About bacteria

Understanding the composting process requires some overview on the different microorganisms responsible for the degradation of organic material.

The main category of organisms at work is bacteria with 3 classes playing different roles in this process, differentiated depending on the temperature ranges where they survive and grow;

– Psychrophiles; grow at low temperatures, down to 20C (59F)
– Mesophiles; grow at medium temperatures, from 20C (68F) to 45C (113 F)
– Thermophiles; grow over 45C (113F), and some can survive to the boiling point of water 100C (212F).

Mesophiles include human pathogens like E.Coli, found in human intestine.

Thermophiles are in high proportion and may represent one of the most ancient form of life. They stay dormant at ambient temperature and will thrive under extreme heat.

Chemical resources

bacterial-degradationBacteria use carbon and oxygen as a source of energy (sugar, carbohydrates) and nitrogen to build protein they need for their bodies. You may see it the following way; Carbon Oxygen and Hydrogen are the chemical fuel, nitrogen is the biological fuel. This is the reason why nitrogen is an important part of a compost pile. A Carbon/Nitrogen ratio ranging between 25:1 and 30:1 is the optimum combination (these ratio will fit with different categories of plants). When adding material high in Nitrogen in the compost pile (e.g. manure) you need to take into account the fact that carbon is present as well, therefore to adjust the ratio to 1/4 or 1/2 to obtain a 1/25 of nitrogen/carbon ratio.

Aerobic degradation is the most rapid, effective and healthy composting process. To perform this the oxygen level should be over five percent. This is the reason why compost piles need to be ventilated. This kind of composting requires some labor or specific techniques for ventilation. Aerobic degradation is as well necessary to kill pathogens.

Decomposer organisms need water to grow and reproduce. Too little moisture will cause bacteria to go dormant. Too much moisture will reduce aerobic decomposition and increase anaerobic one. To identify the right level of moisture few drops of liquid should be expelled when squeezing compost material in your hand.

4-phases-of-compostingComposting phases

There are 2 different types of composting. The cold composting and the one inducing high temperatures (called hot composting, including the famous Berkeley composting method).

The latter happens in compost piles having enough volume to substantiate a critical mass of thermophile bacteria. Usually a compost pile newly formed of 1m3 will have this critical mass. Smaller piles or piles made by small and regular addition of organic matter over time will not generate high temperature reactions.

If we consider a complete process including the high temperature phase we identify 4 different phases;

– mesophilic phase (conducted by related bacteria which proliferate at this stage);
– thermophilic phase (thermophiles bacteria take over while temperature is raising above 45/50c);
– cooling phase;
– curing phase.

During the thermophilic phase an important proportion of mesophile bacteria is destroyed by high temperatures. It is often considered as the way to kill pathogen for a safe compost (and destroy seed weeds for later use of compost in the garden) .

The pile needs to get to 55 C within 3 to 7 days from starting day and stay at this temperature for 10 to 15 days

The Berkeley method (the most famous method of hot composting) proposes following timing;

1) Create the pile

2) Keep the pile untouched for 4 days in a row to start the thermophilic phase in a stable environment

3) Turn the pile every 2 days for a duration of 14 days in order to provide oxygen to the bio-combustion

The pile is then ready to use. The curing phase in this case is quasi nonexistent.


Although the thermophilic phase has a pathogen curing effect the final curing is performed through microbiological competition in aerobic conditions during the last phase (curing phase). Two effects combine; pathogens end up dying over time (limited life span in aerobic conditions) and biological competition happens within the pile (e.g. competition for food, inhibition and antagonism, predation, antibiotics production). This maturing phase is essential and remove not only human pathogens but as well phytotoxins that are toxic to plants. The maturation phase may take 6 to 12 months. An immature compost may as well jeopardize oxygen and nitrogen availability from the soil (due to bacteria competition for food) and highly increase the levels of organic acids, impacting the plant growth. A mature or finished compost is a material where nutrients and chemical energy have been embedded into a stable organic mass. Humus is an example of such a stable material and is usually the result of cold composting.

Some plants have better capabilities to deal with raw compost (if you are in a hurry to grow vegetables). It is the case for Cucurbitaceae (squash, watermelon, cucumbers…).

To summarize; high temperature accelerate the pathogen destruction but aerobic conditions provide with the environment adequate for biodiversity to overwhelm pathogens through different mechanisms based on competition. External resource : Read more about pathogens destruction

In some cases the thermophilic phase by removing most of beneficial mesophile bacteria may prevent as well later microbiological competition and if a pile is re-infected by pathogens those may thrive (not having competitors) and produce a compost with a high concentration of pathogens. This is why, once the cooling period has started, the contamination by pathogen should be avoided. The maturation phase will see the aerobic colonization of the compost pile by beneficial bacteria and macro-organisms increasing biodiversity therefore the diversity of nutrients.

The thermophilic phase is useful as well to detoxify the de-worming substance that may exist in the manure from treated animals. In this situation no more than 5% of this type of manure should be used. Antibiotic does not represent a problem in the compost and is degraded under aerobic conditions.

Once again biodiversity is key in creating a balance and make the system resilient. It is important to note that bacterial infection is not due to the presence of one isolated bacteria but by a certain concentration level. In biological mechanisms bacteria may even communicate to share this information (concentration) and decide or not to deploy an attack against a plant based on a certain density of presence.

Other actors in composting

micro-and-macro-organisms-on-compostingAt the end of the thermophilic phase remains still non degraded material like lignin, too resistant for bacteria, which will be digested by actinomycetes and then fungus during subsequent phases. Avoid branches larger than 1 cm of diameter or hardy vegetation. In tropical climate many tree species have leaves with strong structure which will resist to bacteria alone.

When gathering the material for your pile you may harvest various types of fungus (in the litter) found in the forests around and add them in a humid place of your compost pile during the cooling phase.

Microorganisms (bacteria, fungi, and actinomycetes) are chemical decomposers and the most active in the composting process. However macro-organisms (mites, centipedes, sow bugs, snails, millipedes, spring-tails, spiders, slugs, beetles, ants, flies, nematodes, flatworms, rotifers, and earthworms) which are physical decomposers play as well an important role by reducing materials into smaller pieces, by ventilating the compost pile for aerobic bacterial digestion and by triggering all sort of bacterial transformations.

Worms play particularly an important role in increasing the quality of the nutrients through the mechanisms at work in their digestive system.

Another very important role played by macro-organism is bacteria control through predation. If your compost lacks these organisms not only bacteria will compete with plants for nitrogen but the food web will as well lack the transformation of bacteria in minerals available for plant. Bacteria are the organisms with the most concentrated amount of minerals on earth (NPK; etc..). The Macro-organisms which feed on bacteria need less nitrogen than bacteria and release nitrogen and other minerals when they eat them, which will be used by plants. This mechanism is key in plant feeding.

Compost piles management

The best way to handle the different phases of composting is to have 3 different piles;

– The one you are building
– The one which is maturing
– The mature compost pile you use for your garden or orchard

Dry material can be soaked up into water the day before the pile creation.

In places with heavy rain the pile should be covered to avoid nutrients wash out and anaerobic degradation due to humidity excess. Covering them reduce as well evaporation.

You may use a wire as shown on the photo, covered with the lead of a water tank, a very common equipment;


This solution has many advantages;

– not expensive and very easy and quick to make. The lead can be made using wooden board and plastic sheet. A diameter of 110cm by 100cm high gives roughly 1m3 which is the minimum volume for efficient thermal composting phase.
– portable, meaning that you can make compost in different locations of your garden. You can for example make a pile in a location where once removed you wish to plant vegetables. The compost pile will have then improved the soil where it was settled. It is convenient if you have a source of mulch in a specific location and don’t want to carry the mulch to zone 1.
– good ventilation on the side of the pile for aerobic reaction
– vertical wall of the pile improving the shape of the combustion chamber (pyramidal shape of a pile require more compost for the same result)
– ease the visualization of the pile evolution; material evolution and pile level
– easy removal of the wire when turning the compost or starting the cold composting phase
– you can make slightly different diameters so to embed them one into another like Russian puppets.

In places very dry the pile should be kept humid with regular control of the humidity.

In order to ensure ventilation you may turn 1 to 5 times the pile entirely with at least 3 days of interval between each operation or build a pile having larger branches at the base and in some layers of the pile in order to easy bottom up air passage through the pile. The intervention of macro-organisms will help in creating air pathway through the pile.

Apart from this average indication turning the pile is done;
– When temperature exceed 60C which means that the reaction is going to quick and all oxygen is used in the chemical reaction
– When white ashy actinobacteria appears. It means that the combustion becomes anaerobic with the creation of antibiotics arming fungus and bacteria.
– When bad smells appear, meaning as well that the combustion becomes anaerobic with Nitrogen, Phosphorus, and Sulfur being lost as gases and pathogen thriving.
– When temperature or humidity is not homogeneous in the same layer of the pile

When added to the fields compost should not be mixed with dirt in a ratio of more than 30 percent (compost is an inoculum and does not replace soil).

manureTypes of manure and nitrogen resource

We may identify various types of available animal excretion

– Animals dejection mixing urine and feces (e.g. birds). This type of manure is high in nitrogen since it incorporates the urine component high in nitrogen. As for human and pigs birds eat as well high nitrogen content food compared to pure herbivores.
– Ruminant (e.g. cows). The digestive system of ruminants produce a manure exempt of weed seeds and a high concentration of beneficial bacteria.
– Others, including herbivore and carnivore may have various behavior in relation with the way they are managed in a farm. Sometime only feces are available, other time animal bedding will help gather both urine and feces. This should be taken into account when reading documents which provide with C/N ratios.

Different recipes can apply based on required end result. This mixture is an average;

– 45% material high in carbon; Woody material in the form of cellulose and lignin. Wood chips (better than sawdust since saw dust prevent air to pass through), small stalks, brown paper, cardboard. The C:N ration is 60
– 35% green material. Contain still nitrogen when green but the important part is sugars, proteins, and carbohydrates, an important part of the food for bacteria to thrive. If you store green material it needs to be dried and the C:N ration will go up to 60. If fresh and green the C:N ratio will go down to 30.
– 20% material high in nitrogen. The manure needs to be fresh to keep a high ratio of Nitrogen. C:N Ratio is 10

The compost should be planned depending on fresh manure availability.

In absence of manure the nitrogen can be obtained by using fresh green vegetation and kitchen scraps (grass cuttings, small and soft pruning, remains of annual plant and weed before they have set seed, tea bags and coffee grounds). Special strategies can be developed in places lacking adequate resources (e.g. lack of fresh manure, soft green material) – Read more about making compost with limited supplies.

A mature compost may contain 25% more nitrogen than put initially thanks to nitrogen fixing bacteria.

Usually brown organic matter (e.g. mulch) has lost its nitrogen content. Natural compost found on the forest soil is a combination of carbohydrates coming from the fallen leaves and the nitrogen coming either from the dripping line of the trees during rain or from the activity of the forest soil ecosystem where leguminous are present (mineral exchanges conveyed by soil micro and macro organisms).

Covering an open air garden bed with brown mulch only may create some nitrogen deficiency in the soil not having nitrogen resources available, especially on poor soils.

balanceTypes of compost

Composts vary in their respective quantities of bacteria and fungus. Compost high in fungus are more suitable for trees planting and orchard maintenance (perennials in general). Compost high in bacteria are more suitable for annuals (vegetable garden).

The type of compost will depends;

– on the material used during the composting process. A higher quantity of lignin (branches, fibers) will favor fungus presence.
– and the type of composting; hot composting favors bacterial development and is low in fungus therefore mostly adapted to annuals.

These are some variations on the materials you may use depending on your compost usage;

– Compost high in fungus for orchard or plants needing lignin structure; 10% High Nitrogen, 30% Green Plant Materials, 60% Woody Materials.
– Compost with average fungus and bacteria quantities; 10% High Nitrogen, 40% Green Plant Materials, 50% Woody Materials.
– Compost for vegetable garden; 25% High Nitrogen, 30% Green Plant Materials, 45% Woody Materials. (this type of compost is quicker to make)

When doing kitchen scraps compost, assuming you will get humid incremental material a strategy is to prepare a strata of 30cm high of green and woody material where you will deposit the scraps and recover them to prevent them to dry. over time you cover the surface and start another layer. When you want to start the composting process, at least 2 weeks after the end of the last layer add 10% of manure (high in Nitrogen) and turn the pile.


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