir, with compost and a bit of fine grit or soil so blocks hold together.
- A tub for wetting mix. Blocking mix needs to be much wetter than normal, closer to the texture of thick brownie batter.
- A heat mat for warm-season seeds, and good light once seedlings emerge.
Tips for Clean Blocks
- Wet the mix thoroughly and let it sit so the peat or coir fully hydrates.
- Press the blocker firmly into a deep pile of mix, twisting slightly to pack it.
- Scrape the bottom flat against the edge of the tub.
- Eject blocks into the tray with a steady push on the plunger.
- Dip the blocker in water between presses so blocks release cleanly.
- Water from below by adding water to the tray, never by blasting blocks from above.
Blocks save plastic and grow excellent transplants, but they need more attention to watering than cell trays. Our Soil Blocks vs Seed Trays comparison lays out root health, cost, and labor side by side.
Blocking Mix: Buy or Make Your Own
The single biggest factor in whether blocks hold together is the mix. Commercial blocking mixes save time and give consistent results. Many growers also make their own, typically combining a fibrous base such as peat or coir, screened compost for nutrients and structure, and a small amount of fine sand, grit, or garden soil to help the block bind. Coleman published his own blocking mix recipe in his books, and seed companies that sell blockers often share recipes too.
Whatever you use, screen out large chunks. Twigs and lumps create weak spots that make blocks crumble when you eject them.
| Problem | Likely cause | Fix |
|---|---|---|
| Blocks crumble when ejected | Mix too dry or too coarse | Add water and let it soak in; screen the mix finer |
| Blocks stick in the blocker | Mix too wet or blocker dry | Dip the blocker in water between presses; firm up the mix slightly |
| Blocks slump in the tray | Too much water; too little fiber | Add more peat or coir; water from below only |
| Algae or mold on block surfaces | Constant wetness, poor airflow | Let the tray surface dry slightly between waterings; add a small fan |
Which Crops Suit Soil Blocks
Most seedlings do well in blocks. They are especially useful for crops that dislike root disturbance, such as cucumbers, squash, melons, and beets, because the whole block goes into the ground without breaking the root ball. Onions and leeks can be multi-sown several seeds to a block and planted out as a clump. Lettuce and brassicas thrive in the micro-to-mini nesting sequence.
Very large seeds, such as peas, beans, and corn, can be started in 2-inch blocks with dowel pins that make a deeper hole. They grow fast, so plan to plant them out promptly before roots escape the block.
Blocks grow best under strong, even light that keeps seedlings short and sturdy. If you start seeds indoors, see our Best Indoor Greenhouse Cabinets guide for simple lighted setups.
Our Pick
Start with a Ladbrooke Mini 4, a few solid-bottom trays, and a bag of blocking mix. Add a Micro 20 with cube inserts once you want to start more seedlings in less space, and a 4-inch blocker if you grow large transplants.
Frequently Asked Questions
What size soil blocker should I buy first?
The 2-inch size, such as the Ladbrooke Mini 4. It suits most vegetable, herb, and flower seedlings and is the center of the nesting system.
Can I use regular potting soil in a soil blocker?
It often crumbles. Blocking mixes usually contain more peat or coir, compost, and some fine grit or soil so the blocks hold together when very wet.
How do I water soil blocks?
Water from below by pouring water into a solid-bottom tray and letting the blocks wick it up. Top watering can erode blocks and wash seeds out.
What is block-in-block potting up?
Fitting cube inserts into a larger blocker so each block has a hole sized for a smaller block. You drop the small block in and roots grow straight into fresh mix without disturbance.
Are cheap soil blockers worth it?
They are fine for trying the method, but build quality varies. Growers who stick with soil blocking usually upgrade to a steel blocker like Ladbrooke.