Composting Processes and Stages Explained Simply
Book: Backyard Farming: Composting | Author: Kim Pezza | ISBN: 978-1-57826-587-9 | Hatherleigh Press, 2015
Composting sounds simple because it is. Stuff rots. But Chapter 7 of Pezza’s book gets into the actual science of what’s happening inside your pile, and honestly? It’s way more interesting than you’d expect. There are bacteria working in shifts, temperatures that could cook an egg, and yes, the possibility of spontaneous combustion. We’ll get to that.
What Your Pile Actually Needs
Before anything starts breaking down, your compost pile needs four things dialed in:
- Air. This is why you turn the pile. The bacteria doing the work need oxygen.
- Water. The pile should feel damp like a wrung-out sponge. Moist, not dripping.
- The right carbon-to-nitrogen ratio. We covered this in the previous post. Get it wrong and your pile stalls or stinks.
- Small pieces. Break up big stuff before tossing it in. Smaller particles mean more surface area for bacteria to munch on.
You can also throw in a thin layer of garden soil to introduce microorganisms, but it’s not required. Nature tends to show up on its own.
Two Methods: Aerobic vs. Anaerobic
There are really only two ways composting works.
Aerobic composting is what most people do and what this book focuses on. You build a pile, keep it moist, turn it for air, and let heat do its thing. It’s faster and produces better results.
Anaerobic composting is the lazy cousin. No turning, no air, no heat. You just leave the pile alone and let it sit. It works, but it takes significantly longer. If patience is your thing, go for it.
The Beer Recipe (Yes, Really)
This is one of the more fun things in the book. Pezza includes a DIY compost accelerator recipe, and it reads like a frat party supply list:
- 6 oz beer (the yeast helps speed up decomposition)
- 1/2 cup ammonia (adds extra nitrogen)
- 12 oz non-diet soda (the sugar feeds the microbes)
- 2 gallons warm water
Mix everything together in a 5-gallon bucket and pour it over your pile. The yeast and sugar basically wake up the microbial community and tell them to get to work faster.
If mixing up compost cocktails isn’t your style, there’s a simpler option. Just add a thin layer of finished compost or topsoil on your pile to introduce microorganisms naturally. Then turn the pile every 7 to 10 days.
The Four Stages of Composting
Here’s where it gets genuinely cool. Your compost pile goes through four distinct stages, each with different bacteria running the show.
Stage 1: Mesophilic (The Warm-Up)
First, bacteria called psychrophiles start working at very low temperatures. They oxidize carbon, release heat, and produce amino acids. This phase is so subtle you probably won’t even notice it happening.
Then mesophilic bacteria take over. These guys thrive in moderate temperatures. They colonize the pile within days, eat through everything they can find, and generate enough heat to push temperatures past 100°F. Think of this as the opening act.
Stage 2: Thermophilic (The Main Event)
Once the pile gets too hot for the mesophiles, thermophilic bacteria step in. These are the heavy hitters. They can survive temperatures up to 200°F but only live about 3 to 5 days.
During this stage, the heat kills weed seeds and harmful germs. The thermophiles also produce humic acid, a naturally occurring compound loaded with trace minerals that’s incredibly good for soil.
Now here’s the wild part. These same thermophilic bacteria are responsible for spontaneous combustion in hay bales and very large dry compost piles. The heat they generate can literally start a fire if conditions are right. Before you panic: a typical backyard pile is nowhere near big enough for this to happen. But if your heap is getting crazy hot, turn it to cool things down.
Stage 3: Cooling
Nothing dramatic here. The thermophilic activity slows down, temperatures stabilize, and the pile gradually cools back to ambient temperature. The intense work is done.
Stage 4: Curing (The Finish Line)
The mesophilic and psychrophilic bacteria from Stage 1 become active again now that temperatures are back in their comfort zone. Worms and insects move in and take up residence. This is how you know your compost is nearly ready.
Once this stage wraps up, you’ve got finished compost ready to use in your garden.
A Quick Note on These Bacteria
Both mesophilic and thermophilic bacteria are everywhere in nature. They’re already living on your food waste, in your garbage, in manure, even on human waste. You don’t need to buy them or add special products. When you build a compost pile with the right conditions, they show up and get to work. That’s the whole system.
What’s Next
Your compost has gone through all four stages and it’s done. The next post covers how to clean and actually use your finished compost in the garden.