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.