Soil is the uppermost component of the firm earth's crust and beside water and air a substantial range of our environment. Soil allows growth by plants, it affects considerably the water regime of the earth and by this our climate. Without soil an existence of higher organisms on our earth would be impossible. Soil develop as a function of firm or loose parent rock, relief, climate, soil water, organisms, time and the influence of men.

Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological, and anthropogenic processes on soil parent material resulting in the formation of soil horizons. Soil is always changing. The long periods over which change occurs and the multiple influences of change mean that simple soils are rare.
By the respective parent rock and its chemical composition soils are very different. On granite, sandstone or sandy parent rocks soils develop which over the years tend to strong acidification. On limestone or other parent rocks containing carbonates (like marl) soils rich of bases develop.
The ground development thus depends always first on the composition of the parent rocks and their minerals.
The rocks are divided into smaller components by weathering. Weathering is the decomposition of earth rocks, soils and their minerals through direct contact with the planet's atmosphere.
Two important classifications of weathering processes exist. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals, or biologically produced chemicals (also known as biological weathering), in the breakdown of rocks, soils and minerals.

2.1 Physical (mechanical) weathering
Mechanical weathering is the cause of the disintegration of rocks. The primary process in mechanical weathering is abrasion (the process by which clasts and other particles are reduced in size).
This same phenomena occurs within pore spaces of rocks. They grow larger as they attract liquid water from the surrounding pores. When water becomes ice, it expands. The pressure becomes so large that the rock can be destroyed (see picture).
Rock is warmed up by sun exposure and it expands. Since rocks have different minerals (bright and dark as e.g. with the granite) the different minerals of the rock expand differently. The rock loosens. During cooling in the evening or by a cooler downpour the rock shatters.

2.2 The chemical weathering
Chemical weathering involves the change in the composition of rocks, often leading to a 'break down' in its form. This is done through a combination of water and various chemicals to create an acid which directly breaks down the material. This type of weathering happens over a period of time. Chemical weathering may alter a rocks chemical make up by changing the minerals in the rock or it adds some new minerals.
During the solution decomposition e.g. limestone is dissolved nearly completely in water and supplied with the seeping water to the groundwater. Following the groundwater flow the solved limestone floats into rivers and into the sea, in order to precipitate there again. Thus new rock is formed over long periods.
With the hydration (storage of water molecules) the dipole characteristic of the water comes to carrying, by which water molecules with their differently loaded parts penetrate into the minerals of the rocks and urge them.
During the oxidation the oxygen from air or the seeping water commits itself with the iron minerals to ferric oxides. This gives the affected rock a reddish coloration on the surface which crumbles easily and weakens the rock. This process is better known as 'rusting'.

2.3 The biological weathering
The physically biological and the chemically biological weathering are to be mentioned here.
During the physically biological weathering the roots of the trees are active. By their thickness growth they cut the rock up and thus provide an enlargement of the surface of the rock and again an intensified chemical weathering.
Bacteria, mushrooms and algae contribute to the chemically biological weathering, by excreting acids through their metabolism and thus loosen rock, in order to reach for its nutrients (see figure).

3.1 Grain size
Particles are grouped according to their size into what are called soil separates. For the characteristics concerning the water and air budget and the fertility of soil, apart from many other factors, the sizes of the soil grains are of importance.
The grain size of the ground is subdivided in sizes < 2 mm, the fine ground and in sizes of > 2 mm. Of special importance is the fine ground, which is subdivided again into the separates typically named clay, silt, and sand (see table). Soil texture classification is based on the fractions of soil separates present in a soil.

Diameter in μm	Separate	Abbreviation
< 2.0	Clay	T
2.0 - 63.0	Slit	U
63.0 - 2000	Sand	S

Usually soils have different portions of the different grain sizes. In addition there are soils predominantly built from clay/tone, silt or sand particles. The frequently used term loam covers different constituent amount from all three groups (sand, silt, clay/tone).

Soil types are characterized by the following characteristics:

     Sandy soils
     • low water holding capacity
     • favorable aeration
     • total pore volume relatively small
     • single pores relatively large
     • fast drying
     • snaps warming up barness in the spring
     • high eluvation of plant nutrients
     • low subsequent delivery of nutrients

     Silt soils
     • inclined to the sealing storage and thus to the water rope
     • bad aeration
     • water ascent from damper ground zones is good
     • the ground dredged, cakes and consolidates easily with impact of the rain
     • easy erosion (see 4.2)
     • nutrients are slowly unlocked by weathering
     • nutrients are eluviated little

     Clay soils
     • high water capacity
     • high proportion of not plant-available water
     • unfavorable aeration
     • high total pore volume
     • low water movement
     • low permeability for water and air
     • the natural content of nutrients usually is high and eluviation is low
     • upwelling of soil mass with water absorption
     • shrink and creation of chasms on drainage
     • plants badly penetrate close clay soil through roots

In the clay/tone separates - beside metallic oxides, primary minerals from the original rocks and quartz - are also secondary minerals (new mineral formations), the clay minerals. Clay minerals are of great importance in soils. They are e.g. able to incorporate and provide nutrition ions like calcium, potassium or magnesium.

3.2 Soil moisture and air space in soil
The water regime of a soil depends particularly on its grain size, on the content of organic substance and on the kind of the use. The different grain sizes determine the quantity of the pores, which are present in a ground. The pores are either filled with water or air. The primary pores correspond to the different grain sizes.
So a sandy soil has above all large pores (macropores), which pass the water on fast to the groundwater. A clay soil has predominantly micropores, which hold the water by adhesive powers against the force of gravity. The water in clay soils can be so firmly bound that it is not possible for plants to take up this water. Silt grounds or loamy soils have predominantly medium sized pores in which the plant accessible water is held.
If a soil is frequented under unfavourable conditions, such as being too moisty, it will be compressed. The pores are squeezed and thus an infiltration of the water is made more difficult. The by then mainly existing finer pores do hold the water very strongly and it will be no longer available for the plants. Often under compression a backwater situation develops, which makes the cultivation of soils more difficult and a penetration by plant roots impossible.

3.3 Soil organisms
In and on soils live various organisms. It's them, which convert the organic substance into inorganic basic materials which are at the beginning of all life. All organisms of the ground as a whole are called Edaphon. The classification of the organisms varies, usually they are subdivided by their size. The smallest organisms (microorganisms) are bacteria, mushrooms and algae. The animal organisms cover from the flagellates and ciliates , spiders, woodlice and snails as well as earthworms and moles. Not all soil organisms can be evaluated positively. There are also parasites as e.g. the nematodes (thread worms) which attack sugar beets and causelarge harvest losses. And the soil bacterium Clostridium tetani is a harming organism which causes tetanus (lockjaw) with humans.
The way of life of soil organisms is adapted to their special habitat. Intending for their activity are above all temperature, humidity, season and the pH value, which indicates whether a ground is rather acid or basic.
The meaning of the pH value for the ground organisms is to be recognized with the naked eye in the terrain. In a ground with neutral to weak-acid pH value the arising organic substance, e.g. leaves, will be converted within a year. On grounds with acid pH value (< 5) one can observe a growing organic layer with decreasing pH value. A soil temperature between approximately 10 and 35 degrees is optimal for the organisms and for this reason for the decomposition of organic substance. The temperature must be seen in connection with the seasons. The organisms' activity is different according to the yearly variation.
Most organisms can be found in the upper most 30 cm of the soil. This is justified in the higher offer of oxygen and an increasing load and density growing with the depth. Of course there are also organisms, which evade even into deeper layers, such as the earthworm.
The presence of many easily convertible carbohydrates (sugar, amylum) in the organic substance leads to a fast dismantling.
Plant material with high portions of lignin and tanning agents, e.g. spruces, is not so fast converted by the organisms. Here it requires some specialists (e.g. certain mushrooms), which are able to convert these materials over a long period.

3.4 Soil fertility
Humus is the organic material in soil lending it a dark brown or black colouration. It is of very great importance for the ground. It is able to store water and nutrients.
In soil science, humus refers to any organic matter which has reached a point of stability, where it will break down no further and might, if conditions do not change, remain essentially as it is for centuries, if not millennia.
In agriculture, humus is sometimes also used to describe mature compost, or natural compost extracted from a forest or other spontaneous source for use to amend soil. It is also used to describe a topsoil horizon that contains organic substance.
Humus in interaction with the inorganic ground particles and the ground organisms gives the fertility to the ground. It is provided to the soil by dead plants and organisms as well as by organic fertilizers. During its time in the ground the organic substance is subject to many transforming processes. Soil organisms convert it to the macro nutrients such as calcium, magnesium etc, and to micro nutrients such as zinc, boron etc. as well as to water and CO₂. These nutrients can be taken up then again by plants and/or ground organisms.

3.5 Exchange capacity
One of the most important ground functions is the exchange capacity.
That the ground again and again allows plant growth is because of its ability to deposit nutrients but also pollutants reversibly. This characteristic of reversible exchange of nutrients is called exchange capacity.
Soil particles function as ion exchangers. Ion exchangers in the ground are organic substances, the tone minerals and further small soil particles such as oxides and allophanes. As oxides the ferric oxides or the manganous oxides are to be mentioned. Important ions are above all the cations calcium, magnesium, potassium and sodium. The allophanes are smallest mineral soil particles, which belong to the tone separates. They all are qualified for the change of ions but in different percentages.
The exchange takes place between the soil particles and the soil solution. They exchange the nutrition ions in equivalent quantities (see figure below).

3.6 pH value
It was already mentioned that the pH value has influence on the Edaphon, the soil organisms, as they work best on neutral to weakly acid pH values, and that organic substance is best converted under the same pH values. But also for the nutrient availability the pH value is of special importance.
The pH value is the Briggs logarithm of the hydrogen ion concentration. That means, the hydrogen ions, which are also called H⁺-ions or protons, are responsible for the pH value. Water is as well known H₂O. It is however not exclusively present in this form, but contains also H₃O⁺ and OH⁻ ions (Hydroniumion and hydroxyl ion).
Simplified one writes H⁺ and OH. If H⁺ and OH⁻ are in the equilibrium, a neutral pH value is present, thus pH 7. When the number of H⁺-ions increases in the soil solution, the pH value decreases, the soil acidifies. If the H⁺-ions disappear, the pH value increases.

Reaction	pH
Extremely alkaline	> 11.0
Very strong alkaline	10.1 - 11.0
Strongly alkaline	9.1 - 10.0
Moderate alkaline	8.1 - 9.0
Slightly alkaline	7.1 - 8.0
Neutral	7.0
Slightly acidic	6.9 - 6.0
Moderate acidic	5.9 - 5.0
Strongly acidic	4.9 - 4.0
Very strong acidic	3.9 - 3.0
Extremely acidic	< 3.0

The special importance of the pH value settles in many characteristics of the soil. The availability (mobility) of nutrients and pollutants depends directly on the pH value. The activity of soil organisms and the conversion of organic substances, also the translocation of materials (iron, manganese) or the release of aluminium directly depend on the pH value. Many plants best grow within a specific pH value. Oats, rye and potatoes prefer a lower pH value than e.g. sugar beets and barley.
The pH value affects a many processes in the soil and can be also easily affected. So a pH value can be increased by a liming (addition of lime). Certain nitrogen fertilizers lower the pH value. Here the fertilization with ammonium (NH4⁺), a slowly working nitrogen fertilizer, is to be particularly mentioned. It is subject to conversion processes in the soil which sets free H⁺-ions. So these processes lead to acidification and must balanced by regular liming.
Rainfall also affects soil pH. Water passing through the soil leaches basic nutrients such as calcium and magnesium from the soil. They are replaced by acidic elements such as aluminium and iron. For this reason, soils formed under high rainfall conditions are more acidic than those formed under arid (dry) conditions.

3.7 Structure of soil
Soils are very differently structured according to their developing conditions (parent rock, climate, situation in the area).
Not all soils show these following characteristics. The differences are caused by the parent rock, in the slope inclination (erosion), in the water influence (back water, groundwater), climate (intensity of the decomposition) and many other influences.
So grounds on limestone have only two horizons, i.e. an A-horizon and a C-horizon due to a long development time. This means the permeability for plant roots averages only about 20 cm.



Soil horizons
O) Organic matter: Litter layer of plant residues in relatively undecomposed form.
A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates iron, clay, aluminium, organic compounds, and other soluble constituents.
B) Subsoil: This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.
C) Substratum: Layer of unconsolidated soil parent material. This layer may accumulate the more soluble compounds that bypass the "B" horizon.



A typical brown earth. The 'v' behind the horizons' classification stands for the German 'verwittert'=weather-beaten, the 'h' stands for humus

4.1 Sealing
Soil plays an important role as location for the plants and as base of life for humans and animals. The realization that the buffer, filter and conversion ability of the soils can be finite, led to strengthened soil protection programs during the last years. In these programs above all it is tried, to protect soil from sealing, erosion and the entry of pollutants.
Sealing of soil signifies a coverage with impermeable substances such as asphalt, concrete or buildings. Different reasons to seal a soil are e.g. protection from moisture, improvement of the load-carrying capacity of the underground, creation of "wash and wear open spaces "and protection of the soil from pollutant entries. The sealing has direct effects on the ground-water formation and hives a sinking of the groundwater levels under settlement surfaces.
The evaporation is lowered and thus the climate within sealed ranges will change. From sealed soils surface discharge of water increases and by this the risk of high floods is much higher.
Often soils are not completely sealed. By different surfaces it can be achieved that a part of the precipitation water seeps. It is to be decided in the specific cases whether a water-bound cover (crushed stone surfaces, grass paver) with a permeability of approximately 60%, or small, medium or large cobbled pavement with a permeability between 20 and 40% or asphalt surfaces with only minimum permeability are selected. In many cases it can turn out as necessary to take measures against soil sealing. A sealing as small as possible should be aimed at when building new settlement areas. With strongly sealed surfaces examine an unsealing (which might not be suggestive if the sealing is necessary for the existing use, e.g. for a protection from pollution impact).
Suggestible possibilities can show up when changing the surface pavements and cycle tracks, yard approach roads etc. It also should be examined whether surfaces with higher permeability can be used. In urban places tree grates should be enlarged and oversized roads should be renaturated. Unnecessarily sealed surfaces within the range of yards, front gardens and places should be unsealed. In urban planning the potentials of unsealing or of surface changes are to be determined.

4.2 Erosion
Where ever land masses rise above the sea soil erosion appears. Two processes of the erosion are distinguished: the erosion by water and the erosion by wind.
With the water erosion the soil material, which is cleared away in one place, will be set off in a more lower place (sink) or to be carried with the rivers to the sea (see image).
Over the period of million years new parent rock develops by lifting and lowering the bottom of the sea.
The extent of the water erosion depends among other things on the quantity of the precipitation, on the wind, on slope length, slope inclination and slope form, on the vegetation and on the sealing. With violent and strong precipitation on soils with lacking covering a particularly strong erosion occurs. Rain drops directly impacting on the ground destroy the soil structure and lead to the dredging of the soil surface.
Thus the water absorption capacity of the soil is reduced. The water flowing off on the surface loosens the soil particles and lead them downhill. Something similar is valid for the thaw, if the frozen underground cannot take up the meltwaters.
The vegetation is of great importance with the erosion. An uncovered ground is very much more strongly eroded than ground with a close ground cover. Therefore all kinds of field crops are problematic, which leave the ground almost uncovered during a long period, such as corn or sugar beets.
Not all soils are equivalent susceptible to erosion. The resistance of a soil against erosion rises with the cohesion of the soil particles, the size of the soil particles and the water permeability. Small particles can be more easily shifted than large. Silt-rich grounds accordingly are more easily eroded than sand-rich.
By erosion the base of life for plants, animals and humans disappears. A substantial contribution for the protection of the soil from water erosion is the induction of organic substance (humus) and/or lime, which stabilizes the ground. Soils in hillside areas require adapted cultivation. Farmers must plough during correct soil humidity in a transverse to the hill gradient and avoid soil compactions. Consolidated grounds are subject to faster erosion than uncompressed. In many countries the terrace building of the area is an effective protection from erosion (rice terraces). In Germany one finds terrace building particularly with special cultures such as in the cultivation of wine. An all-season living ground cover (e.g. continuous cultivated pasture), intermediate field fruits (a seed cultivated between the main fruits e.g. between barley and sugar beets), mulching (induction of organic material) decrease erosion just like reducing the slope length (division of a field).
Wind erosion predominantly takes place where strong wind velocities can evolve. For the prevention of wind erosion the same measures are to be adopted as with the water erosion (except terrace building). Here plantings, e.g. hedges, play a main role.

4.3 Pollution impact
Pollutants are brought in by air or water (precipitation, floods). So lead, which was allowed to be added to gasoline until 1998, can be measured in soils even in the far Siberia or in the ice of the Antarctic and Arctic. Nuisance today have reached all soils of this world by long-distance long distance transport.
Pollutants of emitters reach all soils at a distance of 20 to 50 km. By the yield of wastes such as sewage sludge or harbour mud, by dumps or sewage farms the soils were loaded strongly with pollutants. In the range of industrial uses grounds are not only effectively changed, but also loaded with organic and inorganic pollutants. Whether certain materials in the ground can be classified as pollutants is a question of impact. Certain heavy metals (copper, zinc etc.) in small quantities are essential for plants, animals and humans, however with increased ground contents come up with harmful effects.
Whether contents in the soil are normal or too highly is exactly fixed. Since soils contain by nature certain materials and, as already said, a part of these materials are essential, the definition of normal and too high content was preceded by long time research.
Certain organic pollutants such as mineral oil hydrocarbons can be reduced by soil- organisms and thus rendered harmless. Other organic pollutants, as dioxins and furans, which develop with incomplete inefficient burning, are more complicated constructed and not so easily to crack by soil organisms. Soils containing these pollutants must be taken out of the soil unit and burnt in appropriate plants. Through this process pollutants will be destroyed and the soil can be used again.

PDF Version