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 140-175°F (60-80°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 practical 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 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 is the shredded fiber of coconut husks. It holds many times its weight in water but does not decostores 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 also common additives.
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 wood loving mushrooms and pleurotus ostreatus (Oysters).
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.
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.
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 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 is sometimes used in bulk substrates in order to increase the amount of water retention. It usually added at 10-50% by volume.
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.
- Pasteurization in mason jars
Mason jar pasteurization is a good method for pasteurizing up to seven quarts at a time. The advantages to using this method are more precise control of moisture(hydrate the substrate to a little more than field capacity as some moisture will be lost in the process) and temperature control, as you can measure the temperature in each jar.Using a canning pot with the suspension racks, fill seven quart jars with your substrate mix. Then fill the pot until it is a quarter full, and put the lid on. I do not mind water dripping back into the jars as it replaces some of the lost moisture. Heat to 165-180(I prefer 180) for 90 minutes. Let cool until the substrate mix is BELOW 80 DEGREES before spawning your jars.
Always spawn to the substrate as quickly as is allowed by the temperature of the substrate, as it is a race between your mycellieum and whatever contaminants remain in the substrate.
Follow the same proceedures as above, but you are using a pillowcase full of substrate. This method allows for a larger bulk to to be processed, but is at a disadvantage when it comes to moisture regulation. Make sure to check your subsrate's moisture content after using this method. Dry weight vs wet weight is the best way, but the squeeze test is a good rule of thumb as well. To use the the squeeze test, as you load the sub, squeeze every third handfull. The sub should give up a short (less than one second) sream of water, followed by drops, tapering off to nothing. This entire water loss should take no more than five seconds until little or no water can be squeezed out of the substrate. Pressure applied to the squeeze should be the equivilant of a firm handshake.