Start Seeds with Soil Blocks

Skip the seed-starting pots this spring, and instead plant your seeds directly in compressed blocks of soil that move easily from grow lights to garden.

| Spring 2019

Photo by Eliot Coleman.

Finding any technique that’s simpler, more effective, and less expensive than what’s been used before is satisfying. This is true for the production of transplants, and soil blocks meet these criteria. The Dutch have been developing this technique for more than 100 years, but the human history of growing plants in cubes of soil goes back 2,000 years or more. The story of how Aztec horticulturists of the chinampas of Xochimilco, Mexico, used cubes of rich mud to grow seedlings makes fascinating reading. A related technique is the old market gardener’s practice of using 4- to 5-inch (10- to 13-centimeter) cubes of partially decomposed inverted sod for growing melon and cucumber transplants.

How Soil Blocks Work

A soil block is pretty much what the name implies — a block made out of lightly compressed potting soil. It serves as both the container and the growing medium for a transplant seedling. The blocks are composed entirely of potting soil and have no walls as such. Because they’re pressed out by a form rather than filled into a form, air spaces provide the walls. Instead of the roots circling, as they do upon reaching the wall of a container, they fill the block to the edges and wait. The air spaces between the blocks and the slight wall glazing caused by the block form keep the roots from growing from one block to another. The edge roots remain poised for rapid outward growth. When you transplant the seedling to the field, it quickly becomes established. If you keep the plants too long in the blocks, however, the roots do extend into neighboring blocks, so the plants should be transplanted before this happens.

Despite being no more than a cube of growing medium, a soil block is not fragile. When first made, the fibrous nature of the moist ingredients binds it together. Once seeded, the roots of the young plant quickly fill the block and ensure its stability even when handled roughly. Soil blocks are the answer for a farm-produced seedling system that costs no more than the soil of which it’s composed.

The Soil-Block Maker

The key to this system is the tool for making soil blocks — the soil-block maker or blocker. Basically, it’s an ejection mold that forms self-contained cubes out of a growing medium. Both hand and machine models are available. For small-scale production, hand-operated models are perfectly adequate. Motorized block-making machines have a capacity of more than 10,000 blocks per hour, but they’re overscaled for the small vegetable farm.

Photo by Barbara Damrosch.



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