Substrate Preparation

Preparing the bulk substrate your mushrooms will fruit from — CVG, straw, masters mix, and pasteurisation methods.

What Is Bulk Substrate?

Bulk substrate is the nutrient-poor growing medium that you mix with your colonised grain spawn in the monotub. Unlike grain (which is nutrient-rich and must be sterilised), bulk substrate only needs to be pasteurised because its low nutrient content discourages contamination.

The substrate provides structure, moisture retention, and a large surface area for the mycelium to colonise and produce mushrooms from.

CVG — Coco Coir, Vermiculite, and Gypsum

CVG is the go-to substrate for cubensis and most beginner-friendly mushroom species. It is cheap, easy to prepare, forgiving of mistakes, and highly resistant to contamination.

Recipe (enough for one 54–66 qt monotub)

Bucket Tek Preparation

The bucket tek is the simplest and most popular method for preparing CVG. You just pour boiling water over the dry ingredients in a bucket and let it sit.

1. Get a clean 5-gallon (20 L) bucket with a lid. Wipe the inside with isopropyl alcohol or give it a good wash with hot soapy water and rinse well.
2. Break up the coco coir brick. If it is a compressed brick, break it into smaller chunks with your hands. Place it in the bucket.
3. Add the vermiculite and gypsum. Pour them on top of the coir. You do not need to mix yet — the boiling water will do most of the work.
4. Boil 4 litres of water. Bring it to a rolling boil.
5. Pour the boiling water over the dry ingredients. Pour evenly, trying to saturate all the dry material.
6. Seal the bucket with the lid. Close it tightly to trap the heat. The hot water will pasteurise the substrate (killing most contaminants) while hydrating the coir.
7. Wait at least 4–6 hours. Many growers prepare their substrate the night before and let it sit overnight (8–12 hours). The longer it sits sealed, the more thorough the pasteurisation.
8. Mix and check moisture. Open the bucket and mix thoroughly with clean hands or a large spoon. Break up any remaining coir chunks. Check field capacity (see below).

Field Capacity Test

Field capacity is the ideal moisture content for your substrate. Too wet encourages bacterial contamination; too dry and the mycelium will stall. The squeeze test is the standard way to check.

The Squeeze Test

1. Take a large handful of substrate and squeeze it firmly with one hand.
2. Perfect field capacity: A few drops of water drip out between your fingers — not a stream, just a few drops.
3. Too wet: Water streams or pours out when you squeeze. Solution: add more dry coir or vermiculite and mix thoroughly.
4. Too dry: No water comes out at all and the substrate feels crumbly. Solution: add small amounts of boiled and cooled water, mix, and test again.

Pasteurised Straw (for Oyster Mushrooms)

Oyster mushrooms (Pleurotus species) grow exceptionally well on pasteurised straw. This is a different substrate than CVG and is specifically suited to aggressive decomposer species.

Straw Preparation

1. Chop the straw. Cut wheat or barley straw into 5–10 cm (2–4 inch) pieces. A garden shredder, lawn mower (run straw through it on a tarp), or manual cutting with scissors all work. Shorter pieces pack better and colonise faster.
2. Soak in hot water. Fill a large container or bathtub with hot water (65–80°C / 150–175°F). Submerge the straw and hold it under water with a weight. Maintain the temperature for 60–90 minutes. This is the pasteurisation step.
3. Drain thoroughly. After pasteurisation, drain the straw in a mesh bag or colander until it stops dripping freely. The straw should be moist but not waterlogged — similar to the squeeze test for CVG.
4. Cool to room temperature before mixing with grain spawn.

Masters Mix (for Gourmet Mushrooms)

Masters mix is a supplemented substrate used for gourmet species like Lion's Mane, Shiitake, King Oyster, and Maitake. It provides more nutrition than CVG or straw, which these species need to produce substantial fruits.

Recipe

Preparation

1. Hydrate the pellets. Break down the compressed pellets by adding water and mixing. The pellets will absorb water and crumble into sawdust and hulls. Aim for field capacity.
2. Mix thoroughly. Combine the hydrated sawdust and soy hulls evenly.
3. Bag and sterilise. Masters mix must be sterilised (not just pasteurised) because the soy hulls are nutrient-rich and will support contamination if only pasteurised. Pack into autoclavable grow bags with a filter patch and sterilise at 15 PSI for 2.5 hours.
4. Cool completely before inoculating.

Substrate Quick Reference