Soil Science: How Microbes Make Compost to Feed the Soil

Are you looking for a little microbe magic? Think composting.

Composting is a great way to reuse food and plant waste that you would otherwise throw into the trash, which would just end up in a landfill somewhere. During the composting cycle, microbes reduce this organic waste until it can be fed back into the soil as rich, crumbly compost. When returned to the soil, compost feeds plants and improves the nature of life underground. Sound like a great idea? It is — and it's easy.

Top-down composting offers serious benefits to the environment. It takes food and plant scraps off your table and out of your trash so they can be recycled and reused. The food waste you are not sending to the landfill reduces greenhouse gasses and makes you part of the movement to slow climate change. And better yet? The compost you create enriches your house plants, potted outdoor plants, garden, or a nearby farm — everybody, including the planet, wins.

How Does Composting Work?

Although composting is easy, the explanation behind it can be complicated — so let's break it down, right along with the food scraps. Here are the basic steps behind the compost cycle:

Step 1: It Was a Great Dinner

For friends and family, or just you and your dog, it was a great dinner. Salad was involved, so there are fruit and vegetable peels and unused, inedible stalks. You have some eggshells and coffee grounds sitting around from breakfast, too. In a bin under your sink, or in your outdoor composter, you drop off these compostable materials and head out for a walk.

Step 2: Time Passes

Sitting there in your composting pile, the food waste you added starts to decompose. Later, a banana peel hits the pile as well. Regardless of the size of your composter, over time, the addition of nitrogen, in the form of juicy green stuff, and carbon, in the form of dry woody or leafy stuff, starts to add up. That's when the action starts.

Step 3: Decomposition

You can't see them, but bacteria are living in style in your compost pile. Bacteria are your essential decomposers. These aerobic bacteria are so small it would take 25,000 laid out in a straight line to measure an inch.

While aerobic bacteria do the heavy lifting in the composting cycle, they do not do aerobics. Aerobic bacteria thrive in oxygen-rich environments, unlike anaerobic organisms that do not require oxygen for growth. As your green and brown food and garden waste breaks down, mixes, and mingles, different types of aerobic bacteria start to heat things up.

Step 4: Hot Times in the Compost Pile

Like a lot of things, compost goes through phases before it is finished. As the compost cycle gets underway, psychrophilic bacteria step up to the plate. These are bacteria that live in cooler environments, between 50ºF and 70ºF, and they happily nosh your potato peels, freezer-burned veggies, and garden trimmings.

During any phase of the process, bacteria eat nitrogen in the greens to build protein, and they eat carbon in the brown stuff to help them grow and multiply. As part of their digestive and decomposing processes, bacteria oxidize your food waste in order to metabolize it for energy. So, by eating plant waste, bacteria gain energy and give off heat as a byproduct. Meanwhile, your vegetable peels and used tea bags are breaking down and changing form.

Step 5: A Party in the Pile

The heat given off by the bacteria kicks up the temperature of the composting bin, or pile, and puts out a welcome mat for bacteria that like conditions a bit warmer than psychrophilic bacteria. These newcomers in the second phase of composting are called mesophilic bacteria.

As mesophilic bacteria enjoy the buffet in a cozy setting between 70ºF and about 100ºF, the compost gets even hotter, and both of these families of bacteria make way for thermophilic bacteria that like their food served hot, around 113ºF to 160ºF. As these bacteria enjoy their king of the heap status, the heat of the compost kills pathogens and weed seeds. The hotter it is, the faster the food waste is used, and the compost pile eventually cools.

Soil Science: How Microbes Make Compost to Feed the Soil — You can see the steam rising off this extensive compost pile. *Image by John Winfield/Geograph*

The mesophilic bacteria move back in, along with fungi, as the compost continues to attract more rotifers and decomposers. These are tiny and not-so-tiny critters, like fungi, mites, sow bugs, centipedes, worms, and ants. Each micro- and macro-organism use the composting environment to its benefit.

When the compost is eventually added to the soil, these organisms help enrich, fertilize, and nurture the soil and the plants that live in it.

Is It Done Yet?

It heated up, it cooled down — is it ready to go? Several factors determine when your compost, also known as "black gold," will be ready to spread. Those factors are:

The size of your composting bin or pile: Size matters. A large pile of compostables is going to take longer than a micro-composter.
Food input: A steady diet of green and brown food waste ensures your compost, and your bacteria, enjoy balanced nutrition. If you throw a couple of peels and a biodegradable coffee pod in, and that's it — it's going to take a while. Also, one of the first clues that your compost pile is doing well is the smell. Aerobic bacteria and their slightly more refined bacterial friends actinomycetes emit a pleasantly earthy scent. Anaerobic bacteria stink. Does your compost smell bad? Then one of the factors in this list is amiss.
Bacteria need to breathe: Because the type of bacteria that enjoy food waste also like oxygen, you have to turn the compost pile every so often. While composting is easy — it is only a "set and forget" situation when you have a big pile on the North Forty. Otherwise, you need to turn over the compost in your bin every so often. Frequency depends on the type of your composter. With a tumbler, you can just give it a whirl every week. With a standard home composter (between 3' x 3' or 5' x 5'), you should turn it with a pitchfork every couple of weeks. The point is to leave the waste long enough to get some bacterial action going, and then turn it to give the bacteria more air, de-compact your compostables, expose more surface area, and speed your process.
Moisture: If you only throw in greens, your pile is going to get slick, slimy, and stinky. Remember the balanced diet. To get the rich, crumbly stuff at the end of the rainbow, you need to keep the pile moist — not dry, not wet — but moist. In the summer, you'll want to sprinkle the compost every once in awhile as you continue to feed in your yard and food waste.

Composting is a natural process that gives back in good ways. Take some tips from us, composting does not take a lot of effort or time, and you cannot truly mess it up. If you have a compost bin outside, and you occasionally throw stuff in but don't turn it or keep it moist, it will still compost over time — it will just take longer. The only mistake you can really make with composting is not to try it in the first place.