Bulk substrate

Bulk substrates are moderately nutritious materials used in mass mushroom cultivation. Bulk substrates are often used in conjunction with a pre-colonized grain spawn which is used to inoculate the bulk substrate.

Pasteurization vs sterilization

When moderately nutritious bulk substrates are pasteurized at 140-160°F (60-71°C), some beneficial micro organisms, mainly bacteria, stay alive, inhabit the substrate and guard it against other, more aggressive micro organisms. This resistance to contamination is the reason bulk substrate can be inoculated with spawn in open spaces without taking special sterile precautions. Even with these micro organisms, mycelium will still able to grow on this substrate though.

If you sterilize the bulk substrate it becomes as nearly as susceptible to contaminants as highly nutritious spawn substrates like rye berries and brown rice. This means that you would have to inoculate the big amount of bulk substrate in sterile conditions, such as in front of a laminar flow hood and the substrate must stay in sterile conditions until it is fully colonized. This is not practicable if for large amounts of substrates.

The exception to this rule is when large, sterile spawn bags are used, but in that case, the cultivator is better off using entirely highly nutritious spawn substrate instead of the moderately nutritious bulk substrate.

Common bulk substrates

Manure/Compost

Manure is the aged, dried excrement of horses, cows, elephants, etc. It is one of the most effective bulk substrates for dung loving species like psilocybe cubensis, panaeolus cyanescens and agaricus bisporus (Portobello). It is usually cheap or free if it can be located. Many serious cultivators will compost their manure with other additives in order to produce an optimal substrate.

Coconut coir

Coconut coir is the shredded fiber of coconut husks. It holds many times its weight in water but does not decompose for years. It is PH neutral and inexpensive. Hydroponics stores and hardware stores sell it in compressed bales & pet stores sell it in dried bricks. Coir is very low in nitrogen, so coffee grounds or blood meal are common additives.

Straw

Wheat straw is commonly available in farm/fleet and crafts store. It is extremely inexpensive and relatively easy to work with. Due to its high nitrogen content and porous texture, straw makes an excellent bulk substrate for dung loving mushrooms and pleurotus ostreatus (Oysters).

Hardwood

Some mushrooms, such as lentinula edodes(Shiitake) and psilocybe cyanescens, thrive on hardwood based bulk substrates. Wood holds water well and provides a nutritious growing medium. Softwoods (needle bearing trees) are not suitable as substrates since they contain natural fungicides, but the majority of softwoods used for construction will have those chemicals bleached out.

Supplements

Gypsum

Gypsum is used to improve the structure of the bulk substrate and to act as a pH buffer. It is usually added at 5-10% by volume.

Coffee grounds

Used coffee grounds are a common additive to nitrogen deficient substrates (like coconut coir). Some commercial growers even grow purely on a coffee based substrate. When supplementing another substrate, coffee grounds are often added at 15-20% by volume.

Chicken manure

Chicken manure is not suitable as a bulk substrate due to its excessive nitrogen content and muddy consistency, but it is very effective as a supplement to other bulk substrates.

Vermiculite

Vermiculite is sometimes used in bulk substrates in order to increase the amount of water retention. It usually added at 10-50% by volume.

Worm castings

Although worm casting (aka worm poop) can be used as a bulk substrate by itself, it is most commonly used as a supplement to another bulk substrate since it turns into mud when it is hydrated.

Guides

Pasteurization

• Pasteurization in mason jars
• Pillow case pasteurization
• Hot bucket pasteurization

Complete growing guides

• Growing with bulk substrates