Spent Mushroom Substrate Concerns

My soil is decomposed granite which drains incredibly fast. To counter this, I'll be building some hugel beds, but rather than pile the wood on the surface, I'll be burying it a couple of feet deep. Before backfilling however, I would like enrich the DG with some organic matter. I have an unlimited supply of free "Spent Mushroom Substrate" or mushroom compost as some people call it from a local producer. I would also like to add some biochar to this mix. Is there anything nefarious about SMS? The supplier is not organic, but the list of ingredients seems fairly innocuous. Also, I am not sure what the ratio of DG to SMS should be. I generally use 5 gal. buckets and dump them in a wheelbarrow for mixing. I would like to premix the compost with bio char before adding it to the DG, so a ratio of BC to SMS would be helpful also. Thanks for reading!

I'd save your effort on mixing in the biochar - the worms will do that for you if you give them a little while, and the routine cultivation of your plants will do more, expecially if you mulch again in following years.

spent mushroom castings are fairly nutrient poor (the mushrooms have used it all up) so you will want to consider planting some nitrogen fixers and looking for a source of compost or mulch with some nitrogen.

Thank you Michael! My reasoning for the biochar, though I've never used it before, is to help retain moisture and to capture and store as many nutrients as possible. Is this realistic?

Michael Cox wrote:
spent mushroom castings are fairly nutrient poor

But, hey, it's free. So any nutrients that are left are a bargain.

I agree, no problems with spent mushroom compost, but it's only to improve tilth and water retention and not for nutrients. And yes, your expectations for the biochar are reasonable.

Excellent! That is my first concern, to improve tilth and water retention. I will be planting nitrogen fixers and topping up regularly with better compost as it becomes available. Good to know I'm on the right track. Any thoughts on the ratios?

Bruce Drukker wrote: Any thoughts on the ratios?

If your wheelbarrow is 4 cu. ft., that's 30 gallons, so maybe a gallon of biochar per wheelbarrow load? I'd go heavier on the mushroom compost, maybe 5 gallons of it in a wheelbarrow load.*


*Disclaimer: Not an exact science, your mileage may vary, all biochars are not created equal, offer not good in AK or HI.

Sounds like you are on the right track - although neither biochar, nor mushroom casings are providing nutrients in this scenario. As far as improving tilth goes, I'd aim to do small areas really well to begin with and work outwards in later years as you get more materials.

I use mushroom too but must pay for it. I think there are concerns (but I still use it). Mushrooms are grown on horse manure and before that
the manure is sterilized with some chemical nasty - don't ask me which.

John Elliott wrote:

Bruce Drukker wrote: Any thoughts on the ratios?

If your wheelbarrow is 4 cu. ft., that's 30 gallons, so maybe a gallon of biochar per wheelbarrow load? I'd go heavier on the mushroom compost, maybe 5 gallons of it in a wheelbarrow load.*


*Disclaimer: Not an exact science, your mileage may vary, all biochars are not created equal, offer not good in AK or HI.

That sounds about right. This is just to back fill the Hugels. I'll mound up some good stuff on top and plant nitrogen fixers for the first 6 months or so. That DG is like sand...

Michael Cox wrote:Sounds like you are on the right track - although neither biochar, nor mushroom casings are providing nutrients in this scenario. As far as improving tilth goes, I'd aim to do small areas really well to begin with and work outwards in later years as you get more materials.

Thanks Michael, I basically want to get the beds cooking for 6-9 months before doing any serious planting. In the interim, I'll be putting in trees, making biochar and storing rainwater. I've got the long view in mind.

Angelika Maier wrote:I use mushroom too but must pay for it. I think there are concerns (but I still use it). Mushrooms are grown on horse manure and before that
the manure is sterilized with some chemical nasty - don't ask me which.

Well, I read somewhere on this forum that it was mercury. This is what raised my initial concern. I'm hoping the biochar will help with that.

Bruce Drukker wrote:
Well, I read somewhere on this forum that it was mercury. This is what raised my initial concern. I'm hoping the biochar will help with that.

There is no mercury involved in commercial mushroom production. When I worked in the industry 30 years ago the only sterilization involved was a 6 hour steam treatment to bring the substrate to 160 degrees F before cooling and inoculation.

Hopefully that is the same in Australia. Stammets (this mushroom guy) writes something that one should only purchase organic spent mushroom compost.

Milo Jones wrote:

Bruce Drukker wrote:
Well, I read somewhere on this forum that it was mercury. This is what raised my initial concern. I'm hoping the biochar will help with that.

There is no mercury involved in commercial mushroom production. When I worked in the industry 30 years ago the only sterilization involved was a 6 hour steam treatment to bring the substrate to 160 degrees F before cooling and inoculation.

That is reassuring Milo. This is in line with what my supplier says also. I'm starting to feel pretty good about this, thanks!

Milo Jones wrote:

Bruce Drukker wrote:
Well, I read somewhere on this forum that it was mercury. This is what raised my initial concern. I'm hoping the biochar will help with that.

There is no mercury involved in commercial mushroom production. When I worked in the industry 30 years ago the only sterilization involved was a 6 hour steam treatment to bring the substrate to 160 degrees F before cooling and inoculation.

Hi Milo, I am new here. I'm wondering if you checked any of the farms out lately. So many new chemicals have been introduced into farming recently. Chemical production and use, itself, has mushroomed in the last few decades (pun intended). Do you think they could be using more chemicals these days?

I also read here about some other concerns: high salt, may contain pesticide residues, and not finished composting (which we know) http://www.compostjunkie.com/spent-mushroom-compost.html

Bruce Drukker wrote:My soil is decomposed granite which drains incredibly fast. To counter this, I'll be building some hugel beds, but rather than pile the wood on the surface, I'll be burying it a couple of feet deep. Before backfilling however, I would like enrich the DG with some organic matter. I have an unlimited supply of free "Spent Mushroom Substrate" or mushroom compost as some people call it from a local producer. I would also like to add some biochar to this mix. Is there anything nefarious about SMS? The supplier is not organic, but the list of ingredients seems fairly innocuous. Also, I am not sure what the ratio of DG to SMS should be. I generally use 5 gal. buckets and dump them in a wheelbarrow for mixing. I would like to premix the compost with bio char before adding it to the DG, so a ratio of BC to SMS would be helpful also. Thanks for reading!

Hi Bruce, I've been getting my spent substrate from a similar place in Escondido, but I live in Temecula. Would you mind sharing the info about the Fallbrook farm? I heard from a Meetup member of mine that there was a farm in Fallbrook but she didn't have the info on it.

Jan Tucker wrote:
Hi Bruce, I've been getting my spent substrate from a similar place in Escondido, but I live in Temecula. Would you mind sharing the info about the Fallbrook farm? I heard from a Meetup member of mine that there was a farm in Fallbrook but she didn't have the info on it.

Hi Jan

That's my place too! I haven't been to the one in Fallbrook yet, but from what I've read, their stuff sounds pretty good. I might head out there today or tomorrow to check it out. Here is the link.

CCD Mushrooms, Inc

Good luck!

Jan Tucker wrote:

Milo Jones wrote:

Bruce Drukker wrote:
Well, I read somewhere on this forum that it was mercury. This is what raised my initial concern. I'm hoping the biochar will help with that.

There is no mercury involved in commercial mushroom production. When I worked in the industry 30 years ago the only sterilization involved was a 6 hour steam treatment to bring the substrate to 160 degrees F before cooling and inoculation.

Hi Milo, I am new here. I'm wondering if you checked any of the farms out lately. So many new chemicals have been introduced into farming recently. Chemical production and use, itself, has mushroomed in the last few decades (pun intended). Do you think they could be using more chemicals these days?

I also read here about some other concerns: high salt, may contain pesticide residues, and not finished composting (which we know) http://www.compostjunkie.com/spent-mushroom-compost.html

Hi Jan,

Could they be using more chemicals? I suppose so but no, I don't have any recent experience.

One thing to take note of is which kind of mushroom was grown on the substrate.

I believe button mushrooms are unique for commercial mushrooms because the primary ingredients in the compost are horse manure and straw. This is composted before the mushrooms are grown on it. There may be elevated salts in horse manure compost because any excess salts a horse consumes will be excreted. One thing that has been pointed out in the composting forum is that any chemicals, medicines or what-not that the horse had ingested my be excreted and survive the composting process. Any herbicide or pesticide applied to the straw may carry over as well.

I have not worked with oyster or shiitake but know they are grown on logs, sawdust or maybe straw in the case of oysters. These are not composted before inoculation, just sterilized. The company Bruce linked to says they compost their substrate after they finish growing mushrooms on it. I suspect the spent substrate is still too much like sawdust to sell as compost and the composting is to create a value added product from their waste stream. Personally I'd have fewer concerns with this type of compost.

At the end of the day it probably boils down to practices on an individual farm. In an age where 'Natural' and 'Organic' are debatable it helps to know your farmer to decide if you can trust them. Maybe the same thing applies to your composter?

Stay away from button mushroom/portabella compost unless it's organic. They use huge amounts of pesticides as well as crazy mold inhibitor chemicals that can harm your soil biology.

I would think it would be less of a concern with sawdust or straw based substrates; from shiitake or oyster mushroom production. Even so I would do some research and try talking to the grower to make sure they aren't using anything crazy.

For Bruce Drukker: If you used spent mushroom compost, you are adding over 7 times the necessary or acceptable limit of chloride. Potassium chloride is an ingredient in concrete,,, which can form in your veins (one reason that NaCl isn't good to eat...). Take your spent mushroom compost to AgLab in Fallbrook, and for $30 they will show you.

I don't use mushroom anymore. It contains a lot of salt and I killed various citrus trees with it wondering why. I lost a summer of beans too. Take care many landscapers sell this as organic, but what they mean is that it is an organic material not plastic that does not mean it comes from an organic farm.

Thank you Jaret and Angelika! In the end, I did not use it. I've heard similar stories from some locals and it scared me away. Hugels were a bust anyway. It never rains here…ever… Went with raised beds and micro irrigation instead.

One discovery was Ollas. These worked brilliantly, especially for strawberries. Anyone in the arid south west would benefit from these. Thanks for the replies!

The biochar will sequester carbon which will feed the roots of whatever you have.