Aerobic Composting – Creating an Active Compost For Life

The idea of nurturing healthy soil deserves to be consistently imparted as a standard lesson in schools and colleges. The act of enriching the soil not merely boosts the well-being of plants, but also benefits the general environment. The straightforward and impactful technique of composting can mitigate a number of negative environmental impacts resulting from human practices.

Composting is the process of starting a soil food web (soil biota) by causing the proliferation of aerobic microorganisms (bacteria and fungi) in a controlled environment using organic waste materials.

Estimations are that in the next 40 years, we will need to produce as much food (calories) as was grown in the 10,000 preceding years combined (Source). For humanity to survive and prosper, we must have healthy soil. As gardeners, we can improve the health of our soil naturally.

However, not everyone knows that soil fertility is a direct product of soil-dwelling organisms, collectively called the soil biota. We need to understand that compost is more than a soil augmentation. Composting is the development of healthy soil biota – an essential ecosystem for soil productivity and health.

Picture of Tony O'Neill filling wheelbarrow with compost from a compost bin

25 Reasons Why Aerobic Composting is Essential for a Healthy Garden

  1. To advantageously recycle dead plant material
  2. To reduce pathogens and unwanted seeds
  3. To concentrate nutrients in ways that make them available to plants
  4. To support, optimize, and balance soil microbe and soil fauna populations
  5. To improve and repair soil structure
  6. Limit erosion, runoff, leaching, and compaction
  7. Eliminate soil contaminants, rehabilitate sick soil
  8. Improves soil fertility and tilth
  9. Optimize soil pH variability
  10. Maintain optimal ratios of fungi to bacteria in soil
  11. Maintain optimal ratios of predator/prey in the soil biota
  12. Optimize nutrient cycling and micronutrient uptake
  13. Maximize nutrient density
  14. Sequester excess atmospheric carbon
  15. Increase crop immunity
  16. Support plant-pollinator populations
  17. Manage pest populations without pesticides
  18. Manage weeds without herbicides
  19. Improve flavor and nutrition for animals and humans
  20. Concentrate and retain growth forces essential to human existence
  21. Increase soil gas and water holding capacity
  22. Reduce odor, fly, and other vector problems
  23. To advantageously decompose fecal matter
  24. Limit wastage of valuable resources
  25. Limit the production of harmful gasses from landfills (methane. ammonia, hydrogen sulfide, etc.)

Aerobic Composting Process

For microorganisms to grow and reproduce, a diverse population of predominantly aerobic microbes must decompose the organic matter. Carbon-to-nitrogen ratio management, oxygen supply, moisture content, temperature, and pH management all encourage the activity of microorganisms in compost piles.

When done correctly, composting speeds up natural decomposition while generating enough heat to kill weed seeds, pathogens, and fly larvae. Activated composting and curing are the two distinct phases of the composting process.

This composting process decomposes easily degradable and decay-resistant materials like cellulose during a period of high microbial activity.

Cultivation occurs after active composting, reducing microorganism activity and further decomposition of active composting byproducts (nitrogen conversions). Curing compost to its final stage means it has reached stabilization.

During the active composting period, the compost pile experiences a wide range of temperatures. Some microorganisms cannot survive when the temperature changes, while others thrive in new conditions. A home composting system has three temperature ranges during the active composting period.

Infograph of the compost thermal cycle

These temperature ranges are psychrophilic, mesophilic, and thermophilic based on the types of prominent microorganisms in the pile at those temperatures.

Temperatures lower than 50 degrees Fahrenheit are considered psychrophilic, while temperatures in the mesophilic band range from 50 to 105 degrees Fahrenheit, and temperatures in the thermophilic range over 105 degrees Fahrenheit.

However, the defined temperature ranges do not exclude the possibility of the broader presence of other microorganisms. Temperature ranges provide a rough distinction between the temperatures at which different microorganisms reach their maximum growth rates and efficiencies.

When composting first begins, there is usually a short lag time before the temperature rises rapidly. This lag time is required for the microbial population to grow. The self-insulating compost traps heat generated by microbial activity as the population begins to degrade the most readily degradable material and grows.

As the microbial population grows and diversifies, the temperature increases steadily through the psychrophilic and mesophilic temperature ranges. Depending on the operation, compost piles can take anywhere from 2 to 3 days to transition from mesophilic to thermophilic composting.

Because of the diversity of the microbial population, a wide range of materials, from simple, easily degradable ones to more complex, decay-resistant ones like cellulose, can be decomposed.

Picture of a Celcius thermometer

Expect a rise in temperatures, with a maximum of 130 to 160 degrees Fahrenheit. The microbial activity begins to decline as soon as the readily degradable material and oxygen run out or if the temperature rises too high and becomes harmful to their function.

When the substrate becomes depleted, more heat is lost from the pile than is generated as microbial activity decreases and the pile cools.

Various microorganisms repopulate the pile as the temperature drops below thermophilic levels. At the same time, spores germinate as conditions improve and migrate from cooler spots. It’s these microorganisms that keep the decomposition process moving along.

Depending on the process, the compost pile can stay in the thermophilic range for anywhere from 10 to 60 days.

Aerate the pile to reactivate active composting once the temperature drops below 105 degrees Fahrenheit. Active composting is never determined to be finished at a specific point.

When the pile conditions are such that microbial activity cannot increase enough to reheat the pile, it is usually considered complete and ready for curing.

As microbial activity decreases during curing, the composted materials become more stable. When organic acids and decay-resistant compounds continue to decompose, they are stabilized by other processes like the formation of humic compounds and the generation of nitrate-nitrogen.

Curing has the added benefit of introducing beneficial fungi to the pile, which helps the compost’s disease-fighting properties.

Microbiological activity has decreased and is now operating at a lower level, resulting in the pile generating less heat and decreasing temperature. Proper moisture and oxygen management are still required to maintain microbial activity during the curing period.

To avoid recontamination of the pile with weed seeds, further management is required during the curing period – it may be necessary to cover or relocate the curing piles.

Because curing reactions are so slow, you’ll need plenty of time to wait for them to finish. Curing times vary depending on the operation, the amount of time the compost has been actively composting, and the intended final use of the compost. Extensive curing times are needed when composting takes place for only a short period.

When compost is going to be applied to sensitive crops or used in potting media, it needs to cure for even longer. Curing is complete when the pile reaches room temperature again after several mixings.

Cooling caused by adequate curing must be distinguished from cooling caused by insufficient oxygen supply or moisture content. Curing may take from one month to six months.

The Soil Food Web (or Soil Biota)

I have for decades advocated for feeding the soil rather than the plants. Our knowledge regarding the interaction of plants with soil, and vice versa, is ever-improving. While chemistry has long dominated the plant productivity landscape, there is growing evidence that working with nature reduces the need for added chemicals.

A healthy soil food web will produce all the required chemicals (in the best form) in reciprocation to the plant’s biological system’s support. If humans don’t interfere with the balance, there is a win-win relationship between plants and the soil they grow in.

The biological system of roots, organic matter, bacteria, fungi, soil fauna (nematodes, arthropods, and protozoa), birds, and animals creates a balanced symphony of life. Let’s take a look at that ecosystem in some detail.

If you take the jargon away, this is what happens in layman’s terms:

Infograph if the symbiotic cycle between plants and soil organisms

Soil Bacteria

Before there was man, there were bacteria – billions of years old. Bacteria are some of the tiniest, most abundant microbes in the soil. The average person counts 150 in 30 seconds. Counting non-stop, the bacteria in a teaspoon of soil would take more than six years.

There are an estimated 60,000 different species of bacteria, of which about 6 percent have been named. Each of them lives symbiotically with unique roles and capabilities. Most live in your topsoil, where organic matter is present.


In comparison to bacteria, fungi are much larger organisms. Filaments are long strands that connect individual cells to form networks. It’s common for fungi to appear in the composting process as it progresses. In addition to wood and decay-resistant materials like proteins and hemicelluloses, fungi decompose waxes and lignins.

Compared to bacteria, fungi are more resistant to low moisture and pH. Still, because most fungi are obligate aerobes, they are intolerant of low oxygen levels. Fungi are also unable to survive temperatures greater than 140 degrees Fahrenheit.

Composting is a delicate balance between ensuring high enough temperatures, ensuring enough oxygen, and preventing temperatures exceeding 140 degrees Fahrenheit. Managing your compost pile is as much an art form as a science.


The actinomycetes are the compost pile’s third most common microbial species. Although actinomycetes are classified as bacteria because of their structure and size, they are more closely related to fungi. They form filaments and can grow on a wide range of substrates.

Actinomycetes can degrade organic acids, sugars, starches, and proteins. Their extracellular proteases can disintegrate or dissolve other bacteria. Actinomycetes become more prevalent later in the composting process – after the most easily degradable compounds have been degraded, the moisture content has dropped, and the pH has risen.

Higher Organisms

Higher organisms begin to invade once the compost pile cools to an appropriate temperature. It’s important to note that these microorganisms are diverse. The presence of these more complex organisms enhances the disease-fighting properties of compost. Additionally, they help degrade wood by consuming bacterial and fungal biomass.

Chemical Transformations in Composting

Picture of chemical compound

Plants can only absorb certain synthesized chemical compounds, a product of the soil biota. During the composting process, microorganisms degrade the raw material of the compost mix to synthesize new cellular material and obtain the energy for these catabolic processes.

Several chemical transformations occur as complex compounds are broken down into simpler ones and then synthesized into new complex compounds. Before the microorganisms can synthesize new cellular material, they require sufficient energy for these processes.

The two possible modes of energy-yielding metabolism for heterotrophic microorganisms are respiration and fermentation. In compost manufacturing, avoid fermentation.

Respiration can be either aerobic or anaerobic. In aerobic respiration, the aerobic microorganisms use molecular oxygen, O2, to liberate the bulk of the energy from the carbon source, producing carbon dioxide and water. This conversion is achieved through a series of sequential reactions.

These reactions liberate significant quantities of energy and form many organic intermediates that serve as starting points for other synthetic reactions. We should aim for aerobic respiration, avoiding anaerobic respiration, and fermentation for composting. Aerobic respiration is more efficient, generates more energy, operates at higher temperatures, and does not produce odorous compounds.

Aerobes can also use a greater variety of organic compounds as a source of energy, resulting in more complete degradation and stabilization of the compost material. Aerobic respiration forms organic acid intermediates, but these intermediates are readily consumed by subsequent reactions, reducing their potential for odors compared to anaerobic respiration.

Another critical chemical transformation of the composting process is nitrification, the process by which ammonia or ammonium ions are oxidized to nitrates. Nitrification is a two-step process that requires time.

The process of nitrification takes place during the curing process. Plants cannot tolerate nitrites (NO2 ). Nitrates (NO3) are a valuable form of nitrogen for plant metabolism. For the conversion to happen, the curing period must be long enough. Because nitrification necessitates the presence of oxygen, the compost pile must be adequately aerated throughout the curing process.

Nitrogen Loss During The Process

A significant amount of nitrogen is lost during the composting process. However, the amount of nitrogen lost varies widely and somewhat depends on the material, method, and management methods employed.

Nitrogen losses are a concern because of groundwater contamination risks, odor problems, and the valuable nitrogen content of the compost. You can manage the composting operation to reduce the potential for nitrogen loss. Effective heat management and aeration reduce the loss of nitrogen.

How Carbon: Nitrogen Ratios (C: N Ratios) That Affect Operations

Picture of cupped hands holding soil

Specific nutrients are required in significant quantities by microorganisms. Macronutrients like carbon (C), nitrogen (N), potassium (K), and phosphorus (P), are only some of the required supplies.

The composting process is heavily influenced by the waste’s ratio of carbon to nitrogen. They serve as essential nutrient content indicators.

Other nutrients are likely to be present if these are adequately present. Carbon serves two purposes: it is a fuel for microbe growth and an energy source. In aerobic decomposition, carbon dioxide (CO2) is released partly as a product of the decomposition process.

The remainder is used for microbial growth after being combined with nitrogen. Carbon in compost depletes over time due to this process.

Nitrogen is used to synthesize cellular material, amino acids, and proteins. It is continuously recycled through the cellular material of the microorganisms.

When the microorganism dies, any nitrogen it has taken up in the cells will be released. The carbon-to-nitrogen ratio decreases during composting because most of the carbon is continuously released. In contrast, most of the nitrogen is recycled. The C:N ratio can rise if the system suffers significant nitrogen losses. For quick composting, a C:N ratio of 20:1 to 40:1 is recommended.

Oxygen in the Aerobic Process

Aerobic microorganisms require oxygen to survive. Without enough oxygen, anaerobic microorganisms will take over, slow the composting process, and produce unpleasant odors as anaerobic conditions set in.

An atmosphere with a 5 percent oxygen concentration is needed to maintain aerobic conditions. Forced or passive aeration can be used to add oxygen to the pile. Aeration methods aside, the amount of air supplied to a compost pile does not necessarily correspond to the amount of oxygen reaching the microorganisms.

An important consideration is the amount of moisture in the compost pile – too little and limited microbial activity, and too much oxygen is inaccessible for the microorganisms. Water inhibits oxygen diffusion. The oxygen may be entering the pile, but it may not be reaching the microorganisms at a sufficient rate to meet their needs. When aerating the pile and managing the pile’s moisture content, keep this in mind.

Water Requirements to Remain Aerobic

Picture of cupped hands under a stream of water

For the survival of composting microorganisms, water is also required. To transport nutrients, microorganisms need an aqueous environment. Water serves as both a medium for chemical reactions and a solvent that keeps life in motion.

Microorganisms thrive in a watery environment, but oxygen cannot permeate the compost pile sufficiently to maintain aerobic respiration if the material is saturated.

As a result, the ideal compost moisture content must strike a balance between providing the microorganisms with the moisture to function and ensuring sufficient oxygen flow to maintain aerobic conditions.

Generally, a 40 to 65 percent moisture content is recommended for composting. Microbial activity ceases at a moisture content of 15% or less.

If you squeeze a handful, a single drop should be able to be pressed out.

Acidity and Alkalinity In Aerobic Composting (pH Control)

Because compost has a built-in buffering capacity, it is rarely necessary to adjust the pH. When composting nitrogen-rich material, pH becomes an issue. Superphosphate, used in amounts ranging from 2 to 5 percent of the dry weight of the manure, has been shown to conserve nitrogen.

Physical Characteristics of Your Compost

When developing a compost mix, the ingredients’ physical characteristics must also be considered. Aeration, decomposition, and a pile’s ability to maintain aerobic conditions are all influenced by various physical characteristics. Physical characteristics, porosity, texture, and structure are critical for compost mix.

Compost Pile Porosity

The amount of air space in a compost mixture is known as porosity, and it impacts the amount of resistance a pile has to airflow. Airflow becomes more difficult when the pores in a material become clogged with water due to high moisture content.

As the amount of oxygen available to microorganisms decreases, anaerobic activity takes over. A more uniform mixture of materials improves porosity by ensuring air spaces are not interrupted. Larger particles aid airflow, but their reduced surface area makes them undesirable. Decomposition increases with compost surface area because most microbial activity occurs within a thin liquid layer on the particles’ surfaces.


The surface area available to microorganisms is texture, which refers to the relative proportion of different particle sizes in a material. The more abundant the surface area exposed to microbial activity, the more effective the composting process. By using methods like selection and grinding to reduce particle size, you’re also increasing the amount of material exposed to microbial decomposition on the pile’s surface.


Structure refers to a particle’s ability to withstand compacting and settling during transportation. It’s essential for composting because it helps keep the material porous. The composting process slows when the decomposing material settles and closes off air spaces in a pile.

Less porous material (like grass clippings) loses its structure more quickly than highly absorbent material. Even if a composting mixture contains all of the necessary ingredients, it may not be able to support rapid composting without the proper structure. Compost particle size must balance porosity, surface area, and structural enhancement.

If you don’t like bacteria, you’re on the wrong planet

Stewart Brand

Compost Mixtures Design

Natural decomposition occurs in any pile of waste material even if the C:N ratio, moisture content, and aeration are outside the recommended limits for composting. Generally, however, decomposition proceeds too slowly to be readily noticeable and generates putrid odors.

Composting is a directed effort to maximize the rate of natural decomposition, reduce odors’ production, and destroy pathogens, weed seeds, and fly larvae. The compost mix must be designed to optimize conditions within the pile in terms of nutrition, oxygen and moisture content, and pH and temperature levels so that a high rate of microbial activity is achieved.

Compost Mix Components

The three compost mix components are the primary substrates, amendments, and bulking agents.

Primary Substrate

The primary substrate consists of most of the waste that must be treated. The primary substrate’s characteristics can be used to determine the right kind of material to add to the compost mix.


It is possible to balance the C:N ratio, modify pH, improve stability, and achieve an acceptable moisture content by adding an amendment to the primary substrate. Compost can have multiple amendments added to it. Bulking agents are another use for amendments.

Bulking Agent

The primary function of a bulking agent is to give the pile structure and porosity by protecting it from decay. Some bulking agents don’t decompose at all, so separate them from the finished compost and repurpose them as mulch or soil conditioners.

Composting involves adjusting the chemical, physical, and nutritional properties of different raw materials to create the best conditions for microbial growth. The C:N ratio and moisture content are the two most important considerations.

Composting Monitoring and Processing

Monitor the compost pile and make the appropriate adjustments throughout the composting period. Generally, the monitoring process includes observing temperature, odors, moisture, and oxygen and carbon dioxide.

This is necessary to sustain a high aerobic microbial activity for complete decomposition with minimum odors and maximum destruction of pathogens, larvae, and weed seeds.

Microorganisms will give you anything you want if you know how to ask them

Kinichiro Sakaguchi

Composting Temperatures And The Energy it Creates

Temperature is an indicator of microbial activity. By recording temperatures daily, you can establish a typical pattern of temperature development. Deviation from the usual pattern of temperature increase indicates a slowing of or unexpected change in microbial activity.

The temperature should begin to rise steadily as the microbial population begins to develop. Adjust to the compost mix if it does not start to rise within the first couple of days.

A lack of heating indicates that aerobic decomposition is not happening. The cause could be factors such as lack of aeration, inadequate carbon or nitrogen source, low moisture, or low pH. Poor aeration is caused by low pile porosity that, in turn, can result from the characteristics of the material or excessive moisture.

Dense material (such as grass clippings) does not have good porosity. By adding a bulking agent, you could improve the pile’s porosity. A mix of too wet material also lacks good porosity because moisture fills the air spaces, making oxygen penetration of the pores more difficult.

Small piles do not heat. Another possible reason for the failure of a compost pile to heat is that the initial mix is sterile or does not have a sizeable microbial population. If the initial microbial population is small, it will take longer to develop and grow.

This is generally not a problem with waste material, such as manure or sludge, but can be a problem with “clean” material, such as newspaper or potato waste. You can kickstart the process by adding some active composting material, such as manure or finished compost.

Compaction can cause uneven composting in static piles. A pile settles and begins to seal off air spaces at its base as the material in the pile decomposes. As a result, the lower part of the pile decomposes more slowly than the higher part.

Temperatures near the thermophilic range indicate high levels of microbial activity. Exceptionally high pile temperatures (>170 oF) are more of a sign that the pile cannot regulate its temperature than a sign of vigorous microbial activity.

Too much heat in a pile causes it to overheat and collapse, destroying valuable microorganisms. Evaporation cannot adequately cool a large enough pile if it is too dry or too large. Increasing the exposed surface area and maintaining adequate moisture is necessary for heat release to the atmosphere.

The compost pile’s initial hot spot is usually between 12 and 18 inches deep. Over time, it works its way deeper and deeper into the pile. It’s a good indication that the whole pile isn’t heating if the stack isn’t heating between 12 and 18 inches deep. Temperatures at the center of the pile must be monitored.

Odor Management and Digestion

After temperature, the odor is the best way to tell if the pile is aerobic and, to some extent, if nutrient losses are occurring due to the volatilization of ammonia. Odor control is critical in composting operations where the process is close to nearby residences.

Before composting even begins, odors may be detected. The raw material itself typically causes these odors. Materials like fish processing waste and manure, in particular, fall into this category. These odors, on the other hand, usually go away on their own in the process.

Composting masks or eliminates odors because microbes in the mixture use odorous compounds as substrates, which are then consumed by the composting process. Anaerobic activity is indicated by strong, putrid smells, sometimes with a sulfur scent, especially when low temperatures accompany these odors.

High moisture and low porosity environments frequently lead to anaerobic conditions. A large pile can cause compaction and inadequate aeration, even if there is no excessive moisture or the material’s porosity is considered sufficient.

There may be times when nitrogen conservation is a priority if there are concerns about nutrient losses. Reducing the frequency of turning and adding carbon-rich material to the mix are two examples of management techniques.

Detection of odors is highly subjective, making it impossible to quantify or measure. Whatever the case, the human nose is the most effective way to detect odors. Odors are challenging to get rid of once they’ve been detected.

The best strategy is to keep pile conditions under control so that odors are kept to a minimum. Modifying conditions within the pile to prevent odor production should be considered if odors have already developed.

Moisture Content For Dummies To Avoid Anaerobic Conditions

Composting operations face the challenge of maintaining proper moisture levels. As the composting process progresses, the moisture levels in the pile fluctuate dramatically due to the high rates of evaporation and precipitation.

Moisture problems can slow or stop the composting process, create anaerobic conditions, and cause unpleasant odors. The microbial activity in a dry pile is harmed as well. Even so, it makes dust, which can carry odors and pathogens like Aspergillus fumigatus. These potential issues can be alleviated by maintaining a 40-60% moisture level.

Turning a pile after rainfall or spraying water on it while turning are the simplest ways to fix a low moisture problem.

Water can be introduced into a static pile by inserting a hose under the insulating layer and allowing it to drain.

Add water gradually to avoid adding too much water to minimize moisture losses from runoff. Use composting runoff to re-wet the pile.

Anaerobic Composting

The Pros of Composting Without Air

  • Low energy commitment
  • Can produce large quantities of compost without much upkeep
  • Accepts a wide range of biological material

The Cons of Composting Without Air

  • Slow output
  • No guarantee of beneficial soil microorganisms
  • Possible weed seeds and pathogens as temperatures are too low to innoculate

The video below takes you through some additional tips on the composting process. If you are struggling or want to learn more, then check it out.


Microorganisms play a vital role in our well-being. New studies link our gut microbiomes directly to our physical and mental health. (Source) Soil science is continuously evolving, and discoveries continue to emerge regarding the critical role of soil biota. The best thing humans can do to soil is leave it as much as possible. If we do get involved with digging it up, our default must be to add new life.

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