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Alfisols are a soil order in USDA soil taxonomy. Alfisols form in semi-arid to humid areas, typically under a hardwood forest cover. They have a clay-enriched subsoil and relatively high native fertility. "Alf" refers to aluminium (Al) and iron (Fe). Because of their productivity and abundance, Alfisols represent one of the more important soil orders for food and fiber production. They are widely used both in agriculture and forestry, and are generally easier to keep fertile than other humid-climate soils, though those in Australia and Africa are still very deficient in nitrogen and available phosphorus. Those in tropical regions, however, have a tendency to acidify when heavily cultivated, especially when nitrogenous fertilizers are used.
In the World Reference Base for Soil Resources (WRB), most Alfisols are classified as or , but some are classed as or . Aqualfs are mainly or . Alfisols with a natric horizon are mainly Solonetz.
Alfisols occupy around one-tenth of the Earth's ice-free land surface. They are dominant in many areas, such as the Ohio River basin in the United States, southern Europe and unglaciated Western Europe, the Baltic region and central European Russia, the drier parts of Peninsular India, Sudan in Africa, and many parts of South America.
Alfisols have undergone only moderate leaching. By definition, they have at least 35% base saturation, meaning calcium, magnesium, and potassium are relatively abundant. This is in contrast to Ultisols, which are the more highly leached forest soils having less than 35% base saturation. In eastern North America, Alfisols are commonly found in glaciated areas while Ultisols are restricted to the areas south of the limit of maximum glaciation.
The fossil record of Alfisols begins in the Late Devonian. Probably owing to their fertility, they are the oldest forest soils; vegetation on weathered , by contrast, is not known earlier than Guadalupian. Fossil Alfisols remain common from the Carboniferous and all periods since the Eocene.
In upper New York State, there have been Devonian trees found in a large variety of paleosols. These Devonian trees help give clues as to what ecological tolerances for various early trees looked like. The paleosols associated with progymnosperm trunks are interpreted to not have grown in soils similar to the type of soils modern seed plants are grown in. The taxa of progymnosperm trunk fossils reproduced by . For reproduction, they were more dependent on moisture than modern seed plants. Comparing the Wattieza, from New York Devonian paleosols, and the Callixylon to show this dependency on moisture it shows that the Wattieza lived in an ecotone of 571 ± 72 mm mean annual precipitation and the Callixylon lived in an ecotone of 611 ± 46 mean annual precipitation. Because the Callixylon grew taller than the Wattieza it is inferred it was because there was more mean annual moisture available to the Callixylon.
In the lower Walton Formation, near Sidney, New York fossils of Archaeopteris macilenta, Callixylon zalesskyi, and Geminispora lemurata have been found. The tapering geometry of large woody root traces is the most likely cause of the intimate relationship between , Alfisols, and forests. The large woody root traces can transport water and clay using their large pipes. They are efficient at moving soil into a subsurface horizon when the plant dies and the roots decay. The Mata Amarilla Formation, a formation in the Magallanes Basin in southern Patagonia, Argentina is the location known for the preservation of Podocarpus-dominated fossil forests. In the eastern area of the middle section of the Mata Amarilla Formation, there are vertic Alfisols. In these Alfisols, there is a preservation of fossil forests, whose minimum age is 337 years. The fossil forests in this section are made up of in situ trunks. These trunks, in life position, have shallow root systems with well-developed horizontal roots, and have minimal development of vertical roots. A log of Podocarpxylon gargiae roots was found in the formation as well. The total mean diameter of these trees in the vertic Alfisol paleosol is greater than 60 cm and the smallest tree diameter is 26 cm.
While Afisol paleosols are known for their woodland soils and fossils, other types of fossils have also been found. The ichnofacies of the ground-nesting bees, Celliforma, was formed in well drained Alfisols as well as and . The Termitichnus ichnofacies, which included a large array of termite nests and pellets, is found in Oligocene Alfisols, Aridisols, and Oxisols. All the tetrapod bones and tracks from the paleosol sequences of Late Devonian and Mississippian of New York and Pennsylvania, U.S.A. are found in Aridisols and Alfisols. The Alfisols in the Texas High Plains have evidence for fossilized pollen and beetles.
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