Composting 101: How Does Compost Decompose / Break Down?

This guide Composting 101 is all about how does compost decompose and break down from the scrap items that you put into your compost bin to become the “black gold” that is humus.

 

How Does Decomposition Work?

Composting relies on decomposition. That is, the organic matter that you put in your pile decomposes over time.

This work is done by organisms and some of the animals in the soil. But, for them to be able to do so, you need to provide them with the right conditions.

In a perfect world, your entire composting process could be done within 2-3 weeks. However, this is rarely the case in the real world since you’ll often deal with less than ideal conditions which slow down the overall process.

So, how long it ultimately takes for your compost to finish will depend on you being able to provide it with the right combination of the following components.

 

Carbon

Organisms use carbon for energy. They get that from carbohydrates much like we do.

But, for us, carbs include foods like bread, oatmeal, potatoes, rice and pasta.

For them, it comes from cellulose which is present in plants. These are the tough, fibrous components that our bodies can’t digest.

An easy way to identify items with carbon is to remember that most of them are brown or some shade of brown, like tan, in color.

This includes hay, dry leaves, woody plant trimmings, sawdust, straw and paper products, among other things.

 

Nitrogen

Nitrogen supports organism life. It helps build their cell walls and they need it in order to multiply. That’s because nitrogen-rich materials are rich in protein, which like in humans, are the building blocks of life.

In contrast to the brown carbon matter in your pile, those that contain nitrogen are mostly green in nature. These include leaf trimmings, grass clippings, weeds, manure, fruit waste products, eggshells and coffee grounds.

 

Water

In addition to food, organisms need water as well. As such, your compost pile won’t be completely dry. However, it’s important that it’s just slightly moist.

To give you an idea, it feels like a sponge that’s been wrung out.

This is the perfect amount of liquid for your pile. Too much water drowns the organisms. Too little dehydrates them causing them to weaken and eventually die.

One way to make it easier to manage the wetness of your compost pile is to keep it away from the sun. Using some sort of cover really helps as it reduces the effects of evaporation.

 

Oxygen

Finally, organisms need to breathe. So, in addition giving them the proper food and water, there needs to be enough “space” within your compost’s layers for air to flow through. That means making it too compact or pressing down on it it too much isn’t a good idea.

In addition, it’s worth noting that as the organisms do their job, they’ll consume oxygen. This depletes what’s in your pile. When there’s little oxygen, the decomposers will slow down. Thus, increase the time it takes to complete the composting process.

This is why people will turn their compost piles every so often – to aerate it.

 

Carbon:Nitrogen Ratio (C:N Ratio)

One of the things you’ll hear a lot when it comes to composting is the Carbon-to-Nitrogen ratio.

I won’t bore you with more science than you need to. After all, we’re gardeners, not chemists.

So, here’s the simple explanation of this relationship.

Most organic materials contain 2 elements, carbon, and to lesser degree, nitrogen.

According to compost scientists, yes, there’s a line of work that specifically focuses on composting, the perfect C:N ratio for composting is 30:1. That is 30 parts carbon to 1 part nitrogen.

In simple terms, all it means is, if you want your compost to decompose and turn into humus as quickly as possible, you want to put in 30 times as many carbon-rich materials in your pile as you do nitrogen-rich materials.

If you want a little more leeway, you can go with 25-30:1.

At this ratio, organisms are able to breakdown (decompose) organic matter to the point where you end up with the crumbly, chocolate cake soil-like texture that doesn’t stink, fast.

Composting Materials List Chart. The tables shows you items that you can compost that are carbon-rich or nitrogen-rich. And, alongside each material is the Carbon to Nitrogen C:N ratio of each one so you know what you're composting.
Composting Materials List Chart. The tables shows you items that you can compost that are carbon-rich or nitrogen-rich. And, alongside each material is the Carbon to Nitrogen C:N ratio of each one so you know what you’re composting.

 

So Why a 25-30:1 Ratio?

That’s because you need a balance between energy (carbon) and proteins/cell building blocks (nitrogen). Since the entire pile is a “long-term” project, the organisms need to replenish themselves (build cells) as they burn energy.

Too much energy means they burn out and don’t have any left to grow or multiply. Too many building blocks mean they’re growing and multiplying but don’t have enough energy to work. Either way, work slows down, albeit you get different effects in each case.

  • Too Much Carbon slows down the decaying process. So, you don’t want too high of a C:N ratio, for example 100:1 or higher.
  • Too Much Nitrogen and you end up with a smelly dump of trash. So, you definitely want to avoid a low C:N ratio.

 

Why Does This Happen?

When you have a pile where there’s so much more carbon than nitrogen, there’s a lot of energy. But, the nitrogen runs out quickly. When it does, organisms die.

This frees up the nitrogen stored in their bodies. As such, they somewhat “sacrifice” themselves so their fellow organisms can keep going.

Since there are fewer organisms left to do the work, the decomposition process slows down.

But that’s not all.

Too much carbon can create problems for soil.

If your compost pile is on the ground, there’s a chance that the lack of nitrogen will force the cells to draw nitrogen from the soil.

In doing so, it robs the ground it is on of the element.

You already know the value of nitrogen for plant growth, right? So, this means it also compromises the plants near or surrounding your compost.

On the other hand, too much nitrogen in relation to carbon also has an unpleasant effect. But, this time, you’ll be able to smell it.

This awful smell will be the result of the excess nitrogen getting lost into the air. Thus, what you end up with is the yucky ammonia odor.

 

Related

 

So How Does This Work in Real Life?

I remember the first time I tried composting. Like any newbie, I tried reading all I could.

More importantly, I followed the different tips and guides to a “T”.

So, there I was counting the 30:1 ratio to make my compost pile.

You know what?

It sucked!

Gardening was supposed to be fun. And, this, doing all the math each time I added new stuff to the pile made it tedious, not to mention a headache.

Sure, if you want the fastest results, or you’re in a hurry, by all means go for the 25-30:1 C:N ratio.

But, I’ve found much easier (and more fun) ways to compost.

Here are the two that have worked best for me.

  • 3:1 Brown to Green Ratio. This is simple. Just do 3 bags of carbon materials to 1 bag nitrogen material. This makes it very much like taking out the trash where you have the recyclables, kitchen waste and so on in different bags.
  • 1:1 Brown to Green Ratio. Here’s another one that works. It’s a bit slower than the 3:1 ratio. But, it gets the job done without stinking up the backyard. And, all you need to do is have an even pile of browns and greens.

Nowadays, I’ve gotten used to the 3:1 ratio. And, it’s not a bother to me that it does take more time than the 25-30:1 method. I’m not in a hurry anyway.

 

Below is a chart that shows you things you can put into your compost pile.

The left column consists of nitrogen-rich materials (green matter). And the right column shows the brown matter or carbon rich materials.

composting chart showing what you can put in the compost pile
composting chart showing what you can put in the compost pile

That said, there are some things you should never include in your compost pile. That’s because they will attract pests like mice, insects and other animals.

Or they can cause weeds or disease in the resulting compost which you won’t want to use in your soil.

Some are toxic as well which will damage or kill your plants if you incorporate the infected compost to your soil.

Here’s a chart of things you should not include in your compost pile.

chart that shows what you should not include in the compost pile
chart that shows what you should not include in the compost pile

 

Air & Water Balance

If you’ve been gardening a while, you know the importance of loose soil. This allows air to circulate so your plants can breathe.

Similarly, water is able to flow through to reach the bottom of your container or garden soil.

More importantly, there’s a balance between the two.

It’s one of the reasons you don’t want to overwater your plants. Doing so clogs all the spaces with moisture such that air can’t get through.

These same principles apply to your compost pile.

Within all the materials, there are spaces that allow both air and water to circulate. From above, you know the decomposition process needs both.

  • If there’s too little moisture, they get enough air. But, the lack of water will ultimately kill the organisms. So, your decomposition slows down.
  • If there’s too much moisture, the spaces are clogged with water that there’s not enough air getting through. This leaves you with a very smelly pile.

As such, balance is key.

 

How Do You Maintain Enough Moisture in Your Compost Bin?

From my experiments, you don’t want to go under 40% moisture. That is, 40% of the weight of the pile.

When you go under this level, I’ve found that things dried up. And, when that happens, decomposition slows down considerably. I would believe that’s because the organisms starve from lack of water. So, they slow down and some die.

Instead, you want to stay around 50% or so. Between 40-60% works as well.

I know what you’re thinking…

How in the world do I weigh that?

No need.

Just put your hand into the pile and get a handful of material, then squeeze. This should feel like a sponge that’s been wrung out. Still damp but not wet or soaking.

Do this a few times in different areas and you’ll have a good idea of the overall moisture of your pile.

If any area feels dry, then add some water there and re-squeeze.

 

When There’s Too Much Water

On the other hand, anything over 65% of the weight of your pile in water is too much.

Unfortunately, this lack of air is easier to detect.

You’ll smell it. Believe me, it stinks (literally)!

Using the same sponge analogy, if you squeeze a handful of compost and water drips, you know you’ve got too much moisture.

Like water, organisms need oxygen in order to survive. Too much water blocks the circulation of air.

Just as importantly, when the organisms work hard, they use up oxygen and excrete carbon dioxide. If these spaces are water-clogged, the CO2 can’t escape.

Later on below, you’ll also read about your compost pile gets hot. Oxygen likewise plays a role in bringing up the temperature to speed up decomposition.

The bottom line is, the entire process slows down without enough air.

 

How Do You Balance Air & Water in Your Compost Pile?

Here are a few ways.

  1. Monitor your pile by getting handfuls and squeezing them
    • If it feels like it’s drying out, add water
    • If it feels wet, add browns (carbon material). Items like dry leaves, straw and sawdust will soak up the excess liquid.
  1. Turn your pile. Turning introduces new spaces and allows air to get into areas of the pile that didn’t get enough of it. Also, turning helps dry the pile a little.

 

Understanding the Decomposition Process

Now that you know what you need to make your compost pile work, it’s time to understand what happens “under the hood”.

In short, how the actual decomposition of your compost works.

As usual, I’m not a chemist or botanist, so I won’t bore you with the technical jargon. Instead, I’ll give it to you in simple terms.

 

Physical Breakdown

This is stage 1 of the decomposition process.

At this point, you already know that at some microorganisms like bacteria and fungi will breakdown the materials in your compost pile to something else.

But, before that can happen, something else has to breakdown the large materials like tea bags, dead houseplants, leaves, cardboard, paper towels and eggshells.

Only then can the microorganisms go to work. Otherwise, these items are just too big for them.

That’s where the physical decomposers come in. You’re likely very familiar with these crawly creatures. They are the same ones that feast on your garbage when you leave it outside.

Their job is to chew, bite and shred these objects to turn large pieces into very small ones.

 

Physical Decomposers in Your Compost

You can see most of these creatures crawling around. Although some of them, like roundworms, are so small, you likely can’t.

  • Roundworms
  • Mites
  • Springtails
  • Pillbugs
  • Sowbugs
  • Centipedes
  • Millipedes
  • Beetles
  • Slugs
  • Snails
  • Ants
  • Flies
  • Earthworms

 

Chemical Breakdown

Once the materials are broken down physically, the microorganisms like fungi and bacteria get to work.

Here’s where a little bit of chemistry happens. These microbes release enzymes that break down the chemical structure of the items to turn them into simpler compounds. In turn, they use these byproducts as food to sustain themselves.

It is similar to what happens in your stomach with food. (I know, not the best analogy)

Anyways, these microbes along with other organisms work to decompose all the materials in your pile.

When they’re done, you get humus.

 

Chemical Decomposers in Your Compost

These guys do the “heavy lifting”. By that, I mean the most important part of composting, which is turning your waste items into humus.

Unlike physical decomposers, you can’t see these organisms.

  • Bacteria
  • Actinomycetes
  • Fungi
  • Protozoa

 

Giving the Breakdown a Helping Hand

Here’s where you come in.

One of the things I’ve noticed is that if I take a few minutes to cut up the materials before putting them into the compost pile, it speeds up the entire process.

After some trial and error, I noticed that the smaller the items you put in, the faster the overall time gets.

Of course, do consider the extra time and work you need to do to shred, cut and break up some of the items as well.

In the end, anything between 1-3 inches in size works really well. The smaller you can go the better. But, it also means more manual work for you.

 

Your Compost Pile’s Creature Hierarchy

I’m not sure how much you remember from your grade school science classes. But, you probably still know the term “food chain”.

It’s basically a circular image showing you which creatures eat which creatures to survive. (Again, sorry for the imagery)

While that doesn’t look or sound appealing, it’s necessary. Otherwise, we wouldn’t have meat, chicken, pork, eggs and even vegetable to eat.

In any case, your compost pile its own “food chain” so to speak.

That is, organisms within your pile depend on the presence of one another to survive. So, it’s not just about the kind of work they do, although as you’ve seen above, each group of decomposers in your pile often performs a specific duty.

However, in this section, we’re looking at them as consumers.

Why bother?

You need to invite all these consumers (who’ll eventually do the decomposing work for you) to the “party” so to speak. Without one or the other you won’t be able to attract other kinds of consumers.

And, when that happens, one or more stages of decomposition won’t happen later on.

I hope that makes sense.

 

Here’s How It Works

When you dump all the materials into your compost pile, the first ones to arrive are the primary consumers.

 

Primary Consumers

These are the guys that often come to visit your trash bin. They’re composed of both physical and chemical decomposers. So, you want both groups to be present in your pile since they’ll both contribute to the work later on.

  • Roundworms, millipedes, snails, slugs, pillbugs and mites are just some of the creatures that will start breaking down the physical items.
  • Additionally, bacteria, fungi and protozoa also come in.

These guys are attracted to the leaves, manure, grass and woody clippings as well as kitchen waste that you throw into the pile.

 

Secondary Consumers

These prey on the primary consumers. They arrive because they want to find food in the form of primary consumers.

But, they will also play a role in decomposing your compost as well.

Among these include beetles and springtails. Additionally, some kinds of protozoa that eat bacteria also arrive.

 

Tertiary Consumers

These are the biggest creatures in your compost pile. And, you can see them quite easily.

They, in turn, eat the secondary consumers.

Some of the creatures here include centipedes and mites.

 

Bottom Line

Just like air and water, you want to balance the creatures within your compost pile.

These creatures are both consumers and decomposers. So, you need both for your pile to turn into humus.

The right group of consumers will ensure that other creatures and organisms will arrive. Because these creatures and organisms also happen to be the ones working to break down all the materials, your compost will finish faster.

 

Why Does Your Compost Get Hot?

One of the things you’ll notice is that your compost bin will get hot. At first, it might feel scary. Or, you may think that something weird or wrong is happening.

But, a hot compost pile is actually a good thing. It’s a sign that the decomposers are at work.

Interestingly, you can actually tell what’s happening in your compost based on its temperature. That’s because different organisms produce different heat levels from their activities.

Another fun fact is that while there are many microorganisms living in your pile, bacteria is the most dominant one, making up to 90% of the population.

As such, if you look at the stages below, you’ll notice that 3 types of bacteria stand out. They are:

  • Psychrophiles
  • Mesophiles
  • Thermophiles

In any case, here’s what happens in your heap.

 

Stage 1: Psychrophiles Start The Process

Psychrophiles are bacteria that thrive in low temperatures. As such, they like it when the compost pile is still cool.

They enjoy it when the levels are in the 20s. Yet, they’re happy even between 30 to 50 degrees.

As they consume the materials in the heap, they release heat. In the process, in starts pushing up the temperature in your compost pile.

This heat also helps in breaking down the items in the pile. But more importantly, when it gets to a certain level, it begins to invite another kind of bacteria.

 

Stage 2: Mesophilic Microorganisms Take Over

Mesophilic creatures like temperatures between 50 to 110 degrees Fahrenheit. These decomposers do their best work in the 60 to 80-degree range. And, they break down the contents of your compost heap to produce more heat.

These creatures do a large chunk of the decomposition.

During this phase, it’s important that your pile has enough carbon, nitrogen, air and water. Together, they’ll allow the microbes to really work and bring the temperature up to 100 degrees.

Unfortunately (for them), when it gets to over 100 degrees, the heat kills them.

 

Stage 3: Thermophilic Microorganisms Really Turn Up the Heat

Once the temperature reaches 100 degrees, it’s time for the thermophilic microorganisms to dominate the decomposition process.

These microbes work so hard that they’ll push the temperature of your compost pile all the way to between 130 to even 160 degrees. That’s when a lot of the complex compounds are broken up.

Additionally, these high temperatures also sterilize your compost. As such, it eliminates the risk of passing diseases to your soil when you add your compost to it. The heat also kills any weed seeds that might be in the heap.

That said, you don’t need your compost to get to very high temperatures. Although the high levels will mean that your compost will be “finished” earlier.

The high temperature will last a few days. If you want to sustain it, you’ll need to increase the airflow. As such, turning your pile over helps bring the temperature up again when it starts to drop.

 

Stage 4: Mesophilic Bacteria Return to Action

After the microbial activity peaks, it will slow down. And, the temperature will come back down to under 100 degrees.

This final stage is the longest part. It will last between weeks and months.

 

More About Heat

From above, it’s easy to understand that high heat is good. After all, it

  • Kills diseases and pathogens (130-140 degrees)
  • Speeds up the composting process
  • Eliminates weed seeds (over 130 degrees)

That said, too much heat isn’t good.

Reaching temperatures over 175 degrees for hours at a time actually slows down the entire process. That’s because too much heat prevents the microbes from working at their best.

So, when this happens, allowing some air to get into the pile helps cool it a bit.

 

Cold Composting

That said, you can also opt for slow composting. This takes considerably longer, often at least 6 months or longer. It also doesn’t kill diseases or weed seeds.

So why bother?

You almost don’t have to do anything other than dump the materials into the compost pile.

It’s also worth noting that a few other composting methods use cold temperatures. These include worm composting (vermicomposting) and lasagna gardening (sheet composting).