The optimal moisture level depends on the substrate composition and method of raw material processing.
If the main ingredients are cereal straw mixed with legume hay, a good yield is achieved when the substrate’s moisture is within 68-71%.
When pasteurizing the substrate with steam in a tunnel:
- A sufficient moisture level is 69-72%. It is acceptable to raise it to 73-74% if the block colonizes without contamination.
- The minimum moisture level for even a minimal yield is around 66%.
At 65%, a weak first flush is likely, but a second flush will definitely not occur.
When pasteurizing the substrate with hot water, a sufficient moisture level is 67-69%.
If it is lower, it leads to reduced yield.
If the level is above 71-72%, contamination almost always occurs. Generally, uncolonized spots and pooling water appear in the corners of the bag, and primordia often do not form.
In tunnel pasteurization, sunflower seed hulls can be added to the straw mix, but this doesn’t change the optimal moisture level.
Take 50 grams (1.8 oz) of substrate and put it in a microwave.
The first sample is experimental—you need to determine the correct microwave setting so that the sample dries completely without charring. Don’t forget to account for the weight of the dish or plate used for weighing.
First, set the microwave to the shortest time on the "Reheat" setting. Take the substrate out, and test its texture. The straw (or other raw material) should feel dry and slightly crispy. If it’s still moist, heat again for a short time. Repeat this until the substrate dries and rustle when fluffed up.
Record how many minutes were required to reach this state.
Next, weigh the sample and put it back in the microwave on the lowest time and power setting.
Then, take it out and gently stir with a dry stick. Check for any signs of charring. If there are none, weigh the sample again and microwave it once more.
If any charring appears, then the previous weight measurement is the correct one.
If no charring is visible, weigh it again, record the weight, and continue until the sample starts to char.
The final measurement before charring is the true dry weight!
It takes a lot of words to describe, but the process only takes about 3-4 minutes.
Suppose you take 50 grams (1.8 oz) of substrate, and after drying, you’re left with 16 grams (0.56 oz) of straw or hulls.
So, the water weight is 50 - 16 = 34 grams (1.2 oz).
Then, use the formula:
Moisture (%) = (Water Weight / Raw Substrate Weight) × 100%
So, divide the water weight by the raw substrate weight, then multiply by 100%.
In this example: 34 / 50 = 0.68, then multiply by 100% = 68%.
You can determine moisture at any stage of substrate preparation: during soaking, after turning, before bagging, and in colonized blocks.
Soil moisture meters are not recommended, as they give inaccurate readings with loose substrates.
Silage and haylage moisture meters may be used, but they are costly.
Blocks with insufficient moisture yield poorly and often don’t produce a second flush.
With excess water, the substrate colonizes poorly.
After determining moisture, adjust your substrate processing method accordingly.
If we need a mix with a final moisture content of 69%, it is logical to assume that there should be 690 kg (1,521 lbs) of water in it and 310 kg (683 lbs) of dry material.
However, the dry components themselves contain moisture. You need to take the dry material and determine its moisture content using the method described above.
Let’s say your measurements show a moisture content of 9%. To find out how much raw material you actually need, you multiply 310 kg (683 lbs) by 109 (this is the sum of the moisture content you obtained: 9% + 100%) and then divide by 100%.
(310kg * 109%) divide by 100%=338kg (745 lbs)
This means that to produce one ton (2,204 lbs) of finished substrate, you need to take 338 kg (745 lbs) of raw material and add 662 kg (1,461 lbs) of water.
However, this is purely theoretical to understand how much water will be absorbed by the straw or hulls. In reality, you will spend significantly more water, depending on the method of disinfecting the raw materials.
For example, when pasteurizing with hot water, you need 2 to 2.5 times more water. This means that in a two-cubic meter tank (about 70 cubic feet), you would place approximately 150 kg (331 lbs) of raw material and add around 1.6 tons (3,527 lbs) of water.
About 330-350 kg (727-772 lbs) of water will be absorbed into the substrate, and the rest will need to be drained.
The ability to absorb and retain moisture is influenced by the characteristics and quality of the raw materials:
- The presence of soil dust, very fine fractions of straw, or broken sunflower hulls in amounts greater than 5% increases the likelihood of over-moisturization.
- A waxy coating on the straw prevents moisture penetration. In this case, the moisture content may be lower than optimal. Water may not be absorbed into the straw and instead remain on the outside of the stems, condensing under plastic film or running to the corners of the bags.
If you are treating the substrate with hot water and notice defects (green mold, uncolonized spots, unpleasant odors), and there’s a brown sludge collecting in the corners of the bags, you need to find a way to heat the water faster, and after the thermal treatment, immediately drain the water from the tank.
Do not reuse drained liquid for thermal treatment.
Also, check that the edges of the bags are not sticking out of the water during thermal treatment.
In those areas, there will not be adequate heating. Over-saturated areas with spots will later develop due to bacterial activity, which also causes unpleasant odors.
Do not reduce the soaking time or thermal treatment time to decrease moisture!
If your substrate is too wet after hydrothermal treatment, you can use lime or gypsum to partially absorb excess moisture.
However, this is a stopgap measure!
If you consistently end up with excessively moist substrate, change your raw material processing method. These additives do not guarantee the absence of defects.
For more information about lime and gypsum, you can read here.
With the proper incubation conditions, gas exchange occurs through the holes in the block—mycelium releases water vapor and carbon dioxide, while air from the environment enters the substrate. If there is sufficient moisture in the room (at least 80%), the substrate loses almost none. Additionally, some of the water vapor released by the mycelium remains in the block.
During growth, mushrooms use a lot of water—the nutrients in the substrate dissolve in it, which are necessary for fruit body formation.
As a result, the substrate moisture decreases, so it needs to be monitored and maintained at an optimal level during substrate production.
