Starting Seeds in Soil Blocks

By Julia Frisbie

Posted March 18, 2021

In an effort to reduce my garden’s reliance on single-use plastics, I use a soil blocker instead of plastic 72-cell trays to start my seeds. This method was popularized by Eliot Coleman, and you can find lots of good information about it online. But the actual tool that you use to make the blocks ishttps://www.theseasonalhomestead.com/homemade-soil-blocking-mix/prohibitively expensive upfront if you just want to start a few trays of tender annuals. I have a stand-up 35-blocker because I’m a garden tool junkie, and it seems silly to keep it all to myself. You’re welcome to come use it on my front porch. Text me at 503-975-3778 and we’ll work it out. Bring your own trays and soil.

The benefits of soil blocks are:

  • Seedling roots run into air at the edges of the block rather than plastic, so they don’t wind around and around. They just stop growing (this is called “air pruning”) and wait to be plopped in the ground, which means they do better after transplanting.
  • You can fit more seedlings on a tray. For people with indoor setups, space on the heating mat and under the grow lights is often at a premium.
  • I can fill trays faster with my soil blocker than I could if I were hand-packing damp substrate into plastic 72-cell trays.
  • No more throwing away cracked and nasty 72-cell trays at the end of the season!

Here’s how it works:

  1. Dump substrate in a flat-bottomed bin. Some people use complicated recipes, but I just use the Black Gold potting mix with the orange label. Unfortunately, it comes encased in single-use plastic. Even the complicated recipes involve bagged ingredients like peat moss. A locally-sourced recipe that uses zero bagged ingredients would take a lot of experimentation. Sounds like a fun project for a retired master gardener, but I have a full time job and a four-year-old and escaped ducks running all over the neighborhood and a sink full of dirty dishes, so I’m not investing in research and development at this point.
  2. Add water, mixing as you go, until it’s slightly less wet than brownie batter, but wet enough that when you pick up a handful and squeeze, a little water runs out between your fingers.
  3. Push the soil blocker down into the substrate while doing a little twist-and-shimmy until you can hear and feel it scraping against the bottom of the bin.
  4. Lift it up and set it down into the tray where you want the blocks, and squeeze the two handles while gently lifting to release them.
  5. Repeat two more times, and you’ve got a full tray! My blocker makes blocks that measure 1.125” square, and 105 of them fit in a standard 1020 tray.

Newly planted seeds should never be allowed to dry out before they germinate. Soil blocks are best watered from below, since there’s no plastic holding them together. I pack my blocks into mesh-bottomed trays, and then I set them into a solid-bottomed tray with water in it for a few seconds, letting moisture wick up from below. I’ve also put soil blocks on aluminum pans and plastic lunch trays and poured water in from the sides, tilting the lunch tray so that every block has a chance to wick it up.

You don’t need soil blocks for everything. Many vegetables do just fine direct-seeded into your garden. Some even prefer it! Others– especially the ones with big starchy seeds– I like to pre-soak indoors and then plop directly into the ground. The ones I raise in the soil blocks are the real divas, the long-season veggies that can’t handle a frost. Here’s a simplified, non-exhaustive list of who gets what treatment in my garden:

TRANSPLANTS (VIA SOIL BLOCKS)

  • Nightshades (tomatoes, peppers, eggplants)
  • Cucumber family (cucumbers and melons)
  • Squash family (including zucchini and summer squash)
  • Sunflowers
  • Zinnias
  • Cosmos

PRE-SOAK

  • Peas
  • Beans
  • Corn
  • Nasturtiums

DIRECT SOW

  • Root veggies
  • Leafy greens
  • Herbs
  • Wildflowers

Another thing I think about when I start my seeds is whether or not each type of seed has a belly-button. The belly button is the point or the little mark where it was once attached to its mother plant. Think about the little mark on the middle of a bean, or the pointy end of a squash seed. If I can see a belly button, I plant it facing down or sideways, never facing up. That’s the place where the seed’s first rootlet will emerge from, and the rootlet has to find its way downward before it can push its cotyledons out of the soil. I learned this from the book Better Vegetable Gardens the Chinese Way by Chan and Gill, and from my seed-saving mentor Rowen White.

 

 

 

 

 

 

 

 

 

 

 

If you’ve never started veggies inside before, it bears mentioning that setting them next to a window really doesn’t provide enough light. You need a fluorescent shop light, and you need to hang it so it’s inches (not feet) above the top of the plants, adjusting it upwards as they grow. And most of the veggie divas who need to start life indoors germinate fastest when the soil is around 77 degrees, so stick them and their light in a warm place.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It also bears mentioning that indoor- or greenhouse-grown seedlings need to be “hardened off” before planting them out into the garden. Basically you carry them to and fro for a while. It’s a hassle. Actually, this whole seed-starting process is a huge hassle. But can I stop myself? No! Because springtime is too exciting!!! While unearthing supplies in the shed this afternoon, I caught myself humming, “It’s the most wonderful time of the year…”

Julia Frisbie has been gardening in Coast Salish territory for six growing seasons, and is thankful to learn from plants, animals, and people who have been here much longer. She’s grateful to her mom, Anne Kayser, for cultivating her curiosity, and also to Robin Wall Kimmerer for writing the book Braiding Sweetgrass, which transformed her relationship with the more-than-human world. Follow Julia’s micro-farm on Facebook, Instagram, and/or TikTok

1 thought on “Starting Seeds in Soil Blocks

  1. Anne

    Great article, Julia! This is the second season I have helped two CSA farmers start seeds in seed blocks like you describe. We keep ours on heating tables under low hoops, or in a greenhouse for a few weeks, and we use a mist setting to water them. In some cases we up-pot the seedlings into four-inch pots for a few more weeks before transplanting them into the ground or selling the young plants. It’s easy and fast to up-pot soil blocks without damaging the tender seedlings, and like you pointed out, there are no messy single-use plastic trays to deal with. Soil blocks are efficient and eco-friendly.

    Reply

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