| Alfisol | |
|---|---|
 An Alfisol profile | |
| Used in | USDA soil taxonomy |
| Climate | various |
Alfisols are a soil order inUSDA soil taxonomy. Alfisols form in semi-arid to humid areas, typically under ahardwood forest cover. They have a clay-enrichedsubsoil 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 inAustralia andAfrica are still very deficient innitrogen and availablephosphorus. Those inmonsoonal tropical regions, however, have a tendency to acidify when heavily cultivated, especially when nitrogenous fertilizers are used.
In theWorld Reference Base for Soil Resources (WRB), most Alfisols are classified asLuvisols orLixisols, but some are classed asRetisols orNitisols. Aqualfs are mainlyStagnosols orPlanosols. Alfisols with a natric horizon are mainlySolonetz.[1]
Alfisols occupy around one-tenth of the Earth's ice-free land surface. They are dominant in many areas, such as theOhio River basin in the United States,southern and unglaciatedWestern Europe, the Baltic region and centralEuropean Russia, the drier parts ofPeninsular India,Sudan in Africa, and many parts ofSouth 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 toUltisols, which are the more highly leached forest soils having less than 35% base saturation. In easternNorth America, Alfisols are commonly found in glaciated areas while Ultisols are restricted to the areas south of the limit of maximum glaciation.
Thefossil record of Alfisols begins in the LateDevonian. Probably owing to their fertility, they are the oldest forest soils; vegetation on weatheredOxisols, by contrast, is not known earlier thanMiddle Permian. Fossil Alfisols remain common from theCarboniferous and all periods since theEocene.[citation needed]
The fossil record of Alfisols extends back to the Late Devonian. The Alfisol paleosols were woodland soils and early forest soils. The fertile Alfisols were most likely formed by Devonian forests. The oldest of the Alfisol paleosol forest soils are in the paleosols of the Aztec Siltstone inVictoria Land, Antarctica. In the paleosols of the Aztec Siltstone, the Rosemary paleosol, there is evidence for trees from surficial tree-throwing structures and large drab-haloed root tracers. From the fossil wood and spores found in the Aztec Siltstone, there is a strong indication that the trees wereArchaeopteris-Callixylon.[2]
In upperNew 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 withprogymnosperm trunks are interpreted to not have grown in soils similar to the type of soils modern seed plants are grown in. Thetaxa of progymnosperm trunk fossils reproduced byspores. For reproduction, they were more dependent on moisture than modern seed plants. Comparing theWattieza, from New York Devonian paleosols, and theCallixylon'secotones 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 theWattieza it is inferred it was because there was more mean annual moisture available to theCallixylon.[3]
In the lowerWalton Formation, nearSidney, New York fossils ofArchaeopteris macilenta,Callixylon zalesskyi, andGeminospora lemurata have been found. The tapering geometry of large woody root traces is the most likely cause of the intimate relationship betweenArgillic horizons, 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.[4]
TheMata Amarilla Formation, a formation in theAustral Basin in southernPatagonia, Argentina is the location known for the preservation ofpodocarp-dominatedfossil 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 ofin 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 ofPodocarpxylon gargiae roots was found in the formation as well. The total meandiameter of these trees in the vertic Alfisol paleosol is greater than 60 cm and the smallest tree diameter is 26 cm.[5]
While Afisol paleosols are known for their woodland soils and fossils, other types of fossils have also been found. Theichnofacies of the ground-nesting bees,Celliforma, was formed in well drained Alfisols as well asAridisols andOxisols. TheTermitichnus ichnofacies, which included a large array of termite nests and pellets, is found in Oligocene Alfisols, Aridisols, and Oxisols. All thetetrapod bones and tracks from the paleosol sequences of Late Devonian andMississippian 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.[6]
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