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5. The lithosphere and the soil as power equipment and hazard. Basics of pedology. 1. Definition of soil.
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5. • The lithosphere and the soil as power equipment and hazard Basics of pedology
1. Definition of soil Soil is a natural body consisting of layers (soil horizons) that are primarily composed of minerals which differ from their parent materials in their texture, structure, consistency, color, chemical, biological and other characteristics. It is the unconsolidated or loose covering of fine rock particles that covers the surface of the earth.
Among numerous properties and functions of soils and the Pedosphere as the whole, the following ones, having especially important ecological significance, are isolated: - functions of soils as unique habitat for a great diversity of life forms; - function of soil cover as a link between geological and biological substance circulation in terrestrial ecosystems and the biosphere; and - function of the soil fertility in agriculture and biological productivity in natural landscapes.
2. The Pedosphere The Pedosphere is the soil cover of the Earth, which consist of specific natural bodies (elements), i.e. of soils, which are diverse by their compositions and properties. • As soil differ in their properties both in the vertical succession of overlying horizons and spatial variations. The pedon and the solum (SZALAI – JAKAB 2011)
The first aspect requires the observation and study of a soil profile pit, approximately 1m x 1m x 1m in size and being considered representative for the soil around. This small basic entity, from which one can observe variations in properties and extract samples for analytical investigations, is called a pedon. It is the smallest volume that can be called a soil, but large enough to exhibit a full set of horizons. While the combination of various pedons with minor differences within a larger landform is called a polypedon.
3. Geneticsoillevels The soils are differentiated from one another by the characteristics and properties of their profiles. A soil profile is a physical and chemical description of the layers (called horizons) that make up the soil, from the surface to the depth where pedogenic (soil forming) processes are no longer evident. If a person digs a hole in the ground and looks at the wall of that hole, he is looking at the soil's profile. The horizons of the profile have formed and differentiated from the original parent material in place - they are not the result of geologic processes, although some features of a profile may be caused by geologic events. The horizons of a general soil profile
’A-szint’-neknevezzük általában a talajképződés folyamán kialakult jellegzetes réteget. Ez a talaj szerves anyagban leggazdagabb legfelső szintje.A legfelső, döntően kevéssé lebomlott szerves anyagokból álló szintet A0-lal jelöljük. ’B-szint’-neknevezzük az A-szint alatt fekvő felhalmozódási szintet. Itt halmozódnak fel az A-szintből kilúgozott anyagok azokban a talajokban, amelyekben a felszínre jutó csapadékvíz az év nagy részében a mélybe szivárog. A B-szint differenciálódása esetén az agyagban, vagy szeszkvioxidokban gazdagabb felső szintjét B1-gyel, az alatta húzódó szintet B2-vel jelöljük. A ’C-szint’ elnevezést a talajképző kőzet jelölésére használjuk. C-szintnek a talajképző kőzet azon részét tekinthetjük, amelyben a talajképző folyamatok már hatnak, de anyagában még a kiinduló kőzettest jellegzetességeit hordozza. A ’D-szint’ az ágyazati kőzet jele, vagyis annak kifejezésére használjuk, hogy a talajképző kőzet alatt valamilyen más kőzetféleség található, amely azonban nem alapja a talajképződésnek(Stefanovitset al. 1999, Szalai – Jakab 2011).
4 Physicalproperties of soils The physical properties of soils, in order of decreasing importance, are texture, structure, density, porosity, consistency, temperature, colour and resistivity. Most of these determine the aeration of the soil and the ability of water to infiltrate and to be held in the soil. Soil texture is determined by the relative proportion of the three kinds of soil particles, called soil "separates": sand, silt, and clay. The most characteristicsoilpropertiesarethefollowing: 4.1. Texture 4.2. Structure 4.3. Porosity 4.4. Soilwater 4.5. Soilatmosphere 4.6. Temperature 4.7. Colour
4.1. Texture The mineral components of soil, sand, silt and clay, determine a soil's texture (Table 5.1.). In the illustrated USDA textural classification triangle, the only soil that does not exhibit one of these predominately is called "loam" (Fig. 5.3.). While even pure sand, silt or clay may be considered a soil, from the perspective of food production a loam soil with a small amount of organic material is considered ideal. The mineral constituents of a loam soil might be 40% sand, 40% silt and the balance 20% clay by weight. Soil texture affects soil behaviour, in particular its retention capacity for nutrients and water.
Classification of the grains according to Atterberg and the USDA
Sand and silt are the products of physical and chemical weathering; clay, on the other hand, is a product of chemical weathering but often forms as a secondary mineral precipitated from dissolved minerals. It is the specific surface area of soil particles and the unbalanced ionic charges within them that determine their role in the cation exchange capacity of soil, and hence its fertility. Sand is least active, followed by silt; clay is the most active. Sand's greatest benefit to soil is that it resists compaction and increases porosity. Silt is mineralogically like sand but with its higher specific surface area it is more chemically active than sand. But it is the clay content, with its very high specific surface area and generally large number of negative charges, that gives a soil its high retention capacity for water and nutrients. Clay soils also resist wind and water erosion better than silty and sandy soils, as the particles are bonded to each other
Types of soil textures on the base of the USDA classification (FÜLEKY 2011)
Soil components larger than 2.0 mm are classed as rock and gravel and are removed before determining the percentages of the remaining components and the texture class of the soil, but are included in the name. For example, a sandy loam soil with 20% gravel would be called gravelly sandy loam.
4.2. Structure The adhesion of the soil textural components by organic substances, iron oxides, carbonates, clays, and silica, and the breakage of those aggregates due to expansion-contraction, freezing-thawing, and wetting-drying cycles, shape soil into distinct geometric forms. These peds evolve into units which may have various shapes, sizes and degrees of development. A soil clod, however, is not a ped but rather a mass of soil that results from mechanical disturbance. The soil structure affects aeration, water movement, conduction of heat, plant root growth and resistance to erosion. Water has the strongest effect on soil structure due to its solution and precipitation of minerals and its effect on plant growth. Types of the soil aggregates: a) microaggregates ; b) macroaggregates ; c) pores in macroaggregates (STEFANOVITS ET AL. 1999)
Soil structure often gives clues to its texture, organic matter content, biological activity, past soil evolution, human use, and the chemical and mineralogical conditions under which the soil formed. While texture is defined by the mineral component of a soil and is an innate property of the soil that does not change with agricultural activities, soil structure can be improved or destroyed by the choice and timing of farming practices. At the largest scale, the forces that shape a soil's structure result from swelling and shrinkage that initially tend to act horizontally, causing vertically oriented prismatic peds. Clayey soil, due to its differential drying rate with respect to the surface, will induce horizontal cracks, reducing columns to blocky peds. Roots, rodents, worms, and freezing-thawing cycles further break the peds into a spherical shape.
4.3. Porosity Pore space is that part of the bulk volume that is not occupied by either mineral or organic matter but is open space occupied by either air or water. Ideally, the total pore space should be 50% of the soil volume. The air space is needed to supply oxygen to organisms decomposing organic matter, humus, and plant roots. Pore space also allows the movement and storage of water and dissolved nutrients. There are four categories of pores: 1. Micropores: < 2 microns 2. Mesopores: 2-20 microns 3. Macropores: 20-200 microns 4. Megapores: 200 microns-0.2 mm
Classification of soil pores in size The fine and medium-textured soils (e.g. clay loams, silty clay loams, sandy silt loams) are favorable from an agricultural viewpoint because of their high available retention of water and exchangeable nutrients. In fine pores the water is strongly adsorbed in pores but not available for plants, i.e. cohesion and adhesion water occupy the micropore space and they are retained in soil by forces that exceed gravity. In medium-sized pores the available water content is high, whereas in macropores water is more weakly held and percolation is high (gravitational water). In silty soils the distribution of macropores, medium-sized, and fine pores is optimal relating to available water content.
4.4. Soilwater Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth. Water is essential to plants for four reasons: 1. It constitutes 85%-95% of the plant's protoplasm. 2. It is essential for photosynthesis. 3. It is the solvent in which nutrients are carried to, into and throughout the plant. 4. It provides the turgidity by which the plant keeps itself in proper position. In addition, water alters the soil profile by dissolving and redepositing minerals, often at lower levels, and possibly leaving the soil sterile in the case of extreme rainfall and drainage. In a loam soil, solids constitute half the volume, air one-quarter of the volume, and water one-quarter of the volume, of which only half will be available to most plants.
The forces with which water is held in soils determine its availability to plants. Forces of adhesion hold water strongly to mineral and humus surfaces and less strongly to itself by cohesive forces. A plant's root may penetrate a very small volume of water that is adhering to soil and be initially able to draw in water that is only lightly held by the cohesive forces. Model of the capillarity (FÜLEKI 2011)
4.5. Soilatmospher The atmosphere of soil is radically different from the atmosphere above. The consumption of oxygen, by microbes and plant roots and their release of carbon dioxide, decrease oxygen and increase carbon dioxide concentration. Atmospheric CO2 concentration is 0.03%, but in the soil pore space it may range from 10 to 100 times that level. At extreme levels CO2 is toxic. In addition, the soil voids are saturated with water vapour. Adequate porosity is necessary not just to allow the penetration of water but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower. Oxygen diffuses in and is consumed and excess levels of carbon dioxide, diffuse out with other gases as well as water. Soil texture and structure strongly affect soil porosity and gas diffusion. Platy and compacted soils impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce nitrate to the gases N2, N2O, and NO, which are then lost to the atmosphere. Aerated soil is also a net sink of methane CH4 but a net producer of greenhouse gases when soils are depleted of oxygen and subject to elevated temperatures
4.6. Temperature Soil temperature regulates seed germination, root growth and the availability of nutrients. Soil temperatures range from permafrost at a few inches below the surface to 38°C in Hawaii on a warm day. The colour of the ground cover and its insulating ability has a strong influence on soil temperature. Snow cover will reflect light and heavy mulching will slow the warming of the soil, but at the same time they will reduce the fluctuations in the surface temperature. Below 50 cm, soil temperature seldom changes and can be approximated by adding 1.8°C to the mean annual air temperature. Most often, soil temperatures must be accepted and agricultural activities adapted to them to: 1. maximize germination and growth by timing of planting 2. optimise use of anhydrous ammonia by applying to soil below 10°C 3. prevent heaving and thawing due to frosts from damaging shallow-rooted crops 4. prevent damage to desirable soil structure by freezing of saturated soils 5. improve uptake of phosphorus by plants Otherwise soil temperatures can be raised by drying soils or the use of clear plastic mulches. Organic mulches slow the warming of the soil
4.7. Colour Soil colour is often the first impression one has when viewing soil. Striking colours and contrasting patterns are especially noticeable. The Red River (Mississippi watershed) carries sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma. The Yellow River in China carries yellow sediment from eroding loess soils. Mollisols in the Great Plains of North America are darkened and enriched by organic matter. Podsols in boreal forests have highly contrasting layers due to acidity and leaching.
In general, colour is determined by organic matter content, drainage conditions, and the degree of oxidation. Soil colour, while easily discerned, has little use in predicting soil characteristics. It is of use in distinguishing boundaries within a soil profile, determining the origin of a soil's parent material, as an indication of wetness and waterlogged conditions, and as a qualitative means of measuring organic, salt and carbonate contents of soils (Fig. 5.7.). Colour is recorded in the Munsell color system as for instance 10YR3/4. Possible colour of soils on the Munsell scale
5. Chemicalproperties 5.1. Soilsalinity 5.2. Colloids 5.3. Soil pH 5.4. Cation and anion exchange 5.5. Buffering of soils 5.6. Organicmatter The chemistry of soil determines the availability of nutrients, the health of microbial populations, and its physical properties. In addition, soil chemistry also determines its corrosivity, stability, and ability to absorb pollutants and to filter water.
5.1. Soilsalinity Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salination. Salt is a natural element of soils and water. Salination can be caused by natural processes such as mineral weathering or the gradual withdrawal of an ocean. It can also be caused by artificial processes such as irrigation. Salt is a natural element of soils and water. The ions responsible for salination are: Na+, K+, Ca2+, Mg2+ and Cl-. As the Na+ (sodium) predominates, soils can become sodic. Sodic soils present particular challenges because they tend to have very poor structure which limits or prevents water infiltration and drainage. Over long periods of time, as soil minerals weather and release salts, these salts are flushed or leached out of the soil by drainage water in areas with sufficient precipitation. In addition to mineral weathering, salts are also deposited via dust and precipitation. In dry regions salts may accumulate, leading to naturally saline soils
5.2. Colloids Colloids are the most important elements of soils, which determine the chemical properties. They have two types, mineral and organic colloids. Due to its high specific surface area and its unbalanced negative charges, clay is the most active mineral component of soil. It is a colloidal and most often a crystalline material. In soils, clay is defined in a physical sense as any mineral particle less than two microns (2x10−6 inches) in effective diameter. Chemically, clay is a range of minerals with certain reactive properties. Clay is also a soil textural class. Many soil minerals, such as gypsum, carbonates, or quartz, are small enough to be classified physically as clay but chemically do not afford the same utility as do clay minerals.
Clay was once thought to be very small particles of quartz, feldspar, mica, hornblende or augite, but it is now known to be (with the exception of mica-based clays) a precipitate with a mineralogical composition different from its parent materials and is classed as a secondary mineral. The type of clay that is formed is a function of the parent material and the composition of the minerals in solution. Mica-based clays result from a modification of the primary mica mineral in such a way that it behaves and is classed as a clay. Most clays are crystalline, but some are amorphous. The clays of soil are a mixture of the various types of clay, but one type predominates. Colloid model of Stern (SZALAI – JAKAB 2011)
5.3. Soil pH At 25°C an aqueous solution that has a pH of 3.5 has 10-3.5 moles hydrogen ions per litre of solution (and also 10-10.5 mole/litre OH-). A pH of 7, defined as neutral, has 10−7 moles hydrogen ions per litre of solution and also 10−7 moles of OH- per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 is 10-9.5 moles hydrogen ions per litre of solution (and also 10-2.5 mole per litre OH-). A pH of 3.5 has one million times more hydrogen ions per litre than a solution with pH of 9.5 (9.5 - 3.5 = 6 or 106) and is more acidic. Categories of soil acidity (after FILEP GY.and BOHN, H. ET AL.)
5.4. Cation and anion exchange Cation exchange, between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful. The negative charges on a colloid particle make it able to hold cations to its surface. The charges result from four sources. Isomorphoussubstitution occurs in clay when lower-valence cations substitute for higher-valence cations in the crystal structure. Substitutions in the outermost layers are more effective than for the innermost layers, as the charge strength drops off as the square of the distance. The net result is a negative charge. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete at the edges of clay. Hydrogens of the clay hydroxyls may be ionised into solution, leaving an oxygen with a negative charge. Hydrogens of humus hydroxyl groups may be ionised into solution, leaving an oxygen with a negative charge.
5.5. Buffering of soils The resistance of soil to changes in pH and available cations from the addition of acid or basic material is a measure of the buffering capacity of a soil and increases as the CEC increases. Hence, pure sand has almost no buffering ability, while soils high in colloids have high buffering capacity. Buffering occurs by cation exchange and neutralisation. The addition of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydrogen ions from the colloids, and the end product is water and colloidally fixed ammonium, but no permanent change overall in soil pH.
5.6. Organicmatter Most living things in soils, including plants, insects, bacteria and fungi, are dependent on organic matter for nutrients and energy. Soils have varying organic compounds in varying degrees of decomposition. Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat (histosols), are infertile. In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus. Humus refers to organic matter that has been decomposed by bacteria, fungi, and protozoa to the final point where it is resistant to further breakdown. Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to soil health and plant growth. Humus also hold bits of undecomposed organic matter which feed arthropods and worms which further improve the soil. Humus has a high cation exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.
Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants and animals, microbes begin to feed on the residues, resulting finally in the formation of humus. With decomposition, there is a reduction of water-soluble constituents including cellulose and hemicellulose and nutrients such as nitrogen, phosphorus, and sulfur. As the residues break down, only complex molecules made of aromatic carbon rings, oxygen and hydrogen remain in the form of humin, lignin and lignin complexes as humus. While the structure of humus has few nutrients, it is able to attract and hold cation and anion nutrients by weak bonds that can be released in response to changes in soil pH.
6 Edaphon Organisms affect the type of organic matter that is added to the soil, the rate at which the organic matter is decomposed, the part of the soil to which the organic matter is added and translocated, and the types of chemical reactions that occur in the soil. The soil and the organisms living on and in it comprise an ecosystem. The active components of the soil ecosystem are the vegetation, fauna, including microorganisms, and man Characteristic organisms in the soil (STEFANOVITS ET AL. 1999)
