Ink & Penstemon

Observations on plants, gardening, & nature from the Great Basin steppe in the American West.

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    How To Make Soil Blocks

    Since my last post on how to make your own soil blocker, many of you said that you haven’t heard of soil blocking, so I thought it best to write up an introductory primer—

    Soil blocks are compacted blocks of potting soil into which you place seeds to germinate. As a seed grows, as you know, it sends out roots that head down and out. When it’s growing in a pot, the roots will eventually hit the side of the pot, and start circling. In a soil block, they hit air, and the root sends a chemical message back to the plant that there’s nowhere to go, so the plant sends out more roots to compensate. You may have heard this referred to as “air pruning.” More roots means a more vigorous plant, and, as there is no pot to remove the plant from, there is little or no root disturbance. Plants that you wouldn’t dream of starting indoors in pots like carrots, beets, and peas, can be started in blocks and transplanted out with few problems. I find I don’t even have to harden off transplants grown in the blocks.

    To make the blocks, you will need a seed starting mix that says it has long fibers in it, whether it be coir or peat. That is what is going to hold your block together until the plants roots are able to. Many insist you need to make your own custom blended soil, but I’ve had good luck with “Black Gold” organic mixes out of the bag. Whichever you choose, put the soil in a tub or bucket and add water until it is good and wet but not soaking. Let it sit for at least one hour; more is better. You need the medium to be completely rehydrated. If your mix is peat based, it may be a good idea to let it sit overnight. Once your soil is ready, plunge the blocker down to the bottom of the tub 3 or more times, packing in the soil in really well. This is called “charging” the blocker. You can rub the blocker against the bottom or scrape along the side to level it off. Put your blocker onto your seed tray and, if you’re using my homemade blocker, lightly hold the plunger in place and slowly pull the mold up and off. If you’re really confident, you can hold the mold just above the tray and push out the block with the plunger. Whatever works. If you’re using a Ladbrooke blocker, it has a nifty release mechanism that releases the blocks from the mold. Either way, you should end up with a little block of soil with a divot in the top where you can place your seed, which also allows for a little dirt to be added on top if the type of seed you’re growing calls for deeper sowing.

    If you are starting seed in soil blocks I highly recommend investing in capillary matting*. In my experience, it is very hard to keep the blocks together long enough with any form of overhead watering. Also, I’ve found it helps to cover the blocks with a dome or plastic wrap to keep the moisture in until the seeds germinate.

    I hope this helps and I hope you have success with your soil blocks. I tried to make this bare-bones basic, and I’m sure you will have questions, so just submit them below in the comments.

    *After some frustrating experiences with capillary matting, I recommend just placing the blocks directly on a tray and gently watering around them.


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