One of the key elements of successful mushroom cultivation is the use of a suitable bulk substrate that your mushroom mycelia like. A bulk substrate is the material on which the mushroom mycelium (the vegetative part of the fungus) grows. It provides both a food source and a water source for your mushrooms to grow. In this post, we will focus on the use of bulk substrate for mushroom cultivation.

Why bulk substrate?

Bulk substrate is a great option for mushroom cultivation because it is relatively inexpensive, and easy to obtain. It is also a sustainable option as many of the materials used in bulk substrate can be sourced from waste materials. With proper pasteurization, inoculation, and care, bulk substrate can provide a reliable and consistent source of mushrooms.

What is bulk substrate?

Bulk substrate is a mixture of materials that provide the necessary nutrients and environment for the growth of mushroom mycelium. It can be made from a variety of materials, such as sawdust, straw, coffee grounds, or even cardboard. The bulk substrate you choose will depend on the variety of mushrooms you are trying to cultivate, as different species will prefer different food sources.

Which bulk substrate to use?

Some commonly used bulk substrates for mushroom cultivation are hardwood sawdust, coco coir, wheat straw, composted horse or cow manure. Supplements such as wheat bran, hydrated lime, spent coffee grounds, and gypsum can be used to improve the structure and quality of the mushroom final product.

Bulk substrate recipe

The first step to make bulk substrate for mushroom cultivation is to hydrate your chosen substrate to field capacity. Field capacity refers to the maximum amount of water that a substrate can hold without being water-logged. When unsure, the best practice is to UNDER-hydrate your bulk substrate to prevent any standing water from accumulating.

For hardwood sawdust, add a 1:1 ratio by volume of hardwood pellets to boiled hot water. If the sawdust is too wet, simply add some more pellets to balance moisture content. Some mushroom growers have found that a mixture of half hardwood pellets and soy hull pellets have worked really well, and coined in Master's Mix rightfully so. For coco coir, composted horse or cow manure, follow a similar strategy of adding small amounts of boiled hot water to your coco coir until you reach field capacity. For wheat straw, soak the straw in a bucket overnight. The next day, remove the straw and dry off before pasteurization.

You can supplement your bulk substrate by wheat bran, hydrated lime, spent coffee grounds, and gypsum. We recommend adding in the supplements after you have fully hydrated your bulk substrate so that you know the final total weight of bulk substrate you have. The following is recommended supplement ratios:

• Wheat bran: 5-10% of the total weight
• Spent Coffee Grounds 5% of total weight
• Hydrated Lime: 1% of total weight
• Gypsum: 1% of total weight

For instance, if you are making 5 lbs of hydrated bulk substrate, that weights approximately 2268 grams. 1% corresponds to 22.68 grams; 5% corresponds to 113 grams, 10% corresponds to 226 grams, and 15% corresponds to 339 grams. Weigh out your supplements and thoroughly mix it into your hydrated bulk substrate.

Favorite Bulk Substrate Recipes

Here's a quick run-down of our favorite bulk substrate recipes for growing mushrooms at home.

Master's Mix

• 1 lb hardwood pellets
• 1 lb soy hull pellets
• 1.6 oz gypsum
• 2 quarts hot water

An easy way to measure the water is add the same volume of water as to the volume of pellets.

Hardwood Pellets

• 2 lb hardwood pellets
• 1.6 oz gypsum
• 2 quarts hot water

Supplemented Hardwood Pellets

• 2 lb hardwood pellets
• 3.2 oz wheat bran
• 1.6 oz gypsum
• 2 quarts hot water

CVG - Coco Coir/Vermiculite/Gypsum

• 650g brick of coco coir
• 2 quarts of vermiculite
• 1 cup gypsum
• 4.5 quarts hot water

Dung-loving CVG

• 650g brick of coco coir
• 65g Black Kow Composted Cow Manure
• 2 quarts of vermiculite
• 1 cup gypsum
• 4.5 quarts hot water

The easiest way to make bulk substrate is to measure out all dry the ingredients in a tub or mushroom grow bag. Then add the appropriate amount of hot water and watch your bulk substrate expand! After an hour or two, it should be fully hydrated. Mix thoroughly.

Pasteurization

Next, pack your substrate into mushroom grow bags for pasteurization or sterilization, which will kill off any unwanted bacteria or fungi that may be present in the substrate. Whether you will need to pasteurize or sterilize depends on what is in your bulk substrate. If you choose to supplement your bulk substrate with wheat bran or spent coffee grounds, you will need the more thorough sterilization to completely kill off any potential contaminants. Pasteurization is the less stringent method that will only partially kill off some of the contaminants, but not all. However, the surviving microorganisms are beneficial to your mushroom mycelia and will prevent unwanted contaminants such as trichoderma from taking over. You need to sterilize bulk substrate that is supplemented with wheat bran or spent coffee grounds because these supplements have much higher nutritional content that is a readily accessible food source for your contaminants to take advantage of.

See our blog post on pasteurization.

In conclusion, bulk substrate is a great option for mushroom cultivation. It provides the necessary nutrients and environment for the growth of mushroom mycelium. It can be made from a variety of materials, such as sawdust, straw, coffee grounds, or even cardboard. Proper pasteurization, inoculation, and care are key to success with bulk substrate. With the right conditions and a little patience, you will be rewarded with a bountiful harvest of delicious mushrooms.

Updated November 15th, 2023