Tuesday, March 20, 2012

Soil Block Tutorial

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.

Soil blockers

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.

Twist as you make your blocks

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.

Above is what my blocks looked like after making most of them. I think the blocker puts them too close together to root prune well and since a flat gives me the space I rearrange them to be even.

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.

The original flat is labeled on the sides with tape. So I'm careful not to rotate the flat when I put it in and out of the watering flat.

Thongs for moving a block

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

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.


  1. I have never heard of these. What a wonderful option! I will definitely be looking into these!

    Thanks for demonstrating this evil plastic alternative!

  2. Thank you for such a detailed post! I have been thinking about soil blocks for next year and you just made it seem so easy. Now I know I want them. Thanks!

  3. I picked up a homemade soil cube maker and was not very impressed with it. The first batch I made the cubes were not coming out whole. I am very disappointed with it. So I think I might try the commercial ones like you recommend. Thank you!

    1. Just so you know. Making soil blocks is a real learning process. It takes a while before it becomes easy. I've never tried a homemade one so can't vouch for how hard or easy those ones are. But when you do make them again, just remember, wetter than you think and more compressed than you think. When you first start make sure to press in from the bottom with your hands. Eventually it will just become a check that the blocks formed correctly, but initially you will probably have to fill them in a lot. When I first started making them I'd have to remake about half of them. Now I only messed up one in two full flats.

  4. Thanks so much for the detailed instructions!!! I need to get on this right away. I know it's still early but the unseasonal warm weather makes me feel rushed :0)

  5. I really struggle with what type of soil to start my seedings in. I have always kept the containers from plants that I have bought in the past so I usually just use those. I find it best to have a larger container for the tomatoes especially. I will check out your home made soil mix link above :) Thanks!

  6. Excellent instructions!! I think these are the best soil block instructions on the internet. Trust me, I did a LOT in researching before I invested in mine. I wish I had these when I first began using my soil block maker. Brilliant idea to use the tongs!

    Just a few little things that seem to work for me: I plunge my blocker into the prepared mix 3 times and twist each time. Then I scrape the bottom with a straight edge so they lay flat. When I release them I push half way, pull up a little, then push and lift the rest of the way. I don't have a problem with sticking.

  7. This comment has been removed by a blog administrator.

  8. Great post Daphne. Thanks. We made blocks for peppers tonight and, strangely enough, your most helpful advice for us tonight was the "twisting motion" of the blocker to get the soil to pack properly. I have had trouble with this. It was also very helpful to "vibrate" for a clean release of the blocks from the blocker. I've had problems with this too; your detailed thought was very helpful. Thanks again!

  9. Interesting post. Great way to reduce plastic usage. I have been thinking of investing on this for sometime. Hope some time this year, I have time to buy the needed things to get start building our own blocks.

  10. Thank for a great post. I have been thinking about getting a soil block maker. Maybe next year will be the year!

  11. Awesome tutorial. Much I am already doing, but two things jumped out at me as great ideas - the cinnamon sprinkle and the fine mesh secondary bottom. I am using a open mesh bottom tray inserted in a solid tray like you are - which allows for bottom watering, but the openings are too big so using the screening material makes great sense. Thanks for taking the time to post this!

  12. good to know about the NOFA purchase for Fort V mix. i made my own mix because it was too expensive to order Fort V through fedco. i used coast of maine potting mix as a base. so far so good.

    i like the tip using the mesh in the trays for mini blocks.

  13. Oh, I wish I had come across your blog and this post before I started making soil blocks a few weeks ago! I am pretty happy with them, but used the mini blocks for most of my small seeds, and the seedlings outgrew them so fast-- a lot of handling to repot them into larger digs. On the positive side, I could fit 300 in a tray and two trays on my heating mat, starting 600 seeds at a time.

  14. Great article Daphne, hope you don't mind if I link to it from my soil blocker store in Australia. www.soilblocker.com.au

  15. This comment has been removed by the author.

  16. I've been doing this technique for a couple of years, and I guess the thing I would say is that it is not the no muss, no fuss way to go. I love it. But I love it in a 'grind your own paints' kind of way. I like to make my own mix; use my own compost. I like to spend time making blocks. I like the aesthetics of it (the blocks are kind of cute and my kids like them). I above all like the absence of plastic seed starting six packs constantly floating around my life (my house is already bursting to the seams with ill-ordered stuff). Do I get better results? I'm not sure. Because I don't start with a sterile mix, there's more dampening off. I hear different things about transplant shock and air pruning; one author I trust doesn't like seed blocks because he feels they compact soil more than is good for seedlings. On the other hand, you definitely don't get root balling. I have a lean to greenhouse on the side of my house, so I can move from starting to transplanting pretty quickly, which is what you need to do with this technique. (The blocks do hold up reasonably well, but they erode (in my experience) and so I'm not sure I'd hold on to plants in that form for longer than 6 weeks).

    In short, I love the thing but it's kind of for the enthusiast, in my opinion. If you just want to start a small tray of plants and would rather not spend much bother on it, starter trays or yoghurt cups or whatever are also certainly fine.

  17. Thank you so much for this informative tutorial. I'm in the process of transitioning our home garden (which is quite large for a home garden) over to using soil blocks for seed starting and you provided all of the numbers and facts I needed to make the right decisions when ordering my block makers and things.