Gardening with Soil Blocks

Soil blocks have been around for at least 2,000 years, first used by the Aztecs to build the so-called “Floating Gardens” of Mexico City, which were long strips of land surrounded by water used for growing plants and vegetables. The Chinampas system of farming utilized a seeding technique whereby chunks of harder soil were cut from bogs and used for sprouting seeds.

The Aztec idea still resonates in modern soil blocks, which are essentially free-standing, slightly compressed blocks of soil that are used for germinating and starting seedlings, eliminating the need for pots or seed trays. In the process, a tool of some sort is used to cut the blocks.

Reduce Gardening Expenses by Using Soil Blocks

Many gardeners use soil blocks for the simple reason that they save money. You don’t need pots or any plastic containers. Beyond the initial investment of the soil blocker tool itself, there is little expense, and the blocker usually pays for itself within just a couple of years. Blockers are usually made of zinc or some other sturdy, non-corrosive material and last for years.

Beyond the savings on equipment, using garden soil blocks also save you time and money by producing healthy seedlings that can be quickly planted. Seedlings grown in soil blocks form stronger root systems because they get increased oxygen and the roots tend to naturally air prune themselves so there’s no overgrowth. Once transplanted by simply setting the whole block with seedling into the ground, the seedlings adapt more quickly and are less likely to be lost due to damage or transplant shock. Lost plants mean lost money.

Making Your Own Soil Blocks

Making your own soil blocks is easy, but the key to success is in the soil mix recipe. The blocking mix must contain fibrous material — specifically peat moss — in order to bind all the ingredients and help retain moisture, which is essential to seedling development.

Following is a soil block recipe adapted from The New Organic Grower: A Master’s Manual of Tools and Techniques for the Home and Market Gardener, by Eliot Coleman, 1995.

• 3 buckets (standard 10-qt. bucket) brown peat moss
• ½ cup lime (mix well)
• 2 buckets coarse sand or perlite
• 3 cups base fertilizer (blood meal, colloidal phosphate and greensand mixed together in equal parts)
• 1 bucket soil (make sure it is sterile)
• 2 buckets sterile compost

Using a clean large container or a plastic tarp on the ground, mix the soil recipe with enough water to give it the consistency of peanut butter. It should be wet, but not soggy. Then, take your soil block tool, fill it with the mixture and carefully even off the soil using a cement knife or a board. Pushing with the tool, release the blocks onto a tray with a twisting motion. Now you’re ready to plant your seeds.

Making Your Own Soil Blocks Last

The time between when you plant the seeds and when you place your soil blocks into the ground can be about four weeks or longer, so you want to make sure that your soil blocks last, especially in the early days before the roots have really established themselves.

For the sake of longevity, it’s good to keep the following in mind:

1. Experiment with your soil block mix until you get the right consistency of water and soil. This is essential in giving your seeds a good start.
2. Choose a block size that is appropriate for your seeds — a 2-inch block is best for most, while a 4-inch one is good for larger seeds like squash and beans.
3. Place the soil blocks in a tray about a quarter of an inch apart and cover with a clear lid or plastic wrap.
4. Water from below, checking about twice each day. This will allow the soil pods to soak up the water as needed and will prevent both oversaturation and dryness. A steady source of moisture is best for the blocks and the plants.
5. Encourage quick germination and growth by providing a steady source of light and warmth.
6. Try hardening your seedlings by setting them outside for a little bit each day, being careful to keep them from hungry birds and unwanted lunch guests.
7. Use a spatula to make transplanting easier. Lift the blocks carefully from tray into the newly dug hole, being careful not to break the block apart.

Want to learn more about gardening with soil blocks?

Organic Potting Mix Basics from University of Maine Extension Service
Video: How to Make Soil Blocks from Johnny’s Selected Seed