How to Mix CVG Bulk Substrate for Mushrooms: Step-by-Step

About 1 hour active work, plus colonization time

CVG stands for coir, vermiculite, and gypsum. It is the cheapest, most forgiving bulk substrate in home cultivation, and it works for almost every common species you will run in a monotub: cubensis-style genetics, every oyster, wine cap, and most wood lovers that fruit on bulk. CVG holds water like a sponge, drains air pockets enough that the mycelium can breathe, and pasteurizes with nothing more exotic than a kettle of boiling water.

This tutorial covers the full recipe from dry components to a finished tray of substrate at field capacity. You will end with roughly 5 quarts of hydrated CVG ready to mix with grain spawn, which is enough for one 54 quart monotub at a 1:2 spawn-to-bulk ratio. By the end you should be able to hit field capacity on the first try, know how to test for it, and know the two or three small adjustments that turn a wet sloppy mix into a clean run.

What you'll need

Materials

Coir, 650 g compressed brick (one standard brick rehydrates to about 5 L)
Coarse vermiculite, 4 quarts dry (~4 L)
Gypsum (calcium sulfate), 2 to 3 tablespoons
Boiling or near-boiling water, ~5 L total
Filtered or pre-boiled water, ~500 mL reserve, for moisture adjustment
70% or higher isopropyl alcohol, plus a spray bottle and lint-free wipes
Nitrile or latex gloves

Equipment

5 gallon bucket with a snap-on lid, or large clean tote, for hydration
Kettle or stockpot, for boiling water
Long-handled spoon or sterilized fork, for breaking up the coir brick
Reliable kitchen scale, for weighing gypsum and the coir brick if it is uncut
Measuring cup or quart jar, for portioning vermiculite

Step 1: Pick the right components and skip the bad ones

Half the contamination problems beginners blame on technique actually trace back to the wrong product on the shelf at the hydroponics store. CVG is unforgiving about what you put in it. Before you mix anything, confirm each component is the right thing.

Coir (coconut coir). You want plain compressed coco coir bricks, sold as a soil amendment or hydroponics growing medium. The standard size is a 650 g brick that rehydrates to about 5 quarts. Avoid any product labeled with added nutrients, fertilizer, mycorrhizal inoculants, or buffering agents.“Buffered” coir has been pre-rinsed with calcium nitrate and is fine for mushrooms; “charged” coir or anything from the cannabis aisle that mentions feeding the medium is not. Read the label. The ingredient list should say “100% coconut coir” or “coco coir” and nothing else.

Vermiculite. Use coarse-grade or medium-grade horticultural vermiculite. Fine grade clogs and packs into a paste; the coarse stuff has the right air pocket geometry for a mycelium to colonize. Vermiculite is mined and graded in three sizes (fine, medium, coarse); coarse is what gardening stores stock as a soil amendment. Pure vermiculite has no additives. Skip any product blended with perlite, peat, or soil conditioner.

Gypsum (calcium sulfate, CaSO4).Sold as a soil amendment or in brewing supply stores under names like “Brewing gypsum” or “Cal-Sul.” Get food-grade or agricultural-grade. Do not substitute lime, hydrated lime, dolomite, or pickling lime.Those are calcium carbonates and they raise pH past what mycelium can tolerate. Gypsum is calcium sulfate; it buffers pH gently around the neutral range, adds calcium for cell wall strength, and helps the substrate hold structure once mycelium runs through it. The wrong calcium product is the single most common substitution mistake; if a recipe calls for gypsum and you use lime, you will sterilize the tub.

Water. Tap water is fine for hydration in most regions. If your tap water is heavily chlorinated, fill a clean bucket and let it sit uncovered for 24 hours so the chlorine off-gasses, or boil it, or use filtered water. Chloramine (a different disinfectant some municipalities use) does not boil out; if your municipality uses chloramine, switch to filtered or distilled water for the small reserve volumes used to adjust hydration in Step 4.

Safety note before you boil water

You will be pouring 5 L of boiling water onto a coir brick in a sealed plastic bucket. Cheap thin-walled buckets (the kind that ship free with hardware purchases) can warp or crack under sustained 100°C contact. Use a 5 gallon HDPE food-grade bucket or a large stainless tote. Pour slowly; the brick will release a cloud of steam as it expands. Wear long sleeves, eye protection, and oven mitts. Pour away from you, never toward your body.

Once you have all four components confirmed and your bucket is ready, you can move to mixing.

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