What is weathering and pedogenesis

How did the rock garden come about and what is its framework made of?

(à The botanical garden of the University of Maribor was planned in 2000 and 2001 and extends over an area of ​​800m2. 450m3 Earth, 25m3 Granodiorite, 25m3 calcareous rock and 8m3 Serpentinite, amphibolite and cizlakite delivered.

Our planet consists of several layers inside: crust, mantle and core. Due to the increasing temperature and pressure towards the interior of the earth, these layers differ in terms of their chemical structure and their physical properties.


Image: Earth's interior

Crust (oceanic * and continental **) + upper area of ​​the mantle = lithosphere (solid and cold)

* oceanic plates - narrower and denser

** continental plates - thicker and less dense

The outer layer is called crust designated. It consists of solid rock. We know of two types of crust: continental and oceanic crust. The crust follows the coatwhich is still hard just below the crust and becomes more and more viscous towards the earth's interior. It follows the corewhich consists of heavy elements such as nickel and iron. Due to the high temperatures, the core is in the outer layer (= outer core) liquid, the increasing pressure towards the center that prevents melting is the reason that the inner core is solid and consists only of iron.

The Lithosphere does not consist of one piece, but is divided into individual tectonic plates. We know seven larger and several smaller tectonic plates. The larger plates are the Eurasian, North American, South American, Pacific, African, and Indo-Australian tectonic plates, as well as the Antarctic. The area of ​​Slovenia covers a smaller plate called the Adriatic Plate.

The lithosphere is followed by the next layer, called the Asthenosphere. The asthenosphere is the part of the earth's mantle that extends about 400 km into the depth. Due to the prevailing conditions in this layer, the asthenosphere consists of the rock heated to its melting point. The asthenosphere is thus a viscous layer, partly in a liquid and partly in a plastic state. This enables the tectonic plates to float on the asthenosphere, much like icebergs on the ocean.

The plates move at a rate of a few centimeters per year (nail growth rate) up to 15 cm per year (similar to the hair growth rate).

The tectonic plates move in three different ways.

(Image: movement of the tectonic plates):


Source: (http://vedez.dzs.si/datoteke/tektonika.pdf)

Image: Movement of the tectonic plates

  1. The plates can move away from each other. Areas in which two plates move away from each other are called expansion zones. This happens at the bottom of the oceans, along the long submerged mountain ranges (mid-ocean ridges), which are made up of many active and dormant volcanoes. As soon as the plates move away from each other, a crack is created between them, into which magma from inside penetrates. In the oceans, magma hardens quickly and fills the crack again, but because the plates are constantly moving apart, a crack occurs again, which the magma fills in again. The sea floor is therefore expanding more and more, by a few centimeters per year.

The spreading zones, which are also called trenches, are not only located deep in the oceans, but also occur on land. Such an example can be found in Iceland, where the Eurasian and North American plates are diverging. That is why a ditch is created there both on land and in the sea, called the Silfra Fissure (pictures). The tectonic plates are so close together in some places that divers can touch both plates at the same time (video).

The tectonic rifts in Central Africa and East Africa are also known. The Central African Rift was formed when the Arabian plate was torn from the African plate. The Red Sea is located there today. Very recent evidence suggests that the far eastern part of Africa is in the process of separating itself from the rest of the continent.

  1. The plates can be along each other slidewithout creating a new crust or damaging the old one. So-called leaf displacements arise.

The most famous leaf displacement is the San Andreas Fault in California.

  1. Mutual approach and collision of the plates. In this case, the crust is destroyed.

There are three scenarios:

  • O + K = collision of oceanic and continental plates. The thinner and denser oceanic plate slides under the continental plate and sinks into the hot asthenosphere, where it melts and disappears. This creates deep sea channels, such as the rift in the western part of North and South America, where the Pacific sinks under the North and South American tectonic plate.
  • O + O = collision of two oceanic plates. The same thing happens as in the O + K collision - one of the plates slides under the other. A chain of volcanic islands emerges, such as the islands of Japan, the Philippines and Indonesia.

The area where one plate bends down and displaces the other becomes Subduction called.

  • K + K = collision of two continental plates. When two continental plates collide, which are equally thick and both relatively light, there is not displacement, but one collision. At the point of collision, there is great pressure and thus wrinkles and a rise in the mountains. This is how the formation of the Himalayas began 10 million years ago when the Indian and Eurasian plates collided. The Alps were formed when the Adriatic Plate first separated from the African continental plate and then collided with the Eurasian Plate 60 million years ago.

In particular, the movement of tectonic plates, their breaking and, to a lesser extent, volcanic activity lead to earthquake or jerky ground fluctuations. The seismic waves spread from the focal point of the earthquake below the surface (the Hypocenter), i.e. from the point in the earth's crust where the tectonic plate broke. The point on the surface, just above the focus of the earthquake, where the effects of the earthquake are most visible is called center or epicenter designated.

A seismograph is a device used to measure seismic waves. The strength or intensity of the earthquake is measured using a magnitude. This is the energy (seismic) that is released in the earthquake focus of the earthquake and measures the effects of the earthquake on objects, buildings and nature.

The first scale (1935) was introduced by the American seismologist Charles Francis Richter, therefore it is called the Richter scale. The scale is logarithmic, which means that each level of the earthquake is almost 32 times stronger than the previous one. The scale has 9 levels, from 1.0 to 9.0 and higher, so theoretically it has no upper limit. The greatest magnitude (9.5) was recorded in Chile in 1960. Without a source of external energy, earthquakes should theoretically not reach a magnitude greater than the recorded 9.5. Experts and seismologists assume that the earth trembled with a magnitude of 13 when an asteroid with a diameter of 10–15 km hit the planet 66 million years ago. The asteroid's impact is attributed to sudden climate change and the extinction of most (75%) plant and animal species, including some dinosaurs. Experts also believe that no one on earth would survive an earthquake with a magnitude of 15.

In Slovenia the so-called European Macroseismic Scale (engl. European macroseismic scale - EMS), which was supplemented in 1998 (hence the abbreviation EMS-89). The scale has 12 levels (from I - imperceptible earthquake, V - slight damage to buildings, to XII - completely devastating earthquake).

Since Slovenia lies at the intersection of three tectonic plates (the Eurasian in the north, the African in the south and the Adriatic between them), the country lies in a more seismically active area (country with a medium earthquake risk (image: earthquake threat in Slovenia) do not reach large magnitudes in Slovenia, their impacts can be relatively strong due to the rather shallow earthquake centers (5–15 km). The most seismically dangerous areas are the areas of central and northwestern Slovenia. In the past 80 earthquakes with impacts VI-VII have occurred in Slovenia EMS and 5 recorded with earthquake hotspots outside the country.

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Source: ARSO

Image: Earthquake risk in Slovenia

As can be seen from the picture with the locations of the earthquake epicentres, most earthquakes occur where tectonic plates meet.


Image: Locations of the earthquake epicentres between 1900 and 2017

A particularly large number of earthquakes and volcanic eruptions occur at the so-called Pacific Ring of Fire, an interface between numerous tectonic plates (Photo: Pacific Ring of Fire).


Image: Pacific ring of fire, where up to 90% of all earthquakes and 25% of all eruptions take place

Fig. 1: Rock cycle


As can be seen from the scheme of the rock cycle, arises igneous rockby cooling magma or lava.

In the event that the magma hardens in the earth's interior, rock is created, which one Plutonite is called. Under the surface, the magma hardens slowly, which is why plutonites have a uniform structure and large mineral grains.

Formed by the hardening of lava Penetrations. The lava cools quickly on the surface, which is why these volcanic rocks have a more irregular structure, the minerals are smaller.

The igneous rocks also include volcanics, which occur in the form of vessels in crevices shallow under the surface.

Rocks on the earth's surface disintegrate in the process of weathering. Weathering occurs due to the forces of nature (water, ice, warming and cooling, wind, water currents) and therefore the rock breaks down into more or less fine particles.

The second group of rocks are Sedimentary rocks or sediments. As the name suggests, the rocks were created by deposits of various disintegrated rocks and materials of biological origin, due to this They often contain fossils or dead organisms. Sedimentary rocks are formed during the process of lithification when, due to the high pressure, fine particles are compressed and bound and become a solid whole.

The third group is metamorphic rocks. They got their name because they were formed in the process of a transformation (metamorphosis) of the rocks from the other two groups. Metamorphosis occurs in deeper parts of the earth's crust, where exceptional conditions prevail - pressure> 100 MPa (megapascal = 106 Pa) and temperature> 150 ° C.

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Rock types and the rock cycle à video (Three Types of Rocks and the Rock Cycle)

Now we can more easily imagine how the development of these rocks that now form the rock garden came about.







occurs when magma or lava cools down and hardens

poorly permeable to water

has a crystal-like structure and weathers evenly


Magma cools down and hardens deep in the earth's crust









Magma comes to the surface in the form of lava

is also called volcanic rock

Quartz porphyry

Quartz Keratophyr









Magma breaks into the cracks of existing rocks and hardens there




arises from sedimentation, weathering or erosion of other rocks and remains of organisms

has recognizable layers and may contain fossils


formed as a result of erosion and weathering from particles on land

Breccia (breccia)



Siltstone (silt)

Mudstone (clay)

Marl stone (marl)



created by the adherence and settling of volcanic ash and dust

volcanic breccia




caused by the settling of parts of marine organisms








caused by the excretion of minerals dissolved in the water






is formed by metamorphosis from any previous rock (igneous, sedimentary or already formed metamorphic)

is less resistant to weathering and erosion

has the same chemical composition and properties as the preceding rock from which it was formed










tectonic breccia





Granodiorite is an igneous rock, plutonite. It is an acidic rock from which acidic soil is created when weathered.

Granodiorite from Pohorje (Bacher Mountains) was first called Pohorje granite, later tonalite and then quartz diorite. Petrological analyzes have shown that granodiorite predominates in the massif. On the southern slope of the Pohorje it is extracted near Oplotnica near Cezlak in the quarry Cezlak I and on the northern slope of the Pohorje in Josipdol. The rock is hard, firm and very resistant to static mechanical loads. (see Žlender & Dolinar, 2008, p. 40)

Limestone is a common sedimentary rock that is formed by depositing and stacking the houses of deceased animals in the sea. In Slovenia, limestones make up 10–15% of all sedimentary rocks and cover more than 40% of the country's surface.

The limestone consists of calcite microcrystals, chemical calcium carbonate (CaCO3), which contains various admixtures, so that it is not always white. Calcite is extracted from shell fragments and skeletons of molluscs and other marine organisms, so one can find well-preserved fossils in it.

Limestone is found in areas that are karst (engl. Karst). It is a recognized geological term that has spread from the geographical term - after the Slovenian Karst region. The karst is characterized by karst phenomena such as karst caves with the characteristic stalactites, intermittent lakes (Cerknica lake), suction holes or shrinkages and pits. Stalactites are created because the limestone dissolves in water with dissolved carbon dioxide (weak acid) and precipitates again in the caves. The Julian Alps in Slovenia consist mainly of limestone.

One of the most interesting and promising deposits of various types of limestone, which are used in construction and art for their quality, is in the karst. Historical data show that natural stone was extracted in the Karst over two thousand years ago. Detailed field and laboratory studies of these rocks did not begin until after World War II. The occurrences of gray and colored carbonate rocks can also be found in central Slovenia, the Karst on the Inner Carniola and southeastern Slovenia as well as in Bela Krajina. There are deposits of colored limestone in Tolmin and deposits of lithotamysh limestone in the Drau region and the Posavje hills.

Limestone is used in agriculture (when liming the soil - to increase the pH value), calcium oxide (CaO) or living lime in construction (mortar, white) and in the chemical industry (as a neutralizing agent for acidic solutions, in glass and cement production).

Serpentinite is a massive metamorphic rock. It arises from a low degree of metamorphosis of ultrabasic rocks and the presence of water. It consists of the mineral serpentine, is green to black with a characteristic surface that resembles an incorrectly intertwined network.

Occurrences of serpentinite can be found on the edge of the Pohorje and Possruck (Kozjak).

By looking at rocks one can observe how the structure and composition of the earth has changed in different geological time periods.

Like all living things, plants also need nutrients to grow and develop. Because plants are autotrophic organisms, they can self-enrich in the process of photosynthesis in some of the most important macronutrients such as carbon (C), oxygen (O) and hydrogen (H). The plant body consists of 90% to 99% of these elements, while the rest of the plant consists of a mineral component.

Plants extract minerals from the soil. Soils, which are a natural structure on the earth's crust, consist of three phases (soil = three-phase system):

  • mineral particles (~ 45%) and organic matter (~ 5%) (hard phase),
  • Water with dissolved minerals (liquid phase, ~ 25%) and
  • Gas mixtures (gas phase, ~ 25%).

Plants need a relatively large amount of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S). These nutrients are called Macronutrients designated. Since the macronutrients in the soil are usually insufficient, the soil is taken care of through fertilization. In addition to macronutrients, so-called Micronutrients or those nutrients that plants need in small quantities (boron, manganese, copper, zinc, molybdenum, iron). These are most often added to the plants via their leaves in the form of foliar fertilizers (so-called foliar fertilizers).

Soil fertility depends on the content of individual macro and micronutrients in the soil, their proportions and other properties. The most important properties that affect soil fertility include: soil reaction or soil pH, organic matter content, texture and soil structure.

1.1 Soil reaction (pH values)

Knowledge of reactions or the pH of a solution allows us to define the aqueous solution of a substance (in this case the soil), i.e. a soil solution, as an acidic, basic or neutral solution. The reaction of the solutions is based on a pH scale certainly.




Ever more acidic the solution is the more lower is her PH value.

Ever more basic the solution is the more higher is her PH value.

Table 1: Soil classification according to the measured reaction of the soil solution

Soil types

Reaction of the soil solution (pH value)

strongly acidic

< 4,5



moderately sour




basic (alkaline)

> 7,2

The reaction of the soil is a very important characteristic as the pH affects the availability of plant nutrients and the activity of microorganisms, which is crucial for the nutrient cycle.

Most cultivated plants grow well at a pH value between 6 and 7, since most of the plant nutrients in this area are in the corresponding (ionic) form of plant nutrition (picture: availability of plant nutrients depending on the soil reaction). The majority of the most important soil organisms also act in this area (close to neutral).



Source: (http://projects.ung.si/agriknows/img/KGZS_osnove_prehrane_rastlin_in_gnojenja-1.pdf)

(Image: Availability of plant nutrients depending on the soil reaction)

The optimal soil reaction is not the same for all soil types. If the soil is rich in organic matter (> 20% organic matter), most plants will thrive even with a more acidic pH response (5.0–5.5). Also, the optimal pH of a soil with a lighter texture will be lower. Acidic soil (pH <4.0) is not suitable for the cultivation of crops, as earthworms and most bacteria do not like it and this slows down their activity (e.g. fixation of N, nitrification, decomposition of organic matter). In acidic soils, bacteria are replaced by fungi.

Among the plants there are exceptions that thrive better in acidic soils and those that are also suitable for soils with a basic reaction. Plants that do well in acidic soils are called acidophilic Plants (blueberry, lingonberry, rhododendron, some grasses). Those that do better in basal soils are called basophilic plants.

The soil reaction is the result of many factors and processes that take place in the soil. The main factors that affect the pH of the soil are the content of basic cations on the base and the process of pedogenesis (soil formation).

Two types of soil acidity are determined in pedological laboratories:

  • Active acidity, caused by hydrogen ions (H +), dissociates in the soil solution. It is determined after the soil has been extracted with deionized water.
  • Potential acidity demonstrated by free H +, H+ and Al3+ Ions and are bound to the sorptive part of the soil. It is determined by extracting the soil sample with 0.1 N KCl (the hydrogen cations are exchanged for potassium).

1.1 Organic matter in the soil

The organic matter in the soil consists of plant and animal residues in various phases of degradation. The organic matter in the soil eventually turns into humus. Humus is a decomposed, permanent part of an organic substance that has been broken down by microorganisms.

The soil is more fertile when there is more humus (Photo: Soil distribution with regard to the humus content). Humus in the soil represents the mineral content and thus the food source for microorganisms, it improves the airiness and porosity of the soil, ensures the maintenance of soil moisture and, through its ability to bind nutrients, indirectly prevents nutrients from being washed away.

Table: Soil distribution in terms of humus content

Soil classification

Humus in the soil (%)

poor in humus

< 1

moderately humus


containing humus


contains a lot of humus


very rich in humus


1.2 soil texture

The mineral part of the soil consists of mineral particles. They get into the ground through weathering of the mother bed. According to their size, they are divided into the following groups (fractions):

  • Skeletal particles (diameter> 2 mm),
  • Sand (diameter 2-0.05 mm),
  • Silt (0.05-0.002 mm) and
  • Clay (<0.002 mm)

The ratio between their proportions is called the soil texture. The proportion of the individual fractions is determined in the laboratory by mechanical soil analysis. Once you have the results on the proportion of the individual fractions, the so-called texture class of the soil is determined with the help of the texture triangle (shown on the picture: texture classification of the soil).

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Source: (https://sl.wikipedia.org/wiki/Tekstura_tal)

Image: Texture classification of the soil

The texture is a very important physical property of the soil as the specific particle size depends on the size of the mineral particles. Thus, the movement of water in the soil, the soil's ability to store water, its airiness, its ability to work the soil and its fertility all depend on the texture.

Soils in which sand predominates are called light soils because they can be easily worked. The texture classes sand (P), loamy sand (IP) and sandy loam (IP) are classified under light soils. They are considered warm, airy, but retain water and nutrients poorly.

Soils that are dominated by clay (G) and silt particles (M) are dense, compact, hardly permeable to air and water and difficult to work.

Medium-heavy soils are best suited for the growth of most plants (MI - silty to loamy and I - loamy soils), in which sand, silt and clay are roughly equally represented. Such a soil can be worked easily because it retains water and minerals and is well permeable.

With a little practice, we can determine the texture of the floor ourselves with the so-called finger test. The finger test is done by taking a small amount of the soil sample. This is moistened accordingly and by kneading between the fingers (between thumb and forefinger) an attempt is made to form a roller or a ribbon. If the sample is squeezed in the hand and we feel the moisture, but the water does not drip or run between the fingers, it means adequate moisture. However, if the soil is too dry, we have to moisten it properly with water, but if it is too wet, it is kneaded until it loses the excess moisture.

The size (granularity) and the amount of the individual particles can be estimated fairly precisely with the fingers. The mutual connection of these particles, i.e. their stickiness, can also be assessed. The plasticity is assessed in such a way that a roller is modeled from the sample and this is bent into a band.

Since the sand particles are hard and large, we can easily feel them between our fingers. With clay-like particles, on the other hand, we have a smooth feeling of dust or powder between our fingers.

The roller from the soil sample with a larger amount of silt is smooth and slippery, also silky.

Because of the clay, the roller is sticky and plastic, and you can easily roll it thinner without it bursting when you bend it.

When determining the soil texture with the finger test, we can help ourselves with the scheme shown in the picture (picture: determination of the soil structure using the finger test)

1.1 Soil structure

Mineral soil particles such as sand, silt and clay are not in the soil as individual particles (an exception are pronounced sandy soils) and organic substances are not completely separated components. All these substances are in the soil in the form of a spatially ordered system of Adhesions or Aggregates connected, which have clearly identifiable shapes or mutual interfaces.

The arrangement and the connections between these aggregates in the soil describe the soil structure. The soil structure is thus one method, the Soil particles (Sand, silt, clay and organic substances) to form aggregates of various shapes and sizes connect to - in so-called structural aggregates.

The structure has a significant influence on soil fertility, because it determines properties such as soil porosity, the ratio between macro and micropores in the soil (there is water in macropores, air in micropores), access to plant nutrients, microorganism activity and development as well the root growth.

Depending on the condition of the soil structure, a distinction can be made:

  • the unstructured state in sandy soils that are loose and unbound,
  • non-structural condition in clayey soils that are compact, massive, plastic and sticky,
  • the coherent state (is a transition state where the particles are already partially connected, but the correct structural aggregates are not yet formed),
  • the physical state in which the structural aggregates are formed and well defined. Structural aggregates with the following properties have been developed: characteristics, size, shape, durability and internal porosity.

The forms and properties of the structural aggregates are shown in the table.

* Horizons (O – R)

In pedology, one also researches the soil in such a way that one can find a pedological pit digs. The pedological pit is a pit that is about 80 cm wide and 100–150 cm deep, depending on whether it is log-based or water. The pit is dug smoothly with the shovel so that the pedological profile is easy to see. The pedological profile is a vertical cross-section of the soil, on the basis of which the pedological horizons can be defined and described.

Source: (http://www.kis.si/f/pics/Galerija/2013_07-recharge.green.Profil2.webL_s2.jpg)

Image: Pedological profile

The horizon is a layer of soil that is roughly parallel to the soil surface and was created in the processes of soil formation and development (pedogenesis). It differs from the neighboring horizon in its physical, chemical and biological properties.

Horizons have their own names and markings. The main horizons are marked with capital letters and follow one another from the surface downwards, as can be seen in the picture of the soil horizons.

Source: (https://www.slideshare.net/pragnaprathap1/soil-horizons-and-soil-sampling-methods in web.bf.uni-lj.si/cpvo/Novo/PDFs/KlasifikacijaTal.pdf)

Image: soil horizons

The layers in the soil profile can also be described in more detail in such a way that sub-horizons can also be found within the main horizons. The sub-horizons are marked in such a way that an index is added to the marking of the main horizon (upper case letter) (lower case letters). For example, O horizons are divided into the following sub-horizons:

Oil - organic horizon (O) with visible dry foliage, needles and other plant remains (index = leaf fall)

Of - organic horizon (O) with partially decayed plant remains (index = fermentation)

Oh - humified organic matter (index = humification)

The only way to be able to determine exactly what properties our soil has is through analyzes carried out by authorized laboratories.

The basic soil properties are derived from the results of standardized chemical pedological investigations. These determine the soil reaction or the pH value (active or potentized acid), the content of potassium (K2O) and phosphorus (P2O5) in the soil, the total nitrogen content and the content of organic matter in the soil. The content of other nutrients and the determination of further properties are carried out by laboratories on request.

The best time to take soil samples is in autumn after vegetation has ended or in spring before vegetation begins.

Special drilling machines and various pedological probes (Dutch probe, pointed drill) and shovels are used to take the samples.

Samples for standardized pedological examinations are taken, depending on the use of the soil, at different depths and also in layers; shallower in meadows (0–6 cm and 6–12 cm), on arable land to the depth of arable soil (0– mostly 25 cm) and on vineyards and orchards usually in layers (0–20 cm, 20–40 cm) in 0-30 cm).

The soil sample must be representative and is therefore taken from different locations evenly over the property, avoiding the edges of parcels, dykes, heavily fertilized areas and areas with residues from animals.

The samples taken are mixed well in a bucket so that a homogeneous average sample is obtained. The mass of the sample depends on the analyzes you want to perform (0.5–1.5 kg). The sample is placed in a plastic bag and marked accordingly with the required data such as parcel identification, name of the owner, location, date and depth of sampling.

Plants that grow on serpentinite have adapted to this barren mother ground. We can observe changes or serpentine morphoses in them (cf. Krajnčič, 2014), such as:

  • Stenophilia or leaf area reduction,
  • Thick foliage, which occurs in plants due to a lack of nutrients in serpentinite, mainly Fe-Mg-Silicate,
  • stronger development of the sclerchymes,
  • Nanism or dwarfism of the plant,
  • Plagiotropism is a phenomenon that occurs due to a lack of phosphorus in the serpentinite soil and turns the leaves and stems red,
  • Plants have an enlarged root system.