For numerous reasons, I’ve completely switched over from using plastic cell trays for seed starting to using soil blocks. Soil blocks are the best fit for me. To help you determine whether plastic trays or soil blocks are the best fit for your seed starting plans, this week I am breaking down the pros and cons of both methods.
Where I am, in the Atlanta, Georgia, area, we are already in the thick of seed starting season. If you’re not there yet, you will be soon. I’ve been talking a lot about what I’ve been doing with my seed sowing lately and posting videos to my YouTube channel and Instagram account. I’ve been receiving many questions, especially when it comes to soil blocking, so I thought I’d share all my thoughts on the benefits of and hurdles to soil blocking in one podcast.
I’ve gone all in on soil blocking.
You don’t need to choose between soil blocking and plastic trays. You can use both methods simultaneously depending on what your seed starting scenario calls for.
How I Evolved From Total Plastic to All-in on Soil Blocks
For years, I was the plastic cell tray guy, and even worse, I was using single-use plastics. When I started gardening, reusable trays weren’t really a thing. It’s only been in the last 10 years that the industry standard has moved away from single-use plastic trays.
Single-used plastic trays are convenient because they come in various sizes and are readily available. Nurseries, greenhouse suppliers, mail-order businesses and big box stores all offer them, and they are inexpensive.
The standard tray dimensions for cell trays and bottom trays are 10 inches by 20 inches. The cell trays nest into the bottom trays, which catch the water.
As an avid seed starter for a long time, I ended up accumulating many cell trays, of various cell dimensions, that I would reuse. I would do my best to not have to get rid of them after the first year. So they added up, and I was running out of room.
I explored soil blocking after a visit with Eliot Coleman, the king of market farming, for an episode of “Growing a Greener World®.” He lives in Maine and has been at it for 50 years or more. He’s such an iconic figure in market farming and soil blocking.
I observed and helped Eliot as he bumped up thousands of ¾-inch blocks of tomato seedlings into larger blocks. I was just in awe of the beauty of the system, its simplicity, the health of the seedlings and how he would transfer them to larger cells when it was time. It was an art and a science, but it was very zen. That registered with me.
Eliot at work inserting seedlings into soil blocks from the comfort of his greenhouse on a cold winter day.
I started out with the five-block hand-held soil block maker. The process is that you make the wet seed starting mix, you make the blocks, you eject the blocks into the tray that you’re using, and then you go back and start over until you run out of mix — and then you have to remix a new batch and make more cells and so on.
It’s a rotating process that takes time. And because I practice seed starting on a large scale, requiring hundreds of soil blocks, it added up to a lot of time. Another challenge early on was getting the hang of moistening the seed starting mix just right. Too dry or too wet, and the mix falls apart when you eject it from the block.
Eventually, you get it and it clicks, and you get it right every time after that. But until you do, it’s frustrating.
Once you get into the routine, it’s a no-brainer, and you will come to love the process of making soil blocks.
A small soil block maker can make a a lifetime of soil blocks, but it requires frequent refilling if you plan on making many soil blocks at once — hundreds in my case.
Benefits of Winstrip Trays
In between when I was dabbling with soil blocking and when I went all in on it, I was introduced to Winstrip Trays by my friend Connor Crickmore at Neversink Farm.
Winstrip Trays are perpetual plastic trays that mimic soil blocking. The trays have slits down the sides of the cells, in all four corners, which allows airflow, which then stimulates root pruning.
Root pruning is how roots respond to coming in contact with air. They stop growing at that point, and that’s one of the beauties of soil blocking — you don’t have transplant shock and you don’t have any girdled roots because the roots stop growing. When they get to the edge of the block and they come in contact with air, they just go on hold until they come in contact with soil again.
I had success with Winstrip trays, yet I was still dealing with storing lots of trays of various cell sizes. Also, transplanting from a Winstrip tray is not quite as easy as transplanting soil blocks. It’s the difference between having to eject the seedlings out of the plastic cells and just picking up a block and moving it into a larger container.
Winstrip trays work just like soil blockers. When the roots get to the edge of a block or cell and are exposed to air, they stop growing in that direction. They “air prune” rather than becoming rootbound.
Discovering the Swift Blocker
When I gave soil blocking another tray, I discovered a new soil blocking tool called a Swift Blocker. A Swift Blocker can make 50, 72, 128 or 200 cells at a time that fit perfectly in a standard 1020 tray.
My friend Pete Loughney of Michigan, a student in my Online Gardening Academy™, sows thousands of seedlings for his community services and donations. He turned me on to the Swift Blocker and sold me on full-time soil blocking.
I have a 72-cell Swift Blocker. It makes a tray’s worth of soil blocks at a time.
Experience Helps
When it comes to soil blocking, the more you make, the better you get at it.
Making the mix is like the grown-up version of making mud pies. It’s fun and your hands are filthy, but that visceral experience of getting your hands in there and filling that wet, gooey mix and molding it and squeezing it, and breaking up the clumps, is fun. You get to play while you’re working.
You may have beginner’s luck and feel really good about soil blocking from the get-go, or it may take you several attempts before you get the water to seed starting medium ratio right. Stick with it.
Chances are you won’t make perfect soil blocks on your first go, but eventually, you’ll get the hang of it.
Less Clutter and Cleaning
When you practice soil blocking, you won’t have cell trays to store before and after seed starting season. It makes for much less clutter. You also don’t have to worry about sanitizing those trays between seasons; you’ll only need to sanitize your blocking equipment and bottom trays.
Using plastic trays means storing them somewhere when they are not in use and sanitizing them each year to ensure pathogens do not infect seedlings.
Use Soil Right Out of a Bag, and Work Ahead
I came up with a custom soil block mix recipe, but making that mix takes time, and I wasn’t crazy about adding time to the task. I shared with Dan, the inventor of the Swift Blocker, what I was up to, and he said he has success making blocks using mix right out of a bag. So since then, I have used ProMix BX.
Instead of making soil blocks only on the day I will sow seeds into them, I get a bunch of trays done all at once, days or even weeks in advance. I leave them in my seed starting room with the humidity domes on and the heat mats and lights off, and they are good to go when I need them.
Best practice is to use the blocks right away — though I haven’t run into any trouble.
Soil Blocking Cons
One factor that may have you questioning whether to try soil blocking is cost. While a Swift Blocker is a fantastic piece of equipment, it’s also an upfront investment. A 72-cell Swift Blocker is a one-time $345 investment and will last forever. Over the years, it will pay for itself in savings on plastic trays.
A five-cell handheld soil blocker runs $45 and will also last forever. If you’re not making a lot of soil blocks, that may be a real deal for you.
Another con, which I mentioned earlier, is the learning curve. It does take some time to get the hang of getting the water to seed starting mix ratio right so the blocks hold together.
Soil blocks are more sensitive to drying out than soil in plastic trays. The soil is more susceptible to evaporation in a block because it doesn’t have plastic on the sides keeping moisture in.
You should check on soil blocks every day to ensure they have adequate water. When dry, they look gray-brown and become hard. You can also get to know the weight of a tray of moist blocks. When checking on soil blocks, find out whether the tray feels light. If so, it needs more water.
Plastic Tray Pros
Plastic cell trays offer the opportunity to find any quantity of cells in a 10-by-20-inch footprint. You may want a tray with just a few large cells, or with many very small cells. There are some really convenient options.
Single-use plastic trays are also very affordable, though I encourage you to get perpetual plastic trays from Neversink Farm or Bootstrap Farmer. We have way too much plastic in the world now and we don’t need to be adding to it.
Water retention is better with plastic trays so you don’t have to worry as much about checking the trays every single day.
Plastic cells trays come in every cell size you can imagine.
If you haven’t listened yet to the pros and cons of plastic cell trays and soil blocking, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
What’s your preferred way to start seeds? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
Episode 297: Seed Starting Essentials
Episode 454: More Seed Starting Tips You’ll Be Glad to Know
joegardenerTV YouTube: How I Make Thousands of Soil Blocks Quickly (What Works and Why)
joegardenerTV YouTube: The 5 Biggest Soil Blocking Mistakes (And How to Fix Them)
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course. The course is designed to be a comprehensive guide to starting, growing, nurturing and harvesting your favorite vegetables, no matter what you love to eat, no matter where you live, no matter your level of gardening experience.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
Five-block hand-held soil block maker.
Disclosure: As an Amazon Associate, I earn from qualifying purchases. I receive a commission every time you purchase a product through an Amazon affiliate link. Some product links in this guide are affiliate links. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast was based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Milorganite, Soil3, Territorial Seed Company, Cool Springs Press / Quarto Group and Dramm. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.
