Soil: composition, formation, ecology and human significance

From a pedological point of view, soil is the overlapping zone between the lithosphere (the solid rock cover of ­the earth), the biosphere, the earth's atmosphere and the hydrosphere, which is characterised by soil-forming processes. 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 (produced 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 (called interstitial spaces or intergranular spaces) that are filled with aqueous soil solutions (soil water) and soil air. The structure of topsoil is usually relatively loose ("crumbly"), which is why the term crumb is used in this context. Soil plays the most important role as the central basis of life for plants and directly or indirectly for animals and humans.

The definition according to the soil protection concept of the Federal Government is: "Soil is the transformation product of mineral and organic substances interspersed with water, air and living organisms, formed on the earth's surface under the influence of environmental factors and evolving over time, with its own morphological organisation, which is capable of serving as a site for higher plants. As a result, the soil is able to form a basis of life for animals and humans. As a space-time structure, soil is a four-dimensional system."

"Soil is a section of the pedosphere extending from the earth's surface to the bedrock, that is, that region of the earth's crust in which the lithosphere has been transformed by atmospherics (oxygen, carbon dioxide, nitrogen, precipitation, radiation, outgassing ...) and organisms (bacteria, algae, fungi) and in which such transformations continue to occur."

Soil-forming factors

Soils form relatively quickly over geological periods, but relatively slowly by human standards. The soils in today's Central Europe have formed over the last 12,000 years, since the end of the last ice age.

• The primary prerequisite for the formation of a soil is a parent material consisting of a rock (solid rock or unconsolidated sediment).
• The relief must not be too steep, otherwise the parent material will be carried away by erosion before it can form a soil.
• No large quantities of sediment may be introduced into the area concerned, as otherwise the soil that is forming will be buried again and again and soil formation will have to start all over again.
• The climate should be humid (moist) and not too cold, so that on the one hand there are favourable living conditions for plants and animals and on the other hand because water is important for weathering processes.

Even "finished" soil is still subject to these factors. In addition, there are the soil organisms, as part of the soil, and, since about 8,000 years (later in Central Europe), the human (anthropogenic) influences, such as agricultural or forestry use or pollutant inputs.

Soil forming processes

Soil-forming processes are triggered by soil-forming factors. They have two kinds of effects: They change a parent rock and transform it into soil, but they also change an existing soil.

Primary bottom formation

The parent rock, whether it is solid rock or unconsolidated sediment, is physically and chemically broken down into smaller and smaller pieces by weathering. Fissures that are present in a solid rock anyway widen and provide space for blown dust and pioneer plants. The minerals of the parent rock are transformed into other chemical compounds containing elements that are vital for plants and animals and can be more easily absorbed by plants, e.g. salts of calcium, magnesium, phosphorus or iron (see → Silicate weathering). With the participation of animals and microorganisms, dead pioneer plants are first decomposed into organic compounds such as sugars, proteins, cellulose and then into humic substances (humification). In some cases, the organic compounds are also further transformed by microorganisms into simple inorganic compounds (mineralization). Various processes (bioturbation, cryoturbation, peloturbation) ensure the mixing of organic and mineral matter, as well as the introduction of air into the pore space of this mixture (pedoturbation). Thus, over time, a revitalized, aerated mixture of mineral and organic matter - soil - is formed. The organic substance is most strongly enriched in the uppermost soil horizon, the topsoil.

The soil components serve living organisms for energy supply and growth. Clay minerals and humic substances formed by weathering combine, e.g. in the gut of an earthworm, to form the clay-humus complex, which can "store" nutrients and water and makes the soil crumbly.

Secondary bottom formation

Soil, once formed, changes over time, especially when soil-determining factors change. There are many processes that can affect humus formation or otherwise change the soil. These include:

• the leaching of soil components into deeper horizons or into groundwater, e.g.
• water-soluble salts
• carbonated calcium carbonate
• acid-soluble clay minerals (clay displacement),
• acid-soluble iron and aluminium hydroxide as well as humic substances (podsolation)
• gleying, pseudo-gleying

In addition, the primary soil formation in the border area of soil and parent rock usually progresses into the depths, so that an overall development from shallow, stony, weakly weathered, mineral-rich soils to deep, fine-grained, strongly weathered, washed-out soils takes place. From the existing substrate, the soil type, a soil type with specific properties develops over time due to "external" influences (such as climate, vegetation, type of use). The soil form forms the overall picture of soil type and soil type.