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Although the practice of making solid blocks of soil for growing seeds isn’t new (it may go back over 2000 years), soil blocking has gained popularity in the last few years. It’s not a complicated process: You form blocks of compressed seedling mix, lay them out on a tray, then plant into them. It works much like growing seeds in a plastic seed tray, without the tray. Using less plastic is only one benefit—the real reason people love soil blocking is that it creates seedlings with incredibly strong root systems, without getting root-bound. The air around the block acts as a pruning mechanism; the roots grow to the edge of the soil and then stop. The roots hold the soil block together, and can be easily up potted to continue growing inside, or transplanted directly into the garden.
The drawbacks to soil blocking
In order to make seed blocks, you need soil that will compress well and hold together, and most often, that involves peat. Peat, a byproduct of swamps and bogs, is controversial because the harvesting has a dramatic environmental impact. Some people have had success with other mediums like coconut coir, but those people do not include me. I’ve tried coconut coir a few years in a row and have suffered from lack of germination and blocks falling apart, so I have gone back to seedling mixes with peat.
The other major drawback to soil blocking is the actual blocking: It’s a pain in the ass. My first year, I was inspired by one of my gardening mentors, Meg Cowden of Seed to Fork, who speaks of the calm and meditative nature of blocking. Now, I regard blocking as a necessary evil each year that takes a lot of energy and soil. Still, the results speak for themselves. Soil blocks perform exceptionally well in tests against seed trays.
Step one: Get a soil blocker
The first thing you’ll need is a soil blocker. There are all kinds of blockers on the market now, and many are inexpensive. Soil blocking is similar to making a sand castle or snow castle: You pack the soil tightly into a cube; the soil blocker compresses the cube, and helps spit it out. Soil blockers come in different sizes and layouts. There are mini blocks, which are an inch or so large (for flowers), and maxi blocks, which are four by four inches. But most people are going to want to start with two-by-two-inch blocks. A basic blocker will spit out four of these blocks at once.
Step two: the right seed mix, at the right moisture level
Next, you’ll need the soil. Different kinds of plants at different stages all need different soil. When starting seeds, you want seedling mix, which is devoid of nutrients, has a lot of moisture retention, and is made of fine particles, so it can compact into seed trays. This is perfect for seed blocking, as long as that mix also includes peat or coconut coir. Get more soil than you think you’ll need, since by compressing the soil to make the block, you use more soil than you might with seed trays. I was able to make 100 blocks from a 12-quart bag.
Empty the bag into a bin or other watertight container, and mix it with water. You want to use roughly three times the amount of water as soil, adding it slowly, and stopping to mix the water in two to three times. Soil can absorb an absurd amount of water; you want the soil wet, but not sopping. Grab a handful of the mix, and squeeze it in your hand. Does it clump together? If so, you’ve got enough water. If not, add more until it does.
Step three: Have a tray for the soil blocks
The blocks will live on a tray, and it doesn’t matter much what kind of tray, as long as it can hold an inch or so of water, and is watertight. If you were using seed trays, they’d still need something to sit on like a bottom tray, so you’ve likely got some around. I’ve seen setups with standard 1020 trays and setups with cafeteria trays. The blocks sit right next to one another, so a 1020 tray can hold 50 blocks. A cafeteria tray, depending on its size, can hold much more.
Step four: Set up your dibbles
On the left, interchangeable dibbles, and on the right, dibbles installed on the blocker.
Credit: Amanda Blum
Before we start making blocks, you want to set up your dibbles. When you plant seeds, you have to plant them at the right depth, depending on the kind of seed. Sunflower seeds are buried very deep, for instance, while carrot seeds sit close to the surface of the soil. Your seed blocker has different dibbles you can add to your blocker, and they’ll make corresponding indentations in the soil block, so you can insert a seed more easily.
Step five: Start blocking
Now let’s get down to actual blocking. Again, if you’ve ever made a sand castle, this won’t seem foreign, but the technique requires a little finesse. Holding the blocker upright, push it into the seedling mix over and over, rocking it back and forth, until you hit the bottom of the bin. This pushes soil into the blocker. Swiveling the blocker left and right while pressing against the bottom of the bin compresses the soil. After a few passes, lift the blocker upside down, and check that the four chambers are full. If they’re not, keep taking passes, or you can just keep the blocker upside down and pack soil in with your other hand.
Step 6: Line up the blocks on the tray
To dispense the blocks, move to the tray that will hold them. Push the blocker against the tray, while slowly depressing the handle and lifting up. This process should product four perfect blocks, but that doesn’t always happen, and if a block looks like it will fall apart, pick it up and toss it back into the bin. It’s just soil; it will crumble apart. The blocks are delicate at this stage but not immovable, if you’re gentle.
Now, start all over, and dispense more blocks. Keep going until you have as many as you’d like on that particular tray. At this point, it’s time to plant some seeds. Treat each block like a cell in a seed tray. Plant one or two tomato seeds, or a bunch of carrot seeds, or a single pumpkin seed. Cover the seed up with some extra seed mix.
Step 7: Seed the blocks
From here on it, the process is just like growing in seed trays. The soil needs heat, so place your tray on a heated seed mat. The blocks need water, so you should fill the tray every few days with just enough water to cover the bottom of the tray entirely. The blocks will soak up the water and remain moist, ensuring germination. The seeds need humidity, so covering the blocks with a plastic dome will ensure there’s enough moisture. Finally, the blocks need light, so ensure there’s enough light on the tray, especially after the seeds germinate.
Tomatoes grown in seed blocks
Credit: Amanda Blum
The seeds will grow just as they would in a seed tray scenario, and at some point, will be too big for the blocks. Along the way you’ll want to choose one seedling per block, and cut the other seedling away. You can choose to up-pot the blocks into containers or paper pots, or plant the blocks right in the garden, if it’s not too early. Usually I up-pot the seedlings after a few weeks, and they’ll remain in four-by-four-inch pots until it’s time for them to go in the ground. Blocks that don’t germinate for some reason can be recycled for next year by simply tossing the block back into the bin of seedling mix.
