**1. Introduction**

Soil, typically formed from decomposed rock and organic matter, is a mixture of minerals, several gases and liquids, and many organisms that supports life on Earth. Soil is the basis of agriculture on which all crops for human food and animal feed depend. Its properties vary from one place to another, due to bedrock composition, climate, and other factors. Soil and its properties are typically affected by several factors including current and past land use and distance to pollution sources. In certain location or climate conditions, some soil elements may reach toxic levels for humans, animals, or plants [1]. Soil pollution is majorly caused by the large quantities of either natural or man‐made waste products.

Pollution is an undesirable change in the physical, chemical, and biological characteristics of air, water, and soil, which in turn, affects lives of humans, plants, and animals, as well as albeit more indirectly, industrial progress, socioeconomical welfare, and cultural assets. Accordingly, a pollutant can be anything that adversely interferes with health, comfort, property, or environment of living matter; ranging from certain chemical elements naturally occurring in soil as mineral components, to anything that may be produced through human activities. Last point also covers the use of pesticides, fertilizers, and other amendments to soil, as well as accidental spills and leaks of chemicals used for commercial or industrial purposes. Even some contaminants are transported via the air and deposited on plants as dust or by precipitation. Lastly, exploitation of natural resources while contributing to the socioeconomic growth of countries also causes environmental problems, in particular potentially contaminated soil. Additionally, storage, transportation, and distribution of hazardous substances and oil‐ derived liquid fuels; oil activity in the refining phase; and agricultural and forestry activities can be source of pollutants. Controlling the soil pollution is an important problem, needing urgent solution to preserve the soil fertility while increasing the productivity [2].

#### **1.1. Major soil pollutants**

More formally, soil pollution is the accumulation of persistent toxic compounds, chemicals, salts, radioactive materials, or pathogens in soil, with undesired effects on plants, animals, and human health [3]. Soil becomes a significant source of contamination release when combined with the action of air and water. Similarly, several factors affect the mobility and final desti‐ nation of soil compounds, such as the existence, depth, and runoff direction of the ground‐ water; porosity; temperature; absorption capacity and ionic interchange of soil particles; air and water content; and the soil microbiota. For humans, the risk will mainly depend on their exposure to pollutant sources. These can be through direct inhalation, contact, and consump‐ tion of water, meat, or vegetables affected by pollutants [4].

A significant concept in soil pollution is the bioavailable portion, defined as the chemical amount that directly affects plants, animals, or humans as it can be taken up. This depends on several soil and land characteristics, e.g., how the contaminant is kept by soil and the contam‐ inant's solubility: greater solubility typically implies more bioavailability, but in turn, the pollutant may leach out of the soil. Typically, only a portion of a soil contaminant is biologically available and interestingly, certain chemicals exhibit an "aging effect" and a decreased bioavailability over time.

**1. Introduction**

210 Soil Contamination - Current Consequences and Further Solutions

**1.1. Major soil pollutants**

Soil, typically formed from decomposed rock and organic matter, is a mixture of minerals, several gases and liquids, and many organisms that supports life on Earth. Soil is the basis of agriculture on which all crops for human food and animal feed depend. Its properties vary from one place to another, due to bedrock composition, climate, and other factors. Soil and its properties are typically affected by several factors including current and past land use and distance to pollution sources. In certain location or climate conditions, some soil elements may reach toxic levels for humans, animals, or plants [1]. Soil pollution is majorly caused by the

Pollution is an undesirable change in the physical, chemical, and biological characteristics of air, water, and soil, which in turn, affects lives of humans, plants, and animals, as well as albeit more indirectly, industrial progress, socioeconomical welfare, and cultural assets. Accordingly, a pollutant can be anything that adversely interferes with health, comfort, property, or environment of living matter; ranging from certain chemical elements naturally occurring in soil as mineral components, to anything that may be produced through human activities. Last point also covers the use of pesticides, fertilizers, and other amendments to soil, as well as accidental spills and leaks of chemicals used for commercial or industrial purposes. Even some contaminants are transported via the air and deposited on plants as dust or by precipitation. Lastly, exploitation of natural resources while contributing to the socioeconomic growth of countries also causes environmental problems, in particular potentially contaminated soil. Additionally, storage, transportation, and distribution of hazardous substances and oil‐ derived liquid fuels; oil activity in the refining phase; and agricultural and forestry activities can be source of pollutants. Controlling the soil pollution is an important problem, needing

urgent solution to preserve the soil fertility while increasing the productivity [2].

tion of water, meat, or vegetables affected by pollutants [4].

More formally, soil pollution is the accumulation of persistent toxic compounds, chemicals, salts, radioactive materials, or pathogens in soil, with undesired effects on plants, animals, and human health [3]. Soil becomes a significant source of contamination release when combined with the action of air and water. Similarly, several factors affect the mobility and final desti‐ nation of soil compounds, such as the existence, depth, and runoff direction of the ground‐ water; porosity; temperature; absorption capacity and ionic interchange of soil particles; air and water content; and the soil microbiota. For humans, the risk will mainly depend on their exposure to pollutant sources. These can be through direct inhalation, contact, and consump‐

A significant concept in soil pollution is the bioavailable portion, defined as the chemical amount that directly affects plants, animals, or humans as it can be taken up. This depends on several soil and land characteristics, e.g., how the contaminant is kept by soil and the contam‐ inant's solubility: greater solubility typically implies more bioavailability, but in turn, the pollutant may leach out of the soil. Typically, only a portion of a soil contaminant is biologically

large quantities of either natural or man‐made waste products.

Bioavailability of a contaminant can be affected by fluctuations in soil conditions, e.g., soil pH, texture, clay type or organic matter content. Unfortunately, quick determination of bioavailable portion is lacking. Soil tests that are commonly available quantify a considerable part of the total amount of a specific pollutant in the sample, and not just the bioavailable portion, which in turn can be a small fraction. Most direct way of estimation for bioavailability, however, albeit being slow, expensive, or generally not available, are by using bioassay tests whereby uptake of pollutants by plants or microorganisms is quantified. Therefore, only the total or chemically extractable amounts of a particular pollutant are usually quantified.

Several substances contribute to the pollution of soil, major ones accounted as: petroleum hydrocarbons, pesticides, heavy metals, and solvents.

Additional to the potential adverse health effects on humans, elevated levels of soil contami‐ nants negatively affect all living matter, including the plant vigor, microbial processes via enzymatic processes, and animal health. In particular, the effect of contaminants to biochemical reactions can affect all metabolic processes and decrease yield for crops. These can be effective at even relatively low concentrations of contaminants as these can alter soil chemistry and impact organisms that depend on the soil or plants for their nutrition and habitat. The exact effects of contaminants on living matter and soil within a given system will depend on the properties of the soil, the levels of contamination, and the sensitivity of a particular organism to existing contamination. For example, zinc contamination affects nitrogen fixation process in Rhizobium bacteria, which is specifically sensitive to zinc. This in turn affects the nitrogen availability to plants and cause reduced yield for legume plants and crops (including beans, peas, peanuts, and lentils) since these plants fix nitrogen via symbiotic relation with the aforementioned Rhizobium bacteria in their root nodules.

Contaminants mobilize in soil in several forms and this phenomenon depends on many factors. Chemical changes or degradation into less toxic material are observed for organic (carbon‐ based) contaminants. In contrast, metals do not degrade further, but these may undergo chemical changes in such a way to be taken up by living matter. Furthermore, soil pollutants have different *preference* in their final destination: some are transported to water or either present in soil or to groundwater, some others vaporize; or stay bound to the soil. A major factor in the fate of the contaminants is the characteristics of the soil, which in turn is affected by land use and site management and readily available mechanism of uptake of these by plants or animals. Some important soil features that potentially affect the fate of pollutants contain soil texture in the form of its mineralogy and clay content, the pH, temperature, amount of organic matter, moisture level, and the presence (or absence) of other chemicals.

As for the living matter, people are generally exposed to soil contaminants via either ingestion (eating and/or drinking), inhalation (breathing), or dermal exposure (skin contact). Expectedly, human contact to contaminant of soil depends on the pollutant and on the condition and (past) activities at a specific location. Children ingest, typically unintendedly, small amounts of soil (younger children do more than older ones and adults) during playing, gardening, or per‐ forming other yard work, or even during indoor activities if soil is transported in via, e.g., shoes, clothing, or pets. Many pesticides enter the body by passing through the skin, i.e., being touched. Contaminants bound to soil particles or vaporized directly from soil, therefore becoming airborne, e.g., windblown dust, may also be inhaled. Not seldom animals raised for nutrition take in contaminants from soil, and pass these to people via animal foodstuffs such as eggs, milk, and even meat. Lastly, in case contaminants are directly dumped into a water source or reach surface water via overflow, drinking water may also contain contaminants.

#### *1.1.1. Soil contamination by heavy metals*

Heavy metals are mostly found at specific absorption sites, and these typically are strongly retained by organic or inorganic colloids. These are present also in all uncontaminated soils resulting from residues from the parent materials. A list of basal heavy metal concentrations in soils and plants is given in **Table 1**. Heavy metal accumulation is toxic to all living matter. Exposure to heavy metals is typically chronic, i.e., occurs over a long time period, due to food chain transfer. Some chronic problems associated with long‐term heavy metal exposures, e.g., are: lead—mental lapse; cadmium—affects kidney, liver, and GI tract; and arsenic—skin poisoning, affects kidneys, and central nervous system. Immediate poisoning is comparatively rare and typically occurs via ingestion or (dermal) contact.


**Table 1.** Heavy metal basal concentrations in the lithosphere, soils and plants (µg/g dry matter) [2].

From there, these are spread in the environment and to all living matter, e.g., plants and animal tissues as well as in soil. Interestingly, some of the heavy metals are essential for microbes, animals, and plants, but at very low levels. Their deficiency (essential ones) reduces growth and induces physiological abnormalities in plants [5]. The pollution thereof is mostly seen at urban and industrial aerosols from burning off leaded fuels, mining wastes, and chemical residues in both agricultural and farming practices. Heavy metal contamination of urban and agricultural soil depends on many factors, e.g., fertilizers, mining, tailings, and waste sludge, also the use of synthetic products (e.g., pesticides, insecticides containing arsenic as active ingredients), paints, batteries containing heavy metals, industrial waste, and industrial or domestic sludge applied on land and industrial areas where chemicals may have been buried or in areas downwind to these. It should be noted that heavy metals do also occur naturally, but seldom at levels to be considered as toxic [6].

The risk associated to the pollution is when these spread into the food chain, simply because this is closely related to (increased) bioavailability, in particular, phyto‐availability, i.e., availability to plants, which in turn, are the first stage of terrestrial food chain as essential components of natural ecosystems and agroecosystems. Despite its importance in the food chain, plants would be a threat to animals and human, if these are grown on contaminated soils, due to the accumulation of heavy metals up to toxic levels in the tissues. A common example is the Itai‐Itai disease (caused by Cd metal) affecting farmers working with heavy‐ metal contaminated rice on long term.


**Table 2.** Heavy metal content of fertilizers (µg/g) [2].

forming other yard work, or even during indoor activities if soil is transported in via, e.g., shoes, clothing, or pets. Many pesticides enter the body by passing through the skin, i.e., being touched. Contaminants bound to soil particles or vaporized directly from soil, therefore becoming airborne, e.g., windblown dust, may also be inhaled. Not seldom animals raised for nutrition take in contaminants from soil, and pass these to people via animal foodstuffs such as eggs, milk, and even meat. Lastly, in case contaminants are directly dumped into a water source or reach surface water via overflow, drinking water may also contain contaminants.

Heavy metals are mostly found at specific absorption sites, and these typically are strongly retained by organic or inorganic colloids. These are present also in all uncontaminated soils resulting from residues from the parent materials. A list of basal heavy metal concentrations in soils and plants is given in **Table 1**. Heavy metal accumulation is toxic to all living matter. Exposure to heavy metals is typically chronic, i.e., occurs over a long time period, due to food chain transfer. Some chronic problems associated with long‐term heavy metal exposures, e.g., are: lead—mental lapse; cadmium—affects kidney, liver, and GI tract; and arsenic—skin poisoning, affects kidneys, and central nervous system. Immediate poisoning is comparatively

**Heavy metal Lithosphere Soil range Plants** Cadmium (Cd) 0.2 0.01–0.7 0.2–0.8 Cobalt (Co) 40 1–40 0.05–0.5 Chromium (Cr) 200 5–3000 0.2–1.0 Copper (Cu) 70 2–100 4–15 Iron (Fe) 50,000 7000–550,000 140 Mercury (Hg) 0.5 0.01–0.3 0.015 Manganese (Mn) 1000 100–4000 15–100 Molybdenum (Mo) 2.3 0.2–5 1–10 Nickel (Ni) 100 10–1000 1 Lead (Pb) 16 2–200 0.1–10 Tin (Sn) 40 2–100 0.3 Zinc (Zn) 80 10–300 8–100

**Table 1.** Heavy metal basal concentrations in the lithosphere, soils and plants (µg/g dry matter) [2].

From there, these are spread in the environment and to all living matter, e.g., plants and animal tissues as well as in soil. Interestingly, some of the heavy metals are essential for microbes, animals, and plants, but at very low levels. Their deficiency (essential ones) reduces growth and induces physiological abnormalities in plants [5]. The pollution thereof is mostly seen at urban and industrial aerosols from burning off leaded fuels, mining wastes, and chemical residues in both agricultural and farming practices. Heavy metal contamination of urban and

*1.1.1. Soil contamination by heavy metals*

212 Soil Contamination - Current Consequences and Further Solutions

rare and typically occurs via ingestion or (dermal) contact.

Heavy metals do not only cause diseases on plants, animals, and humans, but also sharply reduce the yield of the crops, causing economic damage to farmers, in particular on sites located near smelters or mine spills.

In contrast to naturally present levels of heavy metals in soils, these are typically significantly higher in agricultural soils. This is because of the applications and accumulation of heavy metals thereof of several chemicals, pesticides, increased doses of fertilizers, farm slurries, other agricultural chemicals, sewage sludge, etc. A short list pointing to the heavy metal content of some fertilizers is given in **Table 2**. In particular, some phosphate fertilizers do contain small amounts of cadmium, which in turn accumulates in the soils whereby these fertilizers are applied.


Along the same line, the heavy metal content of sludges is listed in **Table 3**.

**Table 3.** Heavy metal contents in sludges (µg/g) [2].

Physical, microbial, or biological processes will determine the fate of the heavy metal pollu‐ tants in soil. As a result of being transported via natural routes (via water, nitrogen cycle, etc.) and their level at the destination, these may as well be retained in soluble or insoluble form, which in turn affects their bioavailability. It is reported that the soil organic matter has large affinity to heavy metals, which in turn reduced the nutrient content simply because heavy metals form stable complexes with organic matter in plant [7, 8].

The management of polluted soils requires great deal of knowledge on plant pathways in which biochemical reactions use these heavy metals in one way or another. Therefore, all biochemical processes including intracellular transport, adsorption, exchange with environ‐ ment, complex formation with organic and inorganic ligands, subcellular precipitation‐ dissolution upon, e.g., intracellular pH change, and redox reactions need to be investigated [9, 10]. Like all biochemical reactions, the extent of these reactions is a function of mineral content of the soil (e.g., for ionic strength) in the form of available silicate layers, carbonates, affecting in turn soil pH, and/or available organic matter (e.g., humic and fulvic acids, polysaccharides, and organic acids), and temperature and humidity.

An important point is the heavy metal bioavailability, which depends on a wide range of soil properties, including uptake and secretion rates, pH, clay and organic matter content, temperature, and coexistence of other (trace) metals in soil, which itself correlates with the soil redox potential and pH [11, 12]. Trace metal bioavailability is reduced as a result of re‐ duced redox potential. Heavy metals' availability depends also on the soil type: these are typically higher in sandy soils when compared to soils with high clay content. The metals typically form complexes on clay surfaces, the localization (outer layer, inner layer) has been described for SiOH and AlOH groups [13] and for amorphous hydroxides and oxides, gibb‐ site, and allophane clay [14]. Significant differences in Cd uptake, in soils with high Fe and Mn oxides and low organic matter versus soils with low oxides and high organic matter were found [14].

Along the same line, the heavy metal content of sludges is listed in **Table 3**.

Physical, microbial, or biological processes will determine the fate of the heavy metal pollu‐ tants in soil. As a result of being transported via natural routes (via water, nitrogen cycle, etc.) and their level at the destination, these may as well be retained in soluble or insoluble form, which in turn affects their bioavailability. It is reported that the soil organic matter has large affinity to heavy metals, which in turn reduced the nutrient content simply because heavy

The management of polluted soils requires great deal of knowledge on plant pathways in which biochemical reactions use these heavy metals in one way or another. Therefore, all biochemical processes including intracellular transport, adsorption, exchange with environ‐ ment, complex formation with organic and inorganic ligands, subcellular precipitation‐ dissolution upon, e.g., intracellular pH change, and redox reactions need to be investigated [9, 10]. Like all biochemical reactions, the extent of these reactions is a function of mineral content of the soil (e.g., for ionic strength) in the form of available silicate layers, carbonates, affecting in turn soil pH, and/or available organic matter (e.g., humic and fulvic acids, polysaccharides,

An important point is the heavy metal bioavailability, which depends on a wide range of soil properties, including uptake and secretion rates, pH, clay and organic matter content, temperature, and coexistence of other (trace) metals in soil, which itself correlates with the soil redox potential and pH [11, 12]. Trace metal bioavailability is reduced as a result of re‐ duced redox potential. Heavy metals' availability depends also on the soil type: these are typically higher in sandy soils when compared to soils with high clay content. The metals typically form complexes on clay surfaces, the localization (outer layer, inner layer) has been described for SiOH and AlOH groups [13] and for amorphous hydroxides and oxides, gibb‐ site, and allophane clay [14]. Significant differences in Cd uptake, in soils with high Fe and

**Heavy metal Range (µg/g)** Cadmium <60–1500 Cobalt 2–260 Chromium 40–8800 Copper 200–8000 Iron 6000–62,000 Manganese 150–2500 Molybdenum 2–30 Nickel 20–5300 Lead 120–3000 Zinc 700–49,000

214 Soil Contamination - Current Consequences and Further Solutions

**Table 3.** Heavy metal contents in sludges (µg/g) [2].

metals form stable complexes with organic matter in plant [7, 8].

and organic acids), and temperature and humidity.

Organic matter in soil contains negatively charged sites, e.g., humic compounds, suitable for (heavy) metal complex formation [15]. Metals can therefore be either adsorbed on the surface of precipitated organic matter, or in certain cases can dissolve as soluble organic complex with, e.g., organic acids. Expectedly, plant uptake decreases as the amount of insoluble organic matter increases. An important concept investigating the availability of trace elements is the cation exchange capacity (CEC), which itself is a function of organic matter and clay content of soil. Therefore, the metal uptake in plants decrease as CEC increases [16]. Focusing on individual metals, the Cd adsorption is reported to be controlled by calcium, following a competition for available absorption sites at the root surface [17]. Typically, mercury, copper, lead, cadmium, nickel, copper, zinc, and chromium are found as positively charged metal ions. On the other hand, arsenic, selenium, and molybdenum are present in their neutral forms. Both neutral and positively charged heavy metals are found in soil via sewage, industrial waste, or mine washings (USDNCRS 2000). Additionally, radioactive materials such as thorium, uranium, and strontium also constitute as source of dangerous soil pollution as concentrated in sediments [18]. Decontamination procedures include the use of chelate amendments.

This negative correlation between the plant uptake and metal availability have been investi‐ gated for the negative impact of macronutrients on trace element uptake [19]. In that work, phosphate ions are reported to reduce Cd and Zn uptake in plants, and reduce the toxic effects of arsenic, typically observed on soils treated with arsenic pesticides [20]. This is especially important when considering the substantial amounts of trace metals in fertilizers. The long‐ term use of these fertilizers is expected to increase the levels of trace elements in soils and in long‐term accumulation in plants [21]. Similar antagonistic effect among micronutrients is also common. An example is leaf chlorosis resulting from Fe deficiency, which can result from a surplus of other metals such as Zn, Ni, and Cu, which in turn decrease the Fe uptake by plant roots. This is important since Fe in turn affects the toxic metal Cd absorption. Another antagonistic metal couple reported by Smilde et al. is the well‐known Cd/Zn antagonism. These two metals are chemically similar in their electronic configuration and reactivity with organic ligands: Zn lowers Cd uptake [17], while at low concentrations the interaction is reported to be synergistic [22].

Some plants, known as "hyper‐accumulators" adapt quite well to stressful environmental conditions, holding (heavy) metals in their tissues higher than 1% of the metal and up to 25% on a dry matter basis. As a rule‐of‐thumb, fast‐growing plants (lettuce, spinach, carrots) take up more metals than grasses. Similarly, leafy vegetables accumulate trace metals more than root vegetables which, in turn, accumulate metals more than grain crops [10].

#### *1.1.2. Soil contamination by inorganic toxic compounds*

An important class of contaminants is the inorganic residues from industrial waste causing severe problems in their disposal. These typically form complexes with (heavy) metals and therefore have very high toxicity potential. Examples are the arsenic fluorides and sulfur dioxides from industrial wastes, reported in Ref. [23]. These fluorides typically emerge from superphosphate, phosphoric acid, aluminium, steel, and ceramic industries. Along this line, emitted SO2 makes the soil highly acidic, promoting again metal complex formation, causing further leaf injury and hampered vegetation. In addition to the above‐mentioned contamina‐ tion, some of the fungicides containing copper and mercury, as well as exhaust gases from automobiles running in leaded fuel gets adsorbed by soil particles, therefore adding to soil pollution and is toxic for the plants.

#### *1.1.3. Soil contamination by organic wastes*

Various types of organic wastes, e.g. improperly disposed domestic garbage, sewage, indus‐ trial waste, agricultural effluents from animal farms, and drainage of water sources, cause soil pollution and adversely affect human health as well as vegetative growth of plants [24–27]. These typically contain large amounts of borates, detergents, and phosphates. For soil, the main contaminants are coal and phenols, combustible materials, aerosols, H2S, and carbon mono‐/dioxides.

A typical source of organic waste contamination is irrigation with sewage water, which typically causes both physical changes such as leaching, changes in porosity, and humus content, as well as chemical changes such as salinity, changes in nitrogen, and phosphate content. An important effect of sewage sludge is the heavy metal pollution. This further leads into the phytotoxicity of plants. Alekseev reported that solubility and availability of heavy metals increase as a result of decrease in soil pH, which results from the release of soluble organic carbon following sludge decomposition [28].

#### *1.1.4. Soil contamination by organic pesticides*

Pesticides are often used to control pests and may cause harm to microorganisms and to plants and humans accordingly. Generally, pesticides, particularly aromatic compounds, decompose over much longer time and are known as persistent organic pollutants (POPs). They are the main cause of accumulation, which in turn are highly toxic. Chief examples are aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, hexachlorobenzene, toxaphene, chlorde‐ cone, lindane, and endosulfan. Being undecomposed for long periods of time (ranging from months for diuron to tens of years for DDT), these pesticides move into water streams and into food and the food chain thereof. With their high degree of persistence, they can also be easily transported to far away distances from their sources.

These pesticides typically contain heavy metals such as cadmium, mercury, and arsenic, and these are the major problems in pesticide pollution. Currently, several organochlorine com‐ pound containing pesticides, including DDT has been banned from USA, Europe, and other countries [29, 30].

The harmful organochlorines have currently been substituted by alternative pesticides containing organophosphate, more toxic, yet little to no residue is left and therefore do not pollute the soil. Common practice for controlling the pesticidal pollution is to increase the organic matter level of the soil and choose the nonpersistent pesticides.

#### **1.2. Causes of soil pollution**

dioxides from industrial wastes, reported in Ref. [23]. These fluorides typically emerge from superphosphate, phosphoric acid, aluminium, steel, and ceramic industries. Along this line, emitted SO2 makes the soil highly acidic, promoting again metal complex formation, causing further leaf injury and hampered vegetation. In addition to the above‐mentioned contamina‐ tion, some of the fungicides containing copper and mercury, as well as exhaust gases from automobiles running in leaded fuel gets adsorbed by soil particles, therefore adding to soil

Various types of organic wastes, e.g. improperly disposed domestic garbage, sewage, indus‐ trial waste, agricultural effluents from animal farms, and drainage of water sources, cause soil pollution and adversely affect human health as well as vegetative growth of plants [24–27]. These typically contain large amounts of borates, detergents, and phosphates. For soil, the main contaminants are coal and phenols, combustible materials, aerosols, H2S, and carbon

A typical source of organic waste contamination is irrigation with sewage water, which typically causes both physical changes such as leaching, changes in porosity, and humus content, as well as chemical changes such as salinity, changes in nitrogen, and phosphate content. An important effect of sewage sludge is the heavy metal pollution. This further leads into the phytotoxicity of plants. Alekseev reported that solubility and availability of heavy metals increase as a result of decrease in soil pH, which results from the release of soluble

Pesticides are often used to control pests and may cause harm to microorganisms and to plants and humans accordingly. Generally, pesticides, particularly aromatic compounds, decompose over much longer time and are known as persistent organic pollutants (POPs). They are the main cause of accumulation, which in turn are highly toxic. Chief examples are aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, hexachlorobenzene, toxaphene, chlorde‐ cone, lindane, and endosulfan. Being undecomposed for long periods of time (ranging from months for diuron to tens of years for DDT), these pesticides move into water streams and into food and the food chain thereof. With their high degree of persistence, they can also be easily

These pesticides typically contain heavy metals such as cadmium, mercury, and arsenic, and these are the major problems in pesticide pollution. Currently, several organochlorine com‐ pound containing pesticides, including DDT has been banned from USA, Europe, and other

The harmful organochlorines have currently been substituted by alternative pesticides containing organophosphate, more toxic, yet little to no residue is left and therefore do not pollute the soil. Common practice for controlling the pesticidal pollution is to increase the

organic matter level of the soil and choose the nonpersistent pesticides.

pollution and is toxic for the plants.

mono‐/dioxides.

countries [29, 30].

*1.1.3. Soil contamination by organic wastes*

216 Soil Contamination - Current Consequences and Further Solutions

organic carbon following sludge decomposition [28].

transported to far away distances from their sources.

*1.1.4. Soil contamination by organic pesticides*

Soil gets polluted via either man‐made matter or due to natural causes. The natural causes include rupture of underground storage links, water reservoir, while man‐made causes cover application of pesticides, oil and fuel dumping, direct discharge of industrial wastes, or leaching of wastes from landfills. The more industrialized the area, the more polluted the soil gets, which naturally decreases soil quality.

A significant cause of pollution is uncontrolled use of fertilizers to supply soil deficiencies. These are known to contaminate with impurities, such as ammonium nitrate, phosphorus as P2O5, and potassium as K2O. Important pollutants from fertilizers are the heavy metals, such as, As, Pb, and Cd present in traces in rock phosphate mineral being transferred to super phosphate fertilizers. Being not degradable, heavy metals accumulate in soil above toxic levels for crops. The uncontrolled use of NPK fertilizers therefore reduce the overall yield as well as protein content of vegetables and crops grown on that soil [31].

Another cause of pollution is the rampant use of insecticides and herbicides, which are used majorly to protect plants from insects, fungi, bacteria, viruses, rodents, and other animals. Large‐scale use of insecticides dates back to the 1950s and do include DDT and gammaxene. Over time, insects became resistant to DDT and farmers had to use increasing amounts of DDT to be effective against pests. Add to that the fact that DDT does not readily decompose, quickly created significant contamination. Being soluble in fat, DDT biomagnified in the food chain [32].

Solid wastes, including domestic trash, of discarded commercial operations typically contain recyclable material, e.g., paper, cardboards, plastics, glass, old construction material, packag‐ ing material, and toxic or otherwise hazardous substances. However, albeit small, hazardous wastes, e.g., battery metals, organic solvents, and oils are significant soil pollutants [33].

Another point to consider is the pollution of surface soils materials (e.g., vegetables, rotten and decomposed leaves, wooden pieces, animal wastes and carcasses, and papers) and many nonbiodegradable materials (such as plastic bags, bottles and other wastes, cloths, glass pieces, bottles) [34, 35]. In case the pollution is left uncollected and decomposed, they are a cause of several problems such as clogging of drains, including the burst/leakage of drainage lines; barrier to natural waterways, causing damage to nature but also man‐made constructions; foul smell; and elevated microbial activity in particular along with decomposition of organic material. Specifically, if the source is from hospitals, the microbiota would include several pathogens. Lastly, underground soil may be polluted in particular where industrial activities exist, cities by chemicals released and sanitary wastes. Heavy metals in particular are likely to be accumulated.

#### **1.3. Effects of soil pollution**

Although some of them are obvious and have been enumerated above, it is worth noting that soil pollution affects many aspects of life, majorly food chain but not limited to this. To start with, polluted soil causes reduced crop yield and reduced soil fertility. Polluted soil fixes less nitrogen and has increased erodability. Due to the latter, soil loses more nutrients and soil fauna and flora becomes more imbalanced in its nutrients (becomes extremely salty, acidic, alkali, etc.). In particular, as a result of industrial activities, water gets polluted and drinking water becomes more inaccessible to humans. Again with industrial activities, greenhouse and other pollutant gases release to the atmosphere, which decreases the quality of the air, causing an increase in public health and waste management problems.

The rest of this article focuses on enzymes used for soil remediation as a special case of bioremediation via so‐called plant growth promoting rhizobacteria (PGPRs). As such, it represents one of the alternative tools for soil remediation, such as thermal soil remediation, air sparging, encapsulation, chemical oxidation, stabilization, and soil washing.
