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  • Nithin Vinod

Understanding the science behind composting : Factors and Phases

Updated: Oct 26, 2023


phases of composting, thermophilic, mesophilic composting, factors of composting

India, one of the world's fastest urbanizing countries, faces a significant urban waste problem. Each year, over 70 million tonnes of garbage is generated with only a fraction of it being treated effectively, while the remaining ends up in landfills due to inadequate waste management. A huge portion of this waste is biodegradable and can be converted into a valuable resource. Instead, it becomes a potent source of greenhouse gases. This is where composting comes as a crucial component of waste management.


Composting, an age-old practice rooted in civilizations like ancient China and Rome, has a rich history of enhancing soil fertility through the decomposition of organic materials. While it temporarily waned with the rise of industrialization and synthetic agriculture, it saw a resurgence in the 20th century, thanks in part to the pioneering work of Sir Albert Howard.


Composting which is nature’s way of recycling organic matter and nutrients, holds the key to converting our organic waste into incredibly valuable. In the corporate world and even on a national scale, sustainability and eco-consciousness have rightfully garnered significant attention as efforts to reduce ecological footprints intensify. Composting emerges as a simple yet powerful tool to simplify and enhance this process. However, to effectively incorporate this process, it becomes crucial to grasp the various aspects that are responsible for driving it. Implementing composting at a larger scale becomes more manageable when the different phases and factors behind it are clearly understood.

Composting is a process where different organic materials undergo various changes, eventually becoming nutrient-rich soil called humus. Through this article, we will delve into the science behind composting and how it can be controlled by breaking it down into its fundamental phases and critical parameters. Whether you're a sustainability official in a corporation trying to get a better bottom line or a gardener looking for improved soil fertility, understanding the composting process is essential for its efficient management.


Let us delve deeper into the factors that influence the composting process:


Factors influencing the composting process:

  • Temperature: Temperature is a critical factor in composting. It affects the rate of decomposition and the types of microorganisms that thrive in the compost pile. During the initial mesophilic phase (20-45°C), microorganisms break down easily degradable compounds like sugars and proteins. The subsequent thermophilic phase (45-65°C) is vital for eliminating pathogens and ensuring sanitary conditions. If temperatures exceed 65°C, it can harm microorganisms and enzymes, disrupting the composting process.

  • pH: pH levels in the compost pile play a significant role in microbial activity. Most microorganisms thrive in a neutral to slightly acidic pH range (around 6.5 to 7.5). Managing pH is crucial as extreme levels can inhibit microbial activity and slow down composting. Adjustments with materials like lime or sulfur may be necessary to maintain optimal pH.

  • Moisture Content: Proper moisture content is essential for composting. Microorganisms require moisture to carry out biochemical reactions. Insufficient moisture can slow decomposition, while excess moisture can lead to anaerobic conditions and unpleasant odors. The ideal moisture content typically ranges from 40% to 60%.

  • C/N Ratio: The carbon-to-nitrogen (C/N) ratio is a fundamental parameter. It represents the balance between carbon-rich (e.g., leaves, straw) and nitrogen-rich (e.g., food scraps) materials in the compost pile. A balanced C/N ratio (around 25-30:1) ensures efficient decomposition. Too much carbon can slow down the process, while excess nitrogen can lead to odor issues.

  • Particle Size: The size of composting materials impacts aeration and decomposition rates. Smaller particles decompose faster because they offer a larger surface area for microbial activity. However, if particles are too small, they can lead to poor aeration and compaction issues. Therefore, achieving the right particle size balance is essential.

  • Nutrient Content: Adequate nutrient content, including nitrogen, phosphorus, and potassium, is necessary for microbial growth and activity. Managing nutrient levels is crucial to ensure that microorganisms have the resources they need to break down organic matter effectively.

  • Oxygen Supply: Adequate oxygen is crucial for the composting process. Oxygen supports aerobic decomposition, which is more efficient and less odorous than anaerobic decomposition. Turning or aerating the compost pile regularly helps maintain sufficient oxygen levels. Poor aeration can lead to foul-smelling compost.

  • Carbonaceous Materials: Carbon-rich materials (browns), such as dried leaves and straw, provide the energy source for microorganisms. These materials should be balanced with nitrogen-rich materials (greens), such as kitchen scraps and grass clippings, to achieve the optimal C/N ratio.

  • Aeration: Effective aeration through turning or mixing the compost pile helps distribute oxygen and ensure uniform decomposition. Inadequate aeration can lead to localized anaerobic pockets in the pile, causing odor and slowing decomposition.

  • Microbial Activity: The composition and activity of microbial populations in the compost pile are vital factors. Different microorganisms are active at different temperature ranges, and their activities contribute to the breakdown of organic matter.

  • Time: Composting is not an instantaneous process; it takes time. The length of time required for composting can vary depending on factors like temperature, pile size, and materials used. Patience and consistent monitoring are essential.


In summary, composting is a complex biological process influenced by a combination of physical, chemical, and biological factors. Understanding and carefully managing these factors are key to achieving efficient and high-quality compost that can benefit both the environment and agriculture.

The factors discussed above play a significant role in determining the rate and efficiency of the composting process. By understanding and managing these factors effectively, we can optimize the composting process and produce high-quality compost that is rich in nutrients and beneficial to the environment. Now let us discuss the phases through which the composting process goes through.


Phases of Composting

  • Mesophilic Stage (1 - 2 Weeks)

The mesophilic stage is the initial phase of composting and typically lasts for the first few days to a couple of weeks, depending on various factors such as temperature, moisture content, and the composition of the organic material. During this stage, the temperature of the compost pile rises gradually as microorganisms, primarily mesophilic bacteria, start breaking down organic matter. The temperature in this stage usually ranges from 20°C to 45°C.

Mesophilic bacteria are efficient decomposers of organic materials and are active under moderate temperature conditions. They break down simple sugars, starches, and proteins, producing carbon dioxide, water, and heat as byproducts. As microbial activity intensifies, the organic material begins to break down and decompose.

  • Thermophilic Stage (2 - 6 Weeks)

As the compost pile continues to decompose and microbial activity increases, it enters the thermophilic stage. In this stage, the temperature of the compost pile rises significantly, often reaching temperatures between 45°C to 71°C or even higher. The heat generated by microbial activity in this stage is crucial for killing weed seeds, pathogens, and undesirable organisms.

Thermophilic bacteria, along with heat-loving fungi, take over during this stage. They break down complex organic compounds such as cellulose and lignin, which are typically resistant to decomposition. The high temperatures are also indicative of increased microbial activity and efficiency in breaking down organic matter.

  • Cooling Stage (1 - 3 Months)

After several weeks in the thermophilic stage, the compost pile gradually begins to cool down as the readily available organic matter is depleted. The temperature drops back to mesophilic levels, signaling the cooling stage. During this phase, the compost pile may still experience some fluctuations in temperature, but it eventually stabilizes at or near ambient temperature.

The cooling stage is crucial for the development of a stable and mature compost product. As the compost cools, mesophilic microorganisms continue to break down any remaining organic matter and further stabilize the compost. This stage can last for several weeks to several months, depending on various factors.

  • Stabilization Phase

The stabilization stage represents the final phase of composting. During this stage, the compost undergoes a maturing process, becoming more homogeneous and earthy in texture. Microbial activity decreases significantly, and the compost becomes less biologically active. The compost is considered stabilized when it no longer produces significant heat and has a uniform, dark, crumbly appearance.

At this point, the compost is ready for use in gardens, agriculture, landscaping, or other applications. It is rich in valuable nutrients, beneficial microorganisms, and organic matter, making it an excellent soil conditioner and fertilizer.


In conclusion, composting holds immense potential to address India's pressing urban waste challenge. By understanding and managing critical factors such as temperature, pH, moisture content, C/N ratio, and more, we can optimize the composting process for efficient and high-quality compost production. Composting not only converts organic waste into valuable resources but also benefits the environment and agriculture. Embracing composting on a larger scale is essential for sustainable waste management and promoting eco-conscious practices in both corporate and agricultural sectors. As we explore the phases of composting, from mesophilic to stabilization, we recognize its significance in creating nutrient-rich soil amendments for a greener and more sustainable future.



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