Soil blocks #2: How to, Q & A

Well, this isn't quite the “tomorrow” so casually mentioned in my last post; I must learn not to make such promises. Life's been even more than usually busy, for the past week has included both Abdoulaye's departure and a visit from one of Steve's most excellent brothers. The former event leaves us bereft, but I suspect Abdoulaye feels rather differently, as he's returning to his wife of a just a year and a half. He certainly sounded cheerful when he called this afternoon, as did she. More about Jeff's visit tomorrow soon, but for now, on to soil blocks.

Soil blocks from top

The Mini 4 (left; 2" blocks) and Micro 20 (right; 3/4" bocks)

Q: So what the heck are soil blocks,
and how do you make them?

As mentioned in my last rambling post, soil blocks are free-standing (pot-less) compressed chunks of soil used for starting seedlings. They're made using special molds which consist of cubes or sets of cubes with open bottoms and moveable tops attached to a spring. Two are pictured above. Apparently those tools are sometimes referred to as "blocks," while what they produce is called a "soil block," but to avoid confusion, I'll refer to the tools themselves as "molds."

Holding the flat handle that runs across the entire mold, you dig the mold into a wet soil mixture until it's full, then set it on a board and press down what I'll call the “ejection bar,” the silver, threaded bar at the top shaped sort of like a question mark. As you press, the tops of the mold compress the soil. With your palm on the ejection bar and your fingers under the flat handle, you squeeze, bringing the handle towards the bar so that the sides of the mold slide upwards, away from the soil. When you lift the mold, there on the board sit your soil blocks, each with a little divet in its top for seeding. Set the blocks on a damp mat, and you're ready to plant.

Q: Wonderful, but really, why bother?

Well, according to the experts (of whom I am not one), they prevent transplant shock, which means that you have healthier, happier plants once you set them out. They also make transplanting much easier by eliminating the whole business of getting a plant, undamaged, out of a pot, and all the problems attendent on that process, problems about which I waxed so eloquent (or long-winded) in my last post, such as soil falling away from roots. Well before that stage, since you use (in general–there are exceptions–) one seed per block, they save seeds. Finally, since there are no pots, your plants can't get pot-bound, and since the roots don't circle round and round each other, they make better use of the soil available to them.

Q: But don't the roots end up dangling out in the air?

Roots retreat from the edges of the blocks, though they can grow into the mat they're set on or into neighboring blocks if set too close together.

Q: How can such a small piece of soil provide what a plant needs?

When you compress the mold, you're (supposedly) packing in up to 1½ to 2 times as much soil as a similar-sized pot would hold, so each block provides far more soil and nutrients than would a pot of the same size. (Which is a darn good thing, because the little ones are Really Small—about 3/4th of an inch square.)

Q: But doesn't pressing all the air out of the soil deprive the roots of oxygen?

Yeah, well, this is where you need a real soil scientist, because I've been wondering about this. When I voiced this objection, our resident plant specialist, Abdoulaye, immediately said that the plants can get the oxygen they need from the air. Also, since the blocks are surrounded by air (no pots, remember?) I suppose they can get more from that source than can a potted plant. Finally, water contains oxygen too, and the blocks need to stay consistently moist.

Q: What aren't you telling us? What's the downside?

Beyond the expense of the molds (I spent about eighty dollars), the main drawback is that at every stage, using them requires time, patience, and practice. Mixing the special soil may seem like a big deal if you've always relied on commerical potting soil, but it's neither difficult nor time-consuming nor particularly expensive, so I'm not really counting that.

However, using the molds does take a bit of practice, and even with that, some blocks will fall apart. These can just be dumped back into the slurry, but if the occasional "oops" frustrates you, soil blocks may not be the way to go. Seeding the blocks, too, is a painstaking process, as you're trying to fit a single seed into a tiny dent in a small block of soil. Again, it's all about your own tolerance–and about getting the right tools for handling the blocks and seeds.

Q: And those tools are…?

Pretty simple. A couple of saucers, a pencil or toothpick, two different sizes of tweezers, and you're in business. One saucer holds seeds, the other water; the toothpick, once dampened, is used to plant small seeds; ordinary metal tweezers help in planting larger seeds, and wide tweezers make it possible to pick up the smallest soil blocks.

Q: Okay, so what's in this magic soil mix?

Recipes differ, but the key ingredients include a healthy dose of compost; either peat moss or coconut coir; perlite or sand; ordinary garden soil; and finally, some slow-release fertilzer, including trace elements. The compost and coir help hold water, which is essential in a potless mix; one of the biggest problems people have with blocks is keeping them sufficiently moist. Perlite or sand help “lighten” the soil and improve drainage (yes, I know, this seems to contradict what I just said). Both the compost and fertilizer will provide nutrients while the peat or coir holds the block together. Finally, the garden soil helps to prevent transplant shock by ensuring that when you set out your blocks, the plants will not be entering an entirely foreign medium. I'll give recipe details in the next post soon.

Q: Where can I get them?

There are several online sources, but I got mine from Jason Beam Potting Blocks,  the sole source for Ladbrooke molds, which appear to be more or less the definitive type. The site includes some fascinating history, including that of the Aztec “Floating Gardens” near Mexico City, where a soil block technique was used to start seedlings.

Q. Can I make my own molds?

Apparently, yes, as several websites give detailed instructions. I considered doing this for about five seconds, during which I reviewed the many urgent and interrupted projects awaiting my attention, and then I placed my order. If you decide to do it yourself, or if you already have, more power to you, and I'd love to hear how it goes. Or went.

4 Responses to Soil blocks #2: How to, Q & A

  1. I am considering doing this myself, and was very glad to be able to read such a descriptive post.

  2. Glad it was helpful, Sylvana. Let me know if you decide to go ahead.
    –Kate

  3. I bet you could make one of these using a bottle jack like some car shops do for hydraulic presses. Great article, thanks for the info.

  4. Luke –There are instructions online for making your own, but I decided to face my limitations and admit that I wasn’t up to it. Sigh. But you may be more mechanically inclined, in which case I say, go for it.
    Thanks for stopping by.
    –Kate

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