When I first starting growing my own transplants for my garden, I quickly became frustrated with the seedling trays I used. I tried several from major brands, and I always had a hard time getting the seedlings out of the trays without damaging them. The ones that survived well enough to transplant still had a tough time overcoming transplant shock. I had to plant far more seeds than I had room for in the garden simply because I knew I’d mangle quite a few while transplanting. Everything changed when a friend introduced me to soil blocks. Now I start all my seeds in soil blocks and, not only do they all make it into the garden, they don’t suffer from transplant shock. Once you see how easy this is, you’ll never use the compartment type seed starting trays again.
Why should I use soil blocks?
Soil blocks are compressed blocks of soil that can hold their form in an open tray. You don’t have to pry them out of individual tray compartments which can damage the roots and lead to transplant shock. The roots of soil block grown seedlings are air pruned and continue to grow without delay into the surrounding soil once they’re transplanted. This eliminates transplant shock.
Soil Block Making Tools
Tray labeled for use with soil blocks – You’ll need a bottom tray that is solid (no drain holes) and an insert tray that has a mesh bottom. The soil blocks rest on the insert which sits in the bottom tray. Even though they work together, they’re usually sold separately. Sometimes you can find them at a local farm supply store in the spring, but you can also get them at the links above.
Soil blocker – These come in several different sizes. For beginners and if space is limited, I recommend getting the 2” soil blocker. It’s the most versatile. Some people like to start small seeds in smaller soil blocks and then transplant them into the 2” blocks.
Each of the block makers has a pin at the top of each mold that makes an indent in the top of the soil block. This is where you put the seed. If you want to use the smaller soil blocker and transplant into a 2” block, you can change this pin in the 2” block so that the indent is large enough to accommodate the small soil blocks. The roots will continue to grow into the new 2” block. However, I like to keep things simple and just stick with the 2” soil blocker, but there are other size options.
Container for seedling mix – You can buy specially designed tubs meant for gardening but really any container deep enough to hold enough mix to make the 2” blocks will work just fine. I just use a cheap plastic storage container.
Butter knife – This is used to scrape the excess soil from the bottom of the soil blocks.
Small tongs – These can be helpful to separate out the blocks a bit once they’re in the tray. I just use my fingers but sometimes I can be clumsy so tongs are handy. Separating them isn’t necessary but I like to do it when the seedlings get big.
Seed starting mix
Soil Block Seed Starting Mix
The seed starting mix used with soil blocks is different than the usual mixes you can buy in nurseries. It needs to be able to hold together without being in a container. Some nurseries may carry mixes labeled for soil blocks but it’s not be easy to find. I like to make my own. I wanted to start with a recipe instead of creating my own recipe from scratch. I’d rather start with something known to work with soil blocks and then adjust for my own needs. A quick Google search introduced me to Eliot Coleman and how he makes soil blocks.
A Word About Ratios
Alright…a little rant and a math lesson here. I see so many gardening posts ignore the basic rule about not combining parts and exact measurements within a recipe. Use either parts or exact measurements, not both. A part can be any measurement such as a teaspoon or a gallon. Decide what the part is based on your specific needs.
For a small home garden, you may not need to have 1 part = 1 gallon. If the recipe calls for 5 parts of this, 3 parts of that, and 2 parts of this other ingredient, you’d end up with 10 gallons of seed starting mix which may or may not match your needs. But lets say that’s the recipe you follow and it says for fertilizer add 1 cup of this and 2 cups of that. Well, if you made your base recipe with gallons then the cups might make sense. However, what if you made the base recipe with quarts or even cups? You’d have a huge difference in the amount of fertilizer within the base recipe.
This is the problem I encountered when finding Eliot Coleman’s recipe in many different places. Most recipes used parts for the main seed starting mix and exact measurements (like teaspoons or cups) for the fertilizer part of the recipe. Pick parts or exact measurements but not both!
Soil Block Mix Recipe
Thankfully, I found a video in which Eliot shared his recipe, and I was able to see exactly what he used. Even in the video he mixes parts with actual measurements, but at least I was able to see that he used a quart for the part. That gave me a starting point to which I added worm castings. Based on that video, I came up with these proportions. I mix a large batch and keep it in a 5 gallon bucket with a lid so that it’s ready to go because I make several trays each season.
- 3 quarts compost (sifted through 1/4″ screen)
- 3 quarts peat moss
- 1 quart perlite (or vermiculite)
Mix well and then mix in the following for fertilizer.
- 2 tbsp each of greensand, blood meal, and rock phosphate
Mix in 1 quart of worm castings to 6 quarts of the mix when you are ready to make the soil blocks. You can use up to 30% worm castings when starting seeds.
Related post: How to Build a Worm Farm
How to Make Soil Blocks
For one seedling tray of 32 2” blocks, I use 6 quarts of the mix and 1 quart of worm castings. Add this to your container, add filtered water and mix it well. (Our tap water here is horrible, so I use filtered water for my seedlings). It may be helpful to let it sit so the water can be fully absorbed. Just add more water little by little until you get the right consistency. The mix needs to be quite wet so that it sticks together. When you scoop some onto a trowel, it should dump off in a big clump.
Pile up the mix at one end of the container so that you have some depth for the block maker. Put the block maker in the mix and press down hard while rocking it in all directions. Bring it up and press down again in a different location. Keep doing that until the the mix is packed in tight. When you think it’s good, hold the block maker down in the container and press the top handle down slightly. You should see water seeping through the top of all the block compartments. If not, keep packing in more mix until you see a bit of water.
Now you’re ready to eject the blocks. Scrap excess soil off the bottom of the blocks with your butter knife so they’ll sit evenly. Hold the blocker down in the insert tray. While holding it down tight, press the top handle all the way down while gently lifting the blocker. That’s it! You just made your first row of soil blocks. Once you get the hang of it you’ll be able to make a tray of 32 blocks in just a few minutes.
Related Post: Starting Seeds Indoors with Kids
Soil Blocks in tray
Sowing Seeds
Each block will have a small indent in the top for the seeds. I save some of the mix to put on top of the seed. Small seeds will just barely be covered and larger seeds will have a bit of a mound on top of them. As you’re planting them, make sure you label them! You can’t label these with plant labels like you would with compartment seed starting trays so I use a simple chart. I also note when the seeds germinate and when I transplant them into the garden.
Related Post: How to Grow Tomatoes from Seed
Watering the Soil Blocks
Always water seedlings from the bottom if possible. These trays make it super easy. Lift up one end of the insert tray and put about an inch of water in the bottom tray. Set the insert tray down and let the blocks soak up the water. Once they’re done, remove the insert tray completely to dump out any excess water. I usually have to water them every 2-3 days while they’re under lights.
Fertilizing the Soil Blocks
Once the first true leaves appear, you can add water soluble fertilizer when you water. I don’t put a lot of fertilizer in the soil mix so I like to add a bit more once a week or so when watering. I use fish hydrolysate and water soluble kelp. The fish adds nitrogen, phosphorous, and many trace minerals. Kelp adds potassium and trace minerals. I also add a splash of EM1 as a microbial inoculant.
Hardening Off
Harden off the seedlings for about a week before transplanting them into the garden as you would with any other seedlings. Place them outside in the shade for an hour on the first day and gradually increase the time and direct sun exposure until you’re able to leave them out all day at the end of a week. Keep an eye on the moisture level because they will dry out faster than when they were inside under lights.
Lettuce in Soil Block – ready to plant!
Transplanting the Soil Blocks
This is where the magic happens! Simply lift the blocks from the tray and set it in your garden a bit deeper than the block. Lightly cover the top of the blocks with garden soil.
I love to add some goodies in the bottom of the hole while transplanting. I toss in a handful of worm castings, some organic fertilizer like Down to Earth All Purpose, Mineralized Garden’s Transplant Formula (adds minerals for nutrition), Greensand or Azomite (if not added to the whole bed during preparation), and an endomycorrhizal inoculant. I water in the seedlings after planting with water soluble kelp and again in a week.
It may sound like a lot of work to make them but it goes very quickly once you get the hang of it. I no longer lose seedlings while trying to get them out of the compartment type seedling trays. I no longer start double the seeds for what I want in my garden. This was a game changer for me, and I know you’ll find it makes starting seeds so much easier!
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