Mushroom Compost

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Recently on a few of the composting and compost tea listservs, there have been some questions about mushroom compost, and if it is any good. After looking around some, I have come to the conclusion that people tend to be either blindly in favor of it or dead set against it. Well, I'm here to tell you that they are both wrong!

Spent mushroom compost has many uses in the garden, and it does a wonderful job on improving the soil, but it ain't the same as adding good, high quality, living compost either.

To get an idea of what you can use spent mushroom compost for, we should start out with . . .

How Spent Mushroom Compost is Made

Mushroom compost is normally made in a hot composting process with straw, animal manure and gypsum. Sometimes leaves or ground corn cobs are used in place of the straw. As you probably noticed, this is a much simpler recipe than you will find in most home compost piles.

There are usually other nutrients added either while composting or after composting. Many of them are organic in nature, such as blood meal, or cottonseed meal, but sometimes there are inorganic additives such as urea.

The pile is allowed to heat up to about 160 degrees and turned several times, just as you would do with your home compost pile, except on a much larger scale. The mushroom farm down the street from me uses those giant straw bales that are shipped one per semi trailer.

Now, unlike your home garden pile, they don't let the compost sit and age at this point. They take the compost, load it into planting beds and then they steam pasturize everything!

That's right, they kill off all those wonderful micro-organisms that will continue the composting process and are so important to your garden!

Of course, they aren't thinking about your garden, they are thinking about getting rid of the competition for the mushrooms, and there are a lot of strains of wild fungi in that compost.

Then they top the bed off with a couple of inches of peat moss, and innoculate the bed with the mushroom spawn. They get their first harvest in about a month, and continue harvesting for the next 6 to 10 weeks.

After they are done, they steam sterilize the bedding again, and sell it as spent mushroom compost.

Advantages and Disadvantages of Spent Mushroom Compost

Of course, some people delight in pointing out the "spent" part of the name. While it is true, that it has served its useful life as far as growing the mushrooms, that doesn't in any way mean that it is now useless for growing anything else. If you think about normal compost, the aging, or curing stage has bacteria and fungus continuing to feed on the compost. So I'm not buying that argument.

What IS a very good argument against mushroom compost, is that it has been sterilized. Adding microbiology to your soil is one of the primary advantages to compost, and that microbiology is vital to the disease prevention and and supplying nutrients to your plants. Spent mushroom compost simply does not provide these benefits. And don't even bother trying to make compost tea out of it.

But what many people fail to think about is that even though the mushroom compost does not come with the all biology present, it is still provides a good substrate and food source for that biology. It isn't like steam pasteurizing is the same as sterilizing it with a toxic chemical (though some do use fungicides). You can add some of that biology back in before you apply it, or just set it out and let nature take her course.

That does bring up the fact that few mushroom facilities are organic, and they use something called Integrated Pest Management (IPM). What that comes down to is that they will not use pesticides unless they have a reason to, but some are quicker on the trigger than others. Always smell your spent mushroom compost before buying it. If there is a chemical smell, don't buy it. Even better, try and find out from your supplier what products were used in the current batch that you are buying.

Then there is the mystery argument against it, that adding mushroom compost leads to compaction. Since it is rather fine grained, it does not offer you as much protection against crusting, but there is a difference between crusting and compaction. The fact is, that I have seen quite the opposite effect, but I don't try and use mushroom compost as a top layer mulch either. Mix it in with some soil or cover it with some leaves and you should have no problem.

Another complaint is that it tends to have a high salt content. This comes with making a compost using so much manure, not to mention that some of the amendments added for the mushrooms are high in salts. But this is not so different from any manure based compost. The advice here is pretty simple, don't use it in greenhouses where you don't open it up for natural rain to leach away the salts for at least part of the year. You should also avoid using it too heavily year after year.

For the final semi-valid complaint, there is the fact that much of the mushroom compost contains synthetic fertilizer. While I try to go as organic as possible, I do overlook this one at times where the budget wins out. And it is still mostly organic with only traces of the synthetics. I also don't consider it a major issue since it is not my preferred compost for yearly application on my food crops.

As for the advantages, here is a really big one, it is frigging cheap compared to buying quality compost. To the mushroom farms, it is garbage that they are just trying to recoup some money on. I can buy it for as cheap as $5 a cubic yard, picked up. Mediocre mass-produced green waste compost will run me about $13/yd, good high quality compost or vermicompost (worm compost) will run me $25-50/yd. Personally, I think the mushroom compost does more good for a garden than the $13/yd stuff. It is made better, and with better quality control than the cheap stuff, the only advantage the cheap stuff has is that it isn't dead.

By its nature, it is a finer texture than most other compost. You don't have to sift it to apply it to lawns or ground covers. The smaller grains will make it to the ground with the next rain.

The fine texture is also very nice for beds where you will be growing root crops. No chunk of partially decomposed wood to deal with. While this lack of hard structural pieces is a disadvantage in some areas of the garden, I don't need to add anything to give me even more of those entertaining 3 legged carrots, I have more than enough rocks for that. That said, I like to add it to root crop beds at least a year ahead of time, in case there were chemicals used on it.

When do I use Mushroom Compost

I prefer to make my own compost to use, but I rarely make enough to use in my established garden, and I just don't have enough to properly prepare new beds. So when I am going to make some new beds, mushroom compost is a good, cheap choice. Just as with any new bed, I don't expect to get the great results the first year, because the soil biology isn't going to get fully established anyway for at least a year. By that time, the mushroom compost won't be very sterile anymore.

As previously mentioned, it is also my preferred compost for root crops and for spreading on the lawn. There are other ways to get the biology into the soil, I just consider the mushroom compost to be the food for that biology.

I do not use it on an annual basis for my vegetable garden. Between the lack of biology, the salt buildup issues, and the possibility of synthetic fertilizers, I find that there is enough to be concerned about overuse.

Improving your Mushroom Compost

Almost all the downsides to using spent mushroom compost and be somewhat alleviated by getting some living biology back into your compost. This really isn't all that hard to accomplish. Hell, it will happen over time just by mixing it in with your garden soil.

Probably the quickest way to activate it is to make up a good batch of compost tea (from non-sterile compost) and mix it in to your mushroom compost. Just pretend that it is a compost pile that you are letting age for a few weeks, because at this point, that is really what it is, compost settling into a normal mesophilic stage.

You can also mix in some live compost, or mix the mushroom compost in with your compost pile to be brought back to life. You are basically doing the same thing you did with the compost tea, but the microbes just won't spread as quickly.

The last option is one that I am considering, especially over the winter months, which is to use the mushroom compost as feedstock for my worms. As worms crawl through the compost, eating as they go, they leave an incredible diversity of beneficial organisms in their castings. In the process, most of the salts will leach out in the worm bins, and any synthetic fertilizers should be broken down by the worms and the other critters. It would be interesting to get some of this worm processed "spent" mushroom compost tested at a lab to see what sort of results I get.


My conclusion is that you should not consider mushroom compost as a replacement for good proper compost, but it is worthwhile for you to consider it as a cheap soil amendment. Use it when you need lots of already broken down quality organic matter for very little money, but don't plan on using it as your primary source of compost. I use it on my lawn with little concern, but I simply don't trust is for regular use with my food crops.

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mushroom compost

judy from tennessee:
this spring we landscaped our new yard, using amended soil (2/3 topsoil - 1/3 mushroom compost). In the beds with with shrubs, flowers etc. we put 4 - 6 inches of hardwood mulch. We have had a lot of rain, and hundreds of little mushrooms stated popping up (very unsightly). When I looked underneath, the soil looks matted and grey. Has anyone else had this problem, and if so, how do I get rid of this mess?

Mushrooms (Inky Cap) Growing In Veggie Garden

Hi, I have grown a few gardens but am still fairly new at it and still have a lot of learning to do.
What I am concerned with now is what's growing in my food garden and is it still safe for us to eat the friuts of our labors.
We started this food garden in virgin soil and have put local wood mulch around all plants and in the rows. We began to notice just a couple of weeks back that we also have mushrooms growing, an abundance of the little critters! From what I can gather doing online searches, what they are are "Inky Caps".
What I want to know is, do I do anything about them or do I just let them grow, and rake them over each morning? Will they poision my food that I have growing in my garden?
I just wondered, after reading about mushroom compost being good for gardens, if the growing mushrooms breaking down the mulch right along side my veggies would hurt anything. I just want to know what to do, if anything.
I want to know that whatever fruits of our labors (tomatoes, squash, peppers, etc.) that come in from the veggie garden will be safe foods to eat.

Your veggies are safe to eat

Don't worry about the health of your veggies, they are fine to eat. Fungus is an important part of healthy garden soil, so important that your vegetables wouldn't be able to grow without them. Mushrooms are just the fruiting bodies of a much large mass of fungi growing in your soil and mulch. Be thrilled that your soil is healthy enough to support a large fungal population!

Even the most poisonous mushrooms require you to take a serious bite out of it to get any real affect. Wash your veggies and everything will be fine.

Musgrooms growing in Veggie Garden

I also have mushrooms growing in my veggie garden after using the mushroom compost.
I've used it on my garden in the past but never had this happen.
It's quite a crop, too. I'd like to know if they're safe also.
I went on line and while you seem to be able to identify the mushrooms as Inky Caps, I'm not so sure what mine are!
Have you gotten any response to your question?

Don't eat them, and don't worry about your veggies

The mushrooms are a sign of healthy, living soil, so don't worry about the safety of your veggies. But remember not to eat any mushrooms till you have positively identified them, and you are sure they are safe to eat.

mushroom dirt

Hey to all,
You all sound very educated on the subject of mushroom dirt....however I'm certainly glad I didn't find this site last year, befor I had two loads delivered for my garden. I'm new to TN and didn't know squat about mushrooms or the dirt they grow in. A neighbor said I might try it and I was amazed. I didn't ammend it with anything....Had the most wonderful produce I've ever had. I used it in my flower beds in the front of my are all up and the back of the house....where I ran out of the mushoom dirt ....not so good. Needless to say...I'm having two more loads delivered next week. Me thinks somtimes ....analytical
thought is overdone. Just "do it"....After all there is always next spring.
A gramma from TN.

Creating new Veg garden with stall muckings & mushroom compost

I live in Southwest Chester County, PA. I recently built a raised garden bed over an old flower garden and sloped lawn for a new vegetable garden The raised bed is 18" deep on the downhill side, over grass and about 6" deep on the uphill side over the old flower garden. It is 30' long x 18' wide. I loaded the bed up with about 12-16" of horse stall muckings consisting of bedding straw and 1/2" sq. recycled shredded cardboard mixed with horse manure. I took the manure/straw materials from the bottom of the manure pile, so it is partially composted. I then added about 5 yds of spent mushroom soil compost on top of the manure. (I was able to get a couple pick-up truck loads of spent mushroom compost from a mushroom soil making operation in West Grove, PA for $10 per truckload with the entire truck bed filled & piled as high as the top of the passenger cab).

I plan to roto-till the mushroom compost and horse manure into a loose garden bed. I can also add about 1/2 yd. of still very wet composting kitchen veggie scraps from our enclosed food composter and I can also blend in a couple bales of peat moss, if necessary. I figure if I have to adjust pH I can add lime or sulphur, depending on which direction I need to nudge the pH (I'm anticipating it to be on the acidic side) and blend that into the bed as well. I was not planning on adding any soil other than the grass and dirt buried beneath the 18" of manure/straw and mushroom compost.

I'd like this first year of vegetable gardening to be a success. Can anyone tell me if I am on the right track or do I need to be doing something else to make this first growing season as productive as possible?

William Gould

You should add some soil to the mix

Things like compost and manure are called amendments for a reason. You should mix them in with soil instead of filling the bed with them entirely. About 1/3 manure or compost to 2/3 soil is the riches you should go. If your compost isn't mature, you should also grow a cover crop on it for a season before growing your veggies.

where do I purchase mushroom soil or look for in-kind donations


I am involved in a church mission project that will be building a large 1 acre garden in an underprivileged neighborhood in West Philadelphia this summer to raise crops and supply food to the needy. They are in need of quite a lot of mushroom soil for this project and being a church volunteer project are looking for the lowest prices available, including in-kind donations. Can you please direct me to the best sources you know of for $5/yd or perhaps donations for this soil? I would appreciate it.

I thought your article was very informative and well written. A good educational source. Thank You.

Hit the phones

I can't give you any specifics, because I live 2500 miles away, in Washington State. There are a lot of mushroom farms in PA, so you should try giving them a call. Otherwise contact local bulk soil dealers.

While you are at it, look into finding green waste compost instead. It would be more expensive if you bought it, but there is a good chance that you can get donations. It would be a much better solution than spent mushroom compost.

Mushroom Compost

My wife and I have been using mushroom compost for around seven years now. We started by picking up two to three yards at the mushroom farm each year, spreading it on our raised beds and tilling it into the soil. Our garden got larger and for the last three years we have ordered a dump truck load, approximately 22 yards.
We order it in Feb. and it sits in a pile for 4 to 6 weeks before we start using it. During that time it gets hot, over 140 degrees. It's always amazed me that weeks later you can walk over to the pile, shove your hand in and it's still very hot. In the weeks leading up to pulling our flats of seedlings out of the greenhouse we start preparing the beds. Burning the weeds, and plants from the year before, hand digging up the beds to remove root balls, shoveling wheelbarrow load upon wheelbarrow load of compost sometimes still hot upon the beds, spreading with a rake, and then tilling it in.
Our garden always does well. We love the stuff.

NOT sterilized

Actually, pasturization by definition is preformed at temps of only 140-170 degrees F. which directly simulates composting, thus pasturization is NOT sterilization, which is preformed under pressures of 15 psi, at 250 degrees F. This completely tears down this articles point which is false, and potentally dangerous to people who other-wise could use a most abundant resourse. You see, a healthy compost comprises of thermophilic micro-organisms which live at temps of (guess what) 150-170 degrees F. You have seen these, if you have made a compost pile, as whitish flecks which are called actinomycetes. There are many other arobic micro-organisms as this that NEED these high temps to even exist. Further more, the residual mycelium of the mushroom (its root network) is resplendant with nitrogen!!! SPENT compost is only called spent in the mushroom industry because its nitrogenous percentage is less than nessesary for more mushroom growth. CONCLUSION; "spent" compost is still alive. All that is nessesary is a modest addition of manure, and/or cotton seed meal........and before you try to tear down a good thing, do your research. Spreading miss information is not only dangerous for the reader, but also to anyone that the reader spreads this mis-info to, and all the mushroom farms that other wise could contribute quality compost.(not to mention the time I just "spent" putting things to right)

I'll take your points one at a time

Yes, pasteurization is not sterilization, but the purpose of pasteurization is to kill off everything but the thermophilic bacteria. The high temperatures do not simulate composting, because composting is not about the heat, the heat is a byproduct. the composting is the active consumption of the feedstock, with heat as a byproduct. What the externally added heat does is select for the thermophilic bacteria, while killing off all the other important decomposers like insects, fungi and actinomycetes.

The thermophilic temperature range is 113-176, not 150-170. About the only thing that lives above 150 is some species of bacteria and archaea. Thermophilic fungi and actinomycetes are generally found in the outer 6 inches of a hot pile because they cannot handle temps much above 140. The high temperature bacteria are really only good for breaking down the simple carbohydrates, and even they do a much better job once the shredders (like mites) have come through first. You need the fungal diversity and shredders to really break down any carbohydrate that is more complex than cellulose.

If you actually read the whole article, you might have noticed that I wasn't tearing down SMS, I even pointed out that "spent" for mushrooms doesn't mean it is "spent" for the garden.

Mushroom compost is made for growing mushrooms, not for growing a garden. Whether pasteurized or sterilized, it still doesn't have the full range of microorganisms that you find in a good, healthy, properly matured thermophilic or vermicompost. And unless it was made at an organic mushroom facility, you have no way of knowing what products were used in their IPM.

I'm using mushroom compost right now

Great article. I just started a new raised bed for a vegetable garden. It is 8' L x 4' W x 18" deep. I purchased a mixture of topsoil, mushroom compost, and peat moss from a commercial nursery and can't wait to see how it works. I was halfway through wheel-barrowing the stuff from my driveway to the backyard when it started pouring rain. So far, it seems quite moisture retentive without becoming too compacted, but I guess only time will tell the long term structure of this soil. If I experience any problems or surprise perks of this mixture I'll be sure to let you all know. In the future I plan to amend it with home 'grown' organic compost from a compost pile as well as worm castings to avoid salt and synthetic fertilizer build-up in my precious new bed. Wish me luck, and a happy growing season to everyone!

Mushroom compost

What a great article of information and well written. Thanks for the information and keep up the great work.

Mushroom medium as compost

Hi, your info about mushroom waste* [mrw] is helpful and interesting. I had thought that mrw could prove useful as a basis for a complete potting mix/fertiliser for no dig gardening. I am letting my chickens scratch and dung in it for a while before adding some worm castings to it. This is an experiment. What else would you think might be needed? I am interested in the recycling process so wold prefer to use matter that is readily available and preferably cheap.

Mushroom Compost


I enjoyed your article on mushroom compost. You are correct, there is a lot of variation in the opinion people have about mushroom compost and it does tend to be at the extremes. You did a great job pointing most of these things out!

My experience has been that when people dislike mushroom compost, they had misconceptions about it and used it improperly. The main thing to remember, and this needs emphasized, is that it is compost, not soil. In many areas they actually call it "Mushroom Soil." So often people will fill a flower pot and when their plants dies say it is no good. That is like planting something in a box of Miracle Grow and expecting it to live. It is too strong, too many nutrients, too much of a good thing.

A few comments on some of your points. Almost all growers now use only organic materials. In days past ammonium nitrate and urea where often used, but today that is very rare. Composts are also primarily plant based, that is straw or hay based (the hay based formulations are too complicated to cover here). Mushroom growers often talk about using "Horse manure," This has caused a long running misconception that growers are using piles of dung, but what they are referring to is the straw that was mucked from a horse barn. It is mostly straw. There is very little manure.

Poultry manure (really is poultry manure, not like horse manure) is added to the plant based materials to achieve the proper C:N ratio. This is critical to achieving optimal mushroom production. I think you would find no other compost producers formulate to the exacting standards that a mushroom composter does. Typically manure would be around 15% of the bulk dry weight of the compost.

The compost then heats exactly the way you described. This is what we refer to as the Phase I composting cycle. In this stage we are using the microbes that live at these temperatures to concentrate the food energy in the compost. That is, we are turning simple carbohydrates into complex carbohydrates. Ammonium is also produced by various microbes under these conditions.

When we fill it into the growing house, the Pasteurization stage you referred to is actually a seven to ten day composting process we refer to as Phase II composting. This is a lower temperature, 120 to 140 degrees, composting in which microbes consume the ammonia produced in the Phase I, converting it into microbial protein. The pasteurization portion is only 2 hours. During this time, the compost is already hot, but we inject live steam to raise the air temperature. This does not eliminate competitor fungi. That is what the entire process does, the process provides the proper nutrients and makes the compost “selective.” Exacting composting measures make the correct nutrients and environment to grow Agaricus bisporus.

The pasteurization stage is to eliminate any possible pathogenic organisms, both mushroom and human pathogens. But it is also pasteurization, not sterilization. These terms are not interchangeable. With sterilization, killing everything as you described, but you need temperatures of 250 degrees to do that. With pasteurization, 140 degrees, you are killing the pathogenic organisms like E. coli, but permitting the beneficial organisms to survive. These are the guys you want. They also fill up ecological niches in the compost and help to keep the bad guys from returning.

An important point: As you said, the compost pile reaches 160 degrees, a normal temperature for any compost pile, but the pasteurization is only 140, less than normal composting temperatures. We are not killing the good guys.

If you want evidence the compost is still alive, just take some from a house and put it in a large pile. It will very quickly go into the 140's. You need a lot of microbiological activity to do that!

After phase II composting, we grow the mushroom mycelium out for two weeks, add the casing layer you mentioned and in 16 days get mushrooms which we harvest for three weeks. The entire cycle is 10 weeks.

Some points that are worth mentioning:

* Salts: it is high in nutrient salts. (Potassium salts, not chlorides) This is true of any compost, remember it is compost, not soil.

* Composting: The compost piles and/or vessels used in mushroom composting is superior to anything used in any other type of composting where they generally only use windrows. If mushroom compost is made using a standard rick they have vertical sides permitting more air to enter than could ever enter a windrow. In vessel composters, what most are going to, provide positive air pressure through openings in the floor and do not rely on convection currents to provide air to the organisms, providing a superior compost, both for mushrooms and later green plants.

* Formulation: Mushroom compost is formulated to an exact C:N ratio. All materials are tested for N and the proper amount of nitrogen supplement is added to each batch. The ingredient stream is also very consistent, unlike yard waste compost.

* It is pasteurized, not sterilized. All the good guys are still in there!

* Is just compost. There is nothing magic about it because mushrooms were grown on it. There are plenty of fungi and even mushrooms that will grow in your home composting pile. Mushrooms are just another decay organism that breaks materials down into their component parts as far as green plants are concerned. We are just providing the environment to grow the ones we are looking for.

* Stability: It is not stable. If you want a stable compost it needs to compost further. There is still a lot of energy in it. This is also why it is so high in organic matter and is why it should be blended with soil.

* The Pennsylvania State University is the place to look for research on mushrooms and mushroom compost. That is where most of the world's mushroom research is done.

Dave, I would like to invite you to visit our facility. Send me an email if you are interested.


Mushroom compost

Enjoyed your article on mushroom compost-- I just bought 8 buckets of it.

I also raise red wigglers in containers. Is that the kind of worms you add the compost to or are you referring to the ones raised in a pile of leaves,etc.?

I may add a little to mine because I don't seem to have enough kitchen waste to keep them busy.

using spent mushroom medium for composting

I've just gotten hold of a good supply of fresh spent mushroom medium. I found Dave's comments about its inorganicity quite interesting and informative. I had not considered the salts nor the fact that it may have artificial fertilisers. I don't know what is inserted into the medium before it is inoculated with the mushroom spores. However what I have decided to do is to let my chickens scratch in it for a while, drop their poo in it and let it break down for a while before spreading it over the garden. I am also interested in creating a balanced fertiliser to incorporate into a no-dig garden. I was thinking of using the mushroom compost along with other elements such as Chicken manure, worm castings and whatever else is needed. At present I'm not sure what that is. Does anyone have any good ideas to help? Thanks. Peter

Mushroom compost

I made the mistake of using mushroom compost to plant a Diablo Ninebark. Not realizing the need, I did not mix the compost with soil and merely used it to fill the hole to contain the shrub. Within 2 weeks the Ninebark started to die off, the leaves dried and turned brown. Readers should be aware that mushroom compost must be mixed sparingly with topsoil and not used as soil for planting shrubs; there is too much salt in the product and will kill your plants. I won't ever use it again.

You should never use ANY

You should never use ANY amendment without mixing it. Too much of a good thing is always a bad thing. When I use mushroom compost I try to mix it about 50/50 with the ground soil and I've had great results, and I mean GREAT results. The 50/50 mixture will help to balance out any excess in the compost while still leaving behind vital nutrients your plants want from the compost. I'm with the original poster, I use mushroom compost on new beds and then amend with my own homemade compost thereafter. You can not beat the price and results if used properly.

Mushroom compost

That is a very well written article. Thank you. There was just the right amount of information, entertainingly presented, to help me decide on how to use this garden amendment.

Great article, to repeat

Great article, to repeat many of those comments previously.
How much would you rate and trust mushroom compost, i.e. would you recommend putting it through a compost screen or is it ready to go?

It's ready to go as is.

It's ready to go as is.