The Science of DIY Mushroom Substrates

What our research has revealed about growing specialty mushrooms indoors on different materials.

The spaces used to grow mushrooms can be as small as a closet to a retrofitted room, garage, or basement, to a modified warehouse or a building specifically designed for mushroom growing. Cornell Small Farms Program

Mushroom cultivation is an art as much as a science, and for many growers, it starts with a simple ready-to-fruit block. When folks are just starting indoor mushroom production, we often recommend getting a few ready to fruit mushroom blocks. These are available from most mushroom suppliers, and while everyone has their own substrate recipe and block size, the concept is universal: blocks are formulated to provide a good source of carbon and nitrogen, come pre-inocuated with the mushroom species and strain of your choice, and require minimal care beyond maintaining the right temperature and humidity range. Blocks are cost effective, and used exclusively by some growers. But they are not the only way to grow mushrooms indoors.

Indoor mushroom production is one of the most creative, adaptable, and scalable agricultural enterprises. There are manifold experimental tweaks one can try to improve yield, efficiency, or circularity in production, and there is always something new to learn for growers of any experience. Enter DIY mushroom substrates.

Specialty mushrooms (anything cultivated that is not an Agaricus such as button, cremini, and portbello) are decomposers. They need a carbon source to feed on in order to grow and reproduce. The carbon source can be almost anything, especially if you are growing a generalist species like oyster mushroom, which can grow on cardboard, straw, crop residues, sawdust, old clothes, composted manure, bagasse and more (logs and woodchips are the carbon source for outdoor production). Amendments, such as biochar, minerals, and nitrogen, are typically mixed in with the carbon source; these can boost yield and chemical expression of the mushrooms. When you buy a ready to fruit block, the supplier’s recipe optimizes the carbon source, the C:N ratio, and the specific amendments for their strains. If you make your own mushroom substrate, you can optimize it for your needs, and on materials you have on hand.

The two most common substrate ingredients are straw and sawdust, sometimes mixed in different ratios. There is some research that shows the type of straw (e.g., rice vs oat vs hay) or sawdust (oak vs mulberry vs eucalyptus) has an effect on the mushroom success and yield. If you use sawdust, check with a mushroom supplier that it is compatible with the mushroom strain you want to grow. In general steer clear of conifers! Straw is a safer bet, and is usually easier to come by. Whether you are using straw or sawdust as your primary substrate, you must sterilize it before you inoculate with mushroom spawn.

Spawn typically is a mix of sawdust and mycelium, the “body” of the fungal organism that decomposes a substrate, grows – and when conditions are right – produces the reproductive structures we eat: mushrooms. The amazing thing about mycelium is that you can take a piece of it, introduce it to an appropriate substrate, and it will grow. Then you can use as spawn to inoculate more things to produce mushrooms. In outdoor mushroom production, we drill holes into logs, fill them with sawdust spawn, and get mushrooms a year later. For indoor production, we mix spawn and substrate together in a bag or bucket and get mushrooms weeks or months later. Spawn needs the right conditions, and if you add it to a straw substrate that hasn’t been sterilized, it will have to compete with bacteria and fungi that are already established…you might not get mushrooms.

Sterilization of the substrate can be a labor-intensive process (and is one reason why some growers prefer to just use ready to fruit blocks). Sterilization can be done with an autoclave, or steam room, or simply by boiling the substrate before adding the spawn. The downside is that these methods require infrastructure and electricity. And if you have my luck, there is a chance of burning yourself on boiling cardboard. Luckily there is a low-tech option that does not require heat, pressure, or OSHA.

In 2021, my predecessor at Small Farms, Steve Gabriel, got a grant to explore a lime-based sterilization process for indoor mushroom production (the videos are now up on the Cornell Small Farms website!). To be clear we are talking about lime: calcium hydroxide produced from burning calcium carbonate (limestone) then adding water to make a caustic, alkaline material. Not lime: the citrus fruit. In an exemplary case of mushroom creativity, Steve tested this method of sterilization for four strains of oyster mushrooms with and without a nitrogen amendment.

We evaluated two strains of two species of oyster mushroom (Pleurotus citrinopileatus and ostreatus; “Golden” and “Blue”) for the project. Oyster is a versatile species, with a wide range of substrate tolerances and tones of culinary uses; I typically recommend it as a “starter” mushroom. Pan-seared in butter and served over salad is quite excellent.

In order to not be overly pedantic, assume that every tool, receptacle, bag, hand, and device mentioned below is sterile (or at the very least very clean). This minimizes contamination and is essential for indoor mushroom production:

We added 5 pounds of hydrated mason lime (<10% magnesium) to 80 gallons of water in a 100 gallon steel stock tank, then mixed slowly with a paint mixer until a pH strip showed the solution to be between 10 and 11. Next, 50 pounds of dry shredded straw was added in small portions with a pitchfork; each portion was fully submerged before adding more, and once the whole lot was soaking it was weighed down with a plastic grid and cinderblocks overnight. This process kills off most microorganism competitors that might be growing on the straw, so the field is wide open for colonization of our desired mushrooms.

The following day the soaked straw was laid on a wire mesh to drain – all the alkaline water was captured and returned to the stock tank – and the lime solution was neutralized back to pH 7-8 by adding citric acid. Ten-pound portions of the damp straw was mixed with half a pound of dry soybean hulls (as a N amendment) and half a pound of mushroom spawn, then the mix was tightly packed into plastic bags, subsequently perforated for airflow. All the bags were stored in a dark room to incubate for two weeks, with temperatures between 65-75°F and a constant flow of air. After two weeks, you could see the growing mycelium spread throughout the bags. It was time to fruit!

Slits were cut into each bag, and they were all moved into a “fruiting room.” Here, temperatures were kept at 75°C, humidity at 85%, and fresh air was regularly circulated in. Any fruiting room must have these conditions to maximize mushroom growth, and to minimize mold growth. The fruiting room was also equipped with blue spectrum LED lighting. It is a common misconception that mushrooms don’t need light to grow. They do! And there is a growing body of literature to support that the color and intensity of light in a grow room affects the structure, size, color, and chemistry of the mushrooms. Once the bags were set up in the fruiting room, they were checked on every other day for new mushrooms.

Harvesting mushrooms is all about time management, and depending on light, temperature, and mushroom strain, a few hours may be the difference between perfectly salable, and overly mature. You want to harvest when the mushroom cap is still curled under, and ideally before the mushroom has produced spores; as the mushroom ages, the cap flattens out and becomes less palatable. For this project, the mushroom bags were checked regularly so everything harvested was not past prime.

The results were delightful in both consistency and magnitude for all four stains of mushrooms. Take the Grey Dove strain for example:

GD Flush Dynamics; x-axis represents days since inoculation and the y-axis represents cumulative yield (kg). Connor Youngerman / Cornell Small Farms Program

There’s a lot to take in from this plot: the x-axis is the number of days since the bags had been inoculated (they were moved into the fruiting room 14 days after inoculation); the y-axis is the cumulative mushroom yield.

When you look at a given day, you see how much yield has been achieved to date, the higher the curve, the more yield we saw. Steep increases in the curve mean a big jump in yield. Thin lines are the yield curves for each mushroom bag, and the thick lines are the average yield curve for each group. Purple represents bags that had no amendments; green represents bags that had half a pound of soybean hulls mixed in.

So, what can we say about the yields and timing of the yields for all our mushroom strains?

1. Bags that had soybean hull amendments on average produced around twice as much yield as bags that didn’t. There was variability between strains, but this was generally true for all of them.
2.  Bags that had soybean hull amendments fruited sooner than those without amendments, from 4 to 10 days depending on the strain.
3. Bags that had soybean hull amendments reached their total yields sooner than those without amendments.

Based on this experiment, it is a no-brainer to recommend that you add soybean hulls to your DIY mushroom substrate. For just a little extra cost you can potentially double your yield and achieve the yield faster (so you can grow more mushrooms!).

It also raises a number of other research questions: how much would the yields change with a greater amount of soybean hull per volume substrate? Would we see the same trends if we added the soybean hull to a different substrate, such as sawdust? Can we get a similar yield boost with log grown mushrooms?

We are planning to keep pursuing mushroom research here at Small Farms, so stay tuned for more.