While practical knowledge and empirically obtained insights about the soil have their origin in the most ancient history, soil science has developed as an independent science in the mid-nineteenth century. Soil was mentioned in the oldest scientific book Papyrus Ebers from the sixteenth century BC, and also Greek and Roman writers have been working on the issue of soil fertility and classification. Ancient History and the Middle Ages were centuries of empiricism. The entire knowledge on soil from that time remains based on the experience, and that knowledge was collected mainly for agricultural purposes. The progress of chemistry has accelerated the development of knowledge on the soil. In the second half of the eighteenth century, when the chemistry was so advanced, knowledge of the major chemical principles and analytical procedures has enabled the determination of the composition of living and dead inorganic and organic substances. Biochemistry has developed and the first studies of the soil begun.
At first, the knowledge of the soil was obtained as a part of the geologic system – as geochemistry or agrogeology, chemistry or general crop science, and later as a collection of different sciences that can be applied in agriculture and forestry. The first scientific work about the soil, Bodenkunde – oder die Lehre vom Boden by German agrochemist Carl Sprengel (1787–1859), was published in 1837. Soil science had soil fertility in its focus for a long time, as an expression of indisputably the most important productive role of soil in agriculture and forestry. Only recently, more attention is given to the other roles of soil. Based on all the above reasons, all nations classify soil as the most important national natural resource, that is, as a member of the so-called environmental triad: AIR – SOIL – WATER.
2 Basics of soil science
When speaking about the soil it is most important to point out that we obtain more than 95% of food and raw materials for food from the soil. At the same time, a favorable fertile soil for this purpose is a very limited resource (Figure 1).
There are several definitions of the soil. The simplest definition is: soil is the surface layer of the Earth’s crust altered by the combined influence of climate, air, water, plants and animals. From the ecological point of view, soil is the layer of the Earth’s crust that provides optimum conditions for plant growth. Based on the above-mentioned definitions, we can say that soil is loose skin of the Earth, located between the lithosphere and atmosphere, which arose from parent material under the influence of factors of pedogenesis and pedogenesis process (Figure 2). Soil is a natural product created by complex processes, first by fragmentation and decomposition of rocks, i.e. primary minerals of parent material, and by synthesis – the formation of new, secondary minerals. The resulting mass can hold water, which allows colonization, primarily by bacteria, fungi, algae and lichen, and finally by higher plants. The names of soil in different languages are: počv (Bulgarian), tlo (Bosnian and Croatian), púda (Czech), Έδαφος (Greek), jord (Danish, Norwegian and Finnish), Boden (German), soil (English), grundo (Esperanto), suelo (Spanish), sol (French), talaj (Hungarian), suolo (Italian), počva (Russian and Macedonian), bodem (Dutch), gleba (Polish and Belarusian), solo (Portuguese), sol (Romanian), poda (Slovak), prst (Slovenian), zemljište (Serbian), toprak (Turkish) and grunt (Ukrainian).
The biggest difference between soil and rock is in the content of organic matter. Agricultural soil contains 0.5–5% of organic matter, and peat soil up to 50% of organic matter. Although its content in agricultural soils is relatively small, the organic matter brings and maintains life in the soil. Without organic matter, soil would no longer be soil but a lively rock from which the soil was created. In this case, soil would lose its primary role, which is the supply of plants with water, air and nutrients . It is necessary to clearly distinguish between the two terms that are frequently replaced and cause confusion – soil and land. The difference is obvious. Soil was already described as a natural product created by processes of soil formation (pedogenesis), and the term land refers to the land area (Figure 3). Land is a broader concept – it’s a land (terrestrial) surface and bio-productive system that includes soil, vegetation, other biota, and the ecological and hydrological processes that occur in the entire system. We have already mentioned the names of soil in the major European languages, and the names of the land are in French – terre, English – land (landscape), Russian – zemlja, Italian – terreno, Dutch – landen, Polish – kraina or parcela, and German – Erde. And finally, a few words on the concept of pedology. Although it is in many languages a deep-rooted word (pedology), it is the concept that outsiders often replace with the concept of pedology as a science of physical and mental development of children. Therefore, the term soil science is increasingly used in recent years.
3 Soil formation
3.1 Factors and processes of soil formation
There are four main factors of pedogenesis: lithosphere (parent material), atmosphere, hydrosphere and biosphere. Lithosphere has a passive role, and the other three spheres actively participate in development processes of pedosphere. The atmosphere acts in particular via precipitation, heat, wind and gas; hydrosphere acts via movement and strength of water masses; the biosphere acts via most diverse organisms, primarily with flora, fauna and human activity. The formation of soil is a long process which lasts for thousands of years, and in some substrates such as limestone it lasts for hundreds of thousands, or even a million years. To form one centimeter layer of red soil (terra rossa) as the most valuable type of soil on the limestone of the Mediterranean, it requires a flow of 8,000–10,000 years, therefore for 1 m depth it takes 800,000–1,000,000 years. A man can destroy it by improper management in several dozen years or permanently convert it in a few days. We distinguish five basic groups of pedogenetic processes: depletion of lithosphere; formation of organic matter; degradation (decomposition) of organic matter; displacement of components (translocation) and the new formation (neogenesis).
Weathering of parent materials is changing the physical and chemical properties of the lithosphere.
The compact masses of rock disintegrate and are then made from the same mineral compounds and partly of new mineral compounds. Surface mass significantly increases, and their atoms and atomic groups are more mobile than they were in compact rocks of the lithosphere. Processes of formation of organic matter enrich the mineral erosion with the living organic matter that becomes an important factor in the dynamics and development of soil. Processes of decomposition (rotting, decay) of organic matter result in many chemically and physiologically active compounds that enter in reactions with disintegrated minerals. Processes of transfer can change the position of the mass, or soil particles, on the surface of pedosphere or in its interior. Surface displacement is the most important factor in the development of the external morphology of pedosphere. The most diverse morphological forms are developed according to the forces participating in the surface displacement of pedosphere mass (rain, floods, river, sea, glaciers, wind, etc.). Processes of the new formation of the rock weathering products and the products of degradation of organic matter created new inorganic and organic complexes, of which the most important and most active are the colloidal components of soil – the secondary minerals or clay minerals and humus. These components significantly alter the features of erosion.
3.2 Basic soil characteristics
Soil formation was briefly summarized in the previous section. The basic physical, chemical and biological properties of soil and fertility, as a result of these qualities, are presented in this section. According to its physical characteristics, soil is a three-part system that has a solid, liquid and gaseous component. The ratio of individual components is a dynamic value (especially the ratio of liquid and gaseous components), dependent on the mechanical composition of soil, climate, current weather conditions, and the season, as well as on all other external factors. The solid component consists of mineral and organic parts. Mineral part originates from parent material, and the organic is more or less a humified organic matter. The liquid component of soil is water, more specifically, an aqueous solution of soil.
The most important physical parameter of the soil is the granulometric composition of the soil. The solid component of the soil is by nature a polydisperse system that consists of particles of different dimensions, from the ions, molecules and colloids that are invisible to the naked eye, to the particles of stone. Each particle, that can’t continue to break into pieces by weak mechanical forces (or peptization), is called a mechanical or granulometric element. Mechanical components in the soil are rarely found separately and they are connected to the larger particles – structural aggregates. The term mechanical composition or texture means the content of individual fractions in the soil in weight percent (Table 1). As the lower and upper limits for the classification of mechanical fractions according to the particle size are conventional values, different classification is applied in the world. Textural classes are determined by, among other things, the Atterberg triangle that has data on the percentage of silt, sand and clay graphically applied on every side of it, and the intersection can show a texture mark, as shown in Figure 4. According to their characteristics, all classes can be divided into three basic groups, or types of soil: sand, loam and clay soils. It is widely accepted that the clay soils are labeled as heavy soils, and sandy soils as light soils. The increased soil fragmentation (dispersion) in the same conditions increases the holding power of water, adsorption capacity, the amount of related nutrients, swelling, stickiness and plasticity, and decreases the permeability of water – internal natural drainage. Shortly, the chemical properties of the soil are improved and the physical properties deteriorate.
International classification of granulometric composition .
|Soil with a diameter greater than 2 mm||Soil with a diameter less than 2 mm|
|Rock||More than 20 mm||Coarse sand||2.0–0.2 mm|
|Gravel||20–2.0 mm||Fine sand||0.2–0.02 mm|
|Stony soils||Stony soils|
|I. Absolutely stony soils: more than 90% of the stone||I. Very stony soils: 30–50% of the stone|
|II. Very stony soils: 70–90% of the stone||II. Stony soils: 10–30% of the stone|
|III. Stony soils: 50–70% of the stone||III. Poorly stony soils: less than 10% of the stone|
3.2.1 Soil structure
Mechanical elements are not free or loose in the soil, separated from one another, but are connected to larger clusters – structural aggregate. The term structure of the soil includes the size, form and manner of distribution of structural aggregates in the soil. Classification of structure can be carried out on the basis of different criteria – the shape and size of pores, the microstructure and the like. According to the shape, they can be cubical, the ones with all three axes equally developed, prismatic or columnar, the vertical axis is more developed so they are elongated, and flat, the horizontal axis is longer than the vertical (Figure 5).
Structure is one of the most important factors of soil fertility. It can significantly correct the unfair qualities of the soil caused by mechanical composition. Soils with a favorable structure contain more air and are suitable for processing, so that the root system can develop in them optimally. Structural soil is porous, with a favorable ratio of micro-pores and macro-pores, and that relationship is stable even in conditions of increased humidity. Soils with a stable structure have a good water regime and are permeable to water. They receive and retain water well and they can create water reserves in the soil because they are not subject to drought. The excess of water from such soils is drained through a system of macro-pores. Such soils are, therefore, well drained. A stable structure is an important indicator of the air regime of soil, it provides aeration, and thanks to that the favorable thermal regime. Soils with a favorable and stable structure are less subject to the wind and water erosion. Soils with unfavorable structures have exactly the reverse features, bad air-water regime and poor drainage, and, at times, reduction processes occur in them due to prolonged water retention.
Soil density is a number that shows how many times the weight of the soil is denser than the weight of an equal volume of water. There is a difference between soil particle density (SPD) and bulk (dry) density (BD). SPD indicates the number of times that the mass of soil without pores, therefore only the mass of the rigid soil component is denser than the mass of an equal volume of water. This value does not depend on the mechanical composition, structure and compaction of the soil, it is a constant value associated with the content of organic matter. Soils with a higher content of organic matter have a lower SPD than soils with low organic matter. In arable soils SPD values are in the range from 2.2 g/cm3 to 2.9 g/cm3. The term BD stands for how many times the mass of a volume of natural soil with pores is denser than the mass of an equal volume of water. It is determined in the laboratory so that the mass of a known volume of natural soil is divided with that volume. The value of BD is not constant; it changes with the soil tillage (loosening) and compaction and depends on the SPD and the porosity of the soil. The average value of BD in arable soils is from 1.4 g/cm3 to 1.6 g/cm3 and, generally, is decreasing with depth. The total porosity of the soil and the various evaluations of some soil parameters are determined based on the SPD. Based on the data on the volume density of the soil, the weight of arable layer and the known quantity of a particular parameter, the total amount of the specified parameter per hectare is easily determined. Therefore, the volume density of the soil is a fundamental tool in balancing the investigated parameters in the soil. The free spaces between structural aggregates are called pores or voids of the soil, and their total content in volume percentages is called porosity of the soil (P). It is determined by the formula:
The pores can be of different sizes, from large cracks and burrows of fauna, to capillary and micro-capillary pores. Favorable air-water conditions in the soil arise only when there’s optimal ratio of macro-pores and micro-pores. It is considered to be the ratio of 1:1, or 3:2 in favor of capillary pores. The porosity of the soil depends on many factors, primarily on the mechanical composition, structure and content of organic matter.
Air of the soil consists of different gases that come from the atmosphere into the soil or are formed in the soil. Generally, the most important difference in relation to the air from the atmosphere is that it is fully saturated with water vapor for the most part of the year, richer in carbon dioxide and poorer in oxygen. The composition of the air from the atmosphere and the air from the soil is presented in Table 2. This table shows that the composition of air from the soil is very variable and many factors influence on it. Although the air from soil is derived mainly from atmosphere, a part of the gases are also created in the soil as a result of microbiological-biochemical processes. The sum of the content of carbon dioxide (CO2) and oxygen (O2) in the air of the soil and the atmosphere is approximately equal. Thus, if the soil has more oxygen there will be less carbon dioxide, and vice versa. Therefore, the ratio of CO2 and O2 in the soil depends on the soil aeration. CO2 is released into the soil by respiration of the plant roots and microbial decomposition of organic matter. That is why CO2 content is higher in summer than in winter. CO2 content is higher in humic soil because it has a high biological activity, than in mineral soil that has a low biological activity. It’s higher in the wet than in the dry soils, in heavy soils than in lighter soils, in structureless than in well-ventilated, structural soils. The high content of carbon dioxide itself has no adverse effect on plant growth and the damage to plants comes indirectly from lack of oxygen.
The composition of air of the atmosphere and pedosphere .
|Gas||Volume ratio in the atmosphere %||Volume ratio in the pedosphere %|
|Carbon dioxide (CO2)||0.03||0.1–15|
|Nitrogen (I) oxide (N2O)||0.005|
In soils with low aeration in conditions of reduction, in small amounts, the following compounds are formed – ammonia (NH3), methane (CH4), hydrogen sulfide (H2S), hydrogen (H2) and other gases. Their appearance is an indicator of unfavorable conditions in the soil, of disturbed air-water conditions in the soil, because they can occur only under conditions of lack of oxygen. Favorable air-water regime of soil displays shades of brown, yellow and black, while shades of gray, blue, green and yellow indicate on the unfavorable air-water regime of soil (Figure 6).
Basic chemical properties of soil, e.g. organic matter (humus) in the soil, absorptive ability of soil and soil reaction, will be shown in continuation. In a broader sense, the term humus involves all dead organic matter of soil, and in the narrow sense it is a special organic matter of dark color, formed by humification processes. These are very complex degradation processes of the original organic matter and the synthesis of new, more complex compounds under the influence of soil microorganisms. The composition of humus is complex and cannot be expressed by a single chemical formula, although the adequate procedures can excrete chemical compounds known as components of humus. Thereafter remains a certain part of which chemical nature is not fully known nor understood. With regard to the content of individual fractions, there are two major forms of humus in the soil, although in nature there are many transitional forms. These are mild or mature, and acidic or raw humus. Mild or mature humus, composed of humic (gray) acids, saturated with bases, and salts of humic acids, or humates, is resistant to degradation. It accumulates in the soil and gives it the best physical, chemical and biological characteristics. It is created in the presence of high-quality organic matter that is rich in bases, such as the remains of grass steppe vegetation and deciduous trees, and soils in which, of the microorganisms, bacteria dominate. Acidic or raw humus occurs in conditions of humid climate on leached rocks that are poor in bases, especially under coniferous vegetation. It is dominated by fulvic acids, which are mostly free due to the lack of base, and they give soil unfavorable features. The process of humification in such conditions is a result of microbial activity of fungi. Raw humus is not a favorable form of humus. Between these two extreme forms, there is a series of transitional forms, whose characteristics and impact on the soil depend on the soil characteristics, how rich they are in bases, the type of vegetation, the number and type of microorganisms, climate, altitude and other conditions. Another important feature, which is also a criterion for evaluating the quality of humus, is the ratio of carbon and nitrogen, or the C:N ratio. The optimal ratio of C:N in the soil is 10:1. The value of C:N ratio in the soil higher than 15 is an indicator that plants have a limited content of nitrogen in the soil, while the value of C:N ratio lower than 10 is an indicator of a limited decomposition of organic matter in soil.
Although content of humus in relation to the mineral part is significantly smaller, and the depth in which it is produced is relatively small (up to approximately 20 cm deep), due to its characteristics, especially colloidal nature, humus is the most active component of soil, other than clay. In terms of compost, the ideal value of C:N ratio is 30 and it indicates a sufficient amount of food available for micro-organisms. Table 3 shows that, for example, compost consisting of pine needles, straw, coffee grounds and others, have the ideal value of C:N ratio.
|Material||C/N ratio||The ability of degradation||Humidity in the original form|
|The leaves of alder||25–30||Good||Medium|
|The leaves of ash||25–30||Good||Medium|
|The leaves of linden, beech and oak||40–60||Good||Medium|
|The leaves and roots of potato||25–30||Good||Good after wilting|
|Smaller branches from the garden and orchard||30–60||Good||Medium to excessive|
|Grass from the mown lawn||12–25||Good||Good after wilting|
|The bark (of trees)||100–130||Medium||Low|
|Sawdust of fir||200–230||Medium||Low|
|The remains of pruning in the orchard||100–150||Bad||Very low|
|The remains from the garden||13||Good||Excessive|
|The straw of wheat||150||Good||Very low|
|The straw of barley||100||Good||Very low|
|The straw of oat||50||Good||Very low|
|The straw of rye||65||Good||Very low|
|Poultry manure||13–18||Good||Very low|
|Kitchen waste||12–20||Good||Generally excessive|
|The remains of the pressed fruit trees||30–80||Medium to good||Low|
Humus has a multiple positive effect on the soil:
- –It is the main factor of creating and maintaining a stable crumbly structure, high porosity, optimal ratio of micro-pores and macro-pores, and thus the most favorable air-water settings and good drainage – permeability of soil for water;
- –It has a large capacity for water and high adsorption capacity;
- –It has a beneficial effect on warming and thermal properties of soil because of its dark color;
- –It contains nutrients that released during the process of mineralization, increase the nutritional potential of soil;
- –It has a beneficial effect on soil microbial activity.
The term sorption or sorption ability implies the ability of soil to hold in a variety of substances – ions and molecules in the soil solution, colloids suspended in the water, particles of larger dimensions and microorganisms. Considering the forces involved in the process of sorption, sorption is divided into several groups. Mechanical sorption occurs when soil with its system of pores acts as a natural filter. The particles, which have larger dimensions than the dimensions of the pores, are retained in the soil. According to the system of mechanical sorption, a part of clay particles leached from the upper horizons is retained in soil. Physical sorption is a result of retention of molecules of the compounds dissolved in the water, or gases on the surface of soil particles, under the influence of the forces of the surface attraction. These forces arise at the border between the rigid component and the soil solution. Chemical sorption occurs so that the compounds chemically transform from the readily soluble to the less soluble form, or the reaction occurs with the cations that are bound to the adsorption complex of soil. Biological sorption happens as a result of the influence of higher plants and microorganisms. The plant receives nutrients and other substances in soluble form from the soil solution. By building its underground and above-ground organs, the plant accumulates these substances, primarily biogenic elements in insoluble, organic form. Biological sorption prevents leaching of nutrients and other substances from the soil, and thus increases their accumulation in the humus horizon of the soil. Physicochemical sorption, also called sorption of cations or adsorption, is the most important way of sorption in soil. Adsorption is the ability of colloidal particles of negative charge (acidoid) to bind cations from the soil solution on its surface, by using the physical and chemical forces. The plant receives cations through the root so they cannot be washed out of the soil, and they can be replaced by equivalent amounts of cations from the soil solution. All organic and mineral colloids, which have the ability of sorption of cations, are called adsorption or cation-exchange capacity of the soil (CEC). The composition of the adsorption complex includes clay minerals – secondary alumosilicates, humus, organic and mineral colloids, amorphous colloidal particles (allophane, hydroxides of iron, aluminum, silicon, etc.) and smaller fragments of primary minerals of colloidal dimensions.
Soil reaction shows the relation of the concentration of hydrogen ions and hydroxyl ions. If the concentration of H+ ions is greater, the result is a lower pH and the reaction is more acidic. If the concentration of OH- ions is greater, the result is a higher pH and the reaction is more alkaline (Table 4).
Schematic representation of the reaction of some media .
|Car battery acid||Lemon juice||Wine||Normal rain||Distilled water||Soda water||Soft soap||Ammonia||Alkali|
The reaction of soil solution is quantitatively expressed in pH units which represent the negative logarithm of the hydrogen ion concentration in moles per liter of solution (M, or mol/L or mol/dm3).
It can be noticed from Table 5 that neutral soils can be in the range of pH from 5.9 to 6.3 when distilled water was used as the extracting agent; 5.5 to 6.2 if a solution of 0.01 M CaCl2 is used for extraction, and 6.5 to 7.2 in the soils where the reaction of soil is determined in suspension of 1 M KCl. Most cultivated plants correspond to a neutral, slightly acidic or slightly alkaline reaction (Table 6 and Figures 7–10).
|pHH2O||<5.2||very acidic soils||pHKCl||<4.5||very acidic soils||pHCaCl2||<4.3||extremely acidic soils|
|pHH2O||5.2–5.5||acidic soils||pHKCl||4.5–5.5||acidic soils||pHCaCl2||4.3–4.5||very strong acidic soils|
|pHH2O||5.5–5.9||slightly acidic soils||pHKCl||5.5–6.5||slightly acidic soils||pHCaCl2||4.5–4.8||very acidic soils|
|pHH2O||5.9–6.3||neutral soils||pHKCl||6.5–7.2||neutral soils||pHCaCl2||4.8–5,0||medium acidic soils|
|pHH2O||6.3–6.6||slightly alkaline soils||pHKCl||7.2–7.7||slightly alkaline soils||pHCaCl2||5.0–5.5||slightly acidic soils|
|pHH2O||6.6–7.2||alkaline soils||pHKCl||>7.7||alkaline soils||pHCaCl2||5.5–6.2||neutral soils|
|pHH2O||>7.2||very alkaline soils||pHCaCl2||6.2–6.7||slightly alkaline soils|
|pHCaCl2||6.7–7.0||medium alkaline soils|
|pHCaCl2||7.0–7.3||very alkaline soils|
|pHCaCl2||>7.3||very strong alkaline soils|
Optimum and tolerant reaction of the soil for the cultivation of certain crops .
|Plant||Optimum pH||Tolerant pH||Plant||Optimum pH||Tolerant pH|
3.2.2 Soil biology
Living organisms in the soil, or on it, are not considered an integral part of the soil – pedosphere. They belong to the biosphere, although they have a significant influence on all the features of the soil, and they themselves are affected by them and structured, in a certain way, precisely with these features. Animal organisms, fauna or pedofauna strongly affect the important features of the soil. Members of the soil loose the soil, creating in it a number of corridors and thus increasing the permeability of water and air.
Earthworms (Lumbricus terrestris) have a particularly important role in soil. Their number varies from a few thousand to a few million, and they weight from a few hundred kilograms to one or maximum two tons per hectare . Soils that are rich in humus, with a neutral to slightly acid reaction, suit them well. They agglutinant the mass of the soil in the digestive tract and create specific aggregates. It is believed that 30 tons of soil passes through the digestive tract of earthworms per year, and it means that all weight of the soil to a depth of 30 cm passes through the digestive tract of earthworms in 60 to 100 years. Earthworms have a very beneficial effect on the soil, and their number and appearance are good indicators of favorable conditions in the soil. There are no earthworms in dry and sandy soils. Various worms, larvae and insects dominate in such soils. The richness of species and their distribution in the depth of the soil can be seen from the view of the number and types of microorganisms, which are located in the mass of only one gram of the soil at different depths, and are shown in Table 7.
Biological diversity of one gram of soil considering the number of microorganisms .
|The depth of soil,||Number of microorganisms/gram of soil|
|cm||Aerobic bacteria||Anaerobic bacteria||Actinomycetes||Fungi||Algae|
The formation and development of soil, as well as its physical, chemical and biological features were presented so far. Soil is a natural creation and a very dynamic system in which life is ruled by certain laws that today, at the beginning of the twenty-first century, are not quite clear nor scientifically determined. Nothing is accidental in the soil. Many phenomena in pedosphere are just part of the general system of circulation of matter and energy, in which the soil is not an isolated system. On the contrary, all changes in the lithosphere, biosphere, hydrosphere and atmosphere affect the dynamics of soil and its characteristics. Through examining the physical, chemical and biological properties, we stressed the importance of each of them for the soil fertility. Soil fertility is the ability of the soil to satisfy the needs of plants for nutrients, water, air and heat, that is, to ensure suitable conditions for the development of the underground and aboveground parts of the plant. According to this definition, fertility is a general indicator of soil properties – synthesis of chemical, physical, water, air and thermal properties. Soil characteristics that are essential for its fertility are: soil reaction (pH), content and form of humus, sorption capability of soil for nutrients, content of physiologically active nutrients, porosity, mechanical composition – in particular the amount and type of clay minerals. All those indicators are measurable, can be quantified and, on this basis, we can assess fertility as collective feature which is affected by all of these features of the soil.
3.3 Roles of soil
Soil science has developed as a part of agronomy and forestry profession. It is understandable that its focus was those soil characteristics that made it more or less suitable for growing plants, i.e. the focus was its production role. The problems of environmental damage (especially degradation and soil contamination), particularly of natural resources, in a certain way, have pushed this role. Only in recent decades, there is a growing interest in non-productive roles of the soil, in the first place those relating to environmental protection, in particular the protection of natural resources whose quality depends directly on soil and soil management. Below all the roles of soil are presented, although they are hardly separable from one another .
3.3.1 Role of soil in the formation of organic matter
The most important, indispensable and primary role of soil is supplying plants with water, air and nutrients, which enables the production of biomass – production of organic matter through photosynthesis. In this role the soil is an indispensable factor for the maintenance of life on Earth, for the plant/crop production in primary economic branches – agriculture and forestry. Production of organic matter in agriculture and forestry settles food and non-food needs of humans. Specifically, in this role soil allows us to supply food (bread, meat, milk, eggs, and mushrooms), drinks (wine, beer, and tea), beverages (fruit juices, juices of various vegetables and other plants), fiber (wool, textile plants – cotton, linen, and hemp), medicinal plants and herbs and energy (firewood, biodiesel, alcohol as fuel). Furthermore, soil provides raw material for wood and food industry (flour, oil, sugar, fibers, caoutchouc).
3.3.2 Organic-regulatory role of soil
The soil has a significant place in the biological cycle of matter and energy. Soil is situated between lithosphere and atmosphere, and it has a direct contact with hydrosphere, anthroposphere and biosphere. As such, soil acts as an acceptor of substances that are (intentionally or unintentionally) emitted from these spheres into the environment and are ecologically relevant to all members of the biosphere, whether they have positive or negative impact. These substances can accumulate in soil, so soil acts as their collector due to mechanical, physical and physicochemical sorption of the substances. Soil can modify collected substances, especially the organic ones, with the help of the microbial complex. That way, soil has the role of exchanger (transformer) of these substances.
3.3.3 Soil as a filter of water
Soil is an effective universal natural filter for water that penetrates through the soil into underground. For the functioning of terrestrial and aquatic ecosystems, especially for the protection of groundwater from different pollution, this is a particularly important feature of soil. By using colloidal complex, soil binds the different substances that in the process of natural circulation of the substance, or the food chain, arrive in the soil in the form of dry aerodeposition as dust, or rain water as wet deposition or, even more dangerous, as acid rain. This also applies to environmentally risky substances, therefore, different pollutants. The wider significance of the filtration and buffering capacity of soil can be seen from the fact that 65% of the total population in Europe is supplied with drinking water from groundwater.
3.3.4 Climatic-regulatory role of soil
The soil is the central link in the chain of biotransformation of organic carbon; it affects the content and the total amount of CO2 and other gases that cause the so-called greenhouse effect. Globally, the total amount of soil organic carbon (pedosphere) is three times higher than in the above-ground biological mass. It is believed that about 25% of the emitted carbon is derived from the soil – agriculture. Due to the limitations that are provided for carbon dioxide emissions, land management of tomorrow will receive new tasks in regulating the amount of carbon emitted into the atmosphere, and it will be by reducing its emissions.
3.3.5 Soil as a source of genetic wealth and protection of biological variety
Soil is the habitat and genetic reservation of many microorganisms and macroorganisms of the soil. The fertile soil must have the appropriate biological activity and show large biodiversity. Biological soil degradation is inextricably linked to the degradation of the physical and chemical properties of soil. Weight of organic carbon in the soil is estimated to be 1,550 Gt (gigatonnes); it is two times higher than in the atmosphere and three times higher than in all living organisms (biosphere) on Earth .
3.3.6 Spatial role of soil
Soil characteristics play a key role in the use of landscape today, and also in the past (Figures 11 and 12). Pedosphere provides space for the expansion of urban areas, roads, recreational areas, landfills (for waste disposal) and others. It is believed, for example, that about 2% of the total area of soil in Europe is under buildings. The range is from 0.5% in Ireland, 12% in Hungary, 13% in Italy to 14% in the Netherlands .
3.3.7 Soil as a source of raw materials
Soil is an important source of raw materials, especially for the building industry. The exploitation of these raw materials is always associated with damage to soil by open pits, or overlapping other fertile soils with these materials. For example, 0.05 to 0.1% of the soil surface in Europe is destroyed by open pits for mining purposes . According to the Soil Atlas 2015, 24 billion tons of fertile soil is lost every year because of misuse, i.e. because of soil sealing . Soil sealing is one of the most important types of soil degradation today.
3.4 Potential soil contaminants
Soil contamination is one of the most dangerous forms of soil degradation with the consequences that are reflected in virtually the entire biosphere, primarily heterotrophic organisms, and also mankind as a food consumer. The path that any contaminant takes from the soil to the plate is very short and food safety is crucial and increasingly important requirement which is becoming more difficult and complex to implement. In order to define the contaminated soil, it can be said that it is the soil in which human activity or natural phenomena has increased the content of harmful substances whose concentrations may be harmful to human activity, that is, for the production of plants or animals (Table 8). That is why in the last decade the remediation of contaminated soils is becoming an increasingly important subject [15, 16]. In order to achieve satisfaction rate of remediation of contaminated soil, it is very important to determine current condition of soil as accurately as possible. After completing the remediation process, soil sampling should be repeated, that is, the final state should be determined . Based on the determined values of soil contamination after completing the process of remediation, recommendations will be provided for future land use.
Although there are several classifications of soil contamination, in this section potential soil contaminants are divided in next groups: organic contaminants (polycyclic aromatic hydrocarbons and persistent organic pollutants); fuels (hydrocarbons); explosives; inorganic contaminants (metals and metalloids) including radioactive elements.
Polycyclic aromatic hydrocarbons (PAHs) or polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons are a large group of cyclic hydrocarbons consisting of 4, 5, 6 or 7 benzene rings fused together. The most common are PAHs with five or six rings. In accordance with their chemical structure they are categorized as persistent organic pollutants . In nature, PAHs are found in small, almost negligible concentrations while the increased content refers only to different anthropogenic activity . Natural activities that can cause increased content of PAHs in the environment include volcanic eruptions, meteorite and comet falls, summer forest fires or the occurrence of other types of large open flames. Increased content of PAHs is usually a result of pyrolysis processes during the combustion, especially of coal and gas in households and other heating facilities, garbage processing, traffic and in some industries (coke, iron and aluminum plants, power plants, galvanizing plants, petroleum and petroleum products processing, production and uses of asphalt and tar). Wherever a lot of thermal energy is needed in the work process an increased content of PAHs can be expected. The highest concentrations of PAHs are found in chimneys in family houses or in large funnels in industrial plants that use coal as raw material. Increased content of PAHs may be measured in major cities as a result of exhaust gases of automobile traffic. Increased content of PAHs can also be determined in dried meat, that is, in well-roasted meat (more accurately, fried), which is prepared on the grill. Some European countries have developed soil quality criteria according to the degree of PAHs contamination and necessary measures of reclamation . The criteria are based on the effect of PAHs on the ecosystem and human health. Criteria for re-cultivation are defined according to the method of soil exploitation in accordance with the established types of soils and their mechanical structure. Values lower than 200 µg/kg of total PAHs are considered to be natural values. The sum of sixteen PAHs between 600 µg/kg and 10,000 µg/kg is considered to be troubling, and values higher than 10,000 µg/kg total PAHs in soil require re-cultivation measures.
Some of Persistent Organic Pollutants – (POPs) are toxic synthetic organic aromatic compounds derived from the process of chlorination of biphenyl in the presence of a catalyst . (If their content is increased in soil, due to their high affinity for binding, especially to the organic component of soil, it is very difficult to remove them from soil. At the end of the twentieth century, the United Nations Environment Programme (UNEP) launched an initiative to create the Convention on Persistent Organic Pollutants (http://chm.pops.int/default.aspx). The aim of the Convention is monitoring the production and use of some POPs. According to the Stockholm Convention the following 12 are defined as initial POPs and so-called ’dirty dozen’: aldrin; chlordane; dichlorodiphenyltrichloroethan or generally known as DDT; dieldrin; endrin; heptachlor; hexachlorbenzene (HCB); Mirex; toxaphene; polychlorinated biphenyls (PCBs); polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).
The following pollutants are hydrocarbons. Increased content of hydrocarbons in soil may be caused by natural or anthropogenic activity . Unlike other chemicals, especially pesticides, hydrocarbons are not applied to the soil with a specific purpose. Undesirably increased content of hydrocarbons in soil mostly refers to specific incidents (pipeline ruptures, breakdowns during crude oil extraction or breakdowns at refineries), leaks from underground storage tanks for oil and its derivatives, and landfills. Increased content of hydrocarbons in soil can also be caused by processes relating to the extraction of crude oil, natural gas, coal and peat. As noted, the increased content of hydrocarbons is mainly related to human activities, but can sometimes occur by natural processes, for example, decomposition of bituminous shale or other rocks in which the increased content of hydrocarbons was determined.
From the chemical point of view the term heavy metals comprises metals with density which is greater than 5 g/cm3 and with an atomic number greater than 20 . From a biological point of view Nieboer and Richardson (1980) have categorized heavy metals to the essential elements for at least some organisms (Cr, Mn, Fe, Co, Zn, Mo and V), the elements necessary for the growth and development of plants (Mn, Fe, Cu, Zn, Mo and probably Ni) and phytotoxic elements (Cd, Hg and Pb) . Therefore, some heavy metals are micronutrients, but at the same time they are phytotoxic. The problem is that there is a fine line between the role of micronutrients and phytotoxicity. There are 65 elements of the periodic table that have metal properties: high thermal conductivity, high density and malleability. Of the ten most abundant elements in the lithosphere, seven are heavy metals.
Among the listed soil degradations, soil contamination with heavy metals is gaining in importance and draws the greatest attention in the public. This term refers to the content of heavy metals in soil in an amount which causes visible or measurable disturbance of some of the already mentioned soil functions. Thereat, all of the soil functions are taken into consideration and in particular the most important characteristics – fertility, suitability for normal growth and development of natural and/or cultivated plants. The plants that are grown in clean soil are suitable for all common ways of use and are completely harmless for every consumer. Otherwise, if the soil is contaminated, the plans have been contaminated through the soil, because the contaminant can be found in soil in chemical form that plants can absorb in an amount that reflects negatively on their growth, development and quality .
3.5 Remediation technologies for contaminated soils
A widespread and very serious problem of soil contamination by organic and inorganic substances with natural and anthropogenic activities has already been discussed in the previous section of this chapter. Unlike organic contaminants, inorganic contaminants cannot be degraded and they accumulate in/on the adsorption complex or soil organic matter. If their emission occurs from the adsorption complex, they will be absorbed by a plant or they will percolate to groundwater. If the plant ends up in the food chain, there is a risk that a person will consume it directly or indirectly, if a plant is consumed by animals that would later be consumed by humans. The next probable situation is that the contamination can be drained to deeper soil horizons, which will cause groundwater pollution or the contaminant will reach open water courses through the lateral (sub) surface runoff. Therefore, sites that are contaminated by organic and inorganic contaminants, where the rehabilitation was not conducted, should not be used in any form of agriculture. At the same time they are a potential source of pollution of the hydrosphere and atmosphere.
Compared with the atmosphere and hydrosphere, pedosphere has much less and markedly slower possibilities of recovering from toxic effects. Biogeochemical processes in the soil that affect the fate, behavior and bioavailability of potential contaminants are among the most rewarding scientific researches in recent decades. The negative effects caused by pollution are undeniable: reduced agricultural productivity, polluted water sources and raw materials for food are only a few of the effects of soil degradation, while almost all human diseases (excluding AIDS) may be partly related to the transport of contaminants in the food chain or the air to the final recipients – people, plants and animals. Remediation of contaminated soil can be done in three ways by using various remediation technologies:
- –reducing the concentration of total pollution to acceptable levels in accordance with the future land use;
- –physical /chemical /biological /mechanical isolation of contamination to prevent further reaction (the spread) of contaminated soil with the environment;
- –reducing the bioavailability of organic and inorganic impurities.
The questions that are always asked before the implementation of remediation are: To what levels should the soil be cleaned? When is the soil clean? What does clean soil mean for different purposes? If the soil is clean enough for sport or recreation areas, is it clean enough for residential and agriculture uses? When is the soil clean enough for recreational or green areas? The above indicators are variable paradigms of the soil purity that the wider scientific community, land owners and decision makers are facing in the future use of land/area. The framework of soil remediation limit value is shown in Table 9.
Levels of sensitivity for soil contamination limit values at different ways of using .
|Increasing the sensitivity to soil contamination – lower and stricter limit values for certain contaminants in the soil|
|Agriculture||Spaces for living|
|Commercial business premises||Industrial areas|
|Reducing the dependence on soil contamination – higher and more lenient limit values for certain contaminants in the soil|
This section of chapter covers four types of soil remediation and explains their basic principles/mechanisms of action. The main criteria for choosing/adopting/selecting a technology that will be used are the type and amount of contamination, the location where remediation would be implemented, but also the type of soil on the location where this technology should be applied. Potentially possible and economically viable technologies are shown in Table 10.
Remediation technologies of the contaminated soil .
3.6 Types, forms, techniques and technologies of remediation
Choosing a technology depends mostly on: type and category of contamination, spatial distribution of contamination (surface, volume and location of contamination – the proximity of surface/ground water), soil type (soil pH, soil organic matter, texture – type of clay), period of exposure to potential contamination, future land use and defined legal framework on the required level of remediation in a specific country. Based on the above parameters a decision is made whether to apply in situ, on situ or ex situ remediation. In situ remediation is performed directly at the site without contaminated soil excavation, that is, without disturbance to the soil structure. On situ is a form of remediation which is also performed at the site of contamination, but the soil is excavated. In this case, the contaminated soil is more exposed to external factors, and there is a possibility of more rapid spread of pollution, more by wind erosion than water erosion. Ex situ remediation is performed in a way that the contaminated soil is excavated and then transported to a special landfill where one of the appropriate forms of remediation is carried out. Due to many reasons (cost of remediation, environmental safety, public health, etc.) methods of remediation that are carried out in situ or on situ are more rewarding.
Many in situ or on situ technologies are used in order to stabilize the contaminants in soil by adding various additives, for example, lime, clays, zeolites. Such techniques do not reduce the total amount of contaminants in soil (this primarily refers to metals and metalloids), but they act in a way that they minimize the bioavailability of their fractions. Other in situ methods remove contaminants from soil by runoff, or by washing and using surfactants. This process enables collecting the contaminants from the soil by means of various attractants, since the contaminants are now dissolved in water or bound to the solid components. Ex situ remediation is based on the stabilization by creating a less polluted soil that can be disposed of in municipal waste landfills. Soil washing is one of the ex situ techniques of extraction which results in soil that is cleared of contaminants and which can be returned to its original area or soil that can be disposed of in non-hazardous waste landfills, depending on the remediation efficacy.
Biological methods include any type of remediation that uses microorganisms and plants. There are several processes in which microorganisms and plants can be used for remediation of contaminated soil. Depending on the strategy that is used to purify soil, these processes are blocking or removing contaminants from the soil with the help of plants or microorganisms. Bioremediation technology that uses microorganisms is carried out in two ways: in situ or ex situ. In situ bioremediation uses indigenous microorganisms. It is based on treating the soil in the area where contaminants are identified. The process involves soil excavation or pumping water prior to the bioremediation treatment. The main goal of aerobic in situ bioremediation is the supply of oxygen and nutrients necessary for the growth of microorganisms in order to enable the effective degradation, which is achieved by bio-ventilation or by injecting hydrogen peroxide or any other reagent. Ex situ bioremediation involves the removal and transport of contaminants which is then processed at a different location. In comparison with the in situ bioremediation, it is faster and easier to control and it enables degradation of a wider range of substances. The procedure involves excavation, transport to the landfill, soil treatment before and sometimes after a bioremediation process. The most common procedure is to mix the contaminated soil with an appropriate amount of water or liquid fertilizer in a special bioreactor with the addition of microorganisms.
Restoration with plants – phytoremediation is a technology that uses plants and their rhizosphere microorganisms that live on the roots for the removal, degradation or retention of harmful chemical substances in soil, groundwater and surface waters and the atmosphere . It is believed that the progress of biotechnology has slowed down, as a result of insufficient understanding of the complex relationship between rhizosphere and mechanisms that are based on the ability of plants to absorb and translocate metals from contaminated environments. Some plants that are very effective in the laboratory are not good contaminant accumulators in field conditions. Some plants are good contaminant accumulators in one climate, but have failed to give satisfactory results in another. Everyone involved in this issue points out that this is the method of the future, but also, there are still many unknown facts and even more questions than there are answers for wider application of this method in practice. However, it is the most promising method. For example, winter wheat, winter barley, soybean and tillage practice (ploughing and harrowing) can partially solved the problems of PAHs accumulated in soils due to the contamination by crude oil and drilling fluids . Plants decompose organic contaminants or stabilize them acting as filters. Just like any other method, phytoremediation has its advantages and disadvantages (Table 11). The advantage of phytoremediation is the fact that it is the most acceptable method for the environment because it uses plants . It is economically viable because it is one of the cheapest technologies.
|– Cost reduced over traditional methods||– Long remediation time requirement (up to 15 years)|
|– Low secondary waste volume||– Effective depth limited by plant roots|
|– Improved aesthetics||– Phytotoxicity limitations|
|– Habitat creation – biodiversity, green technology||– Fate of contaminants often unclear|
|– More publicly accepted||– Climate dependent/variable|
|– Provide erosion control||– Potential transfer of contaminants (i.e. to animals or air)|
|– Prevent runoff||– Harvesting and disposal of metals in biomass as hazardous waste may be required, although generally not|
|– Reduce dust emission||– Larger treatment footprint|
|– Reduce risk of exposure to soil||– Indigenous plants|
|– Less destructive impact (applied in-situ)||– Groundwater contamination possibility|
Table 12 shows a list of plant species that can be used for phytoremediation. The literature states that 400–500 plants have the ability of hyper-accumulation heavy metals (which is about 0.15% of all known species). Hyper-accumulators are plant species that are tolerant to high concentrations of toxic substances in the roots and above-ground plant mass. Most hyper-accumulative plants are present in soils that have developed on the serpentinite or soils that have a lot of hemimorphite minerals in parent materials. These plant species have the ability to rapidly translocate elements through the root to the above ground parts of the plant. A major problem for these species is that they produce relatively little overground plant mass and are characterized by slow growth. Another problem is that most of them selectively bind a particular metal, and the third problem is that they can be used only in their natural habitats. They come from a broad range of families, and most of them are members of cabbage family (Brassicaceae), legume family (Fabaceae), mint family (Lamiaceae), grass family (Poaceae) and spurge family (Euphorbiaceae).
Plant species for phytoremediation of various soil contaminations .
|Soil contaminant||Plant species|
|Cadmium||Brown or Indian mustard, Ethiopian mustard, some species of willow and birch, Thlaspi arvense, Thlaspi caerulescens|
|Chrome6+||Brown or Indian mustard|
|137Cesium||Amaranthus, Brown or Indian mustard, Sunflower, Cabbage, some species of willow|
|Copper||Brown or Indian mustard, some species of willow, Oilseed rape, Ribwort Plantain|
|Mercury||Brown or Indian mustard, Sunflower, Hybrid poplar, some species of willow|
|Nickel||Brown or Indian mustard, Spinach, Cabbage, Peas, Barley, Beans, Ricinus communis, Thlaspi arvense|
|Lead||Brown or Indian mustard, Black mustard, Sunflower, Peas, Thlaspi rotundifolium), Buckwheat, Hybrid poplar, some types of corn|
|Arsenic||Hybrid poplar, some types of alder, Aspen, Willow, Ferns, Oilseed rape|
|Selenium||Canola, Brown mustard, Kenaf|
|Uranus||Chinese cabbage, Brown mustard, Sunflower, Kale|
|Zinc||Oats, Brown and Indian mustard, Barley, some species of willow, Thlaspi arvense, Oilseed rape, Ribwort Plantain, Thlaspi caerulescens|
|Thallium||Iberis intermedia or Iberis linifolia|
|Volatile and semi-volatile contaminants||Alfalfa, Clover, Rye, Sorghum, some species of willow, Poplar, Alder, Blueberry, Spruce, Fescue grass, Elderberry, Mulberry|
|Agrochemicals||Alfalfa, Hybrid poplar, Willow, Mulberry|
|Explosive||Rye, Sorghum, Clover, Alfalfa, some species of willow, Poplar, Aspen, Cypress, Sedge|
Some species may be more resistant to several heavy metals, for example, colonial bent (Agrostis tenuis) and ribwort plantain (Plantago lanceolata) that are resistant to Zn, Cu, Cd and Ni. Rascio and Navari-Izzo (2011) reported that more than 400 plants can be nickel hyper-accumulators, 26 plants can be cobalt hyper-accumulators, 24 copper hyper-accumulators, 18 zinc hyper-accumulators, 8 manganese hyper-accumulators, and five plants can be lead and cadmium hyper-accumulators . In order for a plant to be classified in group of hyper-accumulators, it must have a minimum of 0.001% Hg, 0.01% Cd and Se, 0.1% As, Co, Cu, Cr, Ni, Pb, Sb, Se and Tl, and 1% Zn and Mn in its dry matter of the above ground plant mass . Also, in order for a plant to be classified in the group of hyper-accumulators, it should not show any changes caused by the increased content of heavy metals in the plant. The list of potential plants that are hyper-accumulators for heavy metals from the soil is not definitive. Almost every day, studies on a new plant species that has the ability to accumulate high amount of heavy metals from soil can be found in literature. For example, two investigations were performed in order to determine the influence of combustion residues from thermoelectric power plant (coal ash) on soil contamination and potential biological remediation. Zgorelec et al. (2008) indicate that high amounts of arsenic (592,6 mg/kg) and nickel (111,6 mg/kg) in pure coal ash have significantly influenced on metal content in soil, but also that some cultivar of soybean can accumulate higher amounts of nickel in grain than edible parts of barley cultivars . In terms of wild plants and contaminated sites by coal ash, Dellantonio et al. (2008) reported remarkable results of hyper-accumulation of boron in leafage of Salix Alba and Salix Caprea . Scientists within International interdisciplinary Project RECOAL («Reintegration of Coal Ash Disposal Sites and Mitigation of Pollution in the West Balkan Area», funded by the 6th Framework Programme of the European Union, European Commission (# 509173 – CORDIS-EU-FP6- Specific Targeted Research)) also investigated pilot filter column using clinker and bauxite-bed filters aimed at reducing the pH of the effluent water and a vertical flow soil filter (Figure 13), planted with reed (Phragmites spp.) and willows (Salix spp.), to remove suspended solids and trace elements. The results showed that arsenic was reduced for up to 90%, boron for up to 37 % and the pH was further reduced for 2pH .
Also, energy crops such as Arundo donax, Miscanthus x giganteus, Panicum virgatum, Pennisetum purpureum, Sida hermaphrodita and Sorghum x drummondii are recommended in phyto-extraction (cadmium, chromium, copper, lead, mercury, nickel and zinc), rhizo-filtration, stabilization and accumulation of heavy metals. Disadvantage of these plants is the fact that they do not translocate significant amount of heavy metals from the rhizosphere into the root system of plants . While some plants have the genetic potential to remove a variety of organic and inorganic contaminants from soil, they show some negative characteristics from the viewpoint of biotechnology. Most hyper-accumulative plants have a modest above ground plant mass and even smaller root network or they are species that grow slowly and their habitats are poorly represented in larger areas. Therefore, as considered by some authors, the advocates of genetically modified organisms (GMOs), we need to refocus on genetic engineering in order to solve this problem. They propose the transfer of genes responsible for the phenotype of hyper-accumulation, from the species that are characterized by small underground habitus and slow growth, into the species that have high biomass production, but low capacity of hyper-accumulation of heavy metals and other contaminants.
Proponents of GMO believe the GMO technology can solve the problem of heavy metals and other contaminants in soil as it would include incorporating genes of plants that are resistant to increased concentrations of heavy metals in soil in agricultural and other cultures. Other authors, opponents of GMO, refute this view by the fact that unforeseeable and unforeseen changes would take place in nutrient substances, flavors and forms of agricultural and other crops, which would result in catastrophic consequences for human health and the environment. This procedure of using GMO plants may potentially give satisfactory results in the near future, but in the long run it would lead to incalculable consequences for human nutrition and especially for the environment.
Electro-remediation was originally developed for high performance conversion of certain radioactive organic waste into the environmentally acceptable waste . Electro-kinetic technologies are being applied since 1930s, and initially they were used for the extraction of heavy metals in landfills of sewage sludge from urban waste water. Also, this method is successfully used for the removal of contaminants, especially heavy metals, from the contaminated groundwater. Of all the types of potential soil contamination, the highest quality results in the removal of heavy metals are achieved by this method . Electro-remediation is the process by which heavy metals are extracted from soil using a low-intensity current through a network of cathode and anode in the contaminated soil, in order to create a voltage gradient (Figure 14). As a result of the electric field, the mobility of metals and their motion toward the electrode occurs, thus enabling their extraction from soil.
Since 1980s they have been used for remediation of soils contaminated with inorganic and organic contaminants . Electrodes can be placed in vertical or horizontal position. By applying a direct current to the electrodes, an electric field is created between the anode and cathode. An electric field affects the soil, water and metals, leading to the following processes in the soil: motion of ions toward oppositely charged electrodes (electro-migration); water flow (electro-osmosis) and motion of charged particles toward the electrodes (electrophoresis). As a result of an electric potential the ions begin to move toward the oppositely charged electrodes: cations toward the cathode, and anions toward the anode. The result of the movement of ions depends on the diffusion coefficient and the ion mobility. Therefore, H+ and OH− ions have a higher diffusion coefficient and increased ion mobility in water, in comparison to metals. Diffusion coefficient and mobility of H+ ions is higher compared to the OH− ions, allowing H+ ions to dominate the system. Transport of H+ and OH− ions from the anode and the cathode has a significant influence on physicochemical properties of soil mass and the metal associated with them.
The success of electro-remediation depends on the knowledge of physicochemical properties of soil, current humidity, the environment of contaminated soil and the properties and concentration of metals. Mineral composition, mechanical properties of soil and the anomalies on the surface, such as gravel and coarse sand, PVC and asbestos cement pipes, large concentrations of Fe and other materials, can effect on the electroosmotic flow. Soils that are more negatively charged will accumulate contamination at the end of the cathode, while positively charged soils will encourage reverse electroosmotic processes and the accumulation of contaminants on the anode. Contaminants in the soil, especially metals that are tightly bound by cation exchange capacity (CEC), are very difficult to be removed by this process. In order to maximize desorption, different radicals, solvents and chelates are used in the process and are introduced into the soil during the electro-kinetic remediation.
Much better results are achieved by applying this method in soils that have lower cation-exchange capacity, two-layer clay minerals (kaolinite), lighter mechanical composition in relation to soil with higher cation exchange complex, heavier mechanical composition and dominance of three-layer clay minerals [36, 37].
Soil flushing is the process of extraction of soil contaminants using appropriate solutions. Contaminated soil is flooded by water or certain solutions that move the contamination in the area of their concentration in the borehole, where it is removed and processed by further procedures. The process begins by drilling injection and extraction wells in the contaminated soil. The number, location and depth of wells depend on a number of geological factors. Flooding apparatus is draining the solution into the injection wells. When the solution passes through contaminated soil it moves the metals toward the extraction well. The eluates are accumulated in the borehole – the solution with the contaminants that is later extracted from the soil to the surface.
The type of solution to be applied in the process is determined depending on the category and type of contamination. In the process of soil flushing it is common to use only water or water with additives, such as acids (low pH), bases (high pH) or surfactants (e.g. detergents, emulsions). Water is used for contaminants that easily dissolve in water. The acidic solution is a mixture of water with an acid, e.g. hydrochloric and nitric acid, etc., and is used to remove metals and organic components. The alkaline solution is a mixture of water with bases, such as NaOH, and it is applied for the removal of phenols and some metals. The application of these chemicals is the main constraint for a wider application of this technology. Even though the mentioned chemicals are diluted, they drastically change the chemical properties of soil, especially soil reaction, and during and upon the completion of a successful remediation process there is always the possibility that a certain amount of these chemicals is flushed into the groundwater.
Solidification and stabilization are based on solidification or reducing the mobility of contaminants in soil. Of all the soil contaminants the best and highest quality results were obtained from solidification of heavy metals. It consists of in situ and ex situ technologies that inhibit or slow down the release of metals from contaminated soil. Solidification immobilizes metals in contaminated soil by binding them to binding agents, while stabilization converts soluble, mobile and toxic metals into less mobile and inert forms [21, 38]. These two methods are a combination of physical (solidification) and chemical processes (stabilization) and are most often used together in order to reduce the impact of metals on the environment. Methods of solidification and stabilization do not reduce the concentration of metals in soil, but they reduce the risk of processes that are harmful to organisms by reducing the potential bioavailability of metals. The process includes immobilization with chemicals that react with metals and introducing reagents (impurities) into the contaminated soil. Immobilization prevents the mobility of metals and reduces their impact on the environment. Preliminary research is necessary to determine which of the metals are found in contaminated soil, and then to determine the appropriate type of stabilization material that will reduce the solubility of toxic contaminants. The procedure can be performed in situ by introducing impurities in contaminated soil or ex situ by excavation and mixing the soil with admixtures using machines and disposing solidified soil in previously prepared landfills or compaction in place, depending on the outcome of the procedure.
From the physical methods for remediation of contaminated soil, this section of chapter will address covering or encapsulation of contaminated soil, i.e. mixing the soil. Unlike other remediation technologies in physical remediation the changes of chemical and biological soil properties are very small. The system of covering the contaminated soil is one of the most common forms of remediation technologies on smaller areas. This technique involves covering the contaminated area by a multilayered cover in order to provide physical and chemical protection for the soil from wind and water erosion and to avoid direct contact of such soil with flora and fauna. Designing a system for covering the land is specific and depends on its future use. The most critical components of the system are covering layers that create a barrier and drainage layers. At the top there is a layer of soil on which the grass or other plants are seeded, while underneath there is a layer of gravel and pipes to promote drainage. This technology is similar (almost the same) to that applied for the closure of municipal landfills. In practice, it is most commonly carried out along with phyto-stabilization and/or phyto-volatilization. Covering of an area of land can consist of a single layer of fertile soil and a complex multi-layered system of soil and geosynthetics. In areas of dry climate, simpler systems are applied and in wet areas more complex systems are applied, while their complexity depends on the type and degree of contamination. The materials in the construction of coverage system include low-pass and high-pass geosynthetic materials. Low-pass materials prevent the water from reaching the contaminated soil. High-pass materials collect the water that filters into a capsule, and it can be temporary or permanent. Temporary capsules are installed before the final coverage in order to reduce the creation of eluate until a more effective remediation is performed. These capsules are usually necessary to reduce the infiltration. For the final cover layer a more stable substance is used (usually the soil) and in this way subsequent maintenance costs are reduced. It is not advisable to use this remediation method for depositing highly contaminated soil, especially for liquid contaminants. Inorganic or liquid pesticides containing about 5% organic material should be first solidified or stabilized, and only then covered using this technology.
Soil mixing reduces the concentration of contaminants by dilution to a level which is not harmful to the environment [39, 40, 41]. The application of such technology most clearly reflects the famous sentence: The solution for pollution is dilution. This can be achieved by bringing in pure soil and mixing it with contaminated soil or by deployment of clean material that is already in the area. Another method of dilution is based on agro-technical operations – plowing, disc harrowing, harrowing, milling or subsoiling, during which the vertical mixing of contaminated surface layer and less contaminated deeper soil layers occurs. This method also reduces surface contamination, which is no surprise considering the technology. As this is a very simple and inexpensive method of remediation of contaminated sites, it is often applied in many contaminated sites. This method is particularly preferred by investors who have caused pollution since it can be carried out by their own human and technical resources. Problems arise when trying to define the sites where this method can be applied. If a contaminated site is located near the surface or groundwater, this method of remediation of contaminated sites should not be applied. By using soil mixing technology very good results were achieved in remediation of soils contaminated with volatile and semi-volatile contaminants and pollution from fuel [26, 42].
Thermal remediation is the use of technology that is performed at very high temperatures, above 1,000 °C. In this way, the contaminated soil is burned and contaminants are converted into less harmful forms or they evaporate from the soil. It is very difficult for the agronomists to accept such a method because it is relatively successful in solving the problem of potential contamination in soil. At the same time, the high temperature in soil destroys the soil edaphon. In this way, the soil loses its essential role – a substrate for growing plants.
Incineration of contaminated soil has the advantage of reducing the volume to the minimum before depositing. To avoid evaporation, the incineration temperature must not be higher than the boiling point of certain pollutants. In most cases, the contaminated soil is burned in cement kilns of recent production, and sometimes in thermal power plants. Extremely alkaline conditions in cement kilns are ideal for degradation of chlorinated organic waste . Due to the complexity of implementation of this method, satisfactory results were achieved only with soil that was markedly contaminated by polychlorinated biphenyls (concentrations greater than 50 mg/kg soil). When all this is done properly, the destruction of chlorinated compounds in cement kilns can be more than 99% successful with no adverse effects on the quality of the discharged gases from cement factories.
The burning process can, for example, reduce the volume of hyper-accumulating plants. It is customary to burn heavy metal hyper-accumulating plants in cement kilns or in steel furnaces. The ashes of the burned biomass containing 20–40% of metals are considered a valuable ore, while a similar ash with only 2–4% of metals is not commercially viable and should be disposed of .
The most effective, least painful and least expensive soil remediation technology is prevention. Unfortunately, there are many papers and lectures on this subject, but no one is respectful of this technology.
According to data presented in table calculate the CN ratio in compost mixture. And answer to the two questions:
- Does compost mixture have optimal CN ratio?
- If not what amount (kg) of old fruits branches should be added in compost mixture in order to achieve optimal CN ratio?
|Material||Carbon (%)||Nitrogen (%)||Moisture (%)||Mass (kg)|
|Old fruits branches||53||1.0||15||70|
Using the soil textural triangle (Atterberg’s triangle) determine the textural classes of soil if soil consists:
- 60% of clay, 20% of silt and 80% of sand ___________________________
- 10% of clay, 30% of silt and 70% of sand ___________________________
Depending on the textural classes in which soil will (a) or (b) inorganic contaminants be more accumulated and why?
- –Swedish scientist Jöns Jacob Berzelius (1779–1848) defined the soil as an enormous chemical laboratory in which countless processes of degradation and synthesis take place simultaneously and continuously.
- –The Egyptian Queen Cleopatra declared earthworms, the inhabitants of soil, saints. Aristotle called them the intestines of the earth, and Charles Darwin felt that they belong to an important position in the development of the world.
- –As already mentioned, pedosphere has much less and markedly slower possibilities of recovering from toxic effects and the fact that soils are an irreplaceable factors of sustaining life on earth, the Food and Agriculture Organization (FAO) of the United Nations declared 2015 the International Year of Soils. The goal was to increase awareness and understanding of the importance of soil for food safety/security and essential ecosystem functions.
- –For all these reasons, in order to continue the efforts made during the International Year of the Soils, the International Union of Soil Sciences (IUSS) proclaimed The International Decade of Soils (2015–2024). International Decade of soils is result of the ‘Vienna Soil Declaration’ of Dec. 7, 2015. In that Declaration the IUSS has identified the key roles of soil in addressing the major resource, environmental, health and social problems which humanity is currently facing.
- –In terms of the adverse effects on humans and animals, which group of pollutants in soil (organic or inorganic) represents the greater threat to health? Explain why.
- –Enumerate three ways to perform remediation of contaminated soils.
- –List the four major technologies of soil remediation and explain the criteria for selection of individual technology.
- –What is the main difference between On situ and Ex situ remediation technologies?
- –Explain why the biological restoration with plants is the most promising method and enumerate several plants that can successfully accumulate heavy metals from soils.
- –Electro-remediation as a method for conversion of heavy metals is used in (circle the correct statement):
- Sewage sludge remediation
- Remediation of groundwater
- Soil remediation
- –An electric field between the cathode and anode leads to some processes in soil. These processes are:
- –Explain the main role of adding different radicals, solvents and chelates in processes of electro-remediation.
- –List some of disadvantages of using the chemicals in the flushing remediation methods.
- –Although the methods of solidification and stabilization do not directly reduce the concentration of metals in soil, they indirectly perform a very important role. Explain advantage of these methods.
- –Enumerate the methods which are integral part of physical methods.
- –In terms of the primary role of soil (production), specify the main negative impact of the implementation the thermal remediation. This method is most effective in removal of ____________________ (organic soil contaminant).
A FEW WORDS ABOUT ANALYTICAL METHODS
Determination of organic contaminants (for example PAHs) or inorganic contaminants (for example lead content) in soils in first step includes extraction or digestion procedures. Second step includes an analysis of extracts or solutions of digested samples. There are various analytical methods that are applicable for these purposes but standards methods for analysis of listed contaminants are described by several ISO norms. For instance some PAHs in soil samples can be extracted with acetone or toluene in a Soxhlet apparatus and determine using high-performance liquid chromatography . Also, PAHs can be determined by means of gas chromatography method with mass spectrometric detection (GC-MS) in combination of two extracts (acetone and petroleum ether) according to . As previously are mentioned, cadmium, mercury and lead are consisting group of phototoxic elements and their determination in soil first include digestion procedure. For instance, lead (Pb) can be extracted in aqua regia  from soils which are containing less than 20 % (m/m) organic carbon and determined by flame and electrothermal atomic absorption spectrometric method  or using inductively coupled plasma – atomic emission spectrometry (ICP-AES) according .
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This article is also available in: Environmental Engineering, Tomašic/Zelic. De Gruyter (2017), isbn 978‒3‒11‒046801‒4