One of the new things we’re trying in Year Three here at stately Black Sheep Manor is a seed-starting system based on a U.K. technique called “soil blocking.” My stepfather, Michael Sasser, gave me his soil blocking equipment in the winter of 2017-18, but I made a total mess of my first attempts. A few months later — after seed-starting season — we attended a Mother Earth News fair workshop with flower farmer Lisa Ziegler, and that finally got the proper concepts firmly planted in my noggin.
Every one of our 2019 seedlings has been grown using this method. And since we’ve been getting questions about it — in person and online — I thought I’d write up this quick introduction for those of you who are interested in giving it a try.
If you’re new to the idea, think of it this way: Soil blocking sideline all your seed-starting trays and “potting mixtures” and replaces them with a spring-loaded device that takes a very different type of soil mix and forms it into 20 small cubes that are supposed to hold together. One seed goes into each of these little blocks — and in most cases, this is the only home your seedling will need.
Pros? You save space. You save money. You get a higher germination rate, and watering becomes much easier (see my caveats below)
Cons? You can’t walk into your average gardening shop or Big Box and walk out with the supplies you need. You’ll need to rethink your gardening calendar a bit. And there’s no “taking a day off” with soil blocking. You can cheat by over-watering a six-cell tray of young seedlings. Those seedlings must be watered daily. Sometimes twice daily.
Your going to need this stuff:
- Soil-blocking mixture
- A one-cup measure
- Styrofoam meat trays
- A mixing container
- Plant labels
- Sharpie marker
- One 20-block Soilblocker
I also recommend you acquire the following:
- Some fish emulsion fertilizer
- A 1/8th or 1/4 TSP measuring spoon
- A static-free metal seed tray
- A wooden toothpick (pay more for the type with one blunt end)
It’s difficult to find blocking mix, because there’s not much call for it. We ordered our first batch from The Gardeners Workshop, but within a month we were making our own by Lisa Zeigler’s recipe:
- 16 cups peat moss
- 4 cups sifted compost
- 1/4 cup Greensand
- 1/4 cup Rock Phosphate
For your first try at this, get your mixing bin (we use an old tupperware-ish thing that lost its lid), then fill it with three cups blocking mix, and add one cup of water. Mix thoroughly. Prepare to block.
You press your blocker into the mix, making sure each chamber seems full. You want these blocks to hold together, so even after you’ve “filled” it, collect some more mix and try to pack more in. Hold the blocker by the sides, and push it down hard, I use a wooden paint stirring stick to scrape off the excess.
Move the blocker to a tray…
If you use these smaller trays, this orientation will give you 40 starts per tray (2 x 20 blocks per tray). Press the blocker down into the tray a bit first, then press the spring-loaded handle on top, lifting as you do. This will eject the blocks you’ve made.
If you’ve got the luxury of enough room that you can apply Lisa Z’s rule, put only one type of plant on any given tray. If you don’t, try to keep them within plant types, since germination and maturity times vary.
Add your seeds
I bought one of these little seeding trays from The Gardener’s Workshop, and for most seeds (brassicas, lettuces, tomatoes, peppers, etc.), it’s invaluable (Swiss chard? not so much). Pour some seeds into the tray from your packet, then lick the end of your toothpick. When you dab it into the the seed tray, the seed you target will come out dangling from the end.
Unlike standard seed starting, where it’s considered a best practice to put at least two seeds in each cell, in soil blocking you add only one seed per block. Most will need to be pressed beneath the surface. Check your seed specs.
When the seed allows it, like these Chinese cabbages do, it’s faster to lay out each seed with the sharp end of the toothpick, and then follow behind with the blunt end to press each into the block.
Be sure to add a plant label with the date of the start on it.
Move to a germination mat
The rule of thumb for germination is: Once you’ve got 50 percent emergence, move your tray off the heat map and over to the grow light.
But the rule of thumb needs a caveat: If you get a few seedlings but aren’t close to 50 percent, stick a grow light over the done while you wait. Otherwise you’re choosing between poor germination rates and dooming your first emerging plants to an awkward and leggy life of shame and poor productivity. We have an ad hoc rig for a single bulb that we use for these hard cases.
You’ll need to start these seeds on a flat surface, and here’s why: You water these blocks onto the tray rather than onto the block. If the surface isn’t flat, the uphill blocks will be dry while the downhill blocks will be soaked. So break out your level and your shims and get that sucker plumb first thing.
Move to the grow light
Once you’ve got 50 percent emergence, move your soil blocks to a growlamp. And take it from me: If you an possible afford NOT skimping on growlamps, DON’T SKIMP ON GROW LAMPS.
Try to hit this minimum standard:
- Metal reflector hood
- No fewer than four T2 bulbs
- Quality suspension frame that can be folded for storage
We have two four-bulb grow lights. In soil blocking terms, we can fit up to 12 trays (480 starts) under direct, optimal light, and twice that many under the lamp if you raise it higher. In our case that’s useful because one of our lights is located in a shop that’s usually unheated — and leaving the light on can be the difference between frost damage and no frost damage.
According to several sources, you want 16 hours a day of grow light exposure per day, which can be handled by timers. Personally, ours typically get between 12 and 16, and I like tending them myself, since it lets me check on their condition twice a day and add water if necessary.
Watering is much easier
If you are used to watering seed starting cells filled with potting soil, you know that watering is a slow process that has a lot of observation and guess work.
With soil blocking, you fill each tray, then start back with the first to see whether all the water has been taken up. If it has, add a bit more, and work your way down the line again. If you get back to the first tray and it still has standing water in the tray, pour the water off into a container. Done.
You’ve successfully provided exactly the right amount of water for each block on each tray, without having to guess at variables like respiration, temperature and humidity.
The Secret Sauce
One of the things I’ve learned over the past couple of years is that it’s not only OK to fertilize seedlings — it’s something I need to do regularly.
But with soil blocking, I can fertilize from the beginning — by mixing fish emulsion into the original soil blocks. Here’s how I do it:
- Replace the plain water (remember: It’s 3 parts dry mix to 1 part water) with water into which you’ve mixed fish emulsion liquid fertilizer. I generally mix up six cups of dry at a time, so that’s two cups of fish fertilizer solution.
- To make one cup of fertilizer solution, you mix in 1/8th of a teaspoon of the concentrated liquid. Since I’m adding two cups of water, I’ll mix 1/4th of a teaspoon in the first cup I add, and then just add plain water for the second cup.
- Mix it all together and block.
- Let your unused mix dry out, and re-wet it next time.
I didn’t use fertilizer on my earliest blocking attempts. The comparison between these earlier seedlings (which are fine) and my more recent starts is significant. And keep up the fertilizer schedule. Every two weeks, give them a spray.
One of the trickier parts of soil blocking is that the ideal is to get a viable seedling at about three to four weeks, and pop it directly into the ground at the right moment.
But some plants — I’m talking about you, tomatoes — do better if they’re a bit larger when they go into the ground. Around here, that’s supposed to be April 15th. Which means the plants I’m really pushing for first-to-market production have been in 1-inch soil blocks for weeks now.
By comparison, you get 40 starts on one tray with the small blocker. There’s only room for four plants with the large blocker — so think in terms of where you’re going to put them.
The procedure is similar, but uses MUCH more mix. Press the large blocker into the mix and squeeze out one per tray. If you’ve gotten the larger blocker that’s set up for transplanting, it will leave a divet at the top that’s perfectly sized to take one small soil block.
Works great. In theory.
- The large blocks will want to fall apart
- The extra roots you get with some small blocks don’t want to fit
- Waiting too long to “Block up” will cause your tomatoes some stress as they readjust to their new block home.
Here endeth the lesson.