Soil Taxonomy
Soils
Soils are portions of the Earth’s crust whose properties have been altered by various soil-forming factors and processes (Jenny, 1941). In edaphology, soil is defined as a mixture of mineral and organic materials capable of supporting plant life. On the other hand, in pedology, soil is seen as a natural product formed from weathered rock under the influence of climate and living organisms.
Soil science quantifies the factors and processes of soil formation, including its quality, extent, distribution, and spatial variability, from microscopic scales (molecular structure, particles) to regional or global scales (landscape) (Fig. 1) (Luzio et al., 2006).
The two most important and well-known models explaining soil-forming factors are H. Jenny’s model (1941) and Simonson’s model (1959).
H. Jenny’s Model
Jenny’s model defines soil as an ecosystem component to be characterized in terms of geological substrate and biological material, proposing the fundamental equation of soil-forming factors (1), where soil (s) is defined by a combination of climatic (cl), biological (o), topographic (r), geological (p), and temporal (t) variables. This equation establishes that soil properties (e.g., pH, clay content, porosity, density, etc.) are determined by these factors, allowing interpretation of the degree of soil evolution. The ellipsis indicates that they may exist additional soil-forming factors that should be included in the equation (Jenny, 1941).
s = f (cl, o, r, p, t, …) (1)
Climate is a dominant factor in soil formation, as it represents the input of energy (water, temperature, radiation) driving soil development. Soil depth varies with humidity and temperature, just as different parent materials and topographies are associated with different climates. Broadly, climates are divided into humid and arid, which describe regional precipitation and water availability patterns (Luzio et al., 2006).
Figure 1. Levels of perception in soil studies (Luzio et al., 2006)
Biological factors or organisms (o) influencing soil formation include animals and plants. Photosynthesis is a process that transforms solar energy into complex organic molecules incorporated into soil particles directly through vegetation or indirectly through animals. Soil microorganisms metabolize organic matter, releasing CO₂ and water used in photosynthesis, a process known as respiration (Luzio et al., 2006).
Topography or relief (r) conditions different levels of land exposure to morphogenetic agents such as wind, rain, and sun (Fig.). For example, on a slope, the same amount of solar radiation covers different areas of soil depending on inclination, thus radiating differently each area. Some slopes are more sun-exposed (solana), generally facing north in the Southern Hemisphere, while shaded slopes (umbría), facing south, receive less exposure. Precipitation also varies in steep reliefs, producing the so-called orographic effect. This, in turn, relates to the slope that is exposed to the wind (windward), as it usually has wetter conditions, while the slope that remains protected from the wind (leeward) is generally the driest.
Geological characteristics (p) determine the initial conditions of soil formation, since it constitutes de original material from wich the soil is formed, also called parent material or mother rock. Lithology therefore influences the soil’s physical and mineralogical nature. Mineralogoly determines properties relevant to ecology such as nutrient supply, water retention, and water movement (Luzio et al., 2006).
R. W. Simonson’s model
Simonson (1985) proposes a model focused on intrinsic soil properties, unlike Jenny (1941), who emphasized external factors (climate, vegetation, etc.) in soil formation. Thus, soil changes are a function of additions (gains), exports (losses), translocations (transfers), and transformations (changes) of materials within the system: s = f (ad, exp., transfer., transfor.)
Pedon
The pedon is a soil column, extending from the soil surface to bedrock.
A polypedon is the smallest, characteristic soil unit in a given area, with features distinct from adjacent polypedons.
Soil Horizons
Soil horizons are horizontal layers that develop in a soil profile and exhibit differences in composition, texture, color, and other features due to formative processes.
Downward-acting processes in soil genesis create two main interrelated horizontal layers: horizon A and horizon B (known as horizontalization). However, in this process of genesis, mixing of soil materials can also lead to horizon destruction (haploidization) (Hole, 1961).
Horizon A: Corresponds to the uppermost soil layer -or the second one in case the organic horizon (O) is present— and wich contains the greatest abundance of organic matter (humus), usually darker in color. It represents a zone of vertical leaching, where water transports solutes and fine materials downward.
Horizon B: Lacks organic matter, acting as a precipitates or accumulation zone for leached materials from upper horizon. Typically lighter (brown or red), composed mainly of clays, oxides, and metal hydroxides.
Horizon C: Composed of partially weathered mineral material, intermediate between mother rock and soil.
Horizon D or mother rock is the parent material, which has not undergone any exogenous alteration. It is the material from which the overlying soil has formed.
Eluvial horizon (E): A leached layer derived from horizon A, where solutes and fine materials are washed downward to illuvial horizon B. Usually light-colored and not always present.
Horizon O: The most superficial soil layer, containing organic material such as leaves, branches, and plant debris, with less decomposition than horizon A.
Soil color
Soil color results from combined factors: the amount and distribution of organic matter, the degree of mineral weathering, parent material, soil moisture or dryness, and aeration conditions.
Dark Color
Dark color indicates organic matter presence (H₂O₂) and high precipitation zones with restricted drainage. Usually present in horizon A, but also as illuvial deposits in horizon B.
Brown colors occur in deep horizons (B), indicating good drainage. In the presence of oxidized Fe and Mn compounds, red to brownish-black colors develop.
Redish Color
Reddish soils or redish and yellowish mottles indicate alternating reduction and oxidation of iron (greenish, yellowish, bluish tones), and poor drainage. Also associated with differential weathering of parent material.
Light Color
Light color suggests abundance of light minerals, often found in horizons without organic matter, or in soils with salts and leaching processes (E horizon).
This hue is rarely found in surface horizons except in arid or semi-arid regions.
Gray Color
Gray color indicates waterlogging and reduction of minerals.
Soil clasification
Various soil classification systems exist, ranging from indigenous terminologies in small areas to complex systems designed to organize soils globally. However, two common principles underlie them: cognitive economy and perceived population structure (Buol et al., 2011). The first refers to the role of classification in providing maximum information with minimal cognitive effort, reducing differences between objects to produce a useful and easy to apply system. The second seeks to structure information into categories or classes that maximize shared features within categories and minimize overlap between different categories.
Historically influential systems include: Kubiena’s natural system (1953), Russian classification system (1997), Dutch clasification system (de Bakker & Schelling, 1989), Canadian soil clasification system (Soil Classification Working Group, 1998), Brazilian soil clasification system (IBGE, 2001; EMBRAPA, 2006), Chinese soil clasification system (CRG-CST, 2001), among others.
Currently, the World Reference Base for soil resources (WRB) is the international standard for soil classification systems, created to provide a universal reference framework and soil naming system. This effort was also part of the FAO/UNESCO World Soil Map project (FAO, 1988). More recently, the International Union of Soil Sciences (IUSS) working group published an update of the system (IUSS Working Group WRB, 2006). The WRB incorporates modern classification concepts, including USDA Soil Taxonomy (1975), FAO/UNESCO World Soil Map Legend (1988), the Référentiel Pédologique, and the Russian system (www.fao.org).
Morphogenesis and Pedogenesis
Morphogenesis studies the processes that gave rise to landscape forms (Llona et al., 2007). Pedogenesis is the process through which soil originates, also referred to as soil evolution or soil formation. It is generally accepted that there is a correspondence between soil genesis and landscape form (Jenny, 1941; Cruickshank, 1979; Birkeland, 1984). Tricart (1965a) was one of the first authors to propose the relationship between geomorphology and pedology. He argued that geomorphology provides the environment for soil formation, while pedology indicates the properties involved in morphogenesis. Although both disciplines study different objects —landscape forms and soils— their approaches are complementary and generate reciprocal contributions (Jungerius, 1985b).
Soil properties such as texture, color, and structure allow us to infer the origin of the deposits from which they developed, as well as the time in which these deposits were formed (Tricart, 1965). This correspondence is particularly observable in the color and granulometric characteristics of geomorphological units: the longer the formation time, the finer the fragments and the darker their color, due to weathering processes (Honorato et al., 1977; Engel et al., 1996; Alonso-Zarza et al., 1998).
In identical geomorphological units, soils with similar characteristics should be found under similar climatic conditions (Cooke et al., 1973).