How to Make Soil Mix for Your Soil Blocks

Soil Blocks for Germinating Seeds and Growing Seedlings

Soil Blocks offer an innovative way to germinate seeds and grow seedlings all in one convenient spot. Soil blocks can eliminate individual pots and reduce root handling, although the blocks still need a waterproof tray and seedlings can still experience transplant stress. Seedling quality depends on the mix, moisture, light, temperature, nutrition, timing, and crop.

What Are Soil Blocks?

Soil blocks are compact cubes of specially mixed soil, created using a tool called a soil blockers. These blocks are free-standing and designed to hold their shape while supporting seed germination and seedling growth. Soil blocks can reduce the number of individual plastic pots, but a durable tray is still needed. Exposed block edges can encourage air pruning, and intact blocks may reduce root disturbance; neither stronger roots nor less transplant stress is guaranteed.

Curious about how to use soil blockers? Watch Tricia’s video, soil blocks, where she demonstrates how simple and effective this tool can be for your gardening needs.

Why Traditional Potting Soil Won’t Work

If you’re new to soil blocks, it’s important to understand that traditional potting soil is unsuitable for making these cubes. Potting soil lacks the necessary consistency and structure, causing the blocks to crumble. Instead, use a cohesive mix that holds its shape when damp. The best texture varies with blocker size, ingredients, and crop, so test a few blocks before preparing the full batch.

The Ideal Soil Mix for Soil Blocks

The recipe for soil blocks comes from Eliot Coleman’s book, The New Organic Grower. Here’s what you’ll need for this starting mix:

• Peat Moss (or Substitute): Essential for retaining water and creating air pockets in the soil. If you’re concerned about sustainability, consider using coco peat (coconut fibers) as an alternative. Use 30 quarts.
• Compost: Provides nourishment for seedlings. Use 20 quarts.
• Sand or Vermiculite: Vermiculite is lightweight and moisture-retentive. It is not a substitute for sanitation, and dust should be minimized according to the product label. Use 20 quarts in this example recipe.
• Oyster shell flour: Enhances soil structure and provides calcium. Use ½ cup.
• Garden Soil: Contributes a small amount of natural nutrients. Use 10 quarts.
• Base Fertilizer: A mix of 1 cup each of blood meal, colloidal phosphate, and greensand.

Mixing Instructions

Thoroughly mix the ingredients and add water gradually until the mix packs and releases from the blocker without slumping. A small amount of water may appear when pressed, but ingredient moisture and blocker design can change the needed consistency.

Tools for Soil Block Creation

To improve the chance of success, you’ll need a few essential tools:

1. Sieve: Use a fine mesh sieve to screen soil mixture ingredients, creating a light, airy texture that’s ideal for seed germination.
2. Potting tray: A sturdy tray helps with measuring, mixing, and moistening the soil.
3. Respirator: Wear a respirator when working with fine powders like peat moss and oyster shell flour to protect your lungs.

Pre-Made Options: Quickroot

If mixing soil isn’t your thing, try Quickroot, an organic soilless seed-starting mix designed for soil blocks, Seedling trays, and Planter trays. Quickroot’s ingredients include:

• Coconut coir fiber
• Vermiculite (no detected asbestos)
• Organic green waste compost
• Bone meal
• Soft rock phosphate

Available in 1-cubic-foot bags and larger 54-cubic-foot bags, Quickroot simplifies the soil preparation process while delivering excellent results.

Alternative Soil Block Recipes

Soil blocks are more popular in Europe, and many recipes originate from British gardeners. Here are two tried-and-true options:

David Tresemer’s Recipe

From Transplants in Soil Blocks:

• 4 parts peat moss
• 1 part well-rotted compost
• 1 handful of ground calcium limestone (per cubic foot of mixture)
• 1 handful of ground basalt rock powder

Thalassa Cruso’s Recipe

From gardening expert Thalassa Cruso:

• 2 parts peat moss
• 1 part vermiculite
• 1 part good garden soil (preferably friable “chocolate cake” soil)
• A small amount of clay dust as a binder

For both recipes, moisten the mixture to a slurry-like consistency before packing it into the soil blocker.

Benefits of Soil Blocks

Soil blocks offer several advantages over traditional seed-starting methods:

• Reduced individual pots: Can reduce reliance on separate plastic pots, while a reusable tray is still required.
• Air-pruned edges: Exposed block surfaces can limit root circling when moisture and spacing are managed.
• Less root handling: Intact blocks may reduce disturbance, but timing and planting conditions still affect transplant stress.
• Customizable Sizes: Soil blockers come in various sizes, allowing you to choose the right block size for your seeds.

Tips for Success with Soil Blocks

• Place soil blocks on a waterproof tray for easy watering.
• Use a bottom-watering method to maintain moisture without disturbing the blocks.
• Label your blocks to keep track of different seed varieties.
• Cover blocks with a humidity dome or plastic wrap to maintain warmth and moisture during germination.

Are You Ready to Try Soil Blocks?

Soil blocks are one option for producing seedlings with fewer individual pots. Whether you mix your own medium or use a labeled block mix, compare labor, trays, watering, nutrition, and transplant timing with your existing system.

Use soil blocks within the Seed Starting resource path

This recipe is specifically for making stable, workable soil blocks. For timing, temperature, light, watering, thinning, and hardening decisions, use the complete Seed Starting guide. Watch How to Start Vegetable Seedlings Using Soil Blocks for the technique, or compare soil blockers and seed-starting media.