THE PRINCIPLES OF LIVING SOIL
BENEATH OUR FEET: UNDERGROUND WORKERS
Et si on parlait un peu du sol ?
You may not be aware of it, but every day you tread on a real treasure: the soil. Soil is essentially a thin layer ranging in depth from less than 10cm to a few dozen meters. Pedology, the science that studies the formation and evolution of soils, focuses on the different layers, also known as horizons, that make up the soil. When talking about agriculture, the term arable land is also used.
In terms of composition, the soil is generally made up of 3 components:
50% hollow spaces, or pores, which are filled with air or water,
45% mineral matter: clay, silt, sand and pebbles,
and around 5% organic matter, either living or dead, derived from plants, animals and microorganisms.
Until recently, in agriculture, soil was considered mainly as an inert growing medium. However, recent studies and the achievements of pioneering farmers have demonstrated just the opposite. Soil plays an active role in providing numerous benefits, also known as ecosystem services, to human society. Let's take a closer look at some of these services, and give back to the soil its rightful place as a key contributor to the green revolution.
A Remarkable Pool of Biodiversity
Beneath our feet, it's teeming with life! And to give you a clearer idea, here are a few figures that should convince you. Looking only at microorganisms, it is estimated that a teaspoon of soil contains around one billion bacterial cells (from 100,000 to 1,000,000 different species), and one million fungi (from 1,000 to 100,000 different species). Added to this are a host of other microorganisms: nematodes, protozoa, microarthropods, etc. In an undisturbed environment, the mass of all these microorganisms is estimated at around 2.4 tonnes per acre.
Turning now to macrobiodiversity, that which is visible, there is one symbol we cannot overlook: earthworms. If we were to weigh all the living beings on Earth, earthworms would represent at least half of the non-aquatic terrestrial animal biomass, including humans and animals. This is considerable, and it means that talking about biodiversity without mentioning earthworms is missing a major part of the picture. In quantitative terms, if we consider a surface area of one acre, the reference population of earthworms there will weigh between 0.4 and 1.2 tonnes, depending on soil and climate conditions.
All this subterranean life must be regarded as the base of a pyramid, a chain of interactions between the eaters and the eaten, forming a gigantic and all too often underestimated underground food web. This thriving subterranean ecosystem supports the entire development of above-ground biodiversity: insects, birds, small and larger mammals. Without a rich and diverse soil fauna, there is no food for small invertebrates, which in turn feed larger organisms, and so on.
Soil and the Fight Against Climate Change
Soils are commonly referred to as "carbon sinks". At an estimated 6,000 gigatonnes, they contain the largest stock of organic carbon in the continental biosphere. In contrast, the atmosphere contains around 800 gigatonnes of carbon in the form of carbon dioxide (CO2), roughly 7 times less.
As we saw at the beginning of this article, soils are made up of around 5% organic matter, more commonly known as humus. Humus is essentially carbon. To fully understand this, we need to take a closer look at the mechanism of photosynthesis. With only a few exceptions, plants are the only living organisms to photosynthesize. This chemical process enables them to synthesize sugars, i.e. organic matter, using energy from the sun, water and mineral carbon, i.e. CO2 from the atmosphere. In other words, thanks to photosynthesis, plants are able to transform atmospheric CO2 into organic matter, which in turn they can use for their own metabolism, to build their tissues, leaves, roots, etc.
When leaves or branches drop to the ground and form a litter layer, they evolve dynamically in the soil, where they are decomposed by a complex array of subterranean organisms and stored for varying lengths of time. Humus is the result of the transformation through biological activity of plant debris returned to the soil. As we have seen, this organic matter is essentially made up of carbon. Fossil fuels, a major issue in today's society, are merely the result of this process. Coal and oil are nothing more than organic matter produced by plants and fossilized in the soil at an age when soil biodiversity was unable to decompose it. The geological era during which the largest stocks of coal were formed in Western Europe is actually known as the Carboniferous. Our interest in fossil fuels stems from the fact that they are very rich in carbon-based molecules from which we can extract large quantities of energy, which we then release back into the atmosphere in the form of carbon dioxide, which can then be captured again by plants through photosynthesis.
By growing photosynthetic plants, farmers have a direct impact on the carbon storage capacity of their land. From this perspective, soils that remain bare for most of the year are a real nonsense, since they drastically reduce the carbon storage potential of the cultivated ecosystem. This means that, at both field and landscape level, we must try to keep soils covered with plants all year round in order to intensify photosynthesis and carbon storage in the soil. In recent years, a number of initiatives have been launched to promote practices that encourage soil carbon storage. One example is the "4 per 1000" initiative launched at COP21 in 2015, which claims that increasing soil carbon stocks by 0.4% a year would make it possible to offset the increase in atmospheric CO2 emissions.
Fertile Soil: The Foundation of Farming
Farming always starts with the soil. The quality and fertility of the soil is a determining factor in crop productivity and yield potential. Fertile soil can be defined as soil capable of providing its designated ecosystem services. These include the ability to support the growth of healthy plants. We could also talk about self-fertility, i.e. a soil's ability to maintain and sustain its own fertility, enabling plants, via their roots, to extract and absorb the substances they need to grow.
Soil fertility is usually studied under three aspects. The first one, physical fertility, is mainly characterized by the soil's mineral fraction. The proportion of sand, silt and clay determines the soil's texture. Texture plays a decisive role in the overall functioning of the soil, directly conditioning its structure, porosity and moisture regime. Chemical fertility is conditioned by the quantity of mineral elements contained in the soil to nourish plants, of which nitrogen, phosphorus and potash are the most famous and closely scrutinized by farmers. And finally, biological fertility is mainly characterized by soil life. Biological interactions, symbiotic relationships and the stimulation of biological activity in general are key factors in plant health.
Farming techniques that stimulate the soil's natural fertility are levers that activate the self-fertilization cycle so that the soil can support the growth of healthy plants:
Water storage and filtering to preserve water quality
Plants that develop symbioses with the soil's biology to defend themselves against external aggression
Yield maximization
Increased resilience to climatic hazards for the production system as a whole
Reduced dependency on inputs.
The Soil Under Threat
All the roles mentioned above depend upon this thin, fragile layer that is essential to support terrestrial life. Although it is fragile, the organic fraction, even when degraded, can be reconstituted over a human lifetime. But this is far from the case for the mineral fraction. In fact, it takes an estimated 10,000 years to form a 1- to 2-metre-thick layer of soil. This means that, overall, soil is a non-renewable resource that needs to be preserved.
It's worth remembering that between 2006 and 2015, an estimated 1.5m acres of land were converted to artificial surfaces in France, equivalent to about three times the size of Tokyo. Similarly, soil erosion is a phenomenon that is all too often underestimated and overlooked, yet it poses an alarming threat to our soils. When you see muddy run-off water coming out of a field, it means that part of the soil's fertility has left the plot for good, and cannot be renewed.
To address this problem, we need to rethink our farming models and adopt tried-and-tested methods for improving the soil's natural fertility: agroforestry, plant cover, organic soil enrichers (manure, compost, shredded green waste, etc.), as well as reducing mechanical tillage and abandoning deep plowing. These are all techniques aimed at protecting and regenerating the soil, thereby activating the cycle of self-fertilization and enabling the soil to provide the ecosystem services that both farmers and society expect.
Alexandre Forgeoux © Agence DLMNS