How much water would you add to this 5L bag of sawdust

A very simple question I know. But ive actually battled with the idea of 60% hydration of substrate concept…

I tried 3 litres of water in the pst and that was too much. Im thinking 1.4 - 2L next time might suffice? But thought id get a second opinion first.

Will be planning to cultivate reishi, oyster and lions mane

Heres the bag for a reference

Ive looked online but most of the recipes are for hardwood sawdust pellets.


Knowing the initial moisture content can help with determining how much to add. If you have a relatively precise scale that can measure 0.1 or 0.01 gram, this can be done relatively quickly. Take a small (5 - 10 g) sample of your sawdust (after mixing/homogenizing), measure the weight, then spread it out in an oven and let dry at a low heat for several hours. More time in the oven will be required the lower the temperature, but be careful not to turn it so hot that it burns the sawdust (you only need it to dry). After it’s dry, weigh it again. You can then calculate the starting moisture percentage:

Moisture (%) = (1 - (Weight After / Weight Before)) * 100

You can use a less precise scale, but you will have to use more material to improve accuracy and also dry it for a longer period.


@KyleGabriel thank for the advice and formula.
I wasnt taking in the inherent moisture into consideration. That helps alot
Ill put 10 gram in the oven and see how much weight is lost.

So once i have the initial moisture inherent in the grains, I will then add enough water to hydrate it to 60%?

That should work fine. Typical moisture content of raw sawdust is very high, but products that are sold are typically around 5 - 20%, but can vary batch to batch (or tree-to-tree) and is also dependent on the drying technique of the manufacturer. The moisture holding capacity of the wood is also something to consider, but it’s hard to determine tree species from sawdust (sometimes it can be done with wood chips).

Calculating the initial moisture, then ammending to reach your final moisture content is the safest way to get consistent hydration, but not everyone has the time to analyze every batch of sawdust. In large operations this is crucial, but most people will hydrate until when squeezed hard only a few drops of water come off the substrate. I’d both weigh to determine initial moisture content and squeeze as you’re hydrating to get a good sense of how this substrate hydrates. It may also take a while to absorb, so add the water, mix it in, give it 15 minutes to absorb, mix it again, then assess. Maybe add 75% of the total water volume you calculated to reach your 60% moisture content, to see what it feels like, then slowly add the remaining 25% while squeezing to monitor if it starts to feel too saturated.

You could also set up some experiments that have varying amounts of water added to reach theoretical moisture contents (50%, 60%, 70%) and compare yields.

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@merlyn @KyleGabriel I was also about to start some experiments to try do dial in our sawdust hydration more so this detailed info is really helpful over here too.

we get our sawdust from waste from a woodshop that uses different wood sometimes, so doing these quick 10 gram experiments with new batches sounds like it could make a lot of sense.

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This past week I tried the sawdust test with a 10 gram sawdust sample – like baking a cake.

I spread it out on a pan and put in the oven for around 5 hours.

afterwards, the sawdust weighed in at 9.28 grams.

Although, I was concerned that some of the sawdust was still stuck to the pan. Wasn’t sure the best way to get all the sawdust off. Could also repeat with a larger amount where the residue is less of a factor.

^ image of sawdust residue stuck to pan at the end

for the first time we also tried just adding distilled water to our sawdust while innoculating it to fine-tune it up to field capacity by touch. this felt good. since the waste sawdust we use is slightly different each time from the woodshop we source it from, the moisture test might not give us perfect information (unless we repeat it with each batch).

for some reason, we’d never tried adding extra distilled water at the end… makes a lot of sense doing this instead of trying to guess/estimate the water content exactly with variable inputs, and I’m feeling quite optimistic this can help us improve our technique. Basically we pasteurize the sawdust in a cooler with a pour-over of boiling water which is roughly the right amount to hydrate it (we’ve been using 40% of water volume to sawdust volume) and then we can just add a little water the next day while innoculating to get it fully to field capacity. Also planning to switch to calculating the initial water amount by weight of sawdust instead of by volume of sawdust, as I think that might also give us a slightly more reliable starting point. That’s the long sawdust-moisture update.

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