What is the process of soil formation

2.2 Soil development

2.2.1 Soil-forming processes

The processes taking place in the soil are influenced and controlled by a large number of factors. This includes the original rock, the climate, the shape of the terrain, the vegetation, the organism, the water, the activities of humans and the time. These factors have an after, side-by-side and with one another. The soil profiles are divided into topsoil (A-horizons) and subsoil (B-horizons).

Soil formation in our latitudes begins with the accumulation of humus and the formation of raw soils. If there is only a weak accumulation of humus (Ai) in the topsoil above the initial system, these soils in Mecklenburg-Western Pomerania with sands, loams and clays on the surface are called loose syrosemes.

The loose syrosemes gradually develop into more differentiated soil types, the so-called A / C soils. They consist of a humus A-horizon that merges directly into the parent rock.

In Mecklenburg-Western Pomerania these are the regosols (Ah / ilC) for lime-free parent rocks (dune sands), the pararendzines (Ah / elC) for carbonate-containing parent rocks (e.g. glacial till) and the rendzines (Ah / cC) on the Rügen chalk limestone.

Today we also often find A / C soils that were created by erosion of the upper soil horizons.

The further soil development with decalcification, browning, clay shifting (lessivization) and podsolization took place under the forest and led to the formation of brown earths, parabrown earths, pale earths and podzols.

During decalcification (carbonate weathering), the carbonic acid dissolved in the seepage water dissolves the carbonates of the substrate and removes them as easily soluble hydrogen carbonates. They fall out again at greater depths than limestone concretions and limestone streaks. However, some of it also reaches the groundwater.

 

In Europe, browning and thus a development towards brown earth (Ah / Bv / C) is most widespread on silicate-rich parent rock (e.g. sand sand, cover sand and loamy sands in the ground and terminal moraines). The process of browning is tied to a temperate climate and takes place mainly under deciduous and mixed forests with a balanced soil water content. The Fe2+- Understood release from silicates by weathering. Brown-colored iron oxides and hydroxides result from air contact (oxidation) of the bottom iron. They accumulate in profile and evoke the characteristic brown color of the Bv horizon. The browning is often associated with a secondary formation of new clay minerals, which is also referred to as silting.

 

Podsolation is a complicated soil genetic process that takes place in an acidic environment and relocation of substances. It is characterized by intensive dissolution and leaching processes in the topsoil, caused by organic acids from litter decomposition in natural vegetation (forest, heather).

An ash-gray washout horizon (Ae) with free quartz grains forms under the humus topsoil. This leaching horizon merges into enrichment horizons at depth. First a gray-black horizon (Bh) enriched with organic matter follows. The washed-out sesquioxides (aluminum, iron, manganese oxides and hydroxides) are deposited again in the following, rusty yellow to rusty brown Bs horizon. The bh and b horizons are often cemented to cast stone.

Podsols (Ah / Al / Bh / Bs / C) develop on low carbonate, permeable, silicate and therefore nutrient poor rocks in cool, damp to cold damp areas under coniferous forests, acidic deciduous forests and dwarf shrub heaths. Compared to browning and shifting of clay, podsolization proceeds relatively quickly. This is why podsoles can also be found on Holocene deposits or areas that were cut down to the subsurface by erosion in the Holocene.

 

On the marl boulder and loamy sands of Pleistocene ground and terminal moraines in Mecklenburg-Western Pomerania, the shifting of clay (lessivation) is one of the most important soil-forming processes.

Clay shift in soils is the downward shift of the finest soil components (clay particles and colloid grain size <2 µm) with the seepage water in the coarse pores from the topsoil (Al horizon) into the subsoil (Bt horizon). The upper horizons (Ah, Al) become impoverished in clay, while the lower horizons (Bt) become rich in clay.


The clay shift (lessivization) is the profile-defining process of the parabrown earths (Ah / Al / Bt / C) and the pale earths (Ah / Ael / Bt / C). Pale earths show a stronger shift in clay (lessivation) than parabrown earths.

 


In Mecklenburg-Western Pomerania the Al and Ael horizons often show a browning. Then one speaks of brown earth-parabroun earth or brown earth-pale earth.

In a special form of the Lessives on sandy parent rock, the enrichment horizons (Bt) are broken up into bands (Bbt). These banded parabroun earths and banded pale earths occur in Mecklenburg-Western Pomerania on the sands of the terminal moraines. In the ground moraines we often find them in the transition areas between sand and clay / till.

 

Figure 4: Scheme of selected development lines on loose rock with balanced soil water content in Central Europe

Source: Bosch (1994)

 

Under the influence of moisture, the soils are characterized by gleying and pseudo-gleying. The moisture reduces the breakdown of the mineralization of organic matter and increases the solubility of the metal and humus compounds. This creates soils with pale, gray or speckled subsoil horizons.

Gleye (Ah / Go / Gr) form when the groundwater comes close to the surface. If the water builds up near the surface, pseudogleye (Ah / Sw / Sd / C) arise.

Gleye have a dark humus horizon, which, depending on the depth of the groundwater, is either underlain by a rust-stained oxidation zone (Go) in the capillary fringe of the groundwater and a gray reduction zone (Gr) temporarily or permanently in the groundwater.

Gleye are developed in Mecklenburg-Western Pomerania in large areas in the valley sand areas in southwest Mecklenburg and on the Holocene sands of the Darß. Otherwise they occur nationwide in depressions, hollows, river and stream valleys.

 

On the other hand, one speaks of pseudo-gleying when seepage water penetrates the ground and is dammed by thicker layers. The soils are characterized by the constant alternation between winter wet and summer dry phases. In the water-filled storage zone (Sw), primarily black concretions develop, while the storage body (Sd) has a gray / rust-colored marbling.

Pseudogleye arise on clay-rich parent rock with low water permeability. There are various transitional forms between the soil types described, such as Parabrounerde-Pseudogley, Gley-Braunerde, Braunerde-Podsol and others.

The processes of soil formation explained so far are processes that take place slowly in the soil and lead to its horizon differentiation.

 


The typical sequences of soil development are changed by the influence of water and wind. They directly counteract the development of the soil. The upper soil horizons are removed or shortened and the starting material moves closer to the surface again.

If the erosion sediments are deposited on lower slopes, in depressions and hollows above the original soils after a short transport route, they are called colluvia. The floors are Kolluvisole (Ah / MIIf). They are widespread in Mecklenburg-Western Pomerania in all flat-wavy to hilly areas.

If the removed sediment is transported over long distances and is deposited in regularly flooded river and stream valleys, it is called floodplain. In Mecklenburg-Western Pomerania they are only developed in the Elbe Valley.

 

The landscape term "moor" is also used for soils in this landscape. The development of moors is not linked to a parent rock. Bogs and bog soils are mainly characterized by the formation and accumulation of peat and mud. Peat is made from aquatic and marsh plants. Their formation depends on an excess of groundwater or rainwater. Mudden are fine sediments deposited at the bottom of lakes.

Moors consist of peat with> 30% by mass of organic matter and are> 30 cm thick, including intermediate mineral layers and muds.

Soils with peat thicknesses <30 cm are called Moorgleye.

 

Fens arise as a result of high groundwater levels or when groundwater leaks on slopes.

Raised bogs owe their formation to an abundance of rainwater that accumulates on impermeable mineral soils or fens or is stored in the bog vegetation and hardly contains any nutrients.

The development and formation of the moors is conditioned by the water conditions, which can be fundamentally changed by humans today.

Soil formation with the formation of horizons only takes place in the bog after the natural water level has dropped. This is usually a result of measures to make the moors usable or to improve the usability.

 

 

2.2.2 Soil development in the Pleistocene and Holocene

Soil development is a process that takes centuries and millennia to take place.

The oldest sediments stored on the surface in large areas in Mecklenburg-Western Pomerania are those from the Saale Ice Age in south-west Mecklenburg.

The Saale Ice Age was followed about 130,000 years ago by a warm period, the Eem. A soil formation with profound weathering and decalcification of the surface sediments took place in the Eem.

With the beginning of the new Ice Age, the Vistula Ice Age about 115,000 years ago, the ice-free areas (old moraine) that were not covered by the new ice advances had a periglacial climate with significant frost and thawing phases on the surface.

The periglacial overprinting with its distortions (cryoturbations), the soil flow (solifluction) and the effects of the wind (aeolian processes) destroyed the soils developed in the Eem and left clear traces in the soil and in the landscape. The relief was compensated by fluid earth, solifluid material sorting on the slope and the formation of periglacial dry valleys.

The upper decimetres of sand and slightly gravelly soil, known as bed load sand, were enriched by flushing and drifting. The fine particles blown out were deposited as flying sands. There are no loops in Mecklenburg-Western Pomerania that were also created by the action of the wind during this time.

 

The periglacial of the young moraine area began around 14,000 years ago when the ice melted back and lasted until the end of the late glacial around 10,000 years ago.

Although there was a much shorter time available between ice melting (late glacial) and boreal warming in the Holocene than in the old moraine area, the periglacial overforming of the Vistula cold-age sediments is of essential importance in this area as well. Their traces, such as periglacial deck series with floor tiles, ice wedges, teardrop soils, bed load sands and stone floors, have been proven many times over. Whereby they are more clearly developed in the areas of the older peripheral locations than in the younger peripheral locations, where they are sometimes hardly recognizable.

The Aeolian processes continued and e.g. the dunes and drifting sands were deposited in the Ueckermünder Heide, in southwest Mecklenburg and on the Darß.

 

The renewed soil formation at the end of the Vistula glaciation thus took place on more or less periglacial changed glacial sediments.

These soil-forming processes began in the warmer sections (interstadials) of the late glacial around 14,000 years ago and continued under tundra vegetation in the post-ice age. Only with a clear improvement in the boreal's climate 9,000 years ago did a closed vegetation cover develop, which led to forest cover in many areas.

 

Figure 5: Soil development on glacial till since the Late Glacial of the Vistula Ice Age in the Atlantic-temperate climatic area of ​​Central Europe

Source: Kuntze, Roeschmann and Schwerdtfeger (1988)

 

The decalcification started and there was the formation of raw floors and thin pararendzines. Decalcification and the formation of raw soils to pararendzinen are the prerequisites for the subsequent phases of soil development, which took place in Mecklenburg-Western Pomerania under changing forest vegetation. Brown earths develop in humid climatic conditions. Beginning with the Atlantic 7,000 years ago, lessivation is increasingly used as a determining, soil-forming process. Parabrown earths are formed.

This simplified representation of soil development is only to be seen as a guideline for soil genesis.

 

Figure 6: Soil development on Soft Ice Age sand since the Late Glacial of the Vistula Ice Age in the temperate climatic region of Central Europe


Source: Kuntze, Roeschmann and Schwerdtfeger (1988)

 


With increasing warming, with the formation of the Holocene cultural landscape and subsequently today's cultural landscape, the development of the soils becomes increasingly differentiated. Even with minor changes in the original rock (lime content or permeability), other soil formation factors can dominate and lead to changed development sequences.

 

 

2.2.3Anthropogenically influenced soil development

The anthropogenic change and formation of the soils is an inseparable part of soil development. The impact of humans on the soils in Mecklenburg-Western Pomerania began around 7,000 years ago with isolated interventions in nature. Vegetation changes after clearing, reforestation, land abandonment and nutrient depletion.

However, it was only the large-scale agricultural use that began in the Middle Ages that led to significant changes in soil properties and soils. The most important anthropogenic influences and their consequences for the soil are:

 

·      The arable farming leads to a mixing of the soil up to the working depth. Instead of the natural horizons, an artificial, relatively homogeneous horizon (Ap horizon) has formed. By loosening the soil during plowing, the soil is aerated and thus the decomposition of the organic matter is promoted. The aggregate stability is reduced, the tendency towards silting and erosion increases and the soil is compacted by mechanical stress during the cultivation.

·      In steeply sloping terrain or with little vegetation cover, soil erosion is a natural phenomenon. However, it is increased by orders of magnitude when the fields are used and then also takes place on flat slopes. The soil horizons are eroded or shortened by erosion and the starting material of the soil formation comes close to the surface again.

·      Interventions in the soil processes through the use of pesticides. If they get into the soil, microbiological activities are influenced.

·      The drainage of soils affected by groundwater and backwater improves ventilation and ensures better navigability. It was possible to use wet to very wet locations first as grassland and, after more extensive drainage, as arable land.

Degradation begins in the moors with drainage.

·      If there is no structural degradation, rewetting of mineral soils restores the original properties of the soil. In the case of moors, the development is irreversible, the degraded horizon is preserved. If it is rewetted, new peatland growth can begin.

·      The application of non-local soil or other materials interrupts soil development. Soil formation begins again on the new surface, which usually leads to raw soil through humus accumulation.

 

The soils of the glacial sedimentation areas of the Pleistocene in Mecklenburg-Western Pomerania react differently to anthropogenic influences. The combination of soil compaction with increasing surface runoff and soil erosion with wetting and nutrient accumulation in the depressions is characteristic of the wavy to domed moraine landscapes. In the more sandy areas, soil compaction dominates over a large area, especially crumb compaction. Wind erosion (deflation) also represents a considerable burden in some areas (Schmidt 1994).