The Great Soil Block Experiment – part 1

2x2 inch soil blocks

2x2" soil blocks, freshly planted with 4 leek seeds each

A couple of years ago, I bought a “mini-20″ and a 2″ x 2″ x 4 block soil maker. Seed starting in soil blocks is supposed to prevent transplants from becoming pot-bound, eliminate transplant shock, and avoid the need to store and sterilize a bunch of potting trays all of the time. And, I never used them – until this year.

There’s a great deal of very positive hype about soil blockers for seed starting, but almost all that I have seen comes from either Eliot Coleman (a man I greatly respect, but just one guy, after all) or people selling the things. Since there isn’t a whole lot of info out there from people actually using them, I thought it might be useful to record my results for folks throughout this season here.

First off, some background on what I’m up to. This will be our first season starting a garden over one acre in size. The veggie field is a bit over 1.2 acres, with salad greens grown in a separate 3600 square foot garden, and we’re starting to establish herb and perrennial gardens at the new farm. I still intend to direct-seed everything that is practical to do so, as there is much less labor involved in direct-seeding, and we have very limited space to start seedlings indoors. We have one seedling heating mat to warm the soil of anything started on top of it, which I have found invaluable for starting things such as peppers and tomatoes that otherwise take forever to germinate.  Because we only moved to the present farm late spring last year, we don’t have any of our own compost ready to make a soil-blocking mix with, and I have no experience with what the soil should look like before being blocked, so I am using Johnny’s 512 Mix this year to learn with (but, man, that shipping is expensive!).  I have also ordered a 4″ x 4″ single block maker.

So, to review (and complete) the list, I have the following equipment for seed starting this year:

  • 3/4″ x 3/4″ x 20 soil mini-block maker
  • 2″ x 2″ x 4 soil block maker
    • both the standard dibble inserts for starting seeds and the block insert for potting up mini blocks
  • 4″ x 4″ x 1 soil block maker on order
  • about 25 trays that were being discarded from the dining hall at work
  • One seedling heating mat, big enough to hold 4 of the dining hall trays (or 4 traditional flats)
  • The usual sundry things like shop lights, water, seeds, etc

Soil Block Advantages

There are several advantages I expect to see using the soil blocks to start my seeds.  First and foremost, I don’t need to buy seedling flats, or store and sterilize them.  This means I can essentially expand the size of my seed starting infinitely without worrying about sourcing containers to start seeds in, or pots to pot transplants up to as they grow.  This has already been helpful, as I have already started over 200 pepper seeds in 3/4″ blocks, 60  2″ blocks with 5 seeds each of shallots, 44 2″ blocks with 4 seeds each of leeks, and 100 3/4″ blocks split between cellery and celeriac.  Cellery, celeriac, and shallots have already germinated.

A second advantage is space saving, at least at the beginning.  Because the 3/4″ cubes are so much smaller than any individual cell pack, I can fit a lot more germinating seeds in a given space.  Even the 2″ cubes, though bigger than an individual cell, waste less space between them, and so pack more closely together.

This is particularly helpful for those seedlings I start on the heating mat, such as peppers and tomatoes.  The seeds really only need to be on the mat until they germinate, but the mat is very pricey real estate.  It cost a lot, and it uses electricity.  So, obviously, I want to get the biggest bang for the buck possible.  Using a traditional cell pack like I had in the past, I could only fit 4 trays on the heating mat, each with 40 cells, for a total of 160 seedlings.  Right now, I have 232 pepper seedlings on one dining hall tray, with room to spare!  And I can fit 4 of those trays on the mat.  If I pack things closely, I think I can fit 260 3/4″ cubes on a tray, and thus 1040 on the heating mat at a time – a 448% improvement in my utilization of that piece of equipment.  That is truly significant!

Of course, you can’t keep a seedling in a 3/4″ cube very long.  Thankfully, or so I expect, as I haven’t had the opportunity to try yet, I can make 2″ x 2″ blocks with a 3/4″  square divot in the top to accept the mini blocks.  Then, I can make 4″ x 4″ blocks with a 2″ square divot in the top to accept the 2″ blocks.  Because the roots are “air pruned,” the seedlings don’t become pot bound.  Rather than coiling roots arround a pot struggling for nutrients, they sense the edge of the world, and the seedling just stops growing for a bit, waiting to be transplanted.  That’s the plan, anyway.  I’ll report back on how successful that is later.  So, the third major benefit is no transplant shock, combined with easy potting up.

Multi-Plant Blocks

Not really a benefit of soil blocks directly (because you could probably do this in traditional pots), but a simultaneous experiment I’m using this year is multi-plant transplants in the blocks.  I have started 5 shallots per block, as well as 4 leeks per block, and have no plans to thin them.  Instead, I’ll transplant them into the garden at a wider spacing, to preserve the total space each plant would normally have.  This should not lead to more competition – the plants will just shove each other aside a bit as they grow.  But, it will make transplanting much easier (I only have 20% of the transplant effort for shallots, and 25% for leeks), and the wider spacing between clumps should make hoeing much easier.  I’m relatively confident this will work out.

Results So Far

3/4" mini blocks, each with a single pepper seed

3/4" mini blocks, each seeded with a single pepper seed

So, how has the experiment turned out thus far?  Well, mixed, and I’m much more pleased with the 2″ blocks than the 3/4″ mini blocks.

Making the 3/4″ blocks is a bit of a challenge.  They just don’t want to come out of the blocker very well, even if I dip it in water as suggested in one instructional YouTube video, the blocks tend to stick to the expeller. The blocks just don’t weigh very much, and thus compete with the vaccuum formed against the expeller due to the water that is squished out of the soil as it is compressed.

While the 2″ blocks have thus far remained perfectly shaped, a certain percentage of the 3/4″ blocks have basically eroded or crumbled apart, no matter how gently I’ve watered them, as seen in the photo above.  From my tray of 232 pepper seeded blocks, probably 15 blocks have collapsed.  Of course, I still hope the seeds will germinate and I’ll be able to transplant them to 2″ blocks with their siblings anyway.

Also, the 3/4″ blocks are so small they just don’t hold much water.  They are drying out quite quickly, even before the seeds have germinated.  It will obviously be critical to pot them up as soon as they no longer need the help of the heating mat.  Meanwhile, they’re drying out more than I like, even through I’m watering twice a day.  Ug.

Are the 3/4″ blocks worth it?  It’s too early to tell yet.  Because I can get so very many more on the heating mat, my instinct is yes they are, but I’ll continue to be annoyed nonetheless.

The 2″ x 2″ blocks have been performing great thus far.  They are very sturdy, and can be moved around and handled with little difficulty.  They are easy to make and easy to expell from the block maker.  I am fairly confident at this point in the 2″ blocks.

The only other disadvantage thus far is the cost of the soil.  You need a soil that will hold itself together when compressed, and unlike traditional starting systems, you don’t use a sterile soil-less mix.  There are several different recipes, but all involve some sort of nutrient-carying ingredient, usually compost.  Elliot Coleman’s recipe even includes a portion of typical garden soil.  The Johnny’s 512 mix I’m using includes compost and blood meal.  I feel pretty good about that, as the nitrogen from the blood meal should help seedlings along, and I don’t need to fuss with diluting organic liquid fertilizers (which often smell bad, and we are starting seeds inside our house…). 

But, the mix isn’t cheap, and the shipping is hiddeously expensive.  In the future, we’ll make up our own soil blocking mix.  I’ll have gained experience with the consistency it should have from my experience this year, and we should have properly aged compost available at the end of this year’s growing season.  Making mix will just be another fall chore.

Stay tuned.  I’ll be back with more results on our soil block experiment as the season progresses.  Meanwhile, if you have any suggestions for more consistent results with the mini blocker, please share in a comment!

3 Responses to “The Great Soil Block Experiment – part 1”

  1. Lincoln Vannah says:

    We’re doing soil blocks first time this year, and are finding the seedlings are spindly, and seem to have slowed down a lot as they get ready to send out their first true leaves. A good percentage have keeled over, despite being watered regularly with a spray mister. How are yours doing now?

    • Chad says:

      I’ve been meaning to post an updated blog entry on the soil block experiment. But, in short:

      Mouse damage has been high. Not really the fault of the blocks, but annoying
      Germination of peppers was quite rapid. But the wait for ture leaves was long. Still, about 2 weeks ahead of where they would otherwise be
      Germination of leeks and shallots has been spotty. Seed tested as 92% and 98% germination rates respectively. But it’s on a block-by-block basis – either a block germinated, or a block didn’t (I multiplanted, 5 shallots or 4 leeks in each 2×2 block). Think it may be mostly related to planting depth (the dibble making a deeper indentation than I would normally use

      I’m also behind on starting things. Badly behind. Look for an updated post sometime or other…

  2. Pratt says:

    Try watering from underneath with Capillary Mat

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