Soil Blocking: Seed Starting Without Pots

What is soil blocking and why does it work?

A soil block is a little cube of pressed compost that’s both pot and compost. A hand stamper packs a wet mix tight enough to hold its shape, and leaves a dent on top for the seed. You sow, the roots fill the cube, and at planting time the whole block goes into the ground.

It’s an old idea. Market gardeners pressed blocks long before plastic trays came along, and organic growers love it because it cuts plastic out of seed sowing.

There are two good reasons to try it. First, roots never get disturbed at planting time, because nothing is pulled or squeezed out of a cell. Second, the open sides of each block stop roots circling, so you get a better root ball. More on that below. The catch is that blocks take longer to make than filling a tray, and they need careful watering. If you already sow seeds indoors each spring, blocking slots right into the same routine.

Which soil blocker should you buy?

Start with a 50mm stamper that makes four blocks at a time. It covers most crops from sowing to planting out. Later, a mini blocker that makes twenty 20mm blocks is handy for starting lots of small seeds before moving them up.

The two tools you need: a 50mm four-block stamper and a 20mm mini blocker.

The block mix recipe that holds together

Each part has a job:

• Compost, 3 parts. Sieve it first. Bark lumps jam the press and leave gaps that crack the cube. Any decent bag from our peat-free compost guide will do.
• Perlite, 1 part. Keeps air in the pressed block so it doesn’t go sour. Sharp sand works instead and adds a bit of weight.
• Loam, 1 part. Sieved garden soil, molehill soil or bagged topsoil. The clay in it glues the block together. Old stacked turf, rotted down, is the traditional choice if you have some.

The squeeze test: the mix should hold its shape and drip a little, much wetter than normal potting compost.

How do you make soil blocks step by step?

1. Fill the press. Push the stamper into the wet mix with a twist, twice, until mix squeezes out of the top. Press it against the bottom of the tub so each chamber fills solid.
2. Scrape the base. Drag the press across the edge of the tub so the bottoms are flat. A wonky base wobbles and crumbles.
3. Pop them out. Set the press down where the blocks will sit, squeeze the handle and lift the frame away in one smooth move. Leave a finger’s width, 1-1.5cm, between rows so air reaches every side.
4. Sow into the dent. One or two seeds per block, then cover to the depth on the packet with dry sieved compost or vermiculite. Then leave them be until they come up.

Warmth and timing are the same as any indoor sowing. Most spring crops come up at 18-21°C, and our guide to seed germination temperatures lists them crop by crop. Blocks dry out faster than trays, so check them daily, especially on a heat mat. They should always look dark and moist. Pale grey edges mean they need water.

How air pruning grows better roots

When a root reaches the dry air at the edge of a block, its tip stops growing. The plant then grows side roots behind it instead. In a plastic cell, the same root hits the wall, turns and keeps going round and round. So a blocked seedling ends up with a dense web of short roots filling the cube, all ready to grow out into the soil once planted.

Roots in the block stop at the edges, while the roots in the module circle round the cell.

Potting on by nesting: 20mm into 50mm

Nesting means dropping a whole 20mm block into a square hole in the top of a 50mm block. No tipping out and no touching stems. Fit the square insert to your 50mm stamper and every big block comes out with a ready-made hole.

Potting on without touching a root: the small block drops straight into the big one.

Soil blocks vs module trays: which is better?

Both work well. Blocks are kinder to roots and cut out plastic, but they take longer to make and need watering from below. Module trays are quicker to fill and easier to stack and carry. Many growers use blocks for crops that hate root upset, like lettuce, and modules for everything else.

Common soil blocking mistakes

• Using compost straight from the bag. Without sieving and loam, the blocks crumble within days. Sieve the compost and add 1 part loam to every 3 parts compost. This one mistake puts most people off blocking.
• Mixing too dry. The mix should feel like putty and drip when squeezed. If it feels like normal potting compost, it’s too dry and the blocks will break as you lift the press.
• Watering from above. A watering can wears away the edges and the blocks soon turn to mush. Stand trays in about 2cm of water for 10-15 minutes instead.
• Pressing blocks touching each other. Sides that touch stay damp, roots grow from one block into the next and the air pruning stops working. Leave a gap between rows.
• Letting mini blocks dry out. Little 20mm blocks hold very little water and can dry out in a day on a heat mat. Check them every morning.
• Sowing in weak light. Leggy seedlings flop before their roots fill the block. If you sow in February, our guide to grow lights for seed starting explains how a simple LED light helps.

The RHS guide to sowing indoors works just the same for blocks: same temperatures, same hardening off. Blocking simply swaps the pot for the compost itself.

Blocks fit into the same spring routine as any indoor sowing, so plan your season with our seed sowing calendar. For everything else raised from seed, our growing section has plenty more. For crops you sow straight into the ground, seed tapes make spacing easy.

Frequently asked questions

What is soil blocking?

It’s growing seedlings in little cubes of pressed compost, with no pot. A metal stamper presses wet mix into blocks with a dent on top for the seed. The block is the pot, so there’s nothing to remove at planting time and no plastic to store.

What mix do you use for soil blocks?

Use 3 parts sieved peat-free compost, 1 part perlite and 1 part loam. Sieve the compost first so bark lumps don’t jam the press. Add water until a squeezed handful holds its shape and drips a little. Compost straight from the bag crumbles, because it has no loam to bind it.

Do soil blocks fall apart when you water them?

Not if you water from below. A watering can soon wears the edges away. Stand the tray in about 2cm of water for 10-15 minutes and let the blocks soak it up. Once the roots grow through, the blocks firm up nicely.

Are soil blocks better than module trays?

Blocks are kinder to roots but slower to make. Seedlings from blocks settle into the garden with less upset. Module trays are quicker to fill and easier to stack. Both work well, so choose what suits your time.

What size soil blocker should I buy first?

Start with a 50mm four-block stamper. It suits tomatoes, cabbages, lettuce, courgettes and half-hardy annuals, from sowing to planting out. Add a 20mm mini blocker later for tiny or pricey seed you want to start in bulk.

Can you make soil blocks without a blocker tool?

Yes, but they’re less tidy. Press the mix firmly into a cut-down yoghurt pot, then tap it out. The blocks vary in firmness and have no seed dent. A proper stamper makes neat, even blocks much faster.