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Composting and the carbon cycle

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Article Index
Composting and the carbon cycle
A brief introduction to Carbon
The Carbon Cycle
The Greenhouse Effect
How plants get carbon
What happens to plant Carbon?
Humus is the key to organic soil management
Stewardship of the soil
Composting and the return of soil carbon
A Sense of Humus
Composting inside?
How to make Bokashi
Compost as a soil conditioner
Benefits of Compost
All Pages

Soil Carbon: the Basis of all life

Plants and animals, without exception, are principally made up of carbon compounds, in which carbon is combined with hydrogen, oxygen, nitrogen, and other elements.

Nearly half of the solid (non water) parts of plants is carbon.  Because carbon is also processed through plants and given off to the atmosphere during respiration, plants need far more carbon than any other nutrient.  Plants get their carbon as carbon dioxide from the atmosphere, which is why it is sometimes called the gaseous nutrient™. Carbon dioxide occurs in the atmosphere at about 0.03% by volume (300ppm), and lack of carbon dioxide is not a limiting factor for growth under normal circumstances (although crop yields can be more than doubled if additional carbon dioxide is supplied).  The nutrient plants need most of, after carbon, is nitrogen, and our recent scientific view of plant nutrition has been rather nitrogen focused.  Carbon, on the other hand, is most undervalued nutrient, and the vital role of carbon in the soil has been too easily ignored.

CO2 smoke ringA brief introduction to Carbon

Carbon is involved in more compounds than any other element.  Carbon compounds include some of the simplest and some of the largest molecules we know. There are several reasons for the great variety and high strength of carbon compounds.

  1. Carbon has a high covalence (4) that permits attachment of a large number and wide combination of other elements
  2. The carbon to carbon bond is very strong and carbon atoms can form chains of unlimited length
  3. Carbon forms branched chains, ring chains, double bonds and triple bonds
  4. All of the above may occur together

Many minerals contain carbon in the form of carbohydrates, including limestone, dolomite, gypsum and marble.  Diamonds are a crystalline form and the most valuable, nut also one of the least useful forms of carbon.

Carbon compounds have many uses in industry; The black fume pigment used in printers ink and automobile tyre rims is carbon.  Graphite is another crystalline for of carbon, used for pencil lead, in dry cells and light arch electrodes, and as a lubricant.  Carbon dioxide is used to aerate drinks, in fire extinguishers and as dry ice, to keep things cold.  Carbon monoxide is used as a reduction agent in many metal working processes. Carbon tetrachloride and carbon disulphate are industrial solvents, and Freon is used in cooling systems. Calcium carbide is used for welding and cutting metals, and to make other carbon compounds.

Carbon dioxide forms three gases with oxygen; carbon monoxide(CO), and carbon dioxide (CO2), and carbon suboxide (C3O2)

Carbon makes up about 44% of the solid (non water) components of plants and compounds containing most carbon as their base are known as ˜organic™ compounds.

There are 100,000 known inorganic carbon compounds and over 8 million organic carbon compounds.  This number is increasing by more than 200,000 per year.

The simplest organic compounds contain molecules composed of carbon and hydrogen.

Simple organic molecules include simple sugars, starch and protein.  More complex carbon materials include polysaccharides, made by the repetition of sugar-type molecules connected in longer chains.  They are produced by micro organisms in the breakdown of fresh residues, and help to hold soil particles together.

The Carbon cycle

Almost all the carbon on earth is locked in rocks, mainly in the carbonate form, including limestone, dolomite, gypsum and marble. Also in the earth are deposits of coal, petroleum and natural gas (carbon and hydrogen), which are actually the decomposed remains of once living plants and animals.

Carbon Cycle Approximate figures

Carbon is slowly extracted from the rocks in the soil, by weathering, which is largely the action of plants and microbes.  Living organisms cycle carbon through their bodies and release it again in the process of respiration, when sugars (carbon compounds) are oxidised to make carbon dioxide and water.  Respiration is a process common to all organisms, by which they release energy stored in sugars and carbohydrates.  The bodies of living things in and on the soil, in various states of decay known as organic matter contain a larger store of carbon than living plants.  When the huge numbers of microorganisms, which are also 50% carbon, are added, the soils store becomes much larger than the vegetation.

The atmosphere contains some carbon (0.03%) and oceans also contain vast amounts, especially the deep sea.

Perhaps only a few tenths of one percent of all carbon is stored in the atmosphere and the biosphere. Although the total quality is very small compared with sediments, it is constantly cycling around, being used by all living things and involved in every activity necessary for the survival of life.

Carbon and the Greenhouse effect

The energy stored in living organisms eons ago is now transformed in coal and petroleum, the so “called fossil Fuels™. The rapid increase in rate of burning of these fossil fuels and the expansion and intensification of farming over the last few centuries has raised the level of carbon in the atmosphere.  There is now concern about the extent to which the increased carbon will trap heat within the atmosphere is called the Greenhouse effect.

The actual level of warming is intensely debated, but the processes are widely agreed.  The disagreements are over details; such as how much will the oceans warm, and how much more carbon dioxide will warm oceans release to the atmosphere, amplifying the problem.  Carbon dioxide is less soluble in warm water; the effect of this is clearly evident if you leave a bottle of lemonade in the sun (carbon dioxide provides the fizz™).  There is also dispute about the extent to which increased carbon dioxide levels will stimulate plant growth, and how effective plants can be at increasing carbon storage.

How plants get carbon

Plants gather almost all the nutrients they need from the soil, including oxygen and hydrogen from soil water, but carbon dioxide comes directly from the atmosphere.

Carbon dioxide taken in by the plant, in the presence of sunlight and water, is converted to sugar, which is then combined into other carbon compounds.  This process is called photosynthesis and is represented by the equation.

Carbon dioxide + water + solar energy + glucose + oxygen
     6CO2           +  H2O  + solar energy -> C6H12O6 + 6O2

Carbon Dioxide is given off by plants, including plant roots, and soil organisms, during respiration.  Because the soil is home to so many living things, all respiring and giving off carbon dioxide, and there is only slow exchange of air through soil pores, the atmosphere within the soil is greatly enriched with carbon dioxide. Even the air immediately above the soil is greatly enriched, by the CO2 generated by microbial activity of the soil.  In still conditions under a leaf canopy, the carbon dioxide concentration of the air near the ground can reach between 3 and 4 percent, or 100 times the percentage of the atmosphere.  This is one of several important reasons why plants have stomata on the lower side of leaves, through which they take in carbon dioxide (they are also protected from the drying effect of direct sun and wind).  The higher carbon levels actually stimulate the growth of plants.

What happens to plant Carbon?

Plants store carbon more than any other nutrient in their tissues. Nitrogen is also critical for life and the ration between carbon and nitrogen is critical.  When plants die or begin to breakdown, the ration between carbon and nitrogen changes.  Some plants can relocate nitrogen from older leaves, before they fall off, and nitrogen is also preferentially lost to the atmosphere as a gas, or scavenged by detritus organisms.  The ideal carbon to nitrogen ratio (C: N) for the organisms that break down organic matter is 25:1.  This figure is a generalisation and there will be, for instance, organisms that are very good at breaking down cellulose that prefer 30:1.

Some of the plant carbon will be converted into the bodies of soil dwelling animals and micro-organisms, and, because there are so many of then, there is a great bulk of carbon stored in this part of the cycle at any time.  Some of the carbon is lost as carbon dioxide, to the soil atmosphere.  Some of it is worked and reworked over and over again by soil organisms, and 20-30% of carbon originally found in th plant parts is gradually concentrated into the breakdown product of organic matter, which we call humus.  If the starting material is mainly carbohydrate, the amount of carbon preserved will be less.  If it contains mainly lignin and tannins, or similar materials, such as are found in wood and eaves, the amount of carbon stored in humus may be more that 50%.

Humus is the key to organic soil management

Humus is a wonderful product for all types of soil, and is both an indicator and promoter of total soil health.  It helps to hold soils together, retains moisture and nutrients, buffers soils against chemical and physical changes, and releases nutrients again when needed by plants and micro-organisms.  Increasing the level of humus in a oil improves ever measurable characteristic of soil.

Humus is not a uniform product: there are many forms of humus, which very depending on soil type, the nature of the raw organic matter, temperature, moisture, the particular soil organisms available and the dominance of particular types of organisms.  It is impossible for humans to produce stable humus synthetically.

The process of breaking carbon bonds to free up minerals from organic molecules so plants can use them again, is called mineralization.  In a healthy soil, humification (manufactures by humus) and mineralization occur at the same time [This is actually the essence of organic growing, to feed the soil, and let natural rhythms and processes within the soil control the rate of mineralization, rather than feeding the plant.]

Humus us the most important single indicator of soil health, and the best guarantee of yield and quality of produce.

Nutrients are not only stored and released by humus, but available minerals can also be significantly increased by humus.  For instance, some soil carbon gets converted to carbonic acid, which seems down into bedrock and softens it, dissolving minerals or preparing the hard surface for the action of plant roots.

Read more about Humus in the full story on A Sensus of Humus.

Stewardship of the soil

It is common assumption that the contribution of agriculture to the greenhouse effect derives from a combination of burning native vegetation to open up land, and the fossil fuels by tractors and machinery.  In fact the destruction of accumulated organic matter and humus in the soil and decimation of soil organisms are much greater factors.  As can be seen from the carbon cycle illustration, at least twice the amount of carbon is stored in soil as in vegetation in a global context.  When land is converted to agriculture, the carbon levels generally drop rapidly, and then continue to fall over time.  Many Australian soils have declined from 3-4% organic matter to 1% since agriculture was introduced.

Our agricultural practices have effectively turned our farmers into carbon exporters.  City dwellers that rely on space-hungry roads and housing developments are also contributors to the carbon export process.

We can help to slow global warming by planting more trees, but the life span for a tree is a hundred years or so. Even if a tree lives longer, most of the carbon stored will be accumulated within a century, so we have to keep finding more land to plant.  Most of the carbon is recycled to the atmosphere again when the tree decomposed.

We can contribute just as much or more carbon storage by learning to deepen and enrich solids with humus.  To do this we must convert as much of our land as possible to organic growing methods.

The effect of doubling the average depth of agricultural land would be much longer lasting than we could achieve with tree planting.  The average age of humus in an Australian soils is well over 1,000 years, and there are plenty of rangelands and croplands available to convert.

Composting and the return of soil carbon

The basic compost recipe below is easy to follow and the compost process is fairly forgiving so the resulting compost will always be beneficial, although the value of the final product can be improved with practice.
  • Carbon and Nitrogen Materials “ food for microorganisms “ there should be enough mass of material to produce high temperatures.  At least one cubic metre is best
  • Microorganisms “ in practise there are enough present in any soil, and compost made direct on the soil, or with ingredients that have been stored on the soil, should have enough to work.  If in doubt, add a compost activator, or few shovels of material from a compost heap that did work well.
  • Moisture = material should be damp, but not saturated “ it should not be possible to squeeze free moisture out of the heap. 
  • Air “ if moisture levels are correct, and you have not trodden on or otherwise compacted the heap, trapped air will be sufficient to start with.  Over time, as the heal collapses, air is excluded.  Making aerating holes or turning the heap replenishes air.
A good heap takes at least seven or eight weeks to finish, in ideal conditions during summer, especially if turned several times. The process may take much longer in winter, or if the heap is not turned, or carbon to nitrogen ratio is not right.

A Sense of Humus

Humus is dark in colour, smells like sweet earth, is gummy, colloidal and has very high water holding capacity.  Humus is made by soil organisms, especially fungi and bacteria, and by slow breakdown of plant parts containing lignin, which does not decompose readily.  These products, reworked over and over again by many soil organisms, interact with each other to form humus.

Depending on the composition of the raw materials, the particular bacteria and fungi involved, the balance between bacterial and fungal activity, and on soil conditions such as moisture, temperature and aeration, the characteristics of humus can vary.

Humus provides the following functions:
  • Water retention: Humus can hold up to 100 times its own weight in water
  • Nutrient holding: Humus holds nutrients in soil and protects them from leaching or erosion, but in a form that can be accessed by plants.
  • Improves soils structure: Humus has an open, lattice structure that holds air and water. It makes soils lighter to work and helps to gum individual soil particles together into aggregates or crumbs, which permits easier entry of water, air and plant roots. Improves sandy and clay soils equally well.
  • Helps soil to warm: Because it is dark in colour and holds air, which has an insulating capacity, humus can modify soil temperatures at either extreme.
  • Balanced pH: Humus provides a buffering function that moderates both acid and alkaline soils back to balanced pH:
  • Immobilising contaminants: Humus can hold in the soil contaminants such as lead, cadmium and pesticide residues, preventing them from entering plants and hence the food chain
  • Feeding the soil food web: Humus is made by soil life and is consumed and broken down by soil life. It is continually being made and destroyed in a healthy soil ecosystem.

Composting inside?

Most composting is done outside in heaps, as described above.  A unique form of composting called Bokashi is now attracting attention.  Bokashi is not limited to small scall systems, but it is very adaptable to small spaces and can be done in closed container, even inside the house or on the veranda.  It does not produce foul odours, and is ideal for flat dwellers, inner city apartments, and even restaurants.

Effective micro organisms (EM) and Bokashi Compost

Bokashi Compost Bin

Effective Micro-organisms or EM is an inoculant of about 800types of microbes, mainly lactic acid bacteria, yeast and actinomycetes, EM is the life work of Professor Teruo Higa, from the Ryukyus University in Okinawa, Japan.  He isolated and selected a set of beneficial, micro-organisms that would be able to survive and deliver beneficial effects in a wide range of soils.

EM can be applied to degrade or undeveloped soils to increase the microbial diversity of the soil, stimulating the natural microbes and creating the mini eco system that drives healthy soil and enhances the growth and quality of crops.  EM can also be used as a starter for the Bokashi compost system.  Bokashi is a Japanese work that means fermented organic matter.

The Bokashi Method is different from normal composting because Bokashi is a ferment, in which yeasts and fermenting fungi play a much larger role than in normal compost.  This prevents the waste from rotting and minimizes foul smells.  The result is compost you can make inside, in a closed container.  Now anyone who lives in a flat or unit can recycle his or her waste, save on landfill, create there own organic fertilizer and return carbon to the soil.

The components of EM are:
  • Photosynthetic bacteria that synthesize useful growth promoting substances such as amino acids, nucleic acids and sugars from roots secretions, organic matter and normally noxious gases like hydrogen sulphide.
  • Lactic acid bacteria that produce lactic acid from sugars and other carbohydrates. Lactic acid suppresses harmful micro-organisms and increases the rate of decomposition of organic matter, especially lignin and cellulose, which are normally resistant to breakdown.
  • Yeasts that synthesize antimicrobial substances and growth promoting hormones.
  • Actinomycetes that also produce antimicrobial substances that suppress harmful fungi and bacteria.
  • Fermenting fungi including Aspergillus and penicillium that rapidly decompose organic matter.

How to make Bokashi Bokashi starter mix

The Bokashi process starts with an inoculant made from ingredients such as water, molasses, micro-organisms, powdered minerals, and rice bran.

The starter can be used directly in compost or cultured further to increase the quantity of organisms.

To increase the Bokashi starter, mix 20kg of wheat bran, 6 litres of warm water, 120cc of molasses and 120ml of EM in an air  tight container, such as an industrial plastic barrel with a lid, then follow these simple steps: 
  1. Spread wheat bran out on a plastic tarp or flat surface
  2. Mix together the warm water, molasses and Em
  3. Spray the liquid mixture over the bran using a watering can or spray bottle
  4. mix the brad and the liquid by hand, until the bran is evenly moist.
  5. Put the mixture in the air-tight barrel. Press it down to remove as much air as possible.
  6. Leave it for about a month in a warm place.
  7. The Bokashi starter is ready to use when the surface of the mixture is covered with a fuzzy white mold-like material and has a sour fermented smell.
  8. The bokashi can be used straight away or dried and stored for at least two years. Spread it out to dry on a tarp, away from direct exposure to sunlight and moisture.  Break up any lumps, so ithas a granular texture.
To make Bokashi compost place any vegetable matter or leftover food in a plastic bucket and add several handfuls of the Bokashi mixture produced by recipe above, Avoid outing large amounts of meat or dairy product and large amounts of liquid waste. Keep going by this method, until the bucket is full.  Start the process over again in a second bucket, as described.  When the second bucket is full, place it aside to mature, and dig the contents of the first bucket into the soil, or place it in an outside bin to finish working.

Compost as a soil conditioner

Compost is the best way to improve the physical condition of soil. The humus which compost provides helps alleviate all soil problems and improve soil structure, strength, erosion, resistance and permeability.  Soils, which have been treated with compost, retain more moisture, recover more rapidly from trauma caused by wheel and foot traffic or cultivation, provide easier passage for earthworms and plant roots, and better insulate for roots from temperature extremes.

Compost improves all soils, sandy, clay and everything in between

Compost helps to moderate high and low pH soil.

Benefits of Compost

Tim in a pile of compostCompost provides all the following services at the same time:
  • Improves the ability of soil to absorb water
  • Reduces run off
  • Increases moisture-holding capacity of soil.
  • Provides food for beneficial soil organisms, such as worms
  • Helps to suppress harmful soil organisms such as nematodes
  • Supplies the full range of macro-nutrients and trace
  • Helps to hold nutrients in the soil and prevent leaching
  • Reduces fertilizer requirements
  • Buffer soil pH and increases plant tolerance to acid or alkaline conditions
  • Stimulates the production of vitamins, growth hormones and other growth stimulating substances
  • Maintains good aeration in soil, which helps good water penetration
  • Improves the structure and drainage of soils, including sand or clay
  • Reduces soil erosion
  • Keeps soil cool in summer and warm in winter
  • Assists with lock-up of toxic substances such as pesticides or heavy metals, keeping them safely away from plant roots


Biosphere: The thin zone in which living organisms thrive, at the margins of the earth.

Calcium Carbonate CaCO3: Limestone, marble, chalk, and oyster shells all contain calcium carbonate.  The can be used to neutralize acid soils and amend the cation exchange capacity (CEC) of soil. Pure calcium carbonate 56% lime (CaO) and 44% carbon dioxide (CO2)

: Compounds containing carbon, hydrogen and oxygen, such as glucose (C6H12O2)

Carbon Dioxide CO2:  A gas formed by oxidation of carbon or burning of coal or other carbon containing material.

Carbonic Acid: A weak acid formed when carbon dioxide combines with water.

Carbon-Nitrogen ratio: the ratio of the weight of organic carbon to the weight of organic nitrogen in soil or organic material

Humic Acid: Alkali soluble components of humus

Humification: The process of decay in which plant and animal remains are decomposed to extent that their original structure can no longer be determined.

Humus: the completely or near completely broken down, more or less stable components of organic matter in soil

Hydrocarbons: Compounds made from only carbon and hydrogen.  The simplest hydrocarbon is methane (CH4). More complex hydrocarbons are built from chains of carbon atoms.  The bonds between carbon and carbon or carbon and hydrogen are not easily broken, but the hydrocarbons will burn if heated in the present of oxygen, to yield carbon dioxide and water.  Most liquid fuel (petrol, diesel, paraffin) are hydrocarbons.

Respiration:  A process in which oxygen is taken into an organism and carbon dioxide is given out.  Internal respiration of cells provides energy, when glucose is oxidised to carbon dioxide and water.
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