How to Make CVG Substrate (and Get Field Capacity Right)

CVG is a mix of coco coir, vermiculite, and gypsum. Here’s the recipe I use, how I prepare it, and the squeeze test that tells me when the moisture is right.

My CVG substrate recipe

For one batch, I use:

• 650 g coco coir—one standard compressed brick
• 8 cups vermiculite
• 1 cup gypsum for the traditional CVG version
• 4 US quarts of boiling water to start—about 3.8 litres

Approximate yield: 8–10 US quarts of finished substrate in my setup. Actual volume and water needs vary with the materials.

The important part: Four quarts is a starting point. Check the hydrated mix and adjust until a firm squeeze releases just a few drops.

Watch the CVG tutorial

Watch the full CVG tutorial on YouTube or follow the steps below at your own pace.

How to make CVG substrate, step by step

1. Gather the ingredients and a heat-safe container

Along with the recipe ingredients, you’ll need a heat-safe 5-gallon bucket with a lid, a way to boil and measure water, and something to mix with. I also wrap a towel around the bucket to help retain heat.

I wear disposable gloves for the hands-on mixing and squeeze test shown below.

A kitchen scale is handy for checking the coir weight, especially if you’re using loose coir or part of a larger block. The vermiculite and gypsum amounts above are measured by volume.

2. Add the ingredients and boiling water

Put the coir, vermiculite, and gypsum into your heat-safe container. Bring 4 quarts of water to a boil, carefully pour it over the ingredients, and close the lid. Protect yourself from hot water and steam, and use a container rated for the heat.

Leave the mix alone until the coir has hydrated and the contents are cool enough to handle. There isn’t a fixed cooling time in this recipe; don’t add grain spawn to hot substrate.

A note on “bucket pasteurization”: You’ll often hear that name for this method. As I explain in the video, pouring boiling water into a bucket does not precisely control the internal temperature and holding time. Treat this as the hot-water bucket method I demonstrate, not a claim that the substrate is sterile or has undergone a verified pasteurization cycle.

3. Break up the coir and mix thoroughly

Once it’s safe to handle, open the bucket and work through the mix. Break apart any large coir chunks, mix in the vermiculite and any pockets of gypsum.

Check more than one area of the bucket. An evenly hydrated batch is easier to judge than one with wet pockets and dry clumps.

4. Check field capacity: a few drops, not a stream

This is the part that matters more than following my water measurement exactly. Coir can vary from batch to batch, so the recipe gets you close; your hands finish the job.

Take a handful of cooled substrate, squeeze it firmly over the bucket, and watch what happens.

• A few drops: That’s the target I use.
• No drops, and the substrate feels noticeably dry: Mix in a little water, give it time to distribute, and test again.
• A stream of water: Mix in a little dry vermiculite, let it absorb moisture, and retest.

Make small corrections. You don’t want to bounce from too dry to too wet trying to fix everything in one pour. The goal is a moist substrate with structure, not a bucket of soup.

5. Prepare your tub and measure by volume

In the video I fit a piece of black trash bag inside the tub and trim it into a liner. This is optional. As the substrate shrinks, the liner can move with it, helping reduce the gaps along the sides where side pins can form.

For the setup I demonstrate, I use 1 part fully colonized grain spawn to about 1.5 parts hydrated substrate, by volume. For example, 1 quart of spawn would use about 1.5 quarts of prepared CVG.

I aim for a total mixed depth of roughly 2–4 inches. Your tub’s footprint determines how much mix you need, so you may not use the entire batch. If you need a growing container, here’s the monotub option I like to use.

6. Mix, level, and lightly cover exposed grain

Break up the healthy, fully colonized grain, add the substrate by the handful, and mix until the spawn is evenly distributed. Then level the surface and give it a light pat. Don’t pack it into a solid brick.

I usually finish with a thin layer of the same prepared substrate, just enough to cover exposed grains. This is an optional pseudo-casing layer, not a separate true casing recipe.

From here, follow the colonization and fruiting conditions for your chosen species and method. Making CVG is one preparation step; it doesn’t replace clean spawn or a suitable fruiting environment.

What do vermiculite and gypsum actually do?

Coir: the foundation

In this coir-based method, the grain spawn brings most of the nutrition. The bulk substrate gives the mycelium space to grow and holds much of the water available for fruiting. Coir already does those jobs well.

Vermiculite: a useful texture and moisture adjustment

Vermiculite is an expanded mineral made into lightweight, absorbent flakes. Think of it as rock popcorn.

Adding it changes how the mix holds water and how it feels. I’m aiming for a hydrated, loose substrate, not a dense, soggy brick. It also gives me a convenient way to correct a batch that ends up too wet.

Gypsum: included, but not magic

Gypsum is calcium sulfate, a source of calcium and sulfur. That doesn’t establish a yield benefit for this mix, and it isn’t a substitute for lime when you need to adjust pH.

I include it for traditional CVG, but I’d treat it as an experiment. Vermiculite’s texture and moisture adjustment are the more practical reasons I use this recipe.

Quick answers before you start

Do I need CVG, or can I use plain coir?

For a grow that is suited to a coir-based method, plain coir can work well. If you’re still learning the basics, I’d keep things simple. CVG gives you extra variables to experiment with once you’re comfortable managing your grow.

Can I inject liquid culture straight into this?

Not if you want to succeed! You should use fully colonized grain spawn, which is mixed into the cooled, hydrated substrate. Liquid culture and bulk substrate are different stages of the grow.

What if I have leftover substrate?

You can store the extra substrate sealed in the bucket and recheck field capacity before using it. Keep storage conditions and cleanliness in mind; a sealed bucket does not make the contents sterile or guarantee a shelf life. Making a batch sized to your next grow keeps things simpler.

Choosing your next mushroom?

Explore the liquid cultures and their growing guides to find a project that fits your space and experience. Check each species’ substrate needs before you start.

Explore the cultures

Keep learning: Watch my guide to monotub fruiting conditions.

Need supplies for a different growing method? My mushroom growing gear list separates coir-based materials from hardwood options so you can choose what fits your project.

Further reading: Penn State Extension on vermiculite’s water-holding properties and Iowa State Extension on gypsum and pH. These references explain the materials’ general properties; they do not establish a yield benefit for this CVG recipe.

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