As you may or may not know, we are starting a market farm this year and we have decided to use soil blocks as our primary method of starting our seedlings. In order to give our seedlings the best chance possible, we are making Eliot Coleman’s soil blocking mix from his amazing book, The New Organic Grower.
Before I get into the specific blocking mix recipe, I just wanted to highlight why, according to our expert Eliot Coleman, preparing a specific blocking mix is important and what each ingredient brings to the table.
Why A Blocking Mix?
When transplants are grown, whether in blocks or pots, their rooting area is limited.
Therefore, the soil they grow in must be specifically formulated to compensate for these restricted conditions.
The composition of blocking mix differs from ordinary potting soil because of the unique requirements of block making.
A blocking mix needs extra fibrous material to withstand being watered to a paste consistency and then formed into blocks.
Unmodified garden soil treated this way would become hard and unpenetrable.
A blocking mix also needs good water holding ability because the blocks are not enclosed by a container.
Blocking Mix Ingredients
The bulk ingredients for blocking mixes are peat, sand, soil, and compost.
Peat
Peat is a partly decayed, moisture-absorbing plant residue found in bogs and swamps.
It provides the fibre and extra organic matter in a mix.
It should be noted that all peats are not considered equal and quality can vary greatly.
Eliot recommends using premium-grade peat.
Poor quality peat contains a lot of sticks and is very dusty. The better quality peats have more fibre and structure.
The peat gives ‘body’ to a block.
Sand/Perlite
Sand or perlite is useful to ‘open up’ the mix and provide more air porosity.
Coarse sand with particulars having a ⅛ to 1/16 inch diameter is most effective.
For a lighter-weight mix, Eliot replaces the sand with coarse perlite.
Whichever coarse product you use, adequate aeration is the key to successful plant growth in any medium.
We decided to use perlite for this ingredient as it was easily purchased online here.
Compost and Soil
Both soil and compost are important to plant growth in a blocking mix.
Together they replace the ‘loam’ of successful old-time potting mixtures.
In combination with other ingredients, they provide stable, sustained-release nutrition to the plants.
With soil and compost included, there is no need for supplemental feeding.
Compost is the most important ingredient.
It is best taken from two-year-old heaps that are fine in texture and well decomposed.
The better the compost ingredient, the better the growth of the plants will be.
Soil refers to fertile garden soil. Eliot uses soil stockpiled from the year before that has previously grown onions. He has found that seedlings grow best when the soil in the blocking mix has grown onions.
The soil and compost should be sifted through a ½” mesh screen to remove sticks, stones, and lumps. For an extra-fine mix, sift through a ¼” screen.
As this is our first year as market farmers, we do not yet have a stockpile of compost or soil so we purchased both from a local landscaping company.
We also made our own soil sifter using a ¼” screen. You can watch Jon make our soil sifter here if you like.
Extra Ingredients
Lime, blood meal, colloidal phosphate, and greensand are added in smaller quantities.
Lime: Ground limestone is added to adjust the pH of the blocking mix.
The quantity of lime is determined by the amount of peat, the most acidic ingredient.
The amount of lime in the recipe gives the soil a pH of around 6 to 6.5 which is good for all the major transplant crops.
Blood Meal: This is the most consistently dependable slow-release source of nitrogen for growing mediums.
Colloidal Phosphate: A clay material associated with phosphate rock deposits and contained 22% phosphorous pentoxide. The finer the better.
Greensand (Glauconite): Greensand contains some potassium but is used here as a broad-spectrum source of micronutrients.
These last three ingredients – blood meal, colloidal phosphate, and greensand – when mixed together in equal parts are referred to as the ‘base fertilizer’ in the blocking mix recipe.
We decided to use a commercial fertilizer that we purchased from Amazon here.
Blocking Mix Recipe
A standard 10-quart bucket is the unit of measurement for the bulk ingredients.
A standard cup measure is used for the supplementary ingredients.
This recipe makes approximately 2 bushels of the mix.
Follow the steps in the order given.
- 3 buckets brown peat
- ½ cup lime
- Mix
- 2 buckets coarse sand or perlite
- 3 cups base fertilizer
- Mix
- 1 bucket soil
- 2 buckets compost
Mix all ingredients thoroughly.
The lime is combined with the peat because that is the most acidic ingredient. Then the sand or perlite is added. The base fertilizer is mixed next.
By incorporating the dry supplemental ingredients with the peat in this manner, they will be distributed as uniformly as possible throughout the medium.
Next, add the soil and compost, and mix completely a final time.
Step-By-Step Blocking Mix Instructions (video down below)
Step 1: Add 3 buckets of brown peat moss to your container.
*Eliot recommends sifting the peat, compost, and soil. We used a 1/4″ mesh for all three components.
Step 2: Add 1/2 cup of lime.
Step 3: Mix
Step 4: Add 2 buckets of coarse sand or perlite.
We used perlite.
Step 5: Add 3 cups base fertilizer.
We used a commercial fertilizer that contains the ingredients of Eliot’s base fertilizer.
Step 6: Mix.
Step 7: Add 1 bucket of soil, sifted.
Step 8: Add 2 buckets of compost. We sifted ours.
Step 9: Mix all ingredients together thoroughly.
In Summary
Overall, it was pretty easy to acquire all of the ingredients for Eliot’s blocking mix recipe.
We purchased what we could from local sources (soil and compost) and purchased the rest online (peat, lime, perlite, and fertilizer).
Combining the ingredients in the blocking mix was pretty straightforward.
However, one recommendation we have is to use a large container, bigger than a large Rubbermaid container (68L), because we did not have enough room in one to make the full recipe, we ended up dividing the mix into two containers.
I would recommend at least a 100L container for this recipe to allow adequate room for mixing thoroughly. Trust me, a little extra room won’t hurt!
It has been a few days since we actually made our blocking mix and started our soil blocks, and some of our seeds have germinated, so we must have done an OK job at making this blocking mix!
Let me tell you, it is kind of stressful when you are just learning something and not really sure what you are doing, and you have to wait to find out if your efforts are successful or not.
My advice is to hang in there and trust yourself. You got this.
This is a learning experience and mistakes will be made. It’s important to learn from those mistakes and do better next time.
Just take a look at our seedlings and know that you can do the same!
Do you use Eliot Coleman’s blocking mix recipe or do you use a different recipe for your blocking mix? Please let us know in the comments below.
Stay Local,
Kathy & Jon
your friendly neighbourhood growers
I ordered the “base fertilizer” you suggested. When it arrived I was disappointed to see it included soy. While it’s organic, I prefer to eliminate soy because it’s an endocrine disrupter. I have no idea how much the plants would take up of it, or how they process it, therefor not knowing it’s residual effects. I like to err on the side of caution. Do you happen to know the ratios of the fertilizers used in Elliot’s recipe? TIA
Sorry you were disappointed with the organic base fertilizer we recommended. The ratios in Eliot’s fertilizer are equal parts blood meal, colloidal phosphate, and greensand. You will need a total of three cups of this base fertilizer for Eliot’s soil-blocking mix. Essentially, one cup of each blood meal, colloidal phosphate, and greensand will yield enough for Eliot’s standard recipe. Hope this helps! Happy Gardening!
Do you recommend a certain size of soil blocks to begin with? I have had a harder time finding the 20 block maker than the 8 block maker ones. Is it okay to start with the larger size?
It’s ok to start with a bigger size soil block maker. You just won’t have to pot up your starts as early. Honestly, when you’re just getting started, use what you have and go for it! The most important thing to do is start!