Soil forming factors
The Russian geologist Vasily Dokuchaev (1846–1903), considered the father of soil science, was the first to identify and discuss, at the end of 19th century, what we know today as the “factors of soil formation”. He introduced the idea that soil is not something inert and stable, but it develops and evolves under the influence of climatic agents and vegetation that operate over time on a given geological substrate. We owe to Dokuchaev the writing of the first equation of soil formation:
- s = f(p, cl, o)t </dd>
where s = soil, p = parent material, cl = climate, r = topography, o = organisms. Time (t) was left out of equation because it is not mass nor energy. Relief was mentioned to play a role in soil formation (pedogenesis) in some situations. In 1941, the Swiss Hans Jenny (1899–1992) redefined the meaning of the factors of pedogenesis, assuming that the factors were independent variables and formulating the now famous equation:
- s = f(p, cl, r, o, t, …) </dd>
where s = any soil property is a function of p = parent material, cl = external climate, r = topography, o = biotic potential, t = time, and the dots = additional factors not as yet known or unique to any particular situation. These variables are the State Factors, in that they determine the state of the soil system. As a consequence, soils can be defined as “those portions of the Earth’s crust whose properties vary with soil forming factors”.
Parent material (p) is the material on which soil forms, namely hard rock, unconsolidated sediments, or organic remnants. For a soil that is reforming after severe disturbances or major climate change, the parent material is the soil that was present at the beginning of the new state factor assemblage. Properties of the parent material that exert a profound influence on soil development include mineralogical composition, degree of consolidation, porosity, and permeability. The minerals found in soil but not formed in soil, thus mainly inherited from the parent material, are referred to as primary minerals. Secondary minerals form through substantial chemical or structural changes of primary minerals or de novo precipitation from ion-rich solutions during pedogenesis. The influence of parent material on soil properties is preponderant in young or relatively immature soils, but it is progressively reduced by the influence of the other factors.
The state factor climate (cl) comprises the meteorological conditions which surround the soil system (temperature, rainfall, humidity, etc.), and is consistent with the concept of regional climate. The real soil climate (pedoclimate) depends on both regional climate and unique features of the soil, such as depth, color, texture, and its position on the landscape.
The factor topography (r) refers to the configuration of the land's surface. The topography of a site incorporates its relief (relative differences in elevation), its aspect (position with respect to cardinal coordinates), and the general shape of the land surface. These attributes mediate how the regional climate, such as solar radiation, precipitation, and wind, impinge upon the site. Relief imparts potential energy, by virtue of gravity, that functions to move water and solids from higher landscape positions to lower ones. The movement of materials on a landscape is influenced by the slope gradient and shape and the degree of connectivity of drainage networks. Thus, from a pedologic perspective, topography is important because it exerts a strong influence on the disposition of energy and matter experienced by soils on the landscape.
The biotic potential (o) was defined by Jenny as the independent factor represented by the biota of the system, namely the microbial, plant, and animal gene flux that enters the system from the surroundings. The vegetation or animals that actually survive or reproduce in the soil system are dependent ecosystem properties, and may not directly reflect the potential biota due to the effect of the remaining state factors. With respect to soil, the activity of the biota contributes to the addition, removal, transformation and translocation of matter via a myriad of processes involving algae, lichens, mosses, fungi, bacteria, nematodes, acari, earthworms, termites, ants, beetles, moles, pocket gophers, prairie dogs, birds, badgers, grasses, brushes, trees and others. Living organisms affect the chemical, physical, mineralogical and morphological traits of soil materials either individually or as communities, and by both their excreted metabolites and their mortal remains. An important implication of Jenny’s model, and the meaning of the biotic factor, is that the definition of soil needs not involve life (namely, the biotic factor may be absent). This key distinction now has implications for inter-planetary geochemical studies, where it has been argued that the term soil should not be used on other planets because it implies the past or present presence of living organisms.
The state factor time (t) is defined as the elapsed period since the system began or was exposed to its present assemblage of state factors. For some systems, this is the starting point immediately after an event such as fluvial or volcanic deposition. In other cases, t = 0 may be the point at the end of a major environmental disturbance or change. Many soils older than the Holocene have experienced one or more major climate changes, and possess properties that may be the complex effect of several stages of soil development, and are called polygenetic or relict soils.
Despite often a state factor affects one or more of the others (e.g. topography vs. climate, climate and parent material vs. biota, etc.), in many locations factors may vary independently of each other. As a result, in nature, through judicious site selection, the influence of a single factor can be observed and quantified. These observational studies are called sequences: lithosequence, climosequence, toposequence, biosequence, and chronosequence.
In an increasingly populated planet, humans must be necessarily included among the state factors of pedogenesis because they take on increasing significance as an ecosystem variable. Jenny originally considered humans under the biotic factor because, like other biota, humans contain a genetic component or genotype. However, unlike the other organisms that are not endowed with the ability to reason, humans possess a cultural component that varies from society to society and which operates independently of genotype, arguably making them worthy of a separate factorial treatment. Nowadays, humans influence ecological processes on a global scale, sometimes on par with the role of climate, geological forces and astronomical variations. The anthropogenic factor (h) can affect soil formation by a plethora of different interventions, such as deforestation and forestation, grazing, agriculture, urbanization, construction of turnpikes and landfills, induced fires and floods, mining, pollution, bombing, burial of pipelines or cables. Global climate change is under suspicion to be caused by overexploitation of fossil fuels and mismanagement of forest resources.
- Jenny H., 1941. Factors of Soil Formation. A System of Quantitative Pedology. McGraw Hill Book Company, New York, NY, USA. 281 pp. ISBN: 0486681289
- IUSS Working Group WRB, 2006. World Reference Base for Soil Resources 2006, 2nd edition. World Soil Resources Report 103, FAO, Rome. E.U. 145 pp. ISBN: 9251055114
- NRCS, 2007. Soil Formation and Classification. Natural Resource Conservation Service-United States Department of Agriculture.
- Soil Survey Staff, 2006. Keys to Soil Taxonomy. Tenth edition. USDA, NRCS. United States Government Printing Office, Washington, DC, U.S.A. 341 pp. ISBN: 1410224740
- Spector C., 2001. Soil Forming Factors: The Story of Rocks and Soil. NASA's Goddard Space Flight Center.