When growing mushrooms using a bulk substrate you will be mixing a colonized grain with a non-nutritious substrate. This method of growing allows you to achieve a greater surface area and in theory higher yields compared to growing from the bag. But how do you know how much substrate you need for the best results? Below is a general reference when growing P. Cubensis.
For the best chances of success, it is recommended that you use a lower spawn to substrate ratio. Using a one-to-one or one-to-two spawn to substrate ratio will result in faster colonization time and subsequently, a lower likelihood of contamination. While this is what I’ve found to be the fastest and most reliable ratio some growers use a higher ratio to get more from their grain spawn. A ratio of 1:4 all the way up to 1:8 will do the job but keep in mind that such a disproportionate ratio will result in longer colonization times and an increased risk of contamination.
Ratios in these terms refer to an amount in volume. For example, a ratio of 1:1 could mean 1 quart of spawn and 1 quart of substrate. This can cause some confusion when calculating the required substrate for a 3-pound bag of grain spawn. Luckily for us, one pound of colonized grain spawn equals roughly .89 quarts and the same is true for most coco-coir and verm-based bulk substrates. Therefore, if you are not concerned about being precise, a one-to-one ratio can be achieved by using one pound of grain spawn to one pound of the bulk substrate.
Now that you have an idea of the ratio you will use, the next step is to find the appropriately sized monotub.
What size monotub do you need?
It is a general rule of thumb that your bulk substrate performs best with a depth of 3 inches. That said, to achieve that depth using your desired substrate and spawn ratio, we will need to determine the total volume of your grain spawn and substrate. We are working on developing a calculator for this, however, in the meantime feel free to use this substrate calculator which will help you find the amount of spawn and substrate needed for any sized monotub.