Soil (soil science)
The soil (from ahd. Bodam ), colloquially also called earth or soil , is the uppermost, usually animate part of the earth's crust . At the bottom the ground is bounded by solid or loose rock , at the top mostly by a cover of vegetation and the earth's atmosphere .
- In soil science , soil geography , biology and geology , it is the heavily animated upper decimetres of the earth's surface , whereby geologists primarily do not come into contact with recent soils, but with soils that were once on the earth's surface before geological time periods and are mostly solidified to rock today (see paleo soil ).
- In construction and soil mechanics , “soil” or “earth” is understood to be the building material for so-called earthworks and the material that forms the subsoil of structures and / or has to be moved for construction work (cf. → earthworks ).
- In spatial planning and geodesy , the ground is the actual surface of the earth.
- For the general public, it is the “land”, i.e. mostly the property (or the building site ) with its local land price .
In the following, the term “soil” is treated from the point of view of geosciences and life sciences , especially from the point of view of soil science. In this sense, certain substrates at the bottom of standing water are counted as soils and referred to as subhydric and semisubhydric soils . Geologists, however, count these substrates as sediments and refer to them as Mudden .
Soil from the perspective of soil science and soil protection
From soil science point of view, characterized by soil forming processes overlap zone between the bottom is called the lithosphere (the solid rock , the earth sheath) biosphere , the atmosphere and the hydrosphere referred. This area - the pedosphere - consists of the inorganic (mineral) soil substance (approx. 47%), the organic soil substance (approx. 3%), the soil water (approx. 25%) and the soil air (approx. 25%).
The inorganic soil substance consists of grains and particles of primary and secondary minerals (resulting from chemical weathering) , the dead organic soil substance is called humus . Mineral grains and humus particles are arranged in a three-dimensional soil structure . This soil structure has cavities (so-called interstitial spaces or intergranular spaces on), with the aqueous soil solutions (soil water) and soil air are filled. The structure of the topsoil is usually relatively loose ("crumbly"), which is why one also speaks of the crumb in this context . The most important role is played by the soil as the central basis of life for plants and directly or indirectly for animals and people .
The definition according to the Federal Government's Soil Protection Concept reads: “Soil is the conversion product of mineral and organic substances with their own morphological organization, interspersed with water, air and living beings, created under the influence of environmental factors on the earth's surface and evolving over time Is able to serve as a location for higher plants. This enables the soil to provide a livelihood for animals and people. As a space-time structure, the floor is a four-dimensional system. "
"The soil is a section of the pedosphere that extends from the surface of the earth to the bedrock, that is, that area of the earth's crust in which the lithosphere is created by atmospheric agents ( oxygen , carbon dioxide , nitrogen , precipitation , radiation , outgassing ...) and organisms ( bacteria , algae , Mushrooms ) and in which such transformations continue. "
Formation of soils
Soils form relatively quickly in geological time periods, but relatively slowly by human standards. The soils in what is now Central Europe have been created over the past 12,000 years, since the end of the last Ice Age .
- The primary prerequisite for the formation of a soil is a raw material made from surrounding rock (solid rock or loose sediment).
- The relief must not be too steep, otherwise the starting material will be removed by erosion before a soil can form from it.
- Larger amounts of sediment may not be introduced into the area concerned, as otherwise the soil that forms will be repeatedly buried and soil formation will have to start again.
- The climate should be humid (damp) and not too cold, so that on the one hand there are favorable living conditions for plants and animals and on the other hand because water is important for weathering processes .
Even “finished” flooring is still subject to these factors. In addition, there are soil organisms , as part of the soil, as well as, for around 8,000 years (later in Central Europe) human (anthropogenic) influences such as agricultural or forestry use or pollutant inputs .
Soil-forming processes are triggered by soil-forming factors. They have two effects: they change a source rock and convert it into soil, but they also change an existing soil.
Primary soil formation
The parent rock, regardless of whether it is solid rock or loose sediment, is physically and chemically broken down into smaller and smaller parts by weathering . Fissures that already exist in solid rock widen and offer space for blown dust and pioneer plants. The minerals of the parent rock are converted into other chemical compounds that contain elements that are vital for plants and animals and can be more easily absorbed by plants, e.g. B. Salts of calcium, magnesium, phosphorus or iron (see → silicate weathering ). Dead pioneer plants are first broken down into organic compounds such as sugar, proteins, cellulose and then into humic substances with the participation of animals and microorganisms ( humification ). In some cases, the organic compounds are further converted into simple inorganic compounds by microorganisms ( mineralization ). Various processes ( bioturbation , cryoturbation , peloturbation ) ensure the mixing of organic and mineral substances, as well as the introduction of air into the pore space of this mixture (pedoturbation). In the course of time, this creates a vital, aerated mixture of mineral and organic matter - soil. The organic matter is most concentrated in the uppermost soil horizon , the topsoil or topsoil .
The soil components serve living beings for energy supply and growth . Clay minerals and humic substances caused by weathering combine, e.g. B. in the intestine of an earthworm , to the clay-humus complex , which can "store" nutrients and water and make the soil crumbly.
Secondary soil formation
Soil that has been created changes over time, especially when the factors that determine the soil change. There are many processes that can influence humus formation or otherwise change the soil. These include:
- the leaching of soil constituents into deeper horizons or into the groundwater, e.g. B.
- Glazing , pseudo-gleying
In addition, the primary soil formation usually progresses deeper in the border area between soil and parent rock, so that overall there is a development from shallow, stony, weakly weathered, mineral-rich soils to deep, fine-grained, heavily weathered, washed-out soils. From the existing substrate, the soil type , a soil type with specific properties develops over time through "external" influences (such as climate , vegetation , type of use) . The soil shape forms the overall picture of soil type and soil type.
Soil can have many functions. These are classified and described in the German Federal Soil Protection Act (BBodSchG § 2 Section 2) as follows:
- natural functions
- Basis of life and habitat for people, animals, plants and soil organisms
- Part of the natural balance, especially with its water and nutrient cycles
- Degradation, compensation and build-up medium for material effects due to the filter, buffer and material conversion properties, in particular also to protect the groundwater
- Archive function
- natural archive of cultural history and natural history (with the soil conception of soil only encompassing the most recent section of natural history)
- Usage functions
- Raw material deposit (in the pedological definition of soil to a very limited extent; so-called mineral resources usually arise and are stored in the rock below the soil formation zone)
- Area for settlement and recreation
- Location for agricultural and forestry use
- Location for other economic and public uses, traffic, supply and disposal
Other functions not listed in the law are possible. The German Soil Protection Act does not weight the functions against one another.
As a rule, a floor has several functions at the same time. Unsealed areas even (as a rule) have all natural functions at the same time and also fulfill archiving and utilization functions such as arable farming or forestry. In Germany, it can be assumed that with the exception of a few core zones in national parks, all of the soils are subject to different functions.
The natural functions and the archive function are, however, changed by human use. This influence ranges from minor changes in properties (e.g. compost input in gardens) to a more or less serious impairment (e.g. soil compaction ) to the complete destruction of some functions. Examples of complete destruction would be the removal of barrows through land consolidation measures (destruction of the archive function) or the exclusion of all natural functions through sealing .
An impairment of at least one soil function is called soil damage.
The ÖNORM L 1076 ( "Fundamentals of soil function assessment ') defined in Austria a methodology for soil function evaluation in terms of habitat function, location function, natural soil fertility, water flow regulation, buffer function and archive function. The soil function maps are published by some federal states in the respective geographic information services (as of April 2018: in DORIS , SAGIS ).
In soil science , the physical, chemical and biological properties of different soils are examined. Overall, soils are extremely complex structures with dozens of different properties that all influence one another. In practice, e.g. For example, with many soil samples in agriculture, very few of these parameters are examined in more detail. The type of soil, the humus content, the pH value and the nitrogen, potassium and phosphorus content are usually of particular importance. Many other variables are only recorded specifically when required.
The vast majority of soil properties can be assessed on site using appropriate instructions and tables of characteristic values (e.g. the soil mapping guide ) without a lot of technology. With a little experience these z. B. adequately determine the starting material and the soil color. The use or vegetation with certain types of plants ( indicator types / bioindicators ) can also reliably provide information on soil properties. Exact values, especially for soil chemical parameters, are only provided by scientific measurements, but often not always in the laboratory.
The fundamental peculiarity of soil as a habitat is that all three phases always occur side by side in it: solid, liquid and gaseous. These are homogeneously interwoven and inextricably linked. The fixed component is mainly composed of the mineral framework, which is often perceived in simplified form as “the soil”. This gives the floor its solid, tangible structure and, through its structure and composition, has a decisive impact on many soil properties.
The entire body of the soil is not massive, but is interspersed with numerous microscopic to visibly large cavities (pores). After all, these make up on average around 45% of the total volume of the soil and thus play a significant role in its functions and properties. The pores are filled with the fluid soil components, the soil water (moisture) and the soil air (ventilation), in a strongly changing composition.
In addition, every soil location worldwide has a more or less pronounced soil life . Basically, soils are not dead matter, but living biotopes. The influence of soil organisms on soil properties and functions as well as their mass fraction, however, fluctuates considerably. It ranges from almost undetectable (extremely arid desert soils or locations with very high heavy metal contents ) to almost 100% of the solid component ( moors ).
Soil physical properties
The physical properties of a soil relate to mechanical properties, grain size, water and air in the soil.
Composition and structure
- The soil type (not to be confused with soil type ) indicates how fine or coarse-grained the soil material is; The terms grain size and (soil) texture are synonymous . It is crucial for almost all soil-forming processes and the distribution of the pore volume. The soil type is determined using a grain size analysis .
- The structure is the spatial arrangement of the solid soil components, which has a decisive influence on the water and air balance, the root penetration, the availability of nutrients and similar properties. In natural soils, the material and soil-forming processes determine the resulting structure. Human use (compaction, plowing, irrigation, etc.) can change the structure.
- The pore volume is the void volume in the soil (the sum of the pore volume is almost always around 45 vol%) that can be "filled" by water, air and living beings. Primary pores depend on the grain size as well as the type and composition of the organic components. Secondary pores depend on the structure of the soil and thus on the chemical properties of the minerals, the influence of plants (root tubes) and animals (tunnels) and humans (compaction).
- The effective storage density indicates how densely the soil is stored. This is e.g. B. crucial for the rootability and the seepage water rate.
- The susceptibility to soil erosion depends heavily on the relief, soil type and vegetation.
- The aggregate stability indicates how stable the soil structure is and how much soils tend to silt up.
Air and water balance
- The water holding capacity ( water capacity ) is a very important soil property, because all soils are able to store and release water. This is related to essential properties of water binding, supply and movement in the soil. The water capacity of a soil is represented with the help of a pF curve ( water tension curve ).
- The total pore volume, which is identical to the pore volume, indicates how much water a soil can contain. This variable is divided into non-storable water, which seeps away , and water retained in the soil, the field capacity. The field capacity, in turn, is made up of the usable field capacity available to plants and the dead water that is not available to plants . The distribution of seepage water, NFK and dead water is essentially related to the type of soil. From her z. B. deduce how much available water a soil can store or how high its infiltration rate is.
- The air capacity is the volume fraction of air in the soil when the non-storable water has completely seeped away. This property gives an initial indication of how well a soil is ventilated.
- The soil moisture is the actual water content of soil in the field. It can be measured precisely in the laboratory or estimated in the field using characteristics such as consistency ( cohesive soil ), color or water leakage. In individual cases, soil moisture can be important for determining the soil type (ecological moisture level), but it is often subject to strong fluctuations (precipitation, dry phases). The average moisture in a soil determines how much air (oxygen) there is in it. With permanently high water saturation, reductive , i.e. anaerobic conditions occur.
- The soil air is the current air content in the soil, i.e. the volume fraction that is taken up by air. This value is as high as the total pore volume minus the current water content. Like the current water content, the current soil air content is often subject to strong fluctuations.
Soil chemical properties
The chemical properties of a soil relate to chemical processes and compositions in the soil such as:
- the carbonate content (often abbreviated as lime content): In mineral soils, this essentially refers to the content of calcium and magnesium carbonates ( calcite and dolomite ). The determination in the field is carried out with 10 percent hydrochloric acid based on the resulting reactions. In Central Europe, calcareous soils are a sign of low soil acidification .
- the pH value : All soils have a pH value. This property has a major influence on almost all other soil properties such as the availability of nutrients, the structure or leaching. A pH value analysis is a crucial part of soil research in order to assess the fertilizer and lime requirements of fields.
- the buffer area : the soil is a chemical buffer system that regulates the acidity of the soil. As a rule, the buffer area is not determined explicitly, only the pH value is measured.
- the ion exchange capacity . Every soil can store ions in so-called exchangers ( clay minerals , humus, some oxides ) and release them again. The exchange capacity indicates how large the exchange and storage capacity of a soil is. The level of the value depends not only on the type and quantity of the exchanger, but also very much on the pH value. A distinction is made between the potential value, i.e. the maximum possible, and the effective value, i.e. the currently available value. In general, both anions ( anion exchange capacity, AAC) and cations ( cation exchange capacity , KAK) can be stored. The AAK is only significant in some tropical soil types. The KAK, on the other hand, can be viewed worldwide as the parameter for soil fertility. The higher it is, the better soils can store nutrients and the more fertile they are. The lower it is, the greater the risk of nutrient leaching.
- the chemical composition, i.e. the frequency of the elements and substances currently present in the soil. Of particular importance are certainly the nutrient contents, e.g. B. nitrogen, potassium and phosphorus, or heavy metal pollution.
- the redox potential . Roughly speaking, this value gives an indication of the availability of oxygen in the soil. This depends on the ratio of oxidized and reduced substances in the soil. Reductive zones are anaerobic while oxidative zones are aerobic.
Soil biological properties
- the root penetration : root penetration depth and root penetration intensity. The root penetration intensity is measured as the mean number of fine roots (roots with a diameter of <2 mm) per dm².
- the soil life : The frequency and occurrence of micro-organisms and animals in the soil. The revitalization of the soil can be recorded directly (counting, estimation) or indirectly ( respiration rates , nitrification ).
- the humus content : the amount of organic matter (except coal and anthropogenic carbon compounds) in the soil. It results from the excretions of living and the residues of dead organisms and is subject, even if the total value is relatively stable, to constant build-up, change and breakdown.
- The soil color is the color, brightness and color intensity of the soil. It arises from the interplay of physical, chemical and biological processes in the soil. Pure sand is naturally colorless white ( quartz ). The most important coloring agents in the soil are humus (black, gray) and iron ions in various forms (oxidative: brown, red, yellow, reductive: gray). The floor color is determined using the standardized Munsell color tables .
- the technogenic carbon content: This indicates the proportion of technogenic, i.e. artificially introduced carbon. This material does not occur in natural soils. Technogenic carbon is of artificial origin ( coal , plastic , soot , mineral oils , tar, etc.) and is typical for human-influenced soils such as dumps, heaps or sinks. In heavily influenced soils, the proportion of technogenic carbon can be very high. The separation of natural and technogenic carbon requires special analytical steps in the laboratory.
The determination of a soil , i.e. the allocation to a soil type , can in many cases take place in the field on the basis of some obvious properties such as soil type, color and raw material. In some cases, more extensive physical and chemical analyzes in the laboratory are necessary. The existing soil horizons are almost always decisive (not to be confused with layers ), i.e. areas within the soil that have characteristic features and properties and are characterized by the same soil-forming processes . Horizons differ from one another in their properties such as structure, soil type or color. In order to recognize them, a soil profile must be created.
There are various international soil classifications, so that a soil can be given different names or classifications depending on the classification. The German soil systematics is used in Germany. The World Reference Base for Soil Resources (WRB) and the USDA Soil Taxonomy are also used internationally . The formerly important FAO soil classification has been replaced by the WRB and is only of scientific historical value.
Land value number
The soil value index (BWZ) or soil number is an index for soil fertility. It is determined as part of a soil estimate for the soil in a specific region. The soil number relates the achievable net yield of a soil to the most fertile black earth soil of the Magdeburg Börde , the value of which was set to 100. The number of arable land or arable land value is determined based on the number of soils, and climatic and topographical location factors are also included in the calculation.
Endangerment and protection of soils
With their properties and functions, soils are of fundamental importance for mankind and nature. For this reason, this environmental medium has been legally protected in Germany since 1999 ( Federal Soil Protection Act ) and its status is therefore equated with the other two protected goods, air and water .
The dangers to which soils are exposed are extremely diverse:
- Leaching or leaching
- Soil degradation
- Soil erosion
- Soil compaction
- Salinization of the soil
- Soil acidification
- Soil pollution or contamination
- Surface sealing
- acid rain
Some of these processes, such as sealing, acid rain or pollution with harmful substances do not occur in nature. Others, on the other hand, are natural processes that have reached harmful dimensions through human activity (misuse and overuse ).
Large areas of land around the world, especially in urban areas and agricultural regions, are affected by one or more of these threats. This leads to extensive soil damage . Around 25 million tons of topsoil are lost annually through soil erosion alone.
Due to the increasing downgrading of the value of soils ( soil degradation ) in combination with the increase in the world population, the usable area per person will halve between 2012 and 2050.
World soil map
From 1971 to 1981 the FAO and Unesco published a world soil map on a scale of 1: 5 million in 10 sheets. It is the first worldwide soil map work on such a scale and serves to this day as the basis for further soil maps. A separate soil classification system was developed to create the maps, the FAO soil classification , which has since been replaced by the WRB . In addition to the dominant soil type, each mapping unit also shows two subordinate soil types.
Soil Atlas of Europe
In 2005 the EU Commission published a soil atlas for Europe for the first time . This describes the state of and potential or real threats to European soils. Janez Potočnik and Stavros Dimas , Commissioners responsible for science, research and the environment at the time, presented the atlas to the European Parliament during the information days on the activities of the Commission's Joint Research Center (JRC). The Commission coordinated the creation of Europe's first soil atlas by bringing together leading European soil scientists . The scientists came from over 40 countries and worked at national soil science research institutes, all of which were represented in the (now no longer active) " European Soil Office Network " of the JRC. Even before the actual establishment of this network in 1996, the then European Community issued a soil map for the first time in the 1970s , at that time on a scale of 1: 1,000,000.
The Soil Atlas of Europe contains soil maps and easily understandable texts, which are supplemented by photos, and is based on the European soil information system MEUSIS , which was developed by the Commission and covers the entire EU including the neighboring countries. The maps show the soil types according to the 1st edition (1998) of the international soil classification system World Reference Base for Soil Resources (WRB). In contrast to the world soil map , however, the mapping units only contain one (the dominant) soil, so that no knowledge about the homogeneity or heterogeneity of the soils of a mapping unit can be derived.
In the following years the JRC published:
- Soil Atlas of the Northern Circumpolar Region (2010)
- Soil Atlas of Africa (2013)
- Atlas de Suelos de América Latina y el Caribe (2014)
Global Soil Map
On the initiative of the “Digital Soil Mapping” working group of the International Union of Soil Sciences (IUSS), an international consortium was founded in 2008 to develop a digital global soil map . This is not a soil map in the narrower sense, in which soil types are shown, but a soil property map in which soil properties such as pH value , soil type and humus content are shown. After the consortium had dissolved, the International Union of Soil Sciences (IUSS) founded the “Global Soil Map” working group in 2016 to continue working on the creation of the map of the same name.
Lobbying and public relations
In order to draw attention to the importance of soils for natural cycles and for human societies, the Food and Agriculture Organization of the United Nations (FAO) declared 2015 the International Year of Soils . The same background have the World Soil Day ( World Soil Day ), which every year is celebrated on 5 December, and in Germany the Kürung the soil of the year .
As a platform for an exchange of ideas between science, politics and business on the subject of global soil protection, the Global Soil Week has been taking place in Berlin at irregular intervals since 2012 .
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