I’ve been reading and watching a LOT of information in the past couple of months about growing mushrooms (any kind). There are forums that have been running for decades, some new ones, and lots of videos on YouTube.
I’ve learned in that time that there is rather a lot of misinformation, beliefs, superstitions, and some very good information based on science and experiments.
Mushrooms often grow on recently dead wood, some grow on manure and wood, some grow on the roots of living trees in a symbiotic relationship. A few even grow on dead bugs or animals. For some reason, people imagine that they “need” all kinds of nutrients that aren’t found in wood. They really don’t. They evolved to eat wood. (Same thing goes for house cats and meat. They just need meat, no veggies, no grain, no potatoes. They eat plants to help themselves vomit.)
When people want to grow mushrooms at home or for a business, they try to find methods and recipes for “substrates” (mixes of growing materials that the mushrooms like to eat) that are easy to make and cheap to source. Others experiment with all kinds of substrates, and throw in things like bee pollen and coffee grounds. Some people absolutely swear that by adding ____ to their mix, they get great results, and have done so for years. Those beliefs don’t really help advance the science and facts of growing mushrooms.
Two items in particular get mentioned often in both spawn and substrate recipes: Lime and Gypsum. The problem with these terms is that they are generic and have multiple meanings, and that leaves a lot of people confused about what is really meant. They are added to “help the pH”, though most aren’t sure how exactly.
Here are some terms and factual meanings:
pH is a measurement of hydrogen ion concentration, on a scale of 0 to 14, 7 is considered neutral (neither acid or base). Below 7 is acidic, higher than 7 is alkaline.
Gypsum is the dihydrate of calcium sulfate (CaSO4 2H2O). Gypsum does NOT usually affect pH. This is contrary to a lot of postings about why it is added to mushroom grain spawn or substrate. Instead what gypsum provides are calcium and sulfur, which mushrooms may like. It is added to grain spawn (rye, oats, wild bird seed, etc) to keep the grains from sticking together, and to provide calcium. Calcium carbonate might also be used as a calcium supplement.
Gypsum is typically used in agriculture to help break up clay soil, provided one also adds significant organic matter. Most gardens don’t need it, just more organic matter (compost).
Here is a link to a PDF by Washington State University about what gypsum does and does not do for soil: https://s3.wp.wsu.edu/uploads/sites/403/2015/03/gypsum.pdf
“Lime” can mean several things, because there are different chemicals each called “lime”. One kind is used to buffer pH, another is used to pasteurize or sterilize straw substrate, other kinds should not be used for mushrooms at all.
The kind of lime usually added to adjust pH is calcium carbonate (CaCO3), and the carbonate ion part is what raises the pH (makes more alkaline) by neutralizing acids (hydrogen ions). This should be used sparingly in mushroom substrate. Most mushrooms apparently like the pH to be fairly neutral, or just slightly alkaline. 2% lime CaCO3 is considered good for oyster mushroom substrate, according to this research abstract:
Here are the various kinds of “lime”:
Calcium Carbonate “lime” (CaCO3) is NOT for pasteurizing – this is limestone, marble, chalk, eggshell, coral or oyster shell. This material is used as a pH buffer in substrate and media. This type of lime does not cause a major change in pH, but acts as a buffer. This means it holds the pH steady as the mushrooms grow. This is the preferred amendment for substrate to ensure the substrate does not go too acid during the growth cycle.
Calcium Hydroxide Ca(OH)₂ or Hydrated Lime or Quicklime will create a pH increase but is used primarily for pasteurizing or sterilizing substrate. Don’t add this as a nutrient.
High Magnesium Lime, Builders Lime, Dolomite Lime, or just Dolomite CaMg(CO3)2 has high magnesium and WILL NOT WORK for growing mushrooms. It will stunt growth (according to the link above).
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Wood ashes, bleach, or soap can also be used to pasteurize straw also. The idea is to kill off bacteria and molds that can hinder your mushroom growth.
This website has great info on growing mushrooms at low cost: