Paleopedology (palaeopedology in the United Kingdom) is the discipline that studies soils of past geological eras, from quite recent (Quaternary) to the earliest periods of the Earth's history. Paleopedology can be seen either as a branch of soil science (pedology) or of paleontology, since the methods it uses are in many ways a well-defined combination of the two disciplines.
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Paleopedology's earliest developments arose from observations in Scotland circa 1795 whereby it was found that some soils in cliffs appeared to be remains of a former exposed land surface. During the nineteenth century there were many other finds of former soils throughout Europe and North America. However, most of this was only found in the search for animal and/or plant fossils and it was not until soil science first developed that buried soils of past geological ages were considered of any value.
It was only when the first relationships between soils and climate were observed in the steppes of Russia and Kazakhstan that there was any interest in applying the finds of former soils to past ecosystems. This occurred because, by the 1920s, some soils in Russia had been found by K.D. Glinka that did not fit with present climates and were seen as relic of warmer climates in the past.
Eugene W. Hilgard, in 1892, had related soil and climate in the United States in the same manner, and by the 1950s analysis of Quaternary stratigraphy to monitor recent environmental changes in the northern hemisphere had become firmly established. These developments have allowed soil fossils to be classified according to USDA soil taxonomy quite easily with all recent soils. Interest in earlier soil fossils was much slower to grow, but has steadily developed since the 1960s owing to the development of such techniques as X-ray diffraction which permit their classification. This has allowed many developments in paleoecology and paleogeography to take place because the soils' chemistry can provide a good deal of evidence as to how life moved onto land during the Paleozoic.
Remains of former soils can either be found under deposited sediment in unglaciated areas or in extremely steep cliffs where the old soil can be seem below the young present-day soil. In cases where volcanoes have been active, some soil fossils occur under the volcanic ash. If there is continued deposition of sediment, a sequence of soil fossils will form, especially after the retreat of glaciers during the Holocene. Soil fossils can also exist where a younger soil has been eroded (for instance by wind), as in the Badlands of South Dakota. (One must exclude areas where present-day soils are relics of former wetter climates, as with Australia and Southern Africa. The soils of these regions are proper paleosols.)
Soil fossils, whether buried or exposed, suffer from alteration. This occurs largely because almost all past soils have lost their former vegetative covering and the organic matter they once supported has been used up by plants since the soil was buried. However, if remains of plants can be found, the nature of the soil fossil can be made a great deal clearer than if no flora can be found because roots can nowadays be identified with respect to the plant group from which they come. Patterns of root traces including their shape and size, is good evidence for the vegetation type the former soil supported. Bluish colours in the soil tend to indicate the plants have mobilised nutrients within the soil.
The horizons of fossil soils typically are sharply defined only in the top layers, unless some of the parent material has not been obliterated by soil formation. The kinds of horizons in fossil soils are, though, generally the same as those found in present-day soils, allowing easy classification in modern taxonomy of all but the oldest soils.
Chemical analysis of soil fossils generally focuses on their lime content, which determines both their pH and how reactive they will be to dilute acids. Chemical analysis is also useful, usually through solvent extraction to determine key minerals. this analysis can be of some use in determining the structure of a soil fossil, but today X-ray diffraction is preferred because it permits the exact crystal structure of the former soil to be determined.
With the aid of X-ray diffraction, paleosols can now be classified into one of the 12 orders of Soil Taxonomy (Oxisols, Ultisols, Alfisols, Mollisols, Spodosols, Aridisols, Entisols, Inceptisols, Gelisols, Histosols, Vertisols and Andisols). Many Precambrian soils, however, when examined do not fit the characteristics for any of these soil orders and have been placed in a new order called green clays. The green colour is due to the presence of certain unoxidised minerals found in the primitive earth because O2 was not present. There are also some forest soils of more recent times that cannot clearly be classified as Alfisols or as Spodosols because, despite their sandy horizons, there are not nearly acidic enough to have the typical features of a Spodosol.
Paleopedology is a very important discipline today for the understanding of the ecology of ancient ecosystems because it gives a clue as to what soils conditions past animals and plants were required to live under and how the plants obtained essential nutrients.
In geochemistry, a knowledge of the structure of former soils is also valuable to understand the composition of certain continental rocks laid down many years ago.