Have you seen the popular indoor seed starting method — soil blocking — where you start seeds indoors without using containers?
Perhaps you’re intrigued like I was at first, but you’re wondering: is starting seeds in soil blocks right for you?
I’ve used soil blocks in my indoor seed starting setup for a few seasons. After seeing both the ways it benefits my seedlings and the challenges that occur, I’ll help you understand the benefits and drawbacks of soil blocking to help you decide if it’s right for you.
If you’d like to listen to this discussion instead of reading, click below to listen to this episode of the Beginner’s Garden Podcast.
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What is Soil Blocking?
Soil blocking is an indoor seed starting method in which a soil block maker compresses a mixture of soil and water into a cube. The soil block maker also indents a small divot into the top of the soil block, to which you drop a seed. The seed remains uncovered and sprouts in place.
What is the purpose of blocking seedlings before transplanting?
While there are many benefits to soil blocking (see below), the primary purpose of blocking seedlings before transplanting is to prevent roots from encircling the container during growth. Instead, the roots pause at the edges of the blocks, awaiting transplant into a larger soil medium. When transplanted into the garden (or a container), soil blocks reduce the trauma to roots at transplant time, resulting in a healthier seedling and plant.
Benefits of Soil Blocks
After using soil blocks for a few seasons, I can personally attest to the major benefits of soil blocking.
Healthier root system.
In a contained environment, roots begin to encircle the growing container in search of water and nutrients. When seedlings become “root-bound” in this way, they struggle to acclimate to garden soil at transplant. In a soil block, the roots retain their vigor without the stunting that can occur in a confined container.
Lack of transplant shock
Seedlings quickly acclimate to the garden soil once transplanted. Compared to transplanting a root-bound seedling they start out healthier and grow faster. Seedlings grown in a traditional container often, following transplant, appear stunted for a short time. They must “heal” from the wounding of the roots and the trauma of the transplant. Only then can they resume growth.
When transplanted at the proper time, the roots escape wounding and the plants avoid trauma normally associated with transplanting. The seedlings enter the ground, ready to adjust to their new space.
In my experience, even seedlings of the most sensitive plants (like squash and its cousins) easily transplant into the garden space. Until I began using seed blocks, I avoided starting my squash, cucumbers, and melons indoors because of their unpredictability at transplant. Although these plants are still prime candidates for direct sowing, I enjoy that I can get a head start on these plants indoors.
Ability to start more seeds indoors
Because of the way soil blocks reduce transplant shock, seeds that normally balk at being transplanted (and often don’t survive it), can be started indoors more easily. As mentioned above, as long as fast-growing seedlings like squash, zucchini, cucumbers, and melons are started at the right time (about 3 weeks before transplant), they can get out in the garden faster for an earlier harvest.
Root crops are another option for indoor seed starting when using soil blocks. Beets and carrots — normally best sown directly in the garden — can be started early (using the micro soil-blocker) and transplanted shortly after germination.
Ease of transplant in the garden
This may be my favorite benefit to soil blocks. When it comes time to transplant, transplanting soil blocked seedlings takes a fraction of the time that transplanting from containers does. It’s actually enjoyable!
You don’t have to worry about prying the seedling from the container, risking injury to the delicate stem. You don’t spend hours transplanting dozens of seedlings — straining your back and knees in the process! — because transplanting seedlings in soil blocks is so quick and easy!
Reduced use of plastic containers
For those wanting to lessen their use of plastic in indoor seed starting, soil blocks provide a promising alternative. There’s no need to purchase seed starting containers each year or sterilize re-used ones.
Drawbacks of Soil Blocks
Starting seeds in soil blocks sounds pretty awesome, right? Well, like most endeavors, this method of seed starting also comes with drawbacks. Thankfully, none of these are insurmountable, but it’s good to be aware of the potential issues so you can prepare to mitigate them early.
Purchasing a soil blocker does carry a one-time cost, but in my experience it pays off in the long-run. Not only will you save money with a decreased need for containers, but you will also reap dividends in healthier plants with better harvests.
I’ve found myself fighting against mold on my soil blocks more than with seeds planted in a traditional way. Because of the high percentage of peat moss in soil blocks, it is imperative you do not allow the soil blocks to dry out. Not only will this stress the plant, but rehydrating the blocks can be more challenging than using regular potting soil in a traditional container.
Therefore, when you keep the blocks moist, you also invite mold to form on top of the blocks. To counteract this issue, a dusting of cinnamon usually keeps the mold in check.
Mold on soil blocks presents more than an unsightly appearance. It stunts the growth of delicate seedlings and inhibits the growth of newly-germinated plants. If you do notice mold growing on your soil blocks, scale back on watering (watering only from the bottom) and dust the top with cinnamon until you can get it under control. (More on watering below.)
Not all plants can stay in the soil blocks until transplant in the garden
Plants such as tomatoes will need to be “potted up” into a larger container prior to transplanting in the garden. While this was a drawback for me at first (I was hoping to skip containers altogether), I found the soil blocks to be very convenient to “pot up” into larger containers. And when I did transplant them into a larger pot, the plants skipped transplant shock and surged in growth.
What supplies do you need for soil blocking?
Starting seeds indoors using soil blocks actually is pretty fuss-free when it comes to supplies required.
Soil Block Maker
The primary soil blocking tool is the soil block maker. This easy-to-use tool compresses the soil blend tightly enough that the soil stays in a structured cube for the duration of a seedling’s growth. The soil block maker is the key component in soil blocking; I haven’t found a way to plant in true soil blocks without it.
Soil Block Mix
Most experienced gardeners and soil blocking purists use Eliot Coleman’s Soil Block recipe. A legendary market gardener and highly-respected in his field, Coleman was the pioneer of this method of seed starting. His soil block recipe — as shared in his book The New Organic Grower — combines ingredients such as peat moss, lime, sand or perlite, organic fertilizer, compost, and topsoil:
Basic Soil Block Recipe (by Eliot Coleman)
- 30 parts brown peat
- 1/8 part lime
- 20 parts coarse sand or perlite
- 3/4 part base fertilizer (like this one)
- 10 parts soil
- 20 parts compost
(Coleman also recommends a different blend for the micro-blocker. If you’re using his mixture, he gives more detailed instructions in his book, which I recommend, not only for the recipe but also for the general organic growing principles; it’s a must-have for organic gardeners.)
Is Eliot Coleman’s Soil Block Recipe the Best Solution for the Home Gardener?
I have used Coleman’s recipe but found it cumbersome to obtain all the ingredients, especially if you’re not making a huge amount. Usually bought in bulk, these ingredients can get expensive, and for most small-scale home gardeners, it will prove unnecessary to have that much volume for your seed-starting requirements.
Soil Block Recipe Alternative for Home Gardeners
As an alternative to Eliot Coleman’s soil block recipe, I use a 50/50 combination of a high-quality organic potting mix and peat moss. I’ve found this combination works great for the compression necessary in the soil blocks. Plus, the organic nutrients present in this potting soil feeds the seedling until transplant — just like Coleman’s mix does. I have used other potting soil but found this Fox and Farms Potting Mix to yield the best results for me.
Once you press your soil blocks, you’ll need a place to put them. Shallow trays with rims allow you to water from the bottom, which is critical for soil-blocked seedlings (keep reading for watering tips).
You can use a large baking sheet for a large number of the same kind of seedlings, like tomatoes or broccoli. You will also need a cover of some type, such as plastic wrap, to cover the blocks prior to germination.
Another option is to re-use plastic containers like clam-shell salad containers. This is handy if you start a variety of different kinds of plants that may need varying degrees of light, water, and indoor growing period. Plus, the lids create the greenhouse effect you need prior to germination.
Can you use potting mix to start seeds in soil blocks?
Potting mix alone in soil blocks will not allow for the compaction and structure that the soil blocking method requires. That’s why you will need to use a custom mix as outlined above.
How do I start my own seed soil?
You have two options. If you want to use Eliot Coleman’s Seed Block recipe, mix the ingredients in the order listed (one after the other), in small batches. If you want to use the 50/50 blend I use, simply mix peat moss with the organic potting soil in small batches.
I recommend you blend small batches in a plastic tote with a lid. Because of the amount of peat moss in both mixture options, particles easily blow around. First, it’s not fun to breathe. Second, you risk wasting the ingredients as they blow away from the mixing area. If you mix the blend indoors, turn off any ceiling fans and close any windows. If you mix the blend outdoors, choose a day without wind, or mix in a sheltered location like a garage.
(One exception: if you plan to use all of the mixture right away, you could moisten the mix while blending to help keep the ingredients in place.)
Keep your dry soil block mix sealed until you’re ready to begin creating your soil blocks.
How to Prepare Soil Block Mix for Planting
Keep your soil block mix dry and covered until you’re ready to begin sowing seeds. A couple of hours ahead of planting, transfer the desired amount of soil block mix into a large bowl. Use a bowl with a flat bottom that’s at least the width of your soil block maker. Then add water and mix until the consistency resembles brownie batter.
You want your soil very wet — you will need more water than you think. Peat moss is highly absorbent and mixing thoroughly is key. When the mixture no longer takes in water and the water just begins to pool in places, you can stop adding water.
Ideally, let the mixture sit for a few hours to continue to absorb. This step is helpful for even absorption but not required if you didn’t plan ahead.
(For a demonstration of the process of making and planting in soil blocks, view the video below. Continue reading and I explain each step more thoroughly.)
How to Make Soil Blocks with a Soil Block Maker
Once your soil block mix is thoroughly saturated and in a bowl, I recommend putting on latex gloves before starting. Not only is soil blocking a bit messy, but peat moss can irritate, scratch, or even get lodged your skin like a splinter.
(I don’t recommend garden gloves for this. First, you’ll want the dexterity of latex gloves. Second, your hands will get caked with wet soil. Save your garden gloves for the garden.)
From here, take your soil block maker and press it down into the soil block mix. Pick it up and do this again, until no more soil will go into the blocks. At this point, I tilt my soil block and press more mixture into the blocks with my hands, pressing until water seeps out. The more compact you can get the blocks, the better of a structure you’ll have.
Once the blocks are as full as you can get them, place the soil block maker into the tray you’re using. Press the handle down and lift the soil block maker as it deposits the blocks into the tray.
Planting Seeds into Soil Blocks
After you have made your soil blocks, you will want to plant your seeds. Each block will contain an indentation at the top. The seed will go here. Assuming a 100% germination rate, only one seed is needed per block. But since my seeds don’t always germinate at 100%, I usually plant two seeds per soil block.
This may seem counter-intuitive when you think about planting seeds, but you do not need to cover the seeds with more soil. As long as each seed nestles into the bottom of the indention, viable seed will germinate. Sometimes I use the eraser end of a pencil to gently press tiny seeds into place.
The beauty of planting seeds in soil blocks is being able to watch the germination process that normally happens unseen. It’s a beautiful way to observe the miracle of a new seedling’s life.
If you planted more than one seed per block, and all seeds germinated, you will need to snip the extra seed(s) at soil level as soon as you can identify the strongest seedling, or even sooner. Never pull out the extra seedling from the root, as it will disturb the growing root system of the one you want to keep. I usually cull extra seedlings when the sprouts are one half inch tall and the first sprouting leaves (cotyledons) fully open.
When the seedlings emerge, immediately remove the plastic wrap or dome and place 2-4″ under grow lights.
Watering Soil Blocks
The biggest mistakes I’ve made with soil blocking have been in my watering practices.
Watering Soil Blocks from the Top
Although soil blocks are strong, they aren’t entirely resistant to collapse. That’s why you never want to water from above with a strong stream of water, even from a small-spouted watering can. The only time you might need to add water from above is when the seedling has just sprouted. In this case, use a fine mist sprayer to mist the top of the blocks to keep them from drying out. However, I’ve found that if your soil blocks were saturated enough to begin with, and you used a clamshell container or plastic wrap, enough moisture will stay in the blocks until you begin watering from the bottom.
Another reason you don’t want to water soil blocks from the top — particularly with a stream of water — is this will dislodge a sprouted seedling trying to grow roots. Because that seed doesn’t benefit from the stability of soil all around it, it needs to be left alone until it can anchor itself with deep roots.
If you do notice the soil block drying out while the seedling remains small (less than 1/2″ tall), use a fine mist sprayer to moisten the blocks.
Watering Soil Blocks from the Bottom
From the time a seedling reaches about 1/2″ in height, you want to water from the bottom only. Capillary action in the soil will bring the water from the bottom of the seed block to the top. You can watch this fascinating phenomenon with soil blocks, as the color of the soil changes from light to dark as the water rises in the blocks.
Only add enough water to the bottom of your soil block tray that the blocks will absorb. You never want your blocks sitting in water. This can cause root rot and will exacerbate mold. If you add too much, remove extra water with a turkey baster or medicine dropper.
How Often to Water Soil Blocks
New seedlings will require little water. Remember, the soil blocks contained a high quantity of water to begin with. Check them every few days, but likely, you may only add supplemental water once a week.
Once seedlings begin growing their true leaves, the growing roots will require more frequent watering. Use the same guide as listed above; add only enough water the soil blocks can absorb. But you may find yourself watering every couple of days instead of weekly.
When to Transplant Soil Block Seedlings
Plants like lettuce, cabbage, broccoli, squash, cucumbers, and melons can be transplanted into the garden when they have several sets of true leaves and when the outside garden conditions allow for it. Be sure to harden off all plants (place them outside for small amounts of time for a week, increasing the time each day) before planting them outside.
Plants like tomatoes and peppers will likely need to be transplanted into larger containers before they can go outdoors. (If you grow peppers in the maxi soil blocker, they probably will not require potting up). The best way to know when tomatoes and peppers need more space is when you see their roots growing out at the bottom of the soil blocks. Also, when their height doubles the height of the soil block, you need to consider potting up.
Soil Block Maker Sizes
Have you decided that soil blocking is right for you? If so, I think you’ll be so glad you did it! For most of us, we never go back to seed starting in containers.
But before you obtain your first soil blocker, you need to choose which size. The mini, micro, and maxi sizes each serve different purposes in indoor seed starting.
The most common soil block size — and the one I use the most — is the mini soil blocker. Making four, two-inch cubes, the mini soil block is perfect for starting almost every seed indoors. When I start lettuce, squash, cabbage, broccoli, and melons, I start them in the mini soil block and transplant them directly into the garden.
The mini soil block can also be used for tomatoes and peppers, but I always have to “pot up” these plants into a larger container before they can be transplanted into the garden.
If you want to give soil blocking a try, start with the mini soil block maker.
The micro soil blocker creates twenty, 3/4-inch cubes. Perfect for starting a large volume of seeds, most seeds started in the micro soil blocker will need to be “potted up” into a larger container shortly after germination. If you own the maxi soil blocker (see below), you can nest the micro soil blocks in the maxi soil blocks for an easy way to “pot up” without using containers.
Another benefit to the micro soil block is less seed and soil waste. If a seed doesn’t germinate, you don’t lose as much soil in that “block.”
Micro blocks are also perfect to start slow-to-germinate seeds in a controlled environment — like carrots and beets — and promptly transplant them into the garden upon germination. Personally, I’ve found this very helpful for growing beets. Carrots may or may not be worth it, though. If you’ve had challenges with carrots germinating in the garden, this might be a method to try. I tried it once and it worked well; I transplanted the sprouted carrots in the garden right away, but I found the process to be much more tedious than direct sowing.
The maxi soil blocker creates one four-inch soil block. As mentioned above, freshly-germinated seedlings from the micro soil blocker can be nested into the soil blocks created from the maxi soil blocker. This eliminates “potting up” into a container.
Maxi soil blockers are perfect to use with plants like peppers, where they require more space indoors than a mini soil blocker provides. Using the maxi soil blocker for these plants eliminates the need of potting these plants up prior to transplanting into the garden.
Is Soil Blocking Right For You?
Gardening trends can be found everywhere you look. But I think soil blocking is here to stay. Many of us have found the benefits outweigh the drawbacks by a longshot. We enjoy the ease of transplanting, and we’ve seen the difference in the health of our seedlings.
If you’re ready to jump on board, or if you’ve already started your journey to soil blocking, I’d love to hear your experience. Comment below!
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