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During my first year of flower farming I was looking for every and any way to accomplish four goals:

1) Grow amazing and healthy flowers

2) Grow flowers on a super tight budget

3) Save space

4) Add to the earth’s health, not deplete it.

One of the ways I felt I could accomplish all four goals was through soil blocking.

Soil blocking, what is this you speak of? Well, it’s exactly what it sounds like. You are making compressed blocks of soil, this is in lieu of using plastic propagation trays. Many propagation trays are flimsy and if not pampered over can easily crack and deteriorate from UV rays. There are propagation trays that are sturdier and can be well worth the investment but those can easily run you $16 per tray! When you are looking at starting thousands of seedlings that cost can skyrocket.

These days at Sierra Flower Farm we have moved from strictly soil blocks to using a mixture of soil blocks and propagation trays. On especially frigid days, filling a tray with potting soil and quickly moving the operation indoors is pleasant and allows more flexibility. Soil blocking has many benefits but that does not mean it doesn’t come with some inconveniences. To soil block or not to soil block really comes down to a heart thing along with budget. Though, I think it is an essential tool for every flower farmer to have in their back pocket of know-hows. With that said, we still have quite a few varieties that we bust out the soil blockers for that I will list at the end of this post!

Soil blocking was frontiered in the US by Eliot Coleman. He has all kinds of books and videos that go into the benefits of soil blocking. He is pretty incredible, if you would like to learn more about organic and sustainable gardening his books are awesome and Youtube videos even more so!

Compressing soil into a bit of brick sounds counterintuitive for little seedling roots. I would agree. That is why you don’t use plain old potting soil; you can purchase specific soil mixes or recipes to make your own. The main difference between the seed-starting and soil-blocking mix is the amount of perlite.

In soil blocking, you need a lot of perlite. The soil will not be fluffy, for the tender young roots to grow, and for some oxygen to be in the soil to prevent an anaerobic environment that can lead to disease, perlite is a must. This will allow the soil to drain and those tiny roots to penetrate the soil block and thrive.

Benefits of soil blocking:

Economical: not needing to reinvest in seedling trays.
- The initial investment in soil blockers can be quite a bit, but if you start a lot of seeds year after year, they will return your investment! Snag some cafeteria trays from a local restaurant supply store, and you are set.
Lessened transplant shock: Even plants that typically do not do well transplanted can thrive with a soil block.
- The idea is that the air prunes the roots. Once the roots hit the air, they stop growing until the plants are transplanted into the garden or bumped to the next size soil block. This inhibits the plant from becoming root-bound (which most varieties are sensitive to becoming root-bound). Because the plant is not root-bound, the roots don’t need to be tampered with when it is time to transplant.
- This lessens transplant shock and allows your plants to continue to thrive. They won’t be sad and will simply continue to thrive, whereas transplanting from propagation trays may set the plant’s growth back a few days to even a few weeks!
Storage: storing a couple of soil blockers and trays takes little space!
- If you live in a small house, storing tons and tons of seedling trays may be overwhelming. If you are limited on space for holding trays, you are most likely limited on shelf space in the greenhouse. You can fit many more tiny soil blocks than the traditional seedling trays.
- I know that wind can be insane in our area! Nicely organized trays can quickly become scattered throughout the yard, but soil blocks are. Plus, at plant out time, heavy and won’t move an inch. You have dramatically less cleanup and trays to sterilize! Also, your husband won’t get upset that you keep stealing butter knives from the kitchen to use to get seedlings out of trays… ask me how I know!
It is more earth-friendly. Flower Farming uses a lot of plastic, from greenhouse plastic to row covers to drip tapes.
- Soil blocks are one way to lessen the use of plastic. Many affordable propagation trays will warp in the sun and crack, making it lucky to get a good year out of one, maybe two, if you are diligent.
- Soil blockers will not break down in the sun, and you can skip the plastic when deciding which trays you use! We have used old cookie sheets or, as of late, biodegradable cafeteria trays, with the idea that they are used once and then composted at planting out time.
Let yourself become a kid again: You get to play in the mud!
- Revisit your childhood, and let's get to making mud pies. But really, they look more like brownie bites. Get your kids involved! They will think you are the best at playing in the mud with them!

What You Need to Soil Block

Soil blockers in your desired size
Soil block mix purchased or DIY (recipe later in this post)
Your choice of trays (such as cafeteria trays or our favorite Bootstrap Farmer’s mesh and flood trays)
Dome lid, if using

Purchasing Soil Blockers

The first thing you need to do is invest in some soil blockers.

I am a huge fan of Bootstrap Farmer’s soil blockers, which have many different sizes. The ¾” micro soil blocker and the 2” soil blocker are the most relevant in my flower farming operation. I did wind up getting a bigger size but found that the first two were more than sufficient, and the larger soil block was not cost-effective in the amount of soil needed to make that size.

If you can only afford one size of soil blocker, I suggest going with the micro 3/4” soil blocker. Soil blocking can be a little more time-consuming than using cell propagation trays. To save time, an excellent soil block seed dropper kit has also been created that pairs with the micro soil blockers Bootstrap Farmer to regain some of that precious time!

DIY Soil Block Recipe

Once you have your soil blockers, it’s time to get the soil. You can purchase the soil pre-made from select vendors. I didn’t want to pay for the crazy shipping, and in my area, no soil-blocking mix was available to purchase locally. If you use the regular potting soil, your soil blocks will not become a block; adding some compost or additional perlite may help. We followed Eliot Coleman's soil-blocking recipe the first year we used soil blocks. We did change some of the ingredients based on the availability of what I could find locally.

These days, I must admit: I take slightly crumbly blocks instead of not having to make my own mix. Using quality potting soil such as Fox Farm’s Lucky Dog K9 Kube or Pro-Mix HP doesn’t make for as stout of a block, but it’s convenient! My making soil blocks using this is to my husband’s horror when he sees imperfect soil blocks, but you know what? They hold up well enough until the roots start growing, and then the roots hold the blocks together just fine! I’m not looking to be pretty! Plus, I keep these seed-starting mixes around for cell or root propagation; it all gets used!

Here’s the recipe soil block recipe we used (it makes a ton!):

3 (five gallon) buckets of peat moss or coconut coir
2 (five gallon) buckets of perlite
2 (five gallon) buckets of compost
1 (five gallon) bucket of garden soil
1 cup blood meal
1 cup bone meal
1 cup wood ash

About the Soil Block Mix Ingredients

Choosing the substrate:

Peat moss is readily available and affordable. Coconut coir is considered a more sustainable product than peat moss, but that comes with a heftier price tag. Peat moss is more acidic, whereas coconut coir is neutral. I find coconut coir holds together slightly better and isn’t as fussy if it dries out. Bonus: I have seen coconut coir don’t grow algae as quickly if overwatered compared to peat moss. Coconut coir can retain salts. We suggest monthly flushing seedlings with yucca to help combat the buildup.

Choosing which one is really a heart and budget thing!

You need perlite

The perlite will allow drainage and space for the roots to grow. Plus, the girls love smashing it between their fingers like popcorn! It’s rather inexpensive and an absolute must for soil blocking!

Quality Compost

Oh compost. It's simply delicious and vital for plants to thrive. In our area, it's not the easiest to find good quality compost at an affordable rate (I envy the farmers who pay $14 a yard for GORGEOUS rich compost!). We opt to make our own and only purchase when necessary. With a full-fledged flower farm in swing, why not make our own? The critical part of using quality compost is that you want it properly cooked. The last thing you need is a soil block full of viable weed seeds to compete with your seedlings or for un-composted manure to burn them! You also want the compost fully broken down; if not, there will be competition between seedlings and those un-composted materials for nitrogen.

Garden soil

The addition of garden soil may sound alarming. Don't seedlings need a sterile mix?! Unless you have a known disease, your soil should be safe. You could also substitute this component with additional compost if you would prefer.

The reason behind the garden soil is that, ideally, it aids in the shock of the transplant; that way, the little plants already have a little taste of what their eventual home will be. Though we did use garden soil, in hindsight, I would go with more compost. We have clay soil, which is made of tiny bricks! If you have healthy, loamy soil, then sure! Go for the garden soil. I would prefer for them to have healthy, thriving seedling roots than for them to have a “taste” of their eventual home. Two sides to the coin: experiment and decide which route you prefer!

Macronutrients

The blood meal, bone meal, and wood ash can be supplemented with other amendments if the plants have some macronutrient needs. Some growers prefer not to have fertilizers in the soil for starting seeds; I haven't seen a downside and love that I don't need to feed the seedlings for a few weeks since that's about how long it takes for them to gobble up all the good stuff in the soil mix once germinated. Messing around with foliar fertilizers on such young plants can be playing Russian roulette if you don’t dilute them properly! Please read my post on plant nutrition, Nitty Gritty, to learn more about plant nutrition.

Putting the Soil Blocks Mix Together

We toss all the ingredients in the wheelbarrow and mix thoroughly. Then, we add lukewarm water in the ratio of one part water to every three parts of the mix, adding more water as needed. Why warm water? The warm water will activate all the goodies in the soil, making them readily available for the seedlings. Plus, it’s cold, and my hands get frozen! Okay, a little less about my fingers: it will bring the soil temperature closer to a range needed for the seeds to germinate. Starting with frigid cold soil temperatures will be more difficult to warm up; start them off warm to give those seeds a jumpstart!

The soil block mix needs to be way wetter than you’re used to. When the seed starts in trays, you want the soil damp like a moist sponge. That’s too dry for soil blocking. There is still a balance. If the mix is too wet, it won’t become a block. If it’s too dry, the block will crumble. It’s very much like Goldilocks and the Three Bears scenario.

Throwing the soil to the side of a shed is a fun way to find out if you have your ratios correct; my girls love this part! If you throw it and it runs down, it’s too wet. If you throw it and it doesn’t stick, it’s too dry. If you throw it and the mix stays stuck, it’s just right. After testing the soil, spray down the side of the shed with a hose. Are you not into mud-flinging? Then, eyeball and try making the soil blocks, then adjust accordingly. If the soil mix is too wet, add more soil mix. If the soil mix is too dry, add more water.

Making the Soil Blocks

Once your soil block mix is ready, it’s time to play in the mud! We have used our cement pad to make a nice pile of soil for the soil blocker, but a high work area will be much easier on your body! You want the pile to be about one and a half inches deeper than your soil blocker is high (because it will compress!). If the pile is too deep, the bottom of your soil blocks won’t be even. You won’t have a complete soil block if the pile isn't deep enough.

When the mix pile is ready, take your soil blocker, push it down until it hits the ground (or other surface you are working on), and twist it back and forth. You want to see water gushing out the top of the soil blocker.

Make sure the soil on the bottom of the soil blocks is even; scrape excess soil if needed to have soil blocks that can sit flat. Lift the blocker to your tray and push the handle to release the soil blocks. Easy peasy. Well, not really. It takes a little practice, but it’s well worth it! Slowly push down on the release in small bursts until the soil blocks dislodge. Keep a bucket of warm water on hand; in between each batch, submerge the soil blocker into the water to rinse.

We find the 2” soil blocker makes pretty blocks, but the micro soil blocker is our most used! To make better blocks using the micro soil blocker, you may need to individually push in the soil for each cell to make a complete soil block. It can get tedious, but these blocks are small enough that you don’t need to work with incomplete ones!

Seed Sowing in the Soil Blocks

Soil blocks have been made, and you can start your seeds as usual. I like the ¾” soil blocks for most seeds. The 2” soil blocks are great for sweet peas and if you need to bump up your 3/4” soil blocks. Once you sow the seeds, don’t push the seed into the soil block. It will most likely break the block; instead, sprinkle some soil mix or vermiculite on top, depending on the seed’s germination needs. Bottom water the soil blocks, but don’t let them get soggy! If you overhead water, you risk the soil blocks crumbling. Once seedling has germinated and has a decent root base, overhead water can be done (if desired).

It takes a bit of practice and watching to get watering down. Soil blocks are great at hanging onto water, but if you let them dry out too much, they can be fussy about rehydrating (and your tender seedlings may die a crispy death before you realize it). In the early stages of germination, I like to lightly mist the top of the soil blocks to keep the seeds hydrated. On warm, sunny days, I bottom-water. On days when it is cloudy and cold, I back off of watering. You can use a heating mat or whichever system you prefer, just like the seedling trays. To learn more about seed starting, read our blog post, Nitty Gritty: Seed Starting for the Flower Farmer.

When you see your seedlings busting at the seams (literally), then simply bump them up to the next-size soil block or harden them off and transplant them out as normal. The neat thing about soil blocks is that they are like matryoshka dolls in that each size nestles nicely in the next size, making bumping up crops a breeze. I realize the 3/4” may seem too tiny to plant out, but rest assured they can handle it! Just make sure to water them enough for those first few days until their roots are established. So whether planting out or bumping up: the soil blockers have you covered!

Troubleshooting Your Soil Block Germination

If you see excess algae growth cut back on the watering and increase air circulation.
If you see that seedlings are getting crispy, you are underwatering. Increase watering if the seedling isn’t too far gone, or sow a fresh batch.

Some of Our Favorite Flower Varieties to Use Soil Blocks For

Though it’s been quite a few seasons since making my first soil block, and I now use a mixture of trays and soil blocking, there are still a few varieties that I prefer to stick with the soil blocks for. If a variety is picky about its roots being handled, I like to soil-block those guys. If they want to be directly sown for optimal growth but reality does not meet the ideal, I soil block those guys. Cold-hardy annuals seem to be the majority of the seeds that appreciate a soil block. I have yet to experience a seed that does not love the soil block.

Examples of varieties that I prefer to soil block if I can’t direct-sow them out in the field:

Bupleurum
Bachelor buttons
Cress
Saponaria
Larkspur
Ammi/dara
Nigella
Chamomile
Orach
Ornamental Grasses
Bells of Ireland

Plus, I like having my soil blockers as a backup if I run out of propagation trays! Between children, wind, and a naughty puppy, I have been using more soil blocks this season!

That’s about it on soil blocking! If you are looking for a green alternative to plastic trays or want to save space, soil blocking may be for you!

Having trouble figuring out when to seed start?

Check out our video Demystifying When to Seed Start, link below!

Watch Now

Until next time, we are looking forward to helping you hand blooms soon!

Jessica & Graham

SHAMELESS PLUG!

FLOWER FARMING BUSINESS

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Nitty Gritty on Soil Blocking