How to Make Your Own Soil Block Mix: Growing Medium Recipes to Try

Soil blocking is an innovative and productive way to start seeds, where the potting media becomes its own growing vessel. These blocks hold developing roots and seedlings without walled supports, unlike cell trays or pots. The result is strong seedlings ready to take off with little disturbance at transplanting.

The reliable method promotes sturdy roots by allowing natural air pruning. As roots reach the edge of the soil wall, they desiccate and hold until transplanted into a bed or larger container. They fill out the volume of the block without circling or becoming root-bound. This technique also allows increased oxygen to roots for healthy upper growth and nutrient uptake.

The space-saving technique also uses fewer resources than some other seed-starting methods, relying on a blocking tool and fibrous growing media. The potting mix is a primary difference between blocking and sowing in cell trays. To create blocks, the potting mix needs to be able to hold its form and requires the right consistency at the get-go. The mix can be very simple, with flexibility in materials.

Soil Blocking Notes

Soil blocking is an uncomplicated technique that uses a blocking tool, either homemade or commercial, to condense soil into moist, plantable cubes or rounds. With a specialized potting mix and pressure mold of the tool, the medium holds as a mini vessel for root growth. It takes practice to get the right consistency and proper tool use, but it becomes a mastered skill in short order.

Each block holds one to two seeds, either thinly covered with potting mix or vermiculite or lightly pressed for surface contact. As they germinate, seedlings root in their plastic-free containers to move either to larger blocks, four-inch pots, or straight into their garden locations when roots fully form.

Soil blocking uses the tool and wet growing media to start, depositing the blocks in flat bottom trays. Watering from the bottom and misting helps retain moisture as seedlings develop. 

Benefits of Making Your Own Potting Mix

You can buy potting media tailored to this method, adding water to create the dense blocks. Or, build your mix with nutrient-rich recipes and materials that have versatile garden applications, like your own compost blend. The mix is easy to compile and, with a few elements, goes a long way.

Make as much or as little as you like using scaled proportions. These recipes are in “parts,” where anything on hand becomes a unit of measuring. Use buckets, gallons, etc., to build your mix, keeping the parts near the ratio indicated. Store the dry mix in a bin, and use a large bowl or pan (like a cement mixing tub) to moisten the mix for blocking.

Soil Block Media

At its core, soil blocking uses a nutritive, fibrous base that takes shaping well. The blocks need to be compact with full, crisp corners and free of air pockets to prevent crumbling. To pack them densely to hold their form, mix water with the dry media for a wet consistency. The feel will be sticky and almost (but not) drippy, given a squeeze. Achieving the best consistency takes trial and error, but blocks are forgiving. Toss any fails back into the mix for a restart. 

Most recipes rely on a peat or coir component (or similar fibrous material) that holds the media together. Coir is somewhat more sustainable than peat, as naturally occurring peat bogs provide tons of valuable carbon sequestration and are non-renewable. To reduce peat usage, coco coir is useful. The fiber derives from the husks of coconuts. Many growers are also working on eliminating coir from the blocking medium for a more sustainable system.

There are numerous organic recipes for nutritive block-friendly potting mix. These need approximately two parts water for the best consistency of being wet but not dripping, but this varies. Add water slowly to avoid oversaturation, and keep a dry mix on hand to level out an overage.

Fundamental Ingredients

The elemental composition of soil block mix involves a balance of moisture retention, drainage, and filler materials with nutrients. A simple starter recipe includes:

A Note on Compost

Compost in blocking formulas can be plant-based, manure, or worm castings – whatever your preference or at-home system. It needs to be completely broken down, sterilized, and fine. Sift it using a wire screen or colander to remove large pieces. Chunky compost can create air pockets or impediments in the blocks.

Basic Mix

These recipes are flexible and allow us to be resourceful. This basic, nutrient-rich soil blend is a healthy foundation for seedlings. Building off the simple starter with added nutrients looks like this:

The perlite improves aeration and drainage. The compost aids in moisture retention and nutrition, as does the blood, alfalfa, or kelp meal (nitrogen). If you don’t have (or don’t want to use) coir or peat, try a leaf mold or leaf mulch. There are commercial peat alternatives, too, made of recycled paper pulp. As you gain experience, you’ll likely hone in on your own best mix.

Peat-Free

For those who rely on soil blocking for abundant seeding, like flower farmers, here is a vetted, productive, peat-free recipe: 

Classic Recipe

Eliot Coleman, author of The New Organic Grower, brought soil blocking to the forefront as a viable seed-starting method for sustainable horticulture. His published recipe includes lime in addition to a blend of minerals.

A twist on the classic recipe:

Leaf Mold

If you’ve been taking advantage of those fall leaves over the seasons to make a leaf mold soil conditioner, bring some into the mix. You’ll be able to play with proportions in relation to coir, but here is a strong seed-starting blend:

Starter Organic Plant Food

To streamline the need to purchase minerals and fertilizers, buying one additive to use in versatile potting applications makes it easy. Espoma Bio-tone Starter is a nutrient-rich plant food for seed starting and transplanting.