Pedogenesis (from the Greek pedo-, or pedon, meaning 'soil, earth,' and genesis, meaning 'origin, birth') (also termed soil development, soil evolution, soil formation, and soil genesis) is the process of soil formation as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry. These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors.
New soils increase in depth by a combination of weathering, and further deposition. An estimated 1/10 mm per year rate of soil production from weathering fits observations rates. New soils can also deepen from dust deposition. Gradually soil is able to support higher forms of plants and animals, starting with pioneer species, and proceeding to more complex plant and animal communities. Soils deepen with accumulation of humus primarily due to the activities of higher plants. Topsoils deepen through soil mixing. As soils mature, they develop layers as organic matter accumulates and leaching takes place. This development of layers is the beginning of the soil profile.
There are two principal methods that the state equation may be solved: first in a theoretical or conceptual manner by logical deductions from certain premises, and second empirically by experimentation or field observation. The empirical method is still mostly employed today, and soil formation can be defined by varying a single factor and keeping the other factors constant. This led to the development of empirical models to describe pedogenesis, such as climofunctions, biofunctions, topofunctions, lithofunctions, and chronofunctions. Since Hans Jenny published his formulation in 1941, it has been used by innumerable soil surveyors all over the world as a qualitative list for understanding the factors that may be important for producing the soil pattern within a region.
The organisms living in and on the soil form distinct soil types. Coniferous forests have acidic leaf litter that form soils classed as inceptisols. Mixed or deciduous forests leave a deeper layer of humus and tend to form soils classed as alfisols. Prairies have very high humus accumulation, which, along with bioturbation can create a dark, thick A horizon characteristic of mollisols.
The location of a soil on a landscape can affect how the climatic processes impact it. The geomorphic effects of relief and topography determine how soil is moved, distributed and retained within a watershed and across the landscape. Soil material is carried to lower elevations by water and with gravity. Bottom lands and low lands that retain and accumulate deposited soil will be deeper and richer with organic matter than their comparable uplands.
All of the above factors assert themselves over time, often thousands of years. Soil profiles continually change from weakly developed to well developed over time. Chronosequences used in soil studies consist of sites that have developed over different periods of time with relatively small differences in other soil-forming factors. Such groups of sites are used to assess the influence of time as a factor in pedogenesis.
A variety of mechanisms contribute to soil formation, including siltation, erosion, overpressure and lake bed succession. A specific example of the evolution of soils in prehistoric lake beds is in the Makgadikgadi Pans of the Kalahari Desert, where change in an ancient river course led to millennia of salinity buildup and formation of calcretes and silcretes.