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I'm starting a vegetable garden this spring with ±50m² effective surface area, and I'm wondering whether it would be useful to use soil blocks to reduce plastic usage. However, in my planning I will be sowing seeds every week from the beginning of February until the end of May, in small amounts at a time. My rough estimate is that I will need ±10 blocks per week in March–May, and less in February. I'm worried that making the potting mix and the pots each week will be a lot of overhead and may not be worth the effort.

It would be interesting for me if I can make blocks ahead of time, say, once every 1–2 months (2–4 times in total in the year), and keep them empty until I need them. Would the blocks stay good long enough?

If this is possible, how do I best preserve the blocks until sowing? I suppose I would have to keep them somewhat moist, but I can imagine they will eventually disintegrate without roots to keep them together. Also nutrients may wash away by repeated moistening.

Would it help to add something to the mix to prevent disintegration? (I'm planning to make my own mix, for example I found people using 2 parts compost, 2 parts sowing mix, and 1 part perlite. I don't want to use a ready mix, because all the ones available in my area have peat in them.)

user65560
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2 Answers2

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I have used soil blocks 2 months after I made the block without any fertility issues at all. It's just a matter of storing something delicate.

I do let them dry out almost completely by leaving them in plastic trays in my garage during winter. When they are dried out they have a hard, crunchy, friable texture that bursts like a snowball when you throw it against a wall. Once they are in this state they are much easier to handle. I move them to clean trays so I can reject any failed blocks and leave behind any loose medium on the bottom. They stay like this in the tray until I need and rehydrate them.

I rehydrate the blocks gently - I mist them with water several times. Bottom watering from this dry state can make them break apart when the bottom swells before the top is even flexible again.

The soil recipe you use will be a big part of your success. I've switched from peat to coconut coir, and unfortunately lost some durability. This year I am considering trying human hair and clay as experimental binders, I'll report back if I do it.

Larger soil blocks also hold together better than small ones. I use a Fiskar soil blocker that makes two sizes of soil block:

A picture of two soil blocks, a larger two inch cylinder with a dimple on top and four smaller blocks that each make up 1/4 of a circle similar in size to the large block

The large 2 inch cylindrical blocks are easy to work with. I will have maybe 1 failure in 25 blocks. The smaller 3/4 inch blocks have about 1 failure in 4 blocks.

It takes a little practice, but the mistakes can always be made into new blocks :-).

MackM
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Soil bocks start falling apart as soon as they are made.

The only thing that works against that is root growth in the blocks.

So stockpile the materials and make the blocks only when you need them.

A different way to avoid plastic is terra-cotta (clay) pots. As any archeologist can tell you, those are forever if you don't break them.

Peat pots are another way, but those are an annual expense. And I guess you're avoiding peat, too.

Ecnerwal
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