Stages of Composting | The Phases of Composting Explained

Creating a composting pile for your garden is a fairly simple procedure that results in nutrient-rich compost to grow your plants and feed your lawn. This simple procedure hides the fact that composting is a complex chemical process. 

During the chemical process, microorganisms decompose the composting materials creating carbon dioxide, water, heat, and humus, an organic end product, throughout the composting process.

Composting occurs in four stages 

  1. a few of days in the mesophilic, or moderate-temperature phase;
  2. the thermophilic, or high-temperature phase; 
  3. a polymeric stage;
  4. the curing and maturation stage.

The mesophilic stage

The mesophilic stage is the first stage of composting. After initial assembly, the compost pile will be at around the ambient temperature for your climate. As the microbes grow this temperature will rise to over 100° F in a short space of time, at this point the mesophilic stage gives way to the thermophilic stage.

The mesophilic stage is brief; mesophilic bacteria’s main job may be to boost the ambient temperature for the thermophilic microorganisms that follow. During their brief existence, they make use of the most easily available carbohydrates and decomposable proteins. It is their activity that generates heat, and since the compost pile is insulating, the temperature increases, and thermophiles begin to predominate.

The thermophilic stage

The time necessary for your compost pile to reach the thermophilic stage varies, although it usually happens in two or three days. The temperature of the compost pile will continue to rise before stabilizing at around 150-160° F, after which it gradually cools back down to the ambient temperature. 

The thermophilic bacteria break down the non-cellulose carbohydrate and protein components of compost. These bacteria will also attack lipid and hemicellulose components, although cellulose and lignin are much more resistant.

During the breakdown cycle, the bulk of the compost decomposition will occur in the thermophilic stage of composting.

The polymeric stage – Humus and the finished compost

Stages of Composting | The Phases of Composting Explained

Compost exhibits further changes in composition once it has cooled to ambient temperature. Polymeric reactions occur at this step, producing humic acids and humus, which are complex ligno-proteins.

The curing/maturing stage

Before applying compost to the soil, it should be cured or “ripened.” Otherwise, further breakdown will occur at the price of existing soil nitrogen. A C/N ratio of around 12:1 for finished compost is normally the top limit beyond which the soil will start to lose nitrogen. However, levels beyond this are acceptable if the additional carbon is in a generally non-biodegradable form, such as paper.

The time necessary to cure compost varies greatly since it is a result of numerous factors such as the initial C/N ratio, carbon availability, aeration, and moisture. Compost can be generated mature compost in as little as 3 weeks, but on average takes 8-12 weeks. 

Temperature and Moisture of the Compost Pile

These temperatures are measured at the compost pile’s core and are typical of both big and small heaps. However, there is a significant temperature difference between the core and surface of big and tiny piles.

Because the heat loss is directly related to the surface area of the compost pile and heat production is proportional to its volume, the bigger your compost pile with a lower surface area to volume ratio loses less heat.

The temperature of big compost heaps progressively rises over time to around 160° F. In smaller piles, the temperature can level off at around occurs at 110° F  while the change from the mesophilic to the thermophilic microorganisms occurs.

If composting in colder weather temperatures may induce a slowdown or even come to a complete stop of decomposition hence the need to insulate your pile in winter.

The thermophilic stage of decomposition is the quickest. Based on oxygen, moisture, and having the ideal temperature

Beneficial Microorganisms

There are two types of composting microorganisms aerobes and anaerobes. Aerobes are bacteria that need above 5% oxygen levels to thrive and are the most significant and efficient composting microorganisms. Aerobes devour organic waste and produce nutrients like nitrogen, phosphorus, and magnesium, which plants need to prosper.

Bacteria that thrive without oxygen are known as anaerobic microbes. They also do not degrade organic material as well as aerobic bacteria do. Anaeorbs create compounds that are sometimes poisonous to plants, and they cause compost piles to smell as they produce hydrogen sulfide.

Bacteria account for between 80 to 90 percent of all microorganisms in compost heaps. Fungi, including molds and yeasts, make up the remaining 10-20 percent of the microorganisms.


The presence of fungi is vital in the composting process because they break down difficult plant compounds, allowing bacteria to continue the decomposition process after the majority of the cellulose has been depleted. The micro-floral population changes continuously throughout aerobic composting.

Initially, there are a huge number of aerobic mesophilic bacteria present. These bacteria grow at first, but as their biological activity increases, the temperature rises, reducing the quantity of mesophilic bacteria.

As the temperature falls below 105-130° F, the number of mesophilic bacteria increases again. The population of thermophilic bacteria follows the reverse trend, peaking during the thermophilic stage at 105° -160° F and then progressively declining as the temperature cools.

Saprophytes are the most common type of fungi present and they thrive on dying or dead materials and gain enzymes that break down organic material in dead organic material. During both the mesophilic and thermophilic stages of composting, fungal species proliferate.

When temperatures are high, most fungus resides in the compost’s outer zones. Molds are aerobes that develop on the compost surface and can be present as both invisible filaments and grayish fuzzy colonies.

Carbon /Nitrogen Ratio of a Compost Pile

The correlation between the Carbon to Nitrogen ratio (C/N) of a compost heap and the rate of decomposition is probably the most significant factor in effective composting.

Microorganisms need both a carbon source for growth and a nitrogen supply for protein synthesis. Because microorganisms typically need 30 parts carbon for every component of nitrogen, a C/N ratio of 30 seems to be the most desired for effective composting. 

Ratios more than 35:1 result in an increasingly longer decomposition process. If you are struggling to find enough nitrogen-rich green materials then yo could consider adding urea to your compost to moderate the C/N ratio

An ideal number is between 26 and 30:1 C/N, allowing for quick and effective composting. Ratios that are less than 25:1 result in nitrogen loss through the production of ammonia, although using the hot composting process and turning regularly can still accelerate the decomposition.  However, where ratios exceed 35:1 C/N, the decomposition process will be considerably extended.

One of the most effective ways to regulate the C/N ratio is to add or remove paper. The presence of paper is often responsible for the high C/N ratios: a change in paper content from 15 to 20 percent may change the C/N ratio from 40 to 55:1 which would result in a very slow decomposition process.

Composting Materials Carbon to Nitrogen Ratios

Materials High in Carbon Carbon:Nitrogen Ratio
straw and hay50-100:1
wood shavings or sawdust100-300:1
tree bark120:1
general paper100-200:1
corrugated cardboard500:1
Materials High in Nitrogen Carbon:Nitrogen Ratio
meat, fish soybean meal5:1
vegetable peelings and scraps20:1
fruit wastes20-50:1
coffee grounds20:1
grass cuttings15-30:1
animal manure5-30:1

Of course, there are some materials that you want to avoid composting. You can view a comprehensive list here of what should and shouldn’t be composted.


Composting is a chemical process that works to speed up the natural decay of organic material by providing the ideal conditions for detritus-eating organisms to thrive. The end product of this concentrated decomposition process is nutrient-rich soil that can help crops, garden plants, and trees to grow. 

As a gardener, the elements you can control to produce the best possible finished compost are C/N ratio, aeration (adding oxygen by turning), and moisture. These elements determine the length and extent of the four stages of composting.