Organic Matter and Life
What is organic matter?
The usual definition of organic matter would go something like this: it is the living organisms and the remains of living organisms in the soil. It would then go on to tell you of its importance in giving the soil structure, and retaining water and nutrients in the soil.
While this is undoubtedly true I believe it understates the central role organic matter plays in terrestrial life on this planet.
The First Life
According to fossil evidence in the earliest rocks found on earth, life first appeared on this planet around 3.8 billion years ago, around the end of the Hadean period and the beginning of the Archean. It is possible that life started even earlier, at some point in the Hadean period, but they’re being no rocks from that period it is not possible to verify. These first single cell life forms photosynthesised and thus are thought to be responsible for the oxygen on the planet. This eventually made terrestrial life possible.
The First Soils
Soil requires rocks to be broken down into their constituent minerals. This process would have been carried out through weather and water erosion but also through the activity of other bacteria and fungi. An important adaptation of these primary colonisers was the ability to fix nitrogen from the air. One example of an early terrestrial coloniser is lichen. Lichen is a composite organism consisting of algae and fungi in a symbiotic relationship. The algae obtain carbon (there was plenty of CO2 back then) and nitrogen from the air while the fungi break down the rocks with certain acids to obtain minerals. Thus, over millions of years, soils were formed by life and with life.
The building blocks of life, the single cell
The earliest life forms on the planet were probably single cell organisms called prokaryotic, which means pre nucleus. At some point (after around 1.2 billion years of just prokaryotic cells) we arrive at eukaryotic, which means good or true nucleus. All multicellular beings and quite a few unicellular are eukaryotic while bacteria are prokaryotic.
The only main difference between the two is that in the eukaryotic cell the DNA is wrapped up in a distinctive membrane or nucleus, which is absent in the prokaryotic cell. All life on the planet is made up of these cells with the only difference being the DNA code, (and even that difference is surprisingly small). The human body has about 60 trillion of them. Interestingly the human body probably has ten times that amount of bacteria cells as well in and on it.
About 40-50% of each living cell’s dry weight is made up of carbon. The biomass of the planet, measured in just carbon and not including bacteria is around 600 billion tonnes, of which humans make up about 100 million tonnes, (Atlantic krill make up 500 million tonnes!). Estimates of how much biomass resides in bacteria suggest that this would make up another 600 billion tonnes, although nobody really knows and more and more bacteria are being found all the time. What does appear true is that a large percentage of those bacteria are found in soil. It is estimated that a gram of soil holds between 40 to 200 million bacteria (I know, a very wide range, but it depends on a lot of variables). The actual function of all the bacteria is still largely unknown. However, thinking of them as “pre” (prokaryotic) to the “good” (eurkaryotic) underestimates them. It is becoming apparent that they often work together to achieve an aim. For instance it requires 600 species of bacteria, working in unison, to a attack and decay the enamel of your teeth, the hardest substance in your body.
Soil and human beings, we need each other!
It would seem to me that we human beings have, and all life has, a symbiotic like relationship with soil organic matter. We could not exist without it and without life there would be no soil, just a bunch of minerals. Soil is the engine which drives the carbon cycle of which we, along with all other life on the planet are a part. Just like life forms, organic matter is around 40-50% carbon. When talking about soil organic matter we are also talking about carbon.
Carbon
Carbon (C) is the fourth most abundant element on the planet and forms the basis for most living organisms. It is fundamental to living organisms because it has the ability to bond with other carbon elements into long chains and thus build complex molecules. It also has the ability to bond with other elements, such as oxygen. In its purest, or nearly purest state, carbon is a diamond, one of the hardest objects, and in a different structure, graphite, which is one of the softest.
When referring to climate change or carbon footprint, we mean carbon dioxide, (CO2) which is one element carbon and two or oxygen. This is the form that carbon takes at one stage of the carbon cycle. The carbon cycle is the movement or rotation of all carbon elements. Carbon isn’t produced, merely recycled. So, for instance the carbon in a piece of wood could be burnt, thereby releasing that element into the atmosphere bonded with 2 elements oxygen, (CO2). Through photosynthesis that same carbon element is separated from the oxygen, which is released back into the atmosphere and becomes part of the plant. Once within the plant, through the actions of various enzymes and hydrogen from water (H2O), the same carbon atom becomes part of a glucose molecule, (while releasing oxygen from the water). The energy for this photosyneytsis of course comes from the sun.
It takes 15MJ of the sun’s energy to produce 1kg of glucose. If that amount of energy weren’t used in this way it would have a warming effect. In this way photosynthesis is a cooling process, (why a town is hotter than the countryside, the heat is reflected rather than absorbed).
How the sun fuels life on earth
Resynthesis is the process by which the simple sugars are reformed into a variety of organic compounds, including carbohydrates, protein and organic acids. The sun’s energy, through the process of photosynthesis, provides the fuel for life on earth. Carbohydrates such as cellulose provide energy for grazing animals, the starch in grains provide the energy for livestock and people and the carbon stored from past eras, (hydrocarbons), in the form of oil, coal and gas provide the energy that runs our transport systems, produces our fertilisers (soluble), pesticides, plastics, metals and so on.
Carbon is basically food for the microbes in the soil. It is microbes that drive the powerhouse that is soil. Through their actions nutrients availability is enhanced. Soil would be lifeless without them and they require carbon to survive. Through a process called exudation around 30-40% of the carbon taken from the air and fixed by the plant is exuded into the soil to provide the food source for microbes that in turn allow the plant to better utilise the nutrients within the soil. The amount of active green leaves present and therefore the volume of roots govern the carbon added to the soil in this way. The more plants the more carbon is added.
Humus
Organic carbon is relatively transient, it moves through the soil. Humificaton is the process by which soil microbes convert carbon into humus. Humus, a gel like substance, is organic matter that has reached a point of stability. It has the ability to hold four times its own weight in water, improves soil structure by allowing spaces between the soil particles and appears to be able to hold plant nutrients in an available form. The exact nature of humus is not, even now, fully understood, however it is known that humuification can only occur if there is a continuous supply of food, (carbon) for the soil microbes. If it does not occur then the carbon exuded by the plant roots (or added to the soil in the form of manures) simply oxidise back into the atmosphere as carbon dioxide.
How fungi in the soil help lock up carbon
As well as soil microbes and bacteria it has become apparent in recent years that soil fungi also play an important part in production of stable organic carbon stored within the topsoil. Most perennial grasses are good hosts for mycorrhizal fungi. This fungus attaches itself to the grass roots and takes some of the sugars that the plant produces. In return the fan like network of the fungus, called hyphae, thinner than a hair, push out into the surrounding bulk soil many metres beyond the plant roots. The absorptive area of the fungus is approximately 10 times more efficient than that of root hairs and about 100 times more efficient than that of roots.
Despite that fact that the plant needs to give the fungus up to 50% of the photosynthate, (soluble carbon), plants with this relationship grow 10-20% faster. It appears that the action of the fungus actually allows the plant to photosynthesis more efficiently. In exchange, the greatly enhanced penetration of the surrounding soil allows more nutrients to be tapped. It is not only the increased penetration; it appears that in combination with various bacteria and enzymes, the fungus is able to solubilise otherwise unavailable plant nutrient. Incredibly the fungus has, within the hypae, a bi-directional flow system. This allows the simultaneous flow of soluble carbon from the plant and nutrients from the soil moving in opposite directions.
The fungus appears to play an important part in the production of humus and also the production of something called Glomalin. It uses Glomalin to coat the hypea, thus protecting it. Glomalin is, in itself a highly stable form of soil carbon, which it is estimated in some area (hotter climates and low clay type soils) to make up a third of all soil carbon. This amazing relationship is inhibited by the use of soluble fertilisers, pesticides and ground that is left bare.
Losing soil carbon
Soil carbon is the biggest terrestrial pool of carbon worldwide. It is estimated at 2700 giga tonnes (Gt), although this figure is continually being revised up. Atmospheric carbon is estimated at 780 Gt. In the UK the latest figure suggest that we are losing nearly 1% of soil carbon per year, around 13million tonnes per year. In warmer climates oxidisation occurs more rapidly, Australia has lost around 70-80% of its soil carbon in the last 100 years. The loss of carbon, the food for the bacteria and microorganisms, also means a decline in their numbers. They are also carbon-based life forms. It is estimated (although this appears to be no more than a guess), that between 50-80% of atmospheric carbon comes directly from losses of soil carbon.
The Impact
Soils are less able to retain water. Last spring we had drought conditions in the eastern half of the UK. The average organic matter content of these areas is around 1% now. The lower rainfall was not the only reason there were drought conditions. Years of mono cropping has led to low organic matter resulting in soils unable to retain moisture.
Increased erosion. At the moment around 3 million tonnes of topsoil are being lost in the UK every year. Globally, a UN report estimated that 10 million ha are degraded or lost as rain or wind erode the topsoil each year.
Increased flooding. It is not just the changing climate that creates more floods. Soils without sufficient organic matter cannot retain water, consequently, run off is greater. This creates problems for watercourses that cannot handle the sudden volume of water.
Increased climate change. If the carbon isn’t locked up in the soils then it is accumulating in the atmosphere as CO2, (one carbon and two oxygen combined). The effect of this is to prevent infrared radiation escaping from the planet which has a warming effect. Methane (CH4, 1 carbon and four hydrogen) has 20 times the Global Warming Potential (GWP). However, methane lasts only 12 years in the atmosphere, compared to CO2 which lasts between 50 and 200 years.
Loss of soil structure. Take away organic matter and you have a bunch of minerals. Take away a significant amount of the organic matter and you have fragile soils that are more easily damaged by machinery, or weather.
Loss of nutrients. Without soil carbon the life activity of the soil declines. The amazing relationships explained above don’t happen. Plants don’t have the ability to fully exploit the soils and even soluble fertilisers pass through the soil and into the watercourses quickly. Few soils are deficient in actual phosphate. Most contain sufficient reserves to support plant growth, if they were made available. However, to make them available we have to reverse the decline in organic matter, we have to start looking at the health of the soil rather than looking at the health of the crop. It is estimated that each year in the UK 172,000 tons of phosphate and 123,000 tons of potassium are flushed into our river and sea. We then import 700,000 tonnes of North African rock phosphate and potash.
A quick look at the fate of a number of past civilisations on this planet should teach us something. They have all declined when they have degraded their topsoils. The difference today is that we are a global civilisation and there are no new soils to exploit.
Since the discovery of a means to fix nitrogen from the air (see nitrogen article) we have started to feed the plant and not the soil. This led, initially to huge gains in yield, which fuelled a population explosion. In 1950 the world population stood at 2.5 billion, it has recently topped 7 billion. Hydrocarbons have made it easy to assume that we can ignore proper crop rotations and feed a crop continuously from a bag, concentrating on the plants nutrition rather than the soils health.
We, as a species, have lost touch with what we are and where we come from. We are a carbon-based life form; we are 65 trillion individual cells and we are a product of the soil. The father of soil science, Friedrich Albert Fallou wrote in 1862:
“There is nothing in the whole of nature which is more important than or deserves as much attention as the soil. Truly it is the soil which makes the world a friendly environment for mankind. It is the soil which nourishes and provides for the whole of nature; the whole of creation depends upon the soil which is the ultimate foundation of our existence”
© Stephen Merritt, The Welsh Poultry Centre, 2011
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About the author
Stephen Merritt is a partner in The Welsh Poultry Centre and an accredited advisor and board member of The Institute of Organic Training and Advice and has spent over 30 years working in sustainable agriculture in developing countries, England and Wales. In the last 8 years Steve has specialised in free range and organic poultry production and now offers on farm advice and training to this sector.
