soils of more recent times that cannot clearly be classified as Alfisols or as Spodosols because, despite their sandy horizons, they are not nearly acidic enough to have the typical features of a Spodosol. Importance Paleopedology is an important scientific discipline for the understanding of the ecology and evolution of ancient ecosystems, both on Earth and the emerging field of exoplanet research, or Astropedology. [Section is currently under construction.] Models The different definitions applied to soils is indicative of the different approaches taken to them. Where farmers and engineers experience different soil challenges, soil scientists have a different view again (Johnson & Watson-Stegner 1987). Essentially, these differing views of the definition of soil are different theoretical bases for their study (Retallack 2001). Soils can be thought of as open systems in that they represent a boundary between the earth and the atmosphere where materials are transported and are changed. There are four basic types of flux: additions, subtractions, transfers, and transformations (Simonson 1978; Anderson 1988). Examples of addition can include mineral grains and leaf litter, while subtractions can include surficial erosion of minerals and of organic matter. Transfers include the movement of a material within soil profile, and transformations are the change of composition and form of the materials within a soil. Soils can also be considered to be energy transformers in that they are physical structures of material that are modified by naturally occurring processes. The sun constitutes the primary energy source for soils, and significantly outweighs any heat generated by radioactive decay flowing up from deep within the Earth's crust. The deposition of sediment, or the addition of groundwater or rain, can also be considered an energy gain because new minerals and water can alter preexisting materials within the soil. These processes, coupled with the amount of energy available to fuel them, are what create a soil profile. Another way to view soils is that they are environmental products that are molded over a period of time from the materials available to them. The large amount of influences that effect the formation of soils can be simplified to five main factors: climate, organisms, topographic relief, parent material, and time (Jenny 1941; Buol et al. 1997). These five factors can be easily remembered using the acronym 'CLORPT'. These categories are useful for mentally considering that aspects that occurred during the formation of a soil or paleosol. More importantly however,