A Compost Project

In addition to a trash dumpster and an oversized recycling bin, my neighborhood has a compost bin. It has two large bins that are big enough that one can be composting while the other handles waste from the entire neighborhood. It also has a central section with a roof that is used to store bales of straw that are to be mixed in with the kitchen waste to achieve the right mixture. It has slats on the front for easy extraction of finished compost and a sturdy wire mesh all the way around to keep the varmints out. All in all, it is a fantastic addition to the neighborhood. But it lacks one thing: maintenance. The neighbor who built it also takes care of it. The problem is that he lives elsewhere all summer, not arriving in town until mid-fall and leaving again mid-spring. That means that all summer, when the compost should be cooking like crazy, it never gets turned and rarely gets watered. In addition, the straw, which should get added when the compost gets smelly, gets added liberally with almost every addition of compost. So in addition to not getting enough oxygen or water, it has too much brown material. It seems to me that this is a problem I can fix. Since this is a blog about engineering with biology, we will tackle this as an engineering problem.

Problem Definition

The first step is to clearly define your problem. In this case, we will define it thusly: The compost bin only gets watered in the winter, has too much brown material and never gets turned.

Parameters

The next thing you look at are the parameters of your problem. Cost is nearly always one of the parameters. For example, those 14’ long giant worms from Australia might be just the thing for this problem (but probably not), but it wouldn’t be reasonable (or legal for that matter) to import a few of them. We need to spend little to no money and use local materials as much as possible. The second parameter has already been mentioned: we will be using biological organisms to solve this problem. The third parameter is a request from the person who built the bins: “I’d rather not modify them any more than I have to; I like them the way they are.” And I agree with him.

Options

The next thing we want to consider is our options. The usual composting organisms sound like a good place to start.

Thermophilic bacteria are the biggest composters out there. They work quickly and could turn that entire pile into black gold in about two months. There is a big problem though, they need a steady supply of oxygen (usually supplied by turning the pile), lots of moisture (only available during the winter), and warm temperatures (not available during the winter).

Redworms are the second biggest composters out there. They also work quickly and would turn that pile into black gold in about 2 or 3 months, if added in sufficient quantities. If worms are added to a dry pile, they will seek moisture deeper in the soil, even if it means leaving a huge source of food behind. They are also slow to eat brown material and they tend to go dormant in the winter, retreating to the bottom of the pile and slowing down their metabolism. Now the pile is in a sunny location, so it will probably not freeze solid during the winter and it probably will thaw all the way out most days, but it will still be too cold to keep worms active.

Mushrooms are another organism that can be used in compost bins, but isn’t used frequently. The problem with mushrooms in compost is that they don’t like to be turned frequently, they prefer a mixture that is heavier on the brown material and lighter on the green material, and they are damaged by high heat. See where I am going with this? Mushrooms will work slower than bacteria or worms, but will do an excellent job of breaking down the brown material. They also have limited ability to transport things like oxygen and can continue to grow a little deeper in the pile than the aerobic bacteria. In addition, they are typically better adapted to cool conditions and can continue to grow in just about anything above freezing. In fact, many mushrooms use winter as an opportunity to get a leg up on the competition, expanding their range and collecting nutrients while the bacteria are dormant.

Choosing a Specific Material

Engineering is all about specifics. Saying mushrooms will work is not good enough. You need to select a mushroom. As I mentioned, cost is certainly an issue, so I will work with the mushrooms I already have access to and see if any of those will be acceptable.

It turns out that I have access to four different kinds of mushrooms: 3 that I am growing and one that I harvested wild from nearby recently.

The first candidate is the elm oyster mushroom (Hypsizygus ulmarius) which I am cultivating on a couple of logs. It might be a suitable mushroom, but neither log has fruited recently, so I have no access to stem butts to make spawn.

The second candidate is the black poplar mushroom (Agrocybe aegerita), which I am also growing on logs and fruited recently. I am attempting to propagate this mushroom currently. However, this mushroom has proven difficult for me to grow. It is a primary decomposer, so it prefers raw wood (not so available in the compost bin) and it has had some difficulty with my dry Arizona climate. I don’t think this is a suitable candidate.

The third candidate is the pearl oyster mushroom (Pleurotus ostreatus). I also have this growing on logs, and it also fruited recently. The pearl oyster mushroom is a primary decomposer, but it is also an aggressive decomposer of all things that used to be plants. It grows well on paper, straw, cloth, wood chips, and much more, including compost. It would rapidly decompose much of the compost, but wouldn’t break it down very far. It would also die out when it ran out of nutrients.

The fourth candidate is the shaggy mane mushroom (Coprinus comatus) I recently found a fresh wild fruiting of this mushroom and harvested both dried mushrooms (they dry quickly in our dry air) and stem butts, which I am currently trying to grow on cardboard. Shaggy manes are tertiary decomposers, meaning they live in dirt. They are also great restorers of disturbed land. They are adapted to decompose anything from sawdust and straw to manure and yes, they do well in compost. They will probably work more slowly on the compost than the oyster mushrooms, but they have the added benefit that they are native to this area. They are also a great addition to garden soil and would get added to the soil with the compost.

At this point, I am trying to decide between the oyster mushrooms and the shaggy mane mushrooms. I think that a sequencing of both mushrooms would probably be best in the long run, with oyster mushrooms added first, followed by shaggy manes a month or two later.. Also, adding worms in the spring would help the compost finish quickly, especially if the mushrooms have pre-digested much of the compost.

Over the next month, I will be propagating and expanding the mushrooms I have before putting them in the compost bin. I will be posting several articles on the different propagation methods I use for the mushrooms. I will also keep you all up to date on further details of my compost remediation project. Also, sometime in January I will be teaching an informal class on how to propagate mushrooms, using the mushrooms I have, so if you live in the Prescott, Arizona area and are interested, let me know.