Urban Spore

Bulk Substrates

Bulk Substrates

king oyster mushrooms

What Is Bulk Substrate?

A bulk substrate is a material used for the cultivation of mushrooms. A bulk substrate is combined with an inoculant, often in the form of grain spawn – which will colonise the substrate before fruiting.

Mulch, straw and hardwood are the primary materials used as a bulk substrate. These are typically low in nutrient value to allow for the growth of the mycelium without encouraging the formation of competing pathogens.

Substrates can be enriched with supplementation to increase the availability of nutrients, this will often lead to faster colonisation and higher yields. Supplemented substrates will often require sterilisation.

masters mix

Straw & Mulch

Straw and mulch are excellent materials to use as a substrate for growing oyster mushrooms, the paddy straw mushroom & psilocybe cubensis among many others.

Pre-cut organic sugar cane mulch can easily be obtained from most hardware stores and garden centres.

Straw can easily be pasteurised with either hot water pasteurisation or cold water lime pasteurisation methods. Pasteurisation is very effective with straw & mulch.

Straw & mulch are often supplemented with small amounts of spent coffee grounds or bran to promote larger yields, this may increase the risk of contamination. 

mycelium growing on sugar cane mulch


Many species of fungi like lion’s mane, reishi and turkey tail are hardwood loving. Although some of these species may slowly colonise straw and sugar cane mulch, they still require a diet of hardwood to produce fruits.

Often the easiest way to purchase hardwood is in the form of hardwood fuel pellets, often used for smoking meats available from barbecue supply stores. These pellets can be weighed and rehydrated with the appropriate amount of water. 

Alternatively ekologs compressed hardwood logs or sawdust direct from the timbermill can be used.

Hardwood substrates are frequently heavily supplemented to assist with growth, for this reason they are usually heat sterilised. 

However, some growers have reported success with using a lesser amount of supplementation and treating hardwood pellets with hot water lime pasteurisation while increasing the spawn ratio to compensate.

hardwood fuel pellets


Kitty litter paper pellets are inexpensive and simple to use. They can be used to make a low-tech spawn substitute without any additional equipment. With paper pellets you will be able to expand grain spawn from one generation to another ensuring that you’ve always got some spawn on hand to keep the greenhouse fruiting all year round.

paper pellets for mushroom growing

Coco Coir & Vermiculite

Coco coir and vermiculite are not often used for the cultivation of gourmet mushrooms due to their low nutrient content in comparison to hardwood, straw or mulch.

For this reason, when using coco coir and vermiculite it is essential to add supplementation or a very high spawn to substrate ratio of 1:1.

Coco coir and vermiculite are able to contain a lot of hydration while allowing for air flow, these qualities in conjunction with it’s low risk of contamination make it a popular candidate for some cultivators.

bulk substrate


Magnificent manure isn’t as smelly as you may be thinking. Bags of aged manure are available from the garden centre or hardware store few a few dollars and can be used to enrich sugar cane mulch for the cultivation of gourmet mushrooms.

Manure is also used in the production of mushroom compost used for growing agaricus or button mushrooms. Manure and straw are allowed to partially decompose for a few weeks before being pasteurised and inoculated with agaricus spawn.

This method is employed for the cultivation of secondary decomposers.

manure for mushroom growing


Supplementation can be added to bulk substrates to increase the availability of nutrients for faster colonisation and larger yields. This is particularly common with hardwood loving varieties such as lion’s mane which can struggle to fully colonise a substrate without supplementation.

Substrates are often supplemented with coffee grounds, bran, soy hulls or alfalfa amongst other similiar products. One of the most popular substrate mixes is the masters’ mix consisting of 50% soy hulls.

Generally heavily supplemented substrates are sterilised in a pressure cooker for 90 minutes at 15PSI before being inoculated with grain spawn. 

However, some growers have reported success with using a much lesser amount of supplementation and treating hardwood pellets with hot water lime pasteurisation while increasing the spawn ratio to compensate.

Some species like shiitake will benefit from being grown on a hardwood bulk substrate with a lesser amount of supplementation. 

Soy hulls for mushroom growing

To Pasteurise Or Sterilise?

We often talk about pasteurisation and sterilisation when prepare substrates for growing mushrooms. These two terms are distinctive and cannot be interchanged.

Pasteurisation refers to processing the substrate with a method that removes large quantities of competing pathogens from the substrate while preparing it for the mycelium.

Sterilisation refers to processing the substrate under extreme heat as to completely eliminates all competing pathogens.

Although sterilisation may appear to be the more effective choice, this will be dependant on a number of factors. Heavily supplemented substrates should always be sterilised due to their propensity for developing contamination whereas substrates with little or no supplementation can be pasteurised.

Due to having no remaining micro-biome, sterilised substrates are at risk of further contamination when inoculated in the absence of a flow hood, for this reason methods requiring pasteurisation are preferable for beginners.

buy Presto 23 quart pressure cooker

Field Capacity

Field capacity refers to the ideal level of hydration of the bulk substrate. Once hydrated fully the substrate can be squeezed with gentle pressure within the palm of one hand without releasing excess water.

Hydration is important to the substrate successfully colonising. Mycelium will grow slowly or stop completely when the substrate is too dry or too moist.

Gradually add water while taking notes when working with new substrates to measure your results and what substrate composition yields the best results for you.

manure for mushroom growing

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