Bulk substrate (2024)

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.


  • 1 Pasteurization vs sterilization
  • 2 Common bulk substrates
    • 2.1 Manure/Compost
    • 2.2 Coconut coir
    • 2.3 Straw
    • 2.4 Hardwood
  • 3 Supplements
    • 3.1 Gypsum
    • 3.2 Coffee grounds
    • 3.3 Chicken manure
    • 3.4 Vermiculite
    • 3.5 Worm castings
  • 4 Guides
    • 4.1 Pasteurization
    • 4.2 Complete growing guides

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[]

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.

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 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.



  • 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.

Complete growing guides[]

Bulk substrate (2024)


How much bulk substrate to spawn? ›

Spawn and bulk substrate should be mixed at a ratio if 1 parts spawn to 2/3 parts bulk substrate. If you have to much bulk substrate it could take to long to colonise and contamination could set in.

What is the ratio of bulk to substrate? ›

The more spawn used the faster the bulk substrate will colonize. Recommended spawn ratios are usually no less than 20% (1 part spawn to 4 parts substrate).

What happens if there is not enough substrate? ›

If an inhibitor is competitive, it will decrease reaction rate when there's not much substrate, but can be "out-competed" by lots of substrate. That is, the enzyme can still reach its maximum reaction rate given enough substrate.

How long does it take for mycelium to colonize bulk substrate? ›

Once inoculated, the mycelium takes upwards of 10 days to jump off the spawn into the substrate. At the same time, the wood is losing any ability to wall off an infection. There is no harm in inoculating stumps, logs, or wood chips immediately after harvesting.

How wet should my bulk substrate be? ›

Your substrate should be slightly acidic, with a PH between 5 and 6.5. (Some mushrooms, such as oyster mushrooms, can withstand a PH of up to 8.) A minimum moisture content of 50-70% is essential for your substrate.

How long can bulk substrate sit? ›

Store your 50/50 substrate in a cool and dry location. 50/50 substrate can keep for up to 4 months in these conditions. There will be no need to rehydrated or sterilize the products a second time. If you need to store the product longer it can be refrigerated for up to 6 months and frozen for up to 1 year.

What does increasing the amount of substrate do? ›

Substrate concentration: Increasing substrate concentration also increases the rate of reaction to a certain point. Once all of the enzymes have bound, any substrate increase will have no effect on the rate of reaction, as the available enzymes will be saturated and working at their maximum rate.

How to calculate Vmax and km? ›

How to determine Km and Vmax. Km and Vmax are determined by incubating the enzyme with varying concentrations of substrate; the results can be plotted as a graph of rate of reaction (v) against concentration of substrate ([S], and will normally yield a hyperbolic curve, as shown in the graphs above.

How to increase enzyme activity? ›

Increasing the temperature of the enzymatic reaction, speeds the reaction as it increases the enzymatic activity increases, similarly the reverse happens and the enzyme activity and reaction speed decrease upon decreasing the reaction temperature.

How to tell when bulk substrate is fully colonized? ›

It will take approximately 5-10 days for the substrate to completely colonize with mycelium (you will be able to tell if it has completed colonization when the substrate has turned completely white).

How long to soak bulk substrate? ›

Before soaking the substrate, check the pH to make sure it falls within the required range. Submerge the substrate in the vinegar solution and let it soak for about 16 to 20 hours. After removing the substrate, let it drain for about an hour, and it's ready for inoculation.

How do you know when mycelium is fully colonized? ›

You can tell when mycelium is fully colonized when the entire substrate (such as a jar or a bag) is covered with a dense, white, and fuzzy network of mycelial threads. There should be no visible uncolonized areas or contaminants, and the mycelium should appear healthy and vigorous.

How much substrate for 3lb spawn? ›

We recommend using 30%-50% grain spawn as compared to the weight of the substrate. So, this 3lb substrate will need about 1.5lb of grain spawn. 9) Make sure the substrate is evenly distributed in your tub and the surface is flat.

How much spawn for monotub? ›

We frequently get asked the question, “how much grains and substrate should I purchase to spawn a gourmet monotub?” The answer is a proper ratio in your vessel of choosing. We feel the best ratio is 1:2 spawn to sub.

How do you know when bulk substrate is fully colonized? ›

It will take approximately 5-10 days for the substrate to completely colonize with mycelium (you will be able to tell if it has completed colonization when the substrate has turned completely white).

When to fruit bulk substrate? ›

Once your substrate is fully colonized you will need to decide how you want to fruit it. This can be achieved by leaving the substrate in the bag to fruit or moving it to an external location like a plastic tote, terrarium or mono tub.


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