Soil Blocks: An alternative way to start seeds in the house


Submitted 2 years ago
Created by
Henry Homeyer

A good gardener can start tomato seedlings in old yogurt tins – or anything else that will hold some soil mix. But most commonly gardeners buy plastic containers designed for starting seeds: each unit or “6-pack” has 6 little compartments that hold a few tablespoons of soil and has holes in the bottom for drainage. They fit inside trays that keep them from peeing on the table top. But there is an alternative, the handmade soil block, and it has several advantages.  

Six-packs used for starting onion seedlings

I’ve been making soil blocks for at least 10 years, and like them. They are more work to make than using standard flats, so I use both – depending on the plants and the time I have to prepare them. Here’s what I do:

Making soil blocks

In a large plastic wheelbarrow I mix up the dry ingredients as described below. Then I mix up smaller quantities with water and press a special metal tool into a bin of the moistened mix. I squeeze the spring-loaded handle and 4 blocks pop out, ready to use.

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                   Soil Block Recipe

In a wheelbarrow mix 10 quarts dry peat moss, 3 quarts sand and ¼ cup agricultural limestone (powdered, not pelletized is preferred, but either is OK)

Then mix in :   

            ¼ cup Dried blood

            ¼ cup Rock phosphate

            ¼ cup Green Sand

             ¼ cup Granite dust or Azomite (optional)

OR      1 cup Pro-Gro (or other) organic fertilizer

Add and mix in:  

            10 quarts Peat humus (available at garden centers in bags)

            10 quarts fine compost (your own or purchased)

            10 quarts top soil (your own is preferred, but purchased is OK)

Place 4 quarts dry mix in a plastic basin or flat-bottomed container, and add about 1 quart water. Mix until gooey but firm, not watery.

 I use a 2-quart plastic juice container for measuring out the dry ingredients. I mix the wet ingredients in storage container, the kind people use for storing sweaters. The block maker produces cubes that are about an inch and a half on a side, and have a small divot on the top where you can place a seed. A standard plastic tray or flat used for 6-packs of plants. The flat will hold 32 cubes. I push down hard on a big pile of the wet mix to make a nice dense cube.

Why bother with all this? As you know, seedlings left in a 6-pack will develop roots that become tangled and encircle the space they are growing in. When planting, you need to tease the roots apart so they can grow into the soil. This disturbance breaks roots and causes a plant to stop growing and rest for a while. In extreme conditions – such as big marigolds already blooming in a six-pack – you can lose over a week before the plant recovers.

In a soil block when the roots come to free air they stop growing. When you put the block in the ground, the roots can take off and start stretching on day one. Not only that, soil blocks are full of great organic nutrition. Most potting mixes are mostly peat moss, which has little nutritional value to a plant. In general, plants growing in soil blocks do better than the same seeds started in a sterile potting mix.

There is a popular myth that plants started indoors need a sterile potting mix. There is a fungal disease called “damping off” that is fatal to seedlings and that most gardeners know about – and fear. We learned not to use garden soil because of the possibility of damping off, but in years of using soil blocks that contain garden soil, I have never encountered it. I think having good rich growing medium promotes healthy plants, though I would never use pure soil as it compacts too much.

I planted celeriac (aka celery root) in mid-March. The seeds are tiny and hard to handle. I used a little plastic planting device I got from Johnnys Selected Seeds. They call it a hand seed sower and sell it for $4.25. It holds seeds, and allows one to jiggle and tap it to get seeds to drop off the tip of the device. I like it.

Johnnys hand seed sower

Whether using soil blocks or plastic six-packs, I plant 2 seeds per unit. That generally ensures me of getting at least one plant since most seeds germinate at a 90% rate or better, assuming that you don’t let them dry out. I use plastic covers over the top of the flats – they are clear domes sold for the purpose. I remove them once most of the seeds have germinated. And I snip off one of the two seedlings if both germinate.

I’ll plant my tomatoes this year on April 7 or 8. Both are “fruit days” on the Stella Natura calendar, which advises me on the proper phase of the moon and location of the stars and planets. Call me woo-woo, but it works! Planting seeds indoors is a lot of work, but it keeps me sane (I think) during the gray rainy days of spring.

Have you used soil blocks? Share your experience below in the comments section. 

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