gypsum is used as a buffer... Adding it won't move the substrate's PH in either direction but it will help in keeping the PH at a level conducive to mushroom growth for a longer period of time. As a substrate ages and breaks down due to being consumed by the mycelium, the substrate gradually becomes more acidic. Contaminates thrive in acidic environments so higher PH levels help to limit/prevent competitors from getting a foot hold in your substrate.
Lime on the other hand can be used to raise the substrate's PH and make it more basic. Lime is often used when you are using peat, for example, in your substrate or casing layer which had been pretty common back in the day... Peat has a PH that is well below neutral (mid 4s) so lime would be added to increase the PH to levels that are optimal for mycelial growth. Peat has been replaced with products like coir which has a neutral PH. I would only consider using lime when I am dealing with substrate components that have a lower PH.
When growing cubes, you want to have a neutral PH, somewhere in the high 6 to low 7 range, for optimal growth. You want to keep it in that optimal range for as long as possible. Starting the spawn run slightly higher than neutral isn't terrible because as the substrate breaks down, the PH will begin to drop. When you add the gypsum, it keeps the PH from dropping as quickly without initially raising it to a level that is not conducive to growth.
One of the main reasons that substrates contaminate in later flushes is that the PH level has dropped and conditions are favorable to the contaminates that thrive in acidic environs. As available nutrients decrease and PH levels decrease, the mushroom mycelium weakens... It is always a fight between the mushrooms and the contaminates. There are always contaminate spores in your grows just waiting for their opportunity to germinate and colonize the nutrient source. When you mix live mycelium for the spawn run, it can colonize the nutrient source faster than the contaminate can germinate and start colonization, PH levels help with this race as well as the fact that our myc doesn't have to germinate and also the amount of spawn we are adding... This is also why field capacity is so important... You want the substrate consistency to be ideal for mushroom growth so it can out run any potential contaminates.
I kinda veered off course a little but I was trying to give you guys the big picture of why gypsum is beneficial so there is an understanding of why it works and not just that it does...
Edited by PJammer24, 15 January 2020 - 10:12 AM.