The Science of Composting: What Happens During Decomposition?

Composting, the process of decomposing organic matter into a nutrient-rich soil amendment, is an integral part of a closed-loop, sustainable garden. But what is compost, and how does the process work that turns veggie scraps and garden waste into it?

Basic Steps of Composting

Composting involves collecting organic waste (like cardboard, vegetable scraps, leaves, and grass clippings), arranging it in a compost pile or bin, and waiting for it to decompose. Turning the pile will provide more oxygen for the microbes and speed the process up.

This decomposition process is facilitated by a variety of microorganisms which work to break down the organic material into compost.

The Science Behind Your Compost Pile

At the heart of composting is a process of biological decomposition, whereby microbes (with some help from insects and other fauna) break down organic matter into a simpler, nutrient-rich substance.

The Biology Behind Composting

These microbes include bacteria, fungi, and other microscopic organisms that are too small to be able to see with your naked eye.

The process takes time and the right conditions. The microbes need warmth, moisture, and oxygen to thrive. Too cold, and they’ll go dormant. Too dry or too wet, and they’ll struggle to survive. And without oxygen, they’ll suffocate. So, managing your compost pile to maintain these conditions is crucial.

During this decomposition process, the microbes generate heat, causing the compost pile’s temperature to rise. This is a good sign that decomposition is happening at a healthy pace.

As the organic matter breaks down, it transforms into a dark, crumbly substance known as humus, which is packed with nutrients essential for plant growth.

The Chemistry of the Composting Process

The science of composting also involves a series of chemical changes. As organic matter decomposes, complex compounds like proteins and carbohydrates are broken down into simpler nutrients including nitrogen, phosphorus, and potassium.

Carbon dioxide is a biproduct of aerobic composting, while methane is a bi product of anaerobic decomposition.

The Nutrient Cycle in Composting

Plants, unlike us humans, can’t simply eat a banana. They need nutrients to be in a form they can uptake through their roots. The composting process breaks down food waste andorganic material into nutrients that plants can absorb, contributing to a closed-loop garden system that recycles nutrients within the environment.

Because despite what you may have heard on Tik Tok, plants can’t absorb the nutrients in your fruit scraps until they’ve broken down into a form they can absorb. And if you’re putting those scraps straight on top of your plants, they’ll probably take even longer to break down than they would in a compost pile.

The Stages of Composting

Composting generally proceeds through four stages: the mesophilic phase, the thermophilic phase, the cooling phase, and the maturation phase. Each stage is characterised by specific temperature ranges and microbial activity, contributing uniquely to the overall decomposition process.

The Mesophilic Phase

The mesophilic phase is the initial stage of composting. This phase is named after the mesophilic microbes, which are most active at moderate temperatures, typically between 20 and 45 degrees Celsius.

These microbes start breaking down the readily available organic matter in your compost pile, such as sugars and simple proteins. As they feast away, they generate heat, causing the temperature of the compost pile to rise.

This phase can last from a few days to a week, depending on the conditions.

The Thermophilic Phase

As the temperature rises above 45 degrees Celsius, we enter the thermophilic phase. In this phase, the mesophilic microbes give way to their heat-loving cousins, the thermophilic microbes.

These microbes are capable of breaking down more complex materials like fats, hemicellulose and cellulose. The high temperatures during this phase also help kill off weed seeds and pathogens, making the compost safer for use.

This phase can last from a week to several months, again depending on the conditions.

The Cooling Phase

After the thermophilic phase, the compost pile begins to cool down, signalling the start of the cooling phase. In this phase, the thermophilic microbes slow down and the mesophilic microbes re-emerge, continuing the decomposition process.

Fungi and larger organisms like earthworms also become more active, helping to break down any remaining organic matter. This phase can last from a few weeks to several months.

The Maturation Phase

The final stage of composting is the maturation phase. At this point, the compost pile has cooled down to near-ambient temperature. The decomposition process slows down and the compost becomes stable.

This is when the humus forms, a dark, crumbly substance rich in nutrients. It’s also during this phase that beneficial organisms like earthworms and insects become most active, helping to aerate the compost and contributing to its final texture.

This phase can last from a few months to a year.

Differences Between Cool Composting & Hot Composting

Cool composting, also known as passive composting, relies on ambient temperatures and slower decomposition processes. In contrast, hot composting involves maintaining higher temperatures to accelerate decomposition and kill off weed seeds and pathogens.

The microbial ecology also differs, with different groups of microorganisms dominating at various stages of the composting process.

Cool Composting

Cool composting, often referred to as passive composting, is a leisurely approach to composting. It involves simply piling up your garden waste and kitchen scraps as you go and letting nature take its course.

Over time, the organic matter gradually breaks down due to the action of microbes, insects, and other organisms.

The process is slower than hot composting, often taking anywhere from six months to two years for the compost to fully mature. The compost pile generally remains at ambient temperatures, hence the term ‘cool’ composting.

As such, it doesn’t get hot enough to kill weed seeds or pathogens, so it’s best to avoid adding diseased plants or weeds to a cool compost pile.

Microbial activity in cool composting is primarily driven by mesophilic microbes, which thrive at moderate temperatures. They work at their own pace, breaking down the organic matter into nutrient-rich compost.

1. Build Your Pile: You can build your compost pile in a corner of your garden or a compost bin.
2. Gather Your Materials: Add your materials as they become available, mixing greens and browns as much as possible. Just like hot composting, you’ll need a mix of green and brown materials. The ratio isn’t as crucial in cold composting, but aim for about 2-3 parts brown to 1 part green.
3. Let Nature Take Its Course: With cold composting, you don’t need to turn the pile regularly or monitor the temperature. Just let it sit and nature will do its thing. However, turning it occasionally (every few months) can help speed up the process. You’ll know it’s ready when the original materials have broken down into a dark, rich soil.

Hot Composting

Hot composting, on the other hand, is a more active approach. It involves carefully managing the compost pile to maintain higher temperatures, typically between 55 and 65 degrees Celsius. This not only accelerates the decomposition process but also kills off weed seeds and pathogens, making the compost safer for use.

The key to hot composting is achieving the right balance of green (nitrogen-rich) and brown (carbon-rich) materials, moisture, and oxygen. Regular turning of the compost pile is beneficial to maintain oxygen levels and ensure even heating.

In the hot composting process, thermophilic microbes dominate, thriving in the high-heat environment and breaking down complex materials more efficiently. This results in faster composting, with the process typically completed in a few weeks to a few months.

1. Gather Your Materials: You’ll need a mix of green (nitrogen-rich) and brown (carbon-rich) materials. Depending who you speak to, the best ratio for browns to greens is 20-30:1.
2. Build Your Pile: Start your compost pile on bare ground. Layer your materials, starting with a layer of browns, followed by a layer of greens. Repeat these layers until your pile reaches about 1 cubic metre.
3. Turn Regularly: To maintain oxygen levels and ensure even heating, turn your compost pile regularly. Aim to turn it every few days, especially when the temperature rises above 65 degrees Celsius.
4. Monitor Temperature: Use a compost thermometer to monitor the temperature of your pile. It should heat up to between 55 and 65 degrees Celsius. If it doesn’t, you may need to adjust your green-to-brown ratio or turn the pile more frequently.
5. Wait for it to Cool Down: After the hot phase, let your compost cool down and mature. This could take a few weeks to a few months. You’ll know it’s ready when it has a rich, earthy smell and crumbly texture.

The choice between cool and hot composting largely depends on your composting goals, available resources, and level of commitment. Cool composting is a more laid-back approach, requiring less effort but more time. Hot composting requires more attention and effort but yields quicker results and a more sanitised end product.

The Environmental Impact of Composting

Composting not only enriches our soils but also brings several environmental benefits. It helps reduce waste and greenhouse gas emissions by diverting organic materials from landfill.

On top of this, it contributes to soil health and biodiversity by adding organic matter and beneficial microorganisms to the soil. This assists your garden in absorbing rainwater into the water table, which saves on water and helps your plants absorb more carbon from the atmosphere.

Further reading:

https://www.ozbreed.com.au/best-composting-methods-pros-cons/
https://www.ozbreed.com.au/composting-for-beginners/

Composting to Recycle Food Waste and Organic Material

Understanding the science behind composting can help us connect with the natural processes in our gardening and landscaping practices.

By harnessing the power of microorganisms and chemical transformations, we can turn waste into a valuable resource, fostering healthier soils and more sustainable gardens.

Image Attribution: 1, 2, 3