Sowing Seeds

Sowing With a Soil Blocker

When I first heard about soil blockers, I was skeptical. As a laidback, minimalist and eco-responsible gardener, I wondered if it was just another gadget? I let myself be convinced to try it out. I confess that it’s now my favorite sowing technique: efficient, economical and by far the most fun!

What Is a Soil Blocker?

The soil blocker is a tool used to shape seedling cubes from potting soil. You then cultivate only in the growing medium, without any pots or supports. This is a very practical way of reducing our use of plastic in the garden.

Seedlings cubed with a soil blocker. Photo: Julie Bussières, www.vertmetal.com

The Materials

Popular with market gardeners and flower farms, this technique enables dozens and hundreds of seedlings to be sown. It makes the most of space and requires very little equipment:

  • Soil blocker. Different sizes are available. If you have to choose just one, I recommend those with 4 or 5 cavities (2“ and 1.5” cubes). These are more than enough for your regular needs at home or if you have a small farm.
Various sizes: 20, 5 and 4 cavities. Photo: Julie Bussières, www.vertmetal.com
  • Good quality potting soil;
  • A flat-bottomed bin (e.g. a dish tray, not a round mixing bowl);
  • One or more shallow trays with little or no grooves at the bottom (ideally a cafeteria cabaret-style tray).

The Steps

1. Start by thoroughly moistening the potting soil in the tray, a little more than for sowing in trays. A little water should run from between your fingers when you squeeze the potting soil in your hand. 

2. Press the soil blocker firmly into the potting soil at the bottom of the tray, leaving it in the relaxed position. This means with the cavities open. These will then be filled with potting soil. The excess can be scraped off with a flat tool or by hand. Make sure all cavities are well filled with no air gaps. If this is the case, fill in by hand or put the substrate back into the tray and start again. Nothing goes to waste! After a few tries, you’ll get the hang of it. You’ll enjoy it and become very efficient.

3. Once the tool has been filled and scraped, place it on the bottom of the tray. Squeeze the handle while simultaneously lifting the soil blocker, you will have your first row of seed cubes.

Shape seedling cubes with a soil blocker.

4. Repeat the operation, arranging the next cubes next to the first. Continue until you have a tray of small cubes. You can then place the seeds in the small holes created on the surface of the cubes by the tool.

Tomato seedlings cubed and pressed (day 0). Photo: Julie Bussières, www.vertmetal.com

The plastic or wooden sticks generally used to identify seedlings break the cubes. So consider making a plan or identifying in other ways. For example, you can stick the names on the rim of the tray.

A Few Tips

I can hear you thinking, “yes, but will the cubes disintegrate when watered?” Well, no, the potting soil cubes stay beautiful right up to the end of the crop! That’s why you should never water over the cubes, but rather beside them. For instance, by adding water to the bottom of the tray. In this way, each seedling cube will absorb water by capillary action. They will stay well in place and well hydrated. If you use trays with thin rims, you can drain off the excess if you put too much water in. If you choose a tray with high rims, don’t try to tilt the tray. This will cause your beautiful cubes to fall over and destroy them. Instead, soak up any excess water with a sponge.

As for fertilization, timing and frequency will depend on the initial fertility of your potting soil. It is the same as you would for box seedlings. Compared to box seedlings, cube seedlings produce a high-quality root system without any spiralization.

Tomato seedlings ready for transplanting. Photo: Julie Bussières, www.vertmetal.com

Once the seedlings are well developed, transplant each cube directly into the garden (e.g. lettuce), in large containers or in cubes made with a larger press (e.g. tomatoes). Some of the different sizes of soil blockers have removable needles. These produce holes shaped to accommodate the cubes of small seedlings to be transplanted.

By using different sizes of balers, you can transplant from a small block to a block produced with a larger baler (left) using square punches (right).

A Great Discovery!

Eco-responsible and durable (soil blockers are generally made of metal), the equipment takes up very little space when stored. You simply clean the press and trays and stack them until the next sowing season. So, looking forward to your next planting season?

Sign up for the Laidback Gardener blog and receive articles in your inbox every morning!

Julie Bussière is a biologist specializing in peatland ecology. She is co-owner of Vert métal, an ecoresponsible horticultural boutique and manufacturer of gardening accessories. She is also the founder and manager of the Jardin communautaire de L'Ancienne-Lorette. Vert métal distinguishes itself by manufacturing plant walls and horticultural objects made of stainless steel. Also specializing in metalwork for landscaping, this eco-responsible company offers practical products for designers, landscapers, florists and all plant lovers.

3 comments on “Sowing With a Soil Blocker

  1. To suck up extra water in trays, pot saucers, etc I use a basting syringe.

  2. heathergrammie

    I, too, was skeptical about soil blockers but in an effort to reduce plastics in the garden I got some. Wow! They really are the best!

Leave a Reply