Paleosol

In soil science, paleosols (spelt palaeosols) in Great Britain and Australia) can have two meanings. The first, used in continents where present-day soils are very young due to glaciation, is simply that of a former soil preserved underneath sedimentary bedrock such as alluvium. Such artefacts can be more accurately referred to as soil fossils.

More generally, paleosols are soils formed long periods ago that have no relationship in their chemical and physical characteristics to the present-day climate or vegetation. Such soils form - apart from small scattered localities in outliers of ancient rocks - on extremely old contimental cratons. Because of the changes in the Earth's climate over the last fifty million years, soils formed under tropical rainforest (or even savanna) have became exposed to increasingly arid climates which cause former Oxisols, Ultisols or even Alfisols to dry out in such a manner that a very hard crust is formed. This process has occurred so extensively in most part of Australia as to restrict soil development - the former soil is effectively the parent material for a new soil, but it is so unweatherable that only a very poorly developed soil can exist in present dry climates, especially when they have become much drier during glacial periods in the Quaternary.

In other parts of Australia, and in many parts of Africa, drying out of former soils has not been so severe. This has led to large areas of relict podsols in quite dry climates in the far southern inland of Australia (where temperate rainforest was formerly dominant) and to the formation of Torrox soils in southern Africa. Here, present climates allow, effectively, the maintenance of the old soils under climates which they could not actually form if one were to start with the parent material on which they developed in the Mesozoic and Paleocene.

Paleosols in this sense are always exceedingly infertile soils, containing available phosphorus levels orders of magnitude lower than in temperate regions with younger soils. Ecological studies have shown that this has forced highly specialised evolution amongst Australian flora (Tim F. Flannery, The Future Eaters: An Ecological History of the Australian Lands and People; published 1994 by George Braziller) to obtain minimal nutrient supplies. The fact that soil formation is simply not occurring makes ecologically sustainable management even more difficult. However, paleosols often contain the most exceptional biodiversity due to the absence of competition (David Tilman; Resource Competition And Community Structure; published 1982 by Princton University Press).

Paleosol

See also