**2. Soil quality**

The quality of the soil is defined by "its ability to function within an ecosystem in a way that it sustains biological productivity, maintains environmental quality, and promotes plant and animal health" [8]. Overall, knowing the soil characteristics involved in the development and sustainability of ecosystems is a useful tool.

Soil quality is assessed through the characteristic of indicators that measure or reflect environmental status or sustainability conditions of ecosystems. Soil quality indicators can be classified as physical, chemical, and biological.

## **2.1. Physical indicators**

of recycling raw materials and nutrients, besides being the habitat of a wide variety of organisms such as bacteria, fungi, viruses, nematodes, insects, and worms, among others. The food

The soil, besides being the substance for most man-made food, holds a large amount of pollutants. Chemical and organic fertilizers, pesticides, and other materials applied to the soil often contribute to water and air pollution. Soil is a key component of environmental chemical cycles and an important part of natural resources on Earth. The quality of the soil and climate,

Ecotoxicological tests are internationally acknowledged as complementary tools to chemically analyze soil contamination. However, countries, such as Brazil, do not require the conduction of ecotoxicological tests with soil organisms to evaluate contamination, since it is only based on chemical indicators [5]. The behavior and toxicity of soil elements, or compounds, should not be only assessed based on chemical parameters. Actually, it should be of the utmost importance to include biological parameters in these investigations, since chemical analyses separately applied to the compounds may not show their behavior in the

Terrestrial ecotoxicology seeks knowledge about the consequences of chemical-substance discharge in the environment on the organisms living in it. Therefore, it is essential understanding, to which extent, how hazardous the use of chemicals, alone or in mixtures, is, as well as where its effects are observed by monitoring the lethal, morphological, behavioral, physiological, cytogenetic, and biochemical effects on organisms exposed to these pollutants [6, 7]. Among many organisms in soil fauna, which is divided into macro-, meso-, and microfauna, one finds bacteria and fungi and species known as bioindicators. These organisms indicate environmental changes at their early stages and identify several modification types before these changes become worse, besides determining the pollution types capable of affecting a given ecosystem. They also have the ability to help monitoring and making more accurate and less impacting decisions on soil management. There are standardized bioindicator species around the world, such as *Collembola*, earthworms, nematodes, and enchytraeids. The use of bioindicators in monitoring programs helps detecting environmental changes at their early

stages or the effectiveness of measures taken to improve environmental quality [7].

The quality of the soil is defined by "its ability to function within an ecosystem in a way that it sustains biological productivity, maintains environmental quality, and promotes plant and animal health" [8]. Overall, knowing the soil characteristics involved in the development and

Soil quality is assessed through the characteristic of indicators that measure or reflect environmental status or sustainability conditions of ecosystems. Soil quality indicators can be

consumed by living organisms is produced in the soil [2].

14 Soil Contamination and Alternatives for Sustainable Development

environment [6].

**2. Soil quality**

sustainability of ecosystems is a useful tool.

classified as physical, chemical, and biological.

which enables productivity, is the most valuable asset of the society [3, 4].

The main physical indicators in place are texture, thickness (horizons), soil density, porosity, water, and structure.

Texture refers to particle proportions or to sand, silt, and clay fractions in the soil mass. It is closely related to water retention and transport, soil structure, nutrient content, and organic matter, besides its strong influence on soil erosion processes [9, 10].

Thickness, mainly that of the surface horizon, is essential, since it is the place where the greatest biota activity takes pace. Consequently, the surface horizon is a suitable place for organic matter and nutrient cycling. Soil depth plays a key role given its water storage ability and influence on nutrient supply for plants [10].

The diversity of mineral and organic components, as well as their proportion, determines the density of soil materials. This density changes depending on texture and on soil structural conditions [9, 11].

Porosity is determined by porous space. Part of the soil after the arrangement of solid components is occupied by water and air under natural conditions. Sand retains little water, because its large porous space allows free water drainage from the soil. Clay adsorbs relatively large amounts of water, and its smaller porous space retains water, even with gravity forces [11, 12].

Water amount in the soil depends on climatic factors and on soil texture, structure, and porosity. Water retention capacity reveals the amount of water the soil can store. Hydraulic conductivity is defined by the speed water reaches when it moves in the soil and by its infiltration rates [11, 12].

Soil structure is determined by the geometric arrangement of primary (sand, silt, and clay) and secondary particles (aggregates maintained by cementing agents such as iron, silica, and organic matter) [13].

#### **2.2. Chemical indicators**

Many soil functions such as nutrient reservoir, filtration of substances dissolved in water, chemical reaction accelerator, and pollutant immobilizer would not be possible without the presence of chemical components. Organic matter (OM), cation exchange capacity (CEC), hydrogen ionic potential (pH), and chemical elements are often used as chemical indicators of the quality of the soil [14].

Organic matter (OM) is the set of all organic materials in the soil. It is correlated to most soil properties, besides being a key indicator of the quality of the soil [15].

CEC stands out as one of the basic functions of the soil that lies on providing nutrients to plants. This indicator is found on the surface of solid particles in the soil due to electrical charges (Al, Ca, K, Mg, Mn, and Na), although it can be changed by pH and OM [14, 15].

pH is the indicator used to measure soil acidity, and changes in it can affect nutrient availability, microbial populations, and the availability of chemical elements in the soil.

Chemical elements play many roles, but it is possible roughly framing them in nutrients or toxic elements.

Besides the aforementioned biological indicators, soil fauna can be used as quality indicator. The biological diversity of the soil is fundamental to maintain soil productive ability; therefore, it is of great importance for the decomposition and mineralization of organic residues, since it favors nutrient availability for plants and even for other individuals. In addition, the soil fauna is sensitive to environmental changes, biological, physical, or chemical. Earthworms (*Oligochaeta*) and isopods (*Isopoda*) are among the organisms most studied as indicators [23].

Ecotoxicological Tests as a Tool to Assess the Quality of the Soil

http://dx.doi.org/10.5772/intechopen.82192

17

Soil contamination is one of the main environmental issues worldwide [1]. The unconscious use of the soil for agricultural activities, for the disposal of waste, chemicals, and industrial waste has been the cause of concern for centuries. Besides huge damages to the environment, a large amount of contaminated soils unviable for agriculture or for construction sites is the consequence of such inappropriate discharge [24]. Agricultural production has led to the increased use of pesticides to control pests and weeds. In addition to active toxic ingredients, many agricultural products contain potentially polluting elements or compounds, such as trace elements and emulsifying surfactants, among others [25, 26]. There has been simultaneous increase in the application of sewage sludge, industrial waste—composed of urban waste—and of agricultural waste for disposal or recycling purposes. These residues have high contents of organic matter and mineral elements capable of improving the chemical, physical, and biological properties of the soil. However, these residues may contain trace elements,

pathogens, and many other substances that cause environmental damages [26, 27].

limits, they also cause imbalance and exhibit some toxicity.

sizes and ecophysiological characteristics [14].

The term "soil contamination" refers to the presence of toxic substances belonging to chemical classes such as inorganic ions (metals), organic solvents, radioactive substances, pharmaceuticals, polycyclic aromatic hydrocarbons (PAHs), and pesticides (herbicide, insecticide, and

According to the International Union of Pure and Applied Chemistry (IUPAC), inorganic pollutants are described as "toxic elements." They comprise metal and metalloid elements formerly called "heavy metals." This nomenclature includes elements that, at low concentration, are biologically essential to living organisms. However, when these essential elements are observed at concentrations higher than the required ones or above the recommended

The displacement of water, soil, and air contaminants, as well as the interface between different compartments, is determined by processes related to the chemical properties of substances in the soil and to environmental compartments. Soil is one of the most complex matrices in the environment due to its heterogeneity. Different soil types present a wide variety of particle

Contaminants that reach the lithosphere move by diffusion, that is, water moves through gaps between soil particles. Contaminant displacement speed depends on molecular weight and on the concentration of the contaminant gradient, as well as on soil characteristics such as humidity, clay type, specific area, cation exchange capacity, pH, redox potential, temperature,

**3. Contamination of soils**

fungicide) [28].

Soil nutrients can be divided into two groups:


Have in mind that the same chemical element, in a certain concentration, can be a nutrient but toxic in another one.

#### **2.3. Biological indicators**

Biological indicators, such as microbial biomass, mineralizable nitrogen, microbial respiration, enzyme activity, and metabolic quotient, are fundamental for nutrient cycling and for estimates on soil ability to influence plant growth. In addition, microorganisms provide rapid responses to changes in the environment due to the abundance of metabolic and biochemical activities; therefore, they have great potential to be used as a tool to assess the quality of the soil [16].

The microbial biomass is represented by living components in soil organic matter [17]. This variable controls functions such as the decomposition and accumulation of organic matter or transformations involving mineral nutrients. Biomass provides information about changes in the organic properties of the soil, about changes caused by crops or by vegetation removal, about regeneration after topsoil removal, and about the effects from pollutants such as heavy metals and pesticides [16, 17].

The nitrogen (N) in the soil is the organic compound indirectly available to plants. The N content conversion from its organic form to its mineral one (mineralization) is mainly carried out by bacteria belonging to genus *Rhizobium*, which convert atmospheric nitrogen (N<sup>2</sup> ) into ammonia (NH<sup>3</sup> ) through an enzymatic complex. From this point on, ammonia can be incorporated to different forms of organic nitrogen available for plants. From a soil viewpoint, this potential organic N conversion into mineral N (potentially mineralizable nitrogen) has been considered an important factor; therefore, it is an indicator recommended to measure the quality of the soil [8].

Soil respiration reflects the microbial activity and is defined as the biological oxidation of organic matter into carbon dioxide (CO<sup>2</sup> ), which is conducted by aerobic microorganisms [18]. This indicator occupies a key position in carbon cycles observed in terrestrial ecosystems due to oxidation [19].

Enzymes rule the biological catabolism of organic and mineral components in the soil. This process is closely related to organic matter, physical properties, and microbial activity and biomass; therefore, it is widely used to assess the quality of the soil, since it indicates changes in microbial activity and the presence of pollutants in the soil [14, 20, 21].

The respiratory coefficient represents the relation between carbon in the microbial biomass and total organic carbon (TOC). This coefficient indicates microbial biomass efficiency in using the available carbon for biosynthesis [21, 22].

Besides the aforementioned biological indicators, soil fauna can be used as quality indicator. The biological diversity of the soil is fundamental to maintain soil productive ability; therefore, it is of great importance for the decomposition and mineralization of organic residues, since it favors nutrient availability for plants and even for other individuals. In addition, the soil fauna is sensitive to environmental changes, biological, physical, or chemical. Earthworms (*Oligochaeta*) and isopods (*Isopoda*) are among the organisms most studied as indicators [23].
