I've been asked a lot this spring for a soil block tutorial. I have some pages that explain some of what I do, but I haven't put it all in one page. So this will be the page I can link to when people ask questions.
What is a soil block and why would anyone want to use them?
Soil blocks are just blocks of soil that have been compressed enough so they don't fall apart easily. They are a way of having a plant grow in soil without a pot. They are more fragile than a typical six pack in plastic. But once the roots fill the blocks they are surprisingly sturdy. Recently I left my onions seedlings with friends to take care of. The onions grew way too tall for the lights and when I got back two of the blocks had been knocked out of the flats by the edge of the light fixture when they were put back, but the plants and blocks were still fine.
There are two reasons that I love to use soil blocks over pots. The first is that I don't have to use all that plastic. I don't like buying more plastic that isn't necessary. I don't like washing out all the plastic at the end of the season. And I hate the plastic six packs floating around my garden during the season. You might be good about putting things away when you should, but I'm not. And for me the little six packs end up everywhere.
The second reason is that there is much less root disturbance when you transplant your seedlings. I've heard some people say that you don't have to harden off your seedlings when you use soil blocks. I don't believe that. They need to get used to the wind and the sun, but the roots are much happier. The roots never get root bound. In a six pack by the time the roots have filled out their container there is always a circle of roots at the bottom that you have to untangle before planting. With soil blocks the roots are air pruned as they grow. There are lots of little root ends at the edge of the block just waiting to grow when they get transplanted.
The first thing you need is some kind of potting mix for your blocks. Just about any commercial potting mix will work, however commercial potting mixes aren't made to be compressed as much as a block is and often don't have enough drainage built in. So they won't work as well. You can make your own with Coleman's mix (recipe is about half way down in the linked page). It is very inexpensive if you make large batches. I've done it all. I've found that my plants grow best with the Vermont Compost Company's Fort V mix. It is hands down better than the homemade mix. It doesn't hold together quite as well, but it is good enough and the plants grow amazingly well in it. They grow much faster and more stocky than with anything else. So they spend less time under my lights.
BTW even here in Massachusetts this mix is hard to find. You can get it mail order from Fedco but the shipping is expensive. The Massachusetts branch of NOFA (Northeast Organic Farmers Association) does a tristate bulk buy every year in March for MA, RI, and CT. You order early in January and February (don't remember the exact times) and have to go somewhere to pick up at a certain date in March. You don't have to be a member or a farmer. You can be a little tiny gardener, but you get a lot of things cheap. Including Fort V mix.
Once you have your soil you need some kind of blocker. If you do an internet search you can find instructions for homemade blockers that are cheap. I recommend buying a commercial one however unless you have lots of time or don't have many plants. With commercial blockers you can make multiple blocks all at once and they are square so more space efficient.
Size does matter. I have three sizes. The smallest is a micro blocker. It is only made for germinating seeds. You put just one seed in each micro block so there is no thinning. After they come up you have to transplant them to the 2" (the largest of mine) right away. Most people love that system and size. Personally I hate it. I find seeds don't germinate well in the micro blocks. They don't have holes that are large enough for bigger seeds. I find the 2" blocks take too much room under my lights. Most of my seedlings I put in the 1 1/2" blockers. They fit 72 into a typical US flat. I would consider this size about equivalent to a six pack when you are growing plants. It is probably more soil and nutrients than you get in a six pack (remember that the soil is very compressed and there is less wasted space). You do get less light since six packs fit 48 to a flat.
I use the 1 1/2" size for onions, lettuce, chard, brassicas, herbs, and flowers. I only use my 2' size (without the mini blocker, I seed directly into the 2" size) for peppers. For tomatoes I start with the 1 1/2" size and grow for three weeks. Then I transplant to some tall bottomless newspaper pots. After a week they have filled out the pots and can be transplanted. Yes I only grow tomatoes for four weeks total. I like my transplants small (about 5" tall) and very well rooted. I'd say my newspaper pots are 4" deep. So the bottom is almost as large as the top. The greens I typically transplant at 3 weeks. The onions are 8 weeks (provided the weather cooperates). Flowers and herbs vary a lot. Some are very slow growers some are fast.
Making the blocks
The first thing you have to do to make blocks is to moisten your soil mix. You are always told as a gardener to not work your soil wet. Well here you want it wet. Very wet. As you can see above. I'm squeezing the soil and water is dripping out. If yours isn't that wet add more water. Beginners have a tendency to not get it wet enough. You aren't going for soup, but it will still work with thick soup, but it won't hold together if it is too dry. Most mixes have a lot of peat moss in them that is hard to wet. So use hot water. It will moisten the soil better. It is also better if you moisten your soil a day in advance. I never plan that well when making blocks, so I just do it right before. I mix well with my hands to break up any peat moss that isn't wet.
Then you push the bocker down into the soil with a twisting motion. I often do it several times to make sure it is totally filled and compact (especially getting more in the two ends which tend to get less full). You are going for very tightly compact soil. Loose soil won't hold together. And here is where I differ from most instructions. I always check the bottom after I do it. I make sure the soil is well pressed in in all the cells. Then I take my finger and make the bottom flat. Usually it is rounded on the bottom. The blocks don't sit flat if the bottom isn't flat. Every other instruction I've ever seen says to not take soil off the bottom. I always do so that the bottom is even with the edges of the blocker.
Then you press out the soil onto your flat. Often at this point the soil wants to stick to the blocker. Other instructions say dip the blocker into water every time before making blocks. I've found that doesn't really help. It sticks just as much for me prewetted. So at the end I vibrate my hand so it releases. It is a very small movement, like you are shivering. This tends to release the block without any flaws.
After you are finished making your blocks wash your blocker. I once let mine sit for a several hours. I made some blocks in the morning and was going to come back in the evening to make more. The blocker had already started to corrode. So wash it right when you are done.
Once the blocks are made I dust the surface with cinnamon. This is helps prevent damping off. Then I seed the little holes in the top of the blocks and cover with vermiculite. Vermiculite is easier for the seeds to push up against and it also is pretty sterile compared to the soil. It doesn't have any of the damping off diseases in it so a good choice for seedlings.
Maintenance and containers for you blocks
You can use just about anything for for containers. I think Coleman recommends that you make your own wooden containers with a side missing (the side missing so they can be transplanted in the field easily). He mists the blocks to water them. I don't like that system. Wood sucks the moisture out of the blocks so they have to be watered more. My big sprayer is too cumbersome to spray neatly in a small area evenly (would work in a greenhouse, but not my room with wood floors). The small hand sprayers would take too much effort. Overhead spraying promotes damping off. His system might work well for a greenhouse and a large field operation, but I'm a home gardener.
So I figured out a system which I've yet to see anyone else use. It uses things I had or things I could get easily. I had flats. I had solid flats and I had mesh bottom flats (though a friend helped me out with more of these). The mesh bottom flat goes inside the solid flat so it doesn't drip. The mesh bottom flats were too uneven to hold the block well so I added some screening. In addition with the mesh bottom it lifted the block off of the plastic and and let the roots air prune a bit. I would like to get some wooden strips to lift the mesh off farther. Right now occasionally the roots grow into the bottom as it is moist enough.
This is a flat that is finished. As you can see the 1 1/2" blocks fit very well at 6 across and 12 down. There is just enough space between them to keep most of the roots from crossing from block to block too often. They do cross occasionally. A good thing to do would be to cut the roots between them about a week before transplanting, but I never bother.
One of the nice things about this set up is that watering is easy. I remove the mesh flat from its bottom solid flat. The put it in another flat that I keep half filled with water. So I bottom water all the seedlings. Once they are moist on the top I move them back to their original flat. You could fertilize them like this too. I've found I don't need to. The Fort V mix has lots of nutrients. Even my onions in the 1 1/2" blocks have no trouble over 8 weeks. Other things need more space for the light when they get bigger, so I always pot up to bottomless newspaper pots for them.
Coleman's flats had a side missing to take out the blocks easily. I don't. So I have a pair of tongs I use that makes picking up the blocks a snap. It isn't uncommon for me to rearrange the flats as time goes on. The cabbage family tends to stay in their flats for about 3 weeks (though future successions will be potted up in newspaper pots as I won't have their space ready yet). Somethings grow fast and shade out the small plants. I try to take this in to account when seeding, but some things are seeded much later than others. So my blocks will be moved around. The tape will come with them as they move.
Hardening off is easier. Usually there is massive transplant shock with plastic pots. The plants can be stressed in several different ways. They can get sunburned if they aren't used to direct sun (white patches on your leaves). They can get shocked by the wind (I pet my plants and use a fan at times which helps out with this). And they can have a shock to their root system. I don't have to worry about the root system. It stays intact and is never root bound. So my main issue with hardening off is the sun. The cool weather crops get a few days since the sun isn't as strong when they are being transplanted. But the tomatoes and peppers get a lot longer. The sun is very strong at that time of the year here. It helps if I plant them and put a row cover over them for a week. That way they get a bit of shade (about 15% with a lot of row covers) and less wind shock too. If I push their hardening off I'll always give them a row cover.
Hopefully I haven't missed too much. If so I'll edit the post after the fact.
Oh and just so I have it here. I grow my transplants under shop lights with cool white fluorescent bulbs. You do not need expensive grow lights for transplants. You need them to get something to flower, but not for vegetative growth. And even for flowers your plants will be healthier if you don't have flowers on them when they get transplanted out.