Soils as a Key Component of the Critical Zone 1: Functions and Services

Foreword

ISTE’s scientific publications include a pluridisciplinary editorial sphere entitled Earth Systems – Environmental Sciences and, within this domain, we are now pleased to release a series of works entitled Soils, coordinated by Christian Valentin, as part of the activities of the working group on soils at the Académie d’Agriculture de France (French Academy of Agriculture).

The general title of this series of works, Soils as a Key Component of the Critical Zone merits a number of comments.

The Critical Zone (CZ), a concept which is now globally recognized, designates the location of interactions between the atmosphere, the hydrosphere, the pedosphere – the outermost layer of the Earth’s crust, made up of soils and subject to the processes for soil formation, derived from interactions with the other surface components – the lithosphere and ecosystems. Within this zone, there are vital exchanges of water, matter and energy, such exchanges interacting with those of other layers, both oceanic and atmospheric, within the Earth system. Its extreme reactivity, whether physical, chemical or biological, is an essential factor of the overall regulation of this Earth system.

Supporting all forms of life, this thin layer has a high level of interaction with human activities. Examples of these are agriculture, urbanization, resource extraction, waste management and economic activities.

This concept of the Critical Zone (CZ) entirely revives the environmental approach, simultaneously enabling an integrated, descriptive, explanatory and predictive view of the Earth system, of its major biogeochemical cycles and their interaction with the climate system. The view becomes dynamic, explaining all interactions, and opens the way for predictive modeling. Such processes are necessarily integrated with given models, paying special attention to the hydrological cycle as well as the carbon and nitrogen cycles.

Within the CZ, soil is a key component, playing a prominent role in the storage, dynamics and conversion of biogenic elements (carbon, nitrogen, phosphorous – C, N, P) and of all inorganic, organic or microbiological contaminants. This contributes to significantly affecting the quantity and the quality of the essential resources for human activity, these being soils, water and air quality.

Soils thus return to the top of the international agenda, as a result of the major challenges for any civilization. These include agricultural production, climate change, changes and conflicts over land use (deforestation, urbanization, land grabbing and others), biodiversity, major cycles (water, carbon (C), nitrogen (N) and phosphorous (P)), pollution, health, waste, the circular economy, and so on. They appear therefore legitimately within the United Nations’ sustainable development goals by 2030 (SDG 15: Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss).

The study of soils, as a key component of the Critical Zone, should thus not only be tackled by soil science but also within the highly numerous disciplines of Earth and life sciences, humanities and social sciences. Soils, being as they are at the center of multiple interactions, are an intricate array of systems, a nexus joining the essential parameters. These are food, water, energy, climate and biodiversity.

Soils, in terms of structure and dynamics, with complex processes, are sensitive to global changes that induce developments, which themselves obey threshold processes and issues of resilience. These involve, with regard to their study, taking into account not only short but also long time spans. This aspect was stressed in a white paper on soils published by the CNRS in 2015 (available at the address: www.insu.cnrs.fr/node/5432). The dynamics of major biogeochemical cycles, in particular with timescale characteristics which can be centuries old, indeed even go further back beyond that and so on.

It is clear that among the major components of the environment discussed earlier, soils are the least understood by the general public, by the authorities and even in academic circles. Consequently, it becomes of prime importance to provide the conceptual bases to the greatest number of university teachers and students so as to tackle soils with the complexity of their nature, their mechanics, their diversity and their interactions with other components, within the Critical Zone.

This is what is achieved with the reflections, analyses and the prospective studies carried out by all of the authors in this series. They are top scientists with a high level of international expertise within their discipline, and are mindful of adopting a holistic approach to soil study. The authors of this series pay specific attention to aspects able to be concluded through an open interdisciplinary science, beyond the single scientific community, policy-makers, managers and to all those who are interested in the evolution of our planet. These authors also support their scientific reflection in line with training demands and, of course, the broadest dissemination of knowledge.

The series takes the form of six volumes:

– Soils as a Key Component of the Critical Zone 1: Functions and Services, a volume which will serve as a general introduction;

– Soils as a Key Component of the Critical Zone 2: Societal Issues;

– Soils as a Key Component of the Critical Zone 3: Soils and Water Circulation;

– Soils as a Key Component of the Critical Zone 4: Soils and Water Quality;

– Soils as a Key Component of the Critical Zone 5: Degradation and Rehabilitation; and

– Soils as a Key Component of the Critical Zone 6: Ecology.

Finally, it is worth mentioning again that this series was prepared essentially within the working group Soils at the Académie d’Agriculture de France, under the debonair, yet tenacious and assertive, stewardship of Christian Valentin. We are grateful to this group of scientists and their leader for producing this series.

André MARIOTTI

Professor Emeritus at Sorbonne University

Honorary Member of the Institut Universitaire de France

Coordinator of the series

Earth Systems – Environmental Sciences, ISTE Ltd

1

Soils as a Key Component of the Critical Zone

1.1. What are soils?

The year 2015 was the International Year of Soils, swiftly followed by the International Decade of Soils 2015–2024. Through this initiative, the UN General Assembly [UNI 13] wished to raise awareness with both civil society and policy-makers as to the crucial importance of soils in humans’ lives. Scientific, political and media interest in soils has appeared to be revived for a number of years [HAR 08a, HAR 08b]. They are now genuinely acknowledged as support for plant production and human activities, but also as an essential land-based system in the biosphere, as regulators of the major equilibria (the water cycle, and the carbon, nitrogen, phosphorous, potassium, sulfur cycles and others, which have a remarkable multi- functionality [JEF 10, GIR 11a, GIR 11b, BIS 16]). This multi-functionality of soils positions them as a key component of the Critical Zone for humanity [NRC 01, LIN 10] where life flourishes. However, do we actually know what soils are and what they do?

The word soil comes from the French sol, itself derived from the Latin solum, meaning ground but also base, bottom, foundation, earth, land, floor and pavement. It also means dirt, originating this time from Old French soillier meaning to make dirty. Another meaning of soil is also where one is born.

These terms are linked: in western culture, that which can be cursed – cursed is the ground because of you (Genesis 3:17) – is that which is dirtied by its sediment, or even upon death and burial. It is also considered as a source of all life, sphere of the gods, provider of wealth, stories and legends, patriotic pride, and other factors. Hence, the various interests, indeed contradictory, or lack of interest for this entity which feeds plants, regulates water flow and shows a fantastic biodiversity containing no doubt 25% of the Earth’s living species [DEC 10], other organisms and other biodiversity upon which we live and work. We should know that clay soils are used in pharmacopoeia (beidellite, attapulgite and smectite) for digestive disorders or in cosmetics (clay masks, hair degreasers, shampoos and other products) [LEF 11]. Do not forget that clay soils are used as construction materials (for example, adobe, tiles and bricks) and for thermal insulation. Did you know that these soils contain microorganisms (for example, bacteria and fungi), which produce antibiotics and vitamins and that studies of microbial population and of their antagonisms led Waksman to discover antibiotics (streptomycin) and to his Nobel Prize for Medicine in 1952 [BER 06]?

Do we know what soils are? How are they perceived? How do we define them? What is their position on the earth’s surface? How are they formed? How do their processes work? What do they do? What purpose do they serve? It is such questions to which this series of six works under the heading Soils strives to respond. This first volume, entitled Soils as a Key Component of the Critical Zone 1: Functions and Services, contains 12 chapters, including this introductory one, focused around definitions, presentations and discussions around soil properties, around the processes and services that they ensure, around the pressures by which they are influenced and the perspectives that open up. The five following works are more specialized and more detailed (Societal Issues, Soils and Water Circulation, Soils and Water Quality, Degradation and Rehabilitation and Ecology) and will approach various major aspects of soil processes and the issues that they incorporate.

1.2. The Earth, land, soils, soil cover and the Critical Zone

The semantics of the term Earth are vast, even without mentioning the term ground, which simultaneously takes account of geographical, economic and property aspects, which is hence situated at the level of the farm, the watershed or the ecosystem. The expression field work is analogous to in vivo versus "in vitro and laboratory study. Man’s level of understanding is such that he rarely appreciates the layers underneath arable land. It is true that, to do this, you must plough the ground: The harvest past, Time’s forelock take, And search with plough and spade and rake… To show by such a measure, That toil itself is treasure." wrote Jean de La Fontaine in Le laboureur et ses enfants (The Ploughman and His Sons).

We can group the various meanings that the word earth takes into four different spheres:

– those attached to the Earth (with a capital letter), which comprises all of the continents and the oceans and the planet;

– those attached to the use of soils (and therefore a link with everything which is within agriculture). This may be:

- a surface area corresponding to a given land ownership – you often come across references such as this is my land, or the price of land (which depends upon its use: a forest, crop production, vineyard, second home and similar terms), and selling his or her land;

- a loose layer of the soil cover where plants grow: arable land, wheat-producing land and unfertile land;

- the behavior, the properties or the qualities of the surface layer used by humans: black loam, soft soil, organic soil, sandy soil, light soil, soft soil, clayey soil, heavy soil, limestone soil, fallow soil, cold ground, poor soil, heathland soil, stony soil and fine soil. Thus, according to the Scandinavian proverb, black soil produces white bread and the Albanian proverb the gardeners hands are blackened with earth but his loaves of bread are white (see Chapters 6 and 7: Soils, a Factor in Plant Production: Agroecosystems and Forest Soils: Characteristics and Sustainability);

– those attached to:

- clay – often confused with the term earth – as with, for example, clay used for pottery;

- construction material: brick-earth, fuller’s earth and clay soil;

- surface characteristics: red earth, black earth, white earth and other such characteristics (see Chapter 9: Soils, Materials and Infrastructure Supports);

– those linked with humanity, for example, using various expressions:

- being known throughout the Earth, with the term earth meaning the entire humanity;

- this is my land indicates the area characterized by the property that a given individual owns;

- native land, an expression which comes closer to myths, by qualifying a given space through its link with a population;

- my ancestors’ soils may be linked in the sense of the previous expression, but it goes further in both the mythical and generational sense (see Chapter 10, Cultural Dimensions of Soils). Soils have an archaeological memory, but also an environmental memory, which associates or distinguishes the influence and history of human and climatic activities (see Chapter 11: Environmental and Societal Memories of Soils).

Soils can be considered as entities with their own constituent characteristics or as complex natural objects, defined by their structural and functional properties and their uses.

Their definition depends upon the perception of them, their uses, their functional processes and the benefits that they provide, the given study routes and the mode of study adopted. The Larousse dictionary of the French language defines soils as the outermost layer for the crust of a telluric planet (Earth, Mars, the Moon, Mercury, Venus). However, there are major differences between the soils on Earth and those on Mars or the Moon, as the latter do not appear to reveal organizational structures linked to the action of living organisms. They might be able to be described as regoliths, which are formations of loose particles [DER 64], which are not fundamentally altered by the effects of living organisms.

Within this work devoted to the Earth’s soils, soils comprise this layer of the Earth where life is highly active and which we describe as the soil cover [GIR 11a]. This quasi-continuous, three-dimensional soil cover, which evolves from the Earth’s surface, is a key component of what was recently defined as the Critical Zone of humanity. This Critical Zone (CZ) has been defined by early authors [NRC 01, LIN 10] as extending from the base of aquifers to the top of plant formations. In our view, it should have, as its sub-stratum limits, unaltered mineral substrates and as its top layer the lower atmosphere, sites where life and the biogeochemical and water cycles are still significant.

The term soil cover insists on the fact that soils form a given part, known as the pedosphere, located on the one hand upon another part which most often has a mineral content, the lithosphere, which marries the landscape, the toposphere, and on the other hand, beneath another part which is essentially gaseous, the atmosphere, or more rarely appearing below stretches of water.

This pedosphere also interacts in time and space with two other parts: the biosphere and the hydrosphere (Figures 1.1 and 1.4) by having highly significant matter and energy exchanges (see Chapters 3–5: Soils and the Regulation of the Hydrological Cycle; Soils as Bio-physicochemical Reactors and Soils are Biosystems, Habitats and Reserves of Biodiversity).

Figure 1.1. The soil cover, pedosphere, at the heart of the Critical Zone of humanity, and integrating sections of other spheres (atmosphere, lithosphere, biosphere, hydrosphere and toposphere) [GIR 88]

1.3. The term soil has various meanings according to use and function processes

The Larousse Agricole (a comprehensive French synthesis of modern agriculture) [LAR 02] defines soil as a natural upper loose formation of the Earth’s crust, resulting from the conversion, upon the contact of the atmosphere and human beings, with the underlying bedrock, under the influence of physical, chemical and biological processes.

A further definition is provided by Girard et al. [GIR 11a]: Soil is an organized structure (within various layers), which evolves, in the presence of life, and the material of which is dirt. It is the location for flow transfers, whether water, air, energy or life.

These first two definitions must not hide the numerous other meanings of the word soil in current or common language [GIR 11a]. Some amusing examples can be presented here. There is, far removed from the subject of these works, the musical note (Sol in French – So in English). The French word sol was also a former unit of currency in France, during the early Middle Ages. In another, rarely used sense (except in physical chemistry), the term sol is a liquid containing dispersed matter within its given part, taking colloidal forms.

Other meanings have a closer relationship with the subject of these works, revolving around surface area, section of landscape, given territory (native soil), habitats for roots and animals and so on and will be presented and discussed hereafter.

1.4. The concept of soil varies according to the user

According to the given concerns of human societies, which have evolved over the course of their history, soils are perceived in very different ways (Figure 1.2). For over a century, the perception of soils has evolved to become almost exclusively agronomic (for example, [BER 15a, HAR 16]).

Figure 1.2. Various concepts and perceptions of the soil [GIR 88]

With the help of archeology, it is possible to read at least part of this history, and particularly when humanity has gone from the gathering phase to regular cultivation and onto the phase of dwelling in urban areas (see Chapter 11: Environmental and Societal Memories of Soils). Soils thus enable the discovery of part of human heritage.

1.4.1. Agricultural sector

Within agricultural societies which are more or less self-sufficient, soil is the means of obtaining food. For the agronomist and the forest ranger, it is designed as a resource comprising a plant nutrient reserve (whether or not this is cultivated) and a place for root growth and activity (see Chapters 6 and 7: Soils, a Factor in Plant Production: Agroecosystems and Forest Soils: Characteristics and Sustainability). It must be protected against erosion and other forms of decomposition (acidification, salinization, pollution and other factors – see volume Degradation and Rehabilitation). It is enriched with plant nutrient substances: water and fertilizers. Soil can thus be viewed as a pantry of a greater or a lesser size, which can be full to varying degrees for the benefit of plant life and other organisms living there. Its quality declines in the quantities of usable nutrients: what size pantry is required to ensure a sufficient good quality plant production, while maintaining the essential soil environmental functional processes?

1.4.2. Scientific communities

The points of view of various disciplines vary in relation to soils and may be summarized as follows. For geologists and geochemists (Allègre and Dars [ALL 09]), soil formation is a stage in the formation of sedimentary rocks.

For the geomorphologist, it is the part of the layered formations of continents transformed by living organisms and organic matter [DEW 08]. The geochemist views it as the ground compartment containing organic matter and newly formed silicate minerals (clays and oxides), which develop and constitute a stage in the formation of sedimentary rocks [ALL 09]. The climatologist views the soil as a screen which uses some of the sun’s energy and rainfall, by reflecting part of them, and which emits a given energy linked to its own surface temperature. The hydrologist (and the hydrogeologist) sees the soil as the environment which transforms precipitation into surface runoff or by infiltration and drainage transports it toward the water table, thus highly influencing both flood response times and the makeup of ground water and water courses. The ecologist perceives it as a source of transfers, production and water storage and of mineral or organic elements, a center for the food chains and organism habitats. The biologist and the microbiologist view it as a reservoir of organisms and genes of great scientific and application value. The biochemist perceives it as a reactor in which the most complex multi-enzyme reactions take place. Lastly, for the pedologist, it is a three-dimensional object, which evolves over time and creates its own organizational form (comprising its structures, layers and systems) which enables the development of life and which is rich in a very large range of plant and animal biodiversity.

1.4.3. Urban communities

For civil engineering and construction, you first have the chief surveyor, then the financier and the lawyer, who consider soil as an area to which fees, rights and obligations are attached. The geotechnical engineer, who constructs the buildings, views the soil as composed of materials of a more or less loose nature, which is situated above the bedrock. The urban planner perceives the soil as a material able to receive the foundations and the construction elements necessary for the construction of buildings [ROS 11]. For the city dweller or the sportsperson, it supports their various activities and it is where they walk or use their soccer shoes. In the view of the industrialist and the businessman, soil, which was often considered as a material resource or as plant sites, or for discharging residues or waste, is now, as with agriculture, an asset to use and manage, by applying regulations which are developed to ensure its protection, the protection of water supplies, and where its natural processes take place.

1.4.4. Current pressures and questions

Within our current societies, soils are subject to pressures, with a greater or a lesser intensity, exerted by human societies. The same societies should ensure great care in using, indeed in managing soils so as to respond to the multitude of questions and aim for harmonious and sustainable development of rural areas, urbanization, landscapes, plant production, water quality and other aspects.

Does the soil cover, the epidermis of the Earth, have the capacity to provide a protective and functional layer between the climate and its changes, water circulation and the availability of plant nutrient materials, but also for the animals and microorganisms which guarantee satisfactory operation of processes? (see Chapters 2–7 of this volume and the Ecology volume in this series).

Can soils provide food security and contribute to climate stability by 2050, while maintaining the functions and services with which they provide us? See Chapters 4–8 of this volume.

Can they maintain a sufficient level of soil fertility and plant production while contributing to maintaining biodiversity? See Chapters 4–7 of this volume and the Ecology volume in this series.

Can they efficiently ensure water quality together with the operation of the water cycle? See Chapter 2 of this volume and the volume Soils and Water Circulation in this series.

How can we avoid landslides and earth flows? What can be done so that the sands do not flood the lands behind the dunes where houses are built? See the volume Degradation and Rehabilitation in this series.

Will soils harness climate change as one research program (the so-called Four for One Thousand) proposes studying? The program which aims to favor the storage of excess atmospheric carbon within the soils? See Chapters 4–6 of this volume.

There are many questions (or issues) and answers that are presented in this series of volumes on soils. Soils are recognized here as a key component of the Critical Zone of continental surfaces, going from the atmosphere down to the bedrock – ensuring the good functioning of the Earth’s ecosystems (or that provide and support the fundamental services and functions of the Earth’s ecosystem).

1.5. The approaches and procedures of soil scientists and pedologists

The responsibility of such scientists is to understand the organization, the processes and development of soils, of their structure and everything from the smallest elements (that is to say, micro-habitats and micro-sites) to the largest elements (land parcels, watersheds and soil landscapes). The traditional method is to sort objects according to different criteria and then to group them together and to associate them so as to move from a state of disorder to general laws of organization. This is what botanists and zoologists do, within their respective disciplines, albeit dealing with individual subjects.

This is not the case for soils. It is difficult to find an individual soil. So as to do this, American authors have proposed the concept of pedon. However, the latter does not genuinely correspond to organizational reality and modes of operation for soil systems. This was evidently a stage or a possible approach to characterizing and studying soils. We can also define soil profiles, which are a picture of a vertical sample of the soil cover, and solums, images of vertical zones of the soil cover which correspond to soil sections (Figure 1.2).

Currently, we may well perceive what we currently call soils as not constituted of given individual aspects, but as a continuum that Russian authors [FRI 75] have called soil cover. This responds to a new paradigm which calls into question all forms of sorting, all typologies and all forms of classification. We have thus gone from the notion of the taxonomic boundary to that of the datum boundary [FAO 06, BAI 09]. If it is not possible to define the limits of a given continuum, we can define, either conceptually or statically, modal entities (in the statistical sense).

As far as the pedologist is concerned, although for a long time we have talked of soils [BOU 88], only for a little over a century have soils been defined as objects of study [AFE 17], which has led to the definition and teaching of pedology [DUC 61]. The term pedology originates from the Greek πέδον (pedon): what is under our feet (as opposed to παιδός (paidós): child), and from λόγος (logos): science or discourse on. Pedology is acknowledged as an Earth science and/or a life science. This interface has not helped its development, which should be resolutely interdisciplinary, relying steadfastly upon the disciplines involved in Earth sciences, physics, chemistry and biology. Its Latin equivalent, which has given rise to soil science, has led physicists, chemists, mineralogists and biologists to apply their given approaches to soil samples. This has favored more disciplinary research, rather than interdisciplinary or multidisciplinary research, integrating the processes and parameters which control the given processes.

It is appropriate to develop studies on soil cover by considering its three-dimensional, evolutionary character and the interactions that play out at various scales of space and time. The mechanisms for soil formation are from its development and its resulting processes, from the transformation of materials (minerals and organic matter) through various fluids (water and gas) and through living organisms (animals and plant life whether aerobic or anaerobic microorganisms or heterotrophic or autotrophic), under the influence of biological, physical and chemical processes. It is a location for matter flows (both organic and mineral) due to various energy sources. These include gravity, pressure gradients, heat, life and solar energy.

1.6. Two principles to take into account: geographical continuity and multi-temporality

1.6.1. Principle of continuity

Transfers of matter and energy occur within all directions throughout the soil cover. Obviously, the force of gravity intervenes as a priority and leads to vertical displacements, but also lateral displacements of materials and solutions. This is far less true at the microscopic level: other forces are intervening and exchanges are moving in all directions, but in an anisotropic way. Fauna moves in directions specific to each species. Roots explore the soil according to the given plant species, by interacting with soil density, its compaction, but also depending upon the nature of sites where both water and the necessary elements to provide food for them are found.

Transfers take place, upon various scales, in far more vast areas. In this section, they prove particularly active in the Critical Zone and condition its development. The links of existing soils and the landscape should therefore be taken into account, so as to understand soil structure and development.

We can thus define pedoclimates and pedolandscapes as follows:

– pedoclimates, which exist under the soil cover, are dependent upon the properties of the soils concerned, their topographical position and the given climate conditions;

– pedolandscape indicates all of the soil horizons and the elements of the landscape (vegetation, the effects of human activities, geomorphology, hydrology and the substratum), including the spatial organization enabling the definition in its entirety of a given soil cover (or a part of the soil cover).

The geographical continuity, at the various levels of understanding, poses the issue of the limits of the given study. What about the materials constituting the soils which are moved: are they products of erosion, transported by the wind or rivers, or do they arrive by other means? Do alluvial and colluvial deposits or eolian silts resort from the work of the geologist, the geomorphologist or the soil scientist? In the interests of making greater progress, they should depend upon the work of all three, not separately but interacting together.

What are the limits between the roots of vegetation and the soil, and within whose competence is the study of the rhizosphere? In that regard, it is as much biologists, agronomists, physicochemists, microbiologists and others who are concerned about developing integrated approaches to such a study. A question of the same type occurs for the leaf litter: the layer including the vegetation deposited on the soil (leaves and twigs and other such material) and soil layer activity in the broad sense, and the biological layers at the surface of the soils made up of blue-green algae, mosses, lichens and other similar materials. Does it apply to materials that are out of the water or flooded, owing to short cycles (tides) or longer cycles (mangroves), around the shores? Does the depth of soil correspond to root depth or biological activity, or even indeed that of physically or chemically altered zones, and where autotrophic bacterial areas can develop? What are the limitations of study between the spheres of the agronomist, research forest ranger, of the botanist, of the zoologist, chemist, the microbiologist and so on and the soil scientist?

A location for matter flows (organic and mineral flows) due to various sources of energy and soil cover is still present at the surface of the continental area. It exists within deserts where microbial communities develop, which may weigh several hundreds of kilograms per hectare [DOM 70], and is also found underneath ice sheets [TED 66]. It can thicken, be covered, submerged or on the contrary be eroded. Some layers are thus buried, flooded, thinned or moved, and a new evolution develops. It is the same if the environmental conditions change. These might include climate, land use and geomorphology. The sections of the soil cover, which are periodically recovered with water (such as given humid marine shore areas), and those permanently covered with water, encourage the consideration that the soil cover is also present in marine and continental waters. Life forms that develop there are very different from those of continental areas, and even more within the deep sea areas, where very high pressures reign. We find products there that have been released, and then carried along during the evolution of the soils of the continental surface, these being principally calcium (Figure 1.3) and clays [WIN 76].

Upon looking at the map of underwater soil materials, we note that those from continental shelves come from transfers of soils that have emerged (Figure 1.3). The distribution of the relative abundance of clay minerals within oceanic sediments [LIS 96] shows that maximum accumulations have chorological links1 with soils in which these types of minerals are found or formed (Figure 1.3). We must also take account of material transported by air which is deposited in the oceans, estimated by Windom [WIN 76] to be 10–30% of marine materials. The key components of these materials originating from soils have undergone an evolution, a conversion and a displacement. We find traces of soil structures such as, for example, dessication cracks (but also traces of animal footprints). There are also gases, as although the oxygen content becomes almost nil below 200 meters deep, on the other hand, the content of H2S becomes significant below 600 meters. The limestone dissolves completely beyond a depth of 5,000 meters and at shallower depths within cold oceans, since the cold increases the solubility of CO2 and carbonates. Therefore, chemical evolutions are altered. These elements indicate that it is legitimate to consider that if they are not soils, strictly speaking but mud flats, or mangroves (nomenclature of the soil system of reference 2009), they are at least sediments of soil origin, which are sorted and some transformed by their reaction with seawater, or even more so by the process of diagenesis. Other sea sediments are clearly not soils, such as coral reefs or chalk.

There are therefore underground soils, which mean that the soil cover is not limited to the continental surface (Figure 1.3). On the other hand, geographically, the soil horizons are discontinuous: they begin and end, but their components also develop and migrate in geographical terms.

Figure 1.3. Distribution of soil materials in marine environments – from [DAV 76] within [SCH 06], where we also find kaolinite, illite, smectite clays and chlorite distributed. White: continents; brown: soil materials of continental shelves coming from the transfers of emerged soils; green: terrigenous sediments, including delta plains; red: red clays of deep sea areas; yellow: carbonated materials; purple: siliceous materials; blue: glacial materials. The distribution of clays (kaolinite, smectites or chlorite), not shown here, also reveal these continental origins (source: http://www.geolsed.ulg.ac.be/sedim/sedimentologie.htm and F. Bouvain (University of Liège: http://www2.ulg.ac.be/geolsed/FB.htm); redrawn by M.-C. Girard). For a color version of this figure, see www.iste.co.uk/berthelin/soils1.zip

1.6.2. Principle of multi-temporality

1.6.2.1. Time

What is the significance of time in relation to soil cover? This is difficult to understand, since, as for the spatial aspect, soil cover is in a state of perpetual activity and movement with variable time steps: this