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Seed Starting
Published January 23, 2023 by Nicole Burke

Soil Blocking 101

Filed Under:
seed starting
seeds
soil blocking
soil
indoor gardening
soil blocking

What Is Soil Blocking?

Soil blocking, or molding seed starting mix into a block, is an efficient way to start seeds indoors. Maybe not efficient in regards to your time (there's a bit of a learning curve involved), but efficient with supplies. You don't need all those little containers and plastic cells and other things we've become accustomed to using to start seeds indoors.

That's because a soil block is both a growing medium and a container for the seedling inside—a pot-less pot, you could say. The Dutch developed the technique used today, but there's evidence of soil blocking technology from Central America as far back as 2,000 years ago!

I'd been wanting to give soil blocking a try for years, and after I got the hang of it, I think it's now my favorite way to start seeds.

Here is what my seedlings look like once they germinated in their soil blocks:

with soil block, watering is done from bottom

The Advantages of Starting Seeds in a Soil Block

One of the main advantages of soil blocking is minimizing transplant shock. There's a general rule of thumb in seed starting, which is to move the seedling as little as possible. If a plant has fragile roots, it’s especially important to minimize disturbances. No matter how careful you may be to push a seedling out of a plastic cell so you can transplant it to the garden or pot it up, you're still disturbing those tender little roots and risking transplant shock.

A seedling that's being grown in a soil block doesn't have to be pushed or pulled to come out of its container because its entire container can be planted. That means it'll face a much smoother transition when it's time to be moved. Overall, these seedlings will be healthier than those grown in cell trays.

Other advantages of soil blocking include:

  • You don't need to buy lots of little plastic cell trays.
  • The roots of your seedlings will have more oxygen than if they were in a plastic cell and can establish themselves faster.
  • Roots will avoid growing outside of the soil block (they're not meant to grow in light), so these roots will stay nice and contained within their block. You don't risk keeping a seedling in its little container for too long and having it become root-bound or stunted.
  • Making each row of soil blocks is fun, though also a bit of a workout!
what is soil blocking

Soil Blocking Tool and Other Supplies

To get started, you'll obviously need a soil block maker and then some simple seed starting supplies.

Before I tell you a bit about the supplies I bought, I just want to say this is a non-sponsored post, but some of the links below are Amazon affiliate links, which means I earn a small profit (at no extra cost to you) if you click on the link and purchase the product.

soil blocking tool and other supplies

Soil Blocking Tool

I ordered my hand-held tool off Amazon, and it came with very clear directions for use. You can find it here.

Soil block makers come in different sizes. I opted for one that makes four 2" soil blocks, which is a great size for the type of seeds I want to start indoors. The maker automatically puts a little dimple right in the middle of the top of each block so that you can place your seed there later.

Soil blocking tools are not the cheapest of gardening products you'd find out there, but this one is made of zinc-coated steel and should last my lifetime as long as I take care of it.

Soil Blocking Mix

To make my soil blocks, I used this coco coir brick from Burpee. You can make your own seed starting mix or buy a pre-made mix like I did for your soil blocks. The important thing is to use a seed starting mix that's light and fluffy, not regular potting soil.

Most of the mixes you'll find use peat moss or coco coir as their main ingredient to bind everything together and hold moisture. I try to avoid peat moss because we're depleting this natural resource too quickly, but coco coir has its issues too.

Soil Blocking Trays

You'll need two types of trays so that you can water your seedlings from the bottom in the coming days and weeks to avoid disturbing your soil blocks. First, you'll need a tray with holes in the bottom to let water through; this will be the tray that holds your soil blocks. Then, you'll need a non-draining tray, or a tray with no holes, to hold the draining tray and water.

Other Soil Blocking Supplies  

You'll need a watering can so that you can pre-moisten your seed starting mix and then add water to the bottom tray as needed while your seedlings germinate and grow.

You'll need a large bowl or bucket to mix your seed starting mix in and some kind of large tray to spread your moistened mix out while you make your soil blocks.

You'll also need a bowl of water nearby so you can rinse off your seed block maker between each round of making blocks.

If you're someone who's extra sensitive to unpleasant sounds like nails on a chalkboard, I recommend wearing some headphones or earplugs to block out the noises that your soil blocking tool will make as you pick up soil and then deposit it—there are lots of scraping sounds and squelches involved.

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Steps to Start Seeds with Soil Blocks

Here's a quick overview of the process:

  • Re-hydrate your seed starting mix in a large bowl or tray.
  • Fill the chambers of your soil blocking tool with moistened seed starting mix by twisting it around in the medium.
  • Press down on the plunger of your blocker to eject the soil blocks onto your soil blocking tray.
  • Fill your soil blocking tray with rows of soil blocks.
  • Sow your seeds in the little dimple made by the tool in the middle of each soil block.

Now, let's look at each of this steps in more detail.

what is soil block?

Soil Blocking Step One

Re-Hydrate the Soil Blocking Mix

Seed starting mix needs to be moistened before you add it to trays and sow seeds, and it's actually best to do this several hours before you plan to make soil blocks or fill your seed starting trays. Taking this step ahead of time gives the water longer to fully hydrate the planting medium.

Mix your seed starting mix with warm water in a large bowl or bucket.

Since I used a coco coir block for my first foray into soil blocking, I had to soak the block in about 4 quarts of water anyway to turn it into a medium for starting seeds. I let this medium sit for a couple hours before moving onto step two. (See, I can follow the directions sometimes!)

Ideally, you'll wet your soil mixture until it's the consistency of soft putty. The directions that came with my soil block tool say, "When you squeeze the mixture, moisture should seep out between your fingers." Your mix should be a bit wetter than you would do for seed starting in cells. Even so, I realized while I was making my first soil blocks that I perhaps over-saturated my seed starting mix.

I recommend keeping some extra un-moistened seed starting mix nearby to add in if you pour too much water over your medium, but since I was using a brick that I had already fully saturated, I didn't have more mix on hand.

he-hydrating the soil blocking mix

Soil Blocking Step Two

Fill the Chambers of Your Soil Blocking Tool with Seed Starting Mix

Press the soil blocker into your seed starting mix several times to fill the chambers fully. Each of those chambers will become a soil block shortly. My directions recommended using a twisting motion to ensure each chamber is full. This is like a more physically demanding version of wiggling a cookie cutter around a bit in the rolled-out dough to make sure your edges are nice and clean.

diy soil block

In the picture below, you can see the tray after I've taken several rounds of soil to make blocks. My medium is just a little too wet.

soil blocking soil mix

After trying a couple of rows, I ended up hand-packing the soil into each chamber to get them extra full because I could see the corners already crumbling on my first sets of blocks.

Once the chambers are full, scrape the soil blocker across the side of your mixing tray to remove any excess mixture and create a flat base for each block.

filling soil blocking tool with seed starting mix

Soil Blocking Step Three

Press Down on Your Soil Blocking Tool to Release Each Soil Block

Place your soil blocker on your seed starting tray (the one with holes for draining) and push down on the plunger to eject the blocks onto the tray.

I have to say, this part ended up being more physically challenging than I expected. I got a good little workout in! You might want to stand over your trays to get more leverage.

soil blocking trays

If you're not happy with a row of blocks that you've made (they're slouching or crumbling—both totally normal), scrape that row back into your mixing tray and try again. (I recommend watching the video I posted at the beginning of this article to see my first terrible attempts.) The good news is that nothing is set in stone here; you can try over and over again until you're happy with the results.

Ideally, you'll end up with blocks that are firm and smooth-edged.

After I did my first tray of soil blocks, I read a tip to rinse the soil blocker between every set of blocks so that the seed starting mix can come into clean contact with the inside of each chamber.

what are soil blocks

Soil Blocking Step Four

Fill Your Soil Blocking Tray with Rows of Soil Blocks

Repeat steps 2 and 3 to fill your entire tray with soil blocks.

The picture below was my first attempt ever at soil blocking. There are many rows I probably should have redone!

Quality aside, I ended up with 44 little spots to plant seeds. I can start 44 plants in this one tray without any extra plastic or containers, which I think is pretty cool!

making soil blocks

Soil Blocking Step Five

Sow Seeds

Plant one to two seeds per soil block. Thanks to the dimple made by your seed blocker, you should already have a little hole to place your seeds in, right in the middle of each block.

I planted my first tray of soil blocks with my favorite winter greens seeds (kale, Swiss chard, etc.). I plan to transplant these out to the garden sooner rather than later. I will harden off the entire tray and plant the soil blocks in my garden. When it comes time to start seeds for my warm-season plants that need a little bit longer to grow indoors (tomatoes, peppers, etc.), I will pot up the seedlings into 4-inch containers once they have several sets of leaves before trying to transplant them.

As soon as you plant your seeds, it's time to get them under grow lights.

grow lights for seed starting indoors

Try Soil Blocking with Me!

Once you've got your handy dandy little soil blocks made, you'll want to avoid disturbing them. That means bottom-watering by using that non-draining tray underneath your draining tray. (Watering from the top could completely destroy your hard-earned blocks!) You probably won't need to add more water to the tray for several days since your growing medium will be nice and saturated at first.

I'm very excited for this new adventure in soil blocking that I've embarked upon. I hope you'll try soil blocking alongside me!

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Soil Blocking 101