Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel. 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
Alfisols: High Base Status Soils with Finer-textured Subsoil Horizons
Approximately 10% of the land area of the planet is occupied by Alfisols, which because of natural fertility, location in humid and subhumid regions, and responsive-ness to good management are widely used for agriculture and forestry. Alfisols occur under a wide range of environmental conditions and are found in temperate, tropical, and boreal regions of the world. The central concept of Alfisols is that of forest soils that occupy relatively stable landscape positions and thus have a subsurface zone of clay accumulation. In addition, base-rich parent materials or less-intense weathering and leaching regimes have resulted in subsoils that contain relatively abundant supplies of exchangeable calcium, magnesium, potassium, and in some cases, sodium.
SettingAlfisols are widely distributed globally and occupy approximately 13,156,000 km2 (USDA-NRCS database). They are found on every continent with the exception of Antarctica (Hallmark and Franzmeier 2000). Several prerequisites are met by soils of Alfisol-dominated landscapes. There is sufficient landscape stability to allow accumulation of enough layer-lattice clay (of any species) in the subsoil (often a Bt horizon) to form argillic, kandic, or natric horizons (Figures 8.1B and 8.1C). Alfisols form in parent materials to allow relatively high base (calcium, magnesium, sodium, and potassium) status, with base saturation greater than 35% in the lower part or below the argillic or kandic horizon and usually increasing with depth (Figure 8.1A). Alfisol profiles exhibit rather contrasting horizonation, which under deciduous forest typically include O, A, E, and Bt, with the possibility in various ecosystems of the presence of natric, petrocalcic, duripan, and fragipan horizons, and plinthite (Figures 8.2 and 8.3). Favorable moisture regimes provide available water to mesophytic plants more than half the year, or for three consecutive months in a warm season. Relatively little accumulation of organic matter occurs in mineral soil horizons (most organic matter is naturally cycled in the O horizons), particularly in cultivated areas (Figure 8.1B). Data illustrating these morphological, physical, and chemical properties for two Alfisols are presented in Table 8.1.
Alfisols are present on older landscapes (generally earliest Holocene or older) wherever ample supplies of primary minerals, layer-lattice clays, and available plant
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nutrients are abundant in parent materials. Original vegetation included broadleaf deciduous forest, both unmixed and mixed with needle evergreen forest (North America, Europe, China); grass with and without patches of broadleaf evergreen trees (Africa, South America, California); and broadleaf evergreen forest (Africa, India, Australia, California). Areas of transition between Alfisols and Mollisols are in ecotones between forest and grassland. Transitions between areas of Alfisols and Spodosols lie in ecotones between mixed deciduous and needle evergreen forest. Under warmer temperature regimes, areas of Alfisols are found between Aridisols of
Figure 8.1. (A) Curves for base saturation, by sum of cations, for six soils: Ultisols#1 Paleaquult (Profile #113, N.C.), #2 Fragiudult (Profile #117, Miss.), #3 Rhodudult (Profile #119, Tenn.); Alfisols#4 Glossaqualf (Profile #45, Wis.), #5 Hapludalf (Profile #50, Ill.), #6 Haplustalf (Profile Kaduna). The uppermost horizontal bar on each curve marks the upper boundary of the argillic horizon; the lowermost horizontal bar marks the level 125 cm below that. Dashed vertical lines show unsampled portions of profiles #4 and #6. Data for first five profiles are from Soil Survey Staff 1975; for profile #6, from Harpstead 1974. (B) Curves for clay and organic carbon (O.C.), Profile #50. (C) Six tracings of representative (not average) views of thin sections from the following horizons ( from top down; with depths of horizons in cm, and percent by volume of argillans) of a Hapludalf in Wisconsin (Buol and Hole 1961): E, 1425 cm, 0.03%; BE, 2548, 0.69%; Bt, 4878, 2.69%; BC, 78113, 3.15%; C1, 113148, 5.27%; C2, 148173, 2.05%. The key at lower right shows patterns to represent voids (white), soil matrix (non-clay-skin soil), and argillans (clay-skin).
201 2 43 5 6
0 1 2 3
Percent base saturation Percent clay
0 1 mm
0.079 mm(avg. thickness ofclay-skins in BC)
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Figure 8.2. Vitrandic Fragixeralf (Santa series) from northern Idaho. Soil has formed in loess and reworked silty sediments and supports forest dominated by grand fir (Abies grandis). A well-developed fragipan is present a depth of ~80 cm. Scale is in decimeters. For color detail, please see color plate section.
Figure 8.3. Mollic Hapludalf from Iowa County, Wisconsin. Soil has a mollic epipedon and argillic horizon; it has been leached free of CaCO
courtesy of Dr. Randy Schaetzl, Michigan State University) For color detail, please see color plate section.
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the arid regions and Inceptisols, Ultisols, and Oxisols of the more humid areas (Soil Survey Staff 1999). Transitions between Alfisols and Aridisols may also occur in ecotones between cool, temperate shrub/grassland and drier steppe vegetation in the western United States (Blank and Fosberg 1991).
In cooler regions affected by glaciation, Alfisols are generally found on late Pleistocene surfaces (Hallmark and Franzmeier 2000). Under warmer, subhumid and semi-arid conditions, Alfisols occupy older, stable landscapes that have undergone long-term weathering under fairly constant climatic conditions or are polygenetic, and have experienced climatic changes during the Pleistocene. Under humid, temperate climates, Alfisols may occupy most of the landscape except for very steep slopes, alluvial floodplains, and very poorly drained depressions. They also are common on the borders of depressions where slight concentrations of water have favored migration of sodium and clay to form natric horizons. Where Mollic Albaqualfs are associated with Mollisols in Illinois, their presence has been attributed to lower organic matter production caused by nutrient deficiencies associated with a detrimental moisture regime (Smeck and Runge 1971). Higher elevations, limited rainfall, and higher concentration of bases in parent rock favor Alfisol formation in the tropics (Guerrero 1963). Alfisols can also occur in highly weathered tropical landscapes on dissected side slopes where less-weathered parent materials are exposed(Lepsch et al. 1977b). These Alfisols exist in areas that have been geomor-phi cally rejuvenated on an otherwise highly weathered landscape (Schaetzl and Anderson2005).
Pedogenic ProcessesThe dominant processes in most Alfisols are those that produce subsurface horizons relatively enriched in layer silicate clays and those that cycle nutrients in upper horizons, mostly by littering and organic matter decomposition. Although the genetic concept of argillic and natric horizon formation emphasizes to some degree the process of clay translocation (lessivage), the formation of these horizons and the kandic horizon involves other processes as well. These processes include the selective loss of clay from A and E horizons by dispersion and lateral transport, dissolution of clays in A and E horizons and leaching of dissolved constituents, neosynthesis of clays in the B hori-zon from dissolved constituents, clay production in the B horizon by weathering of primary minerals, and residual concentration of clay by selective dissolution of more soluble minerals (e.g., loss of calcium carbonate in calcareous alluvium, loess, or till).
Translocation of clay from the A and E horizons into the B horizon, in an aqueous suspension (lessivage), with or without aid of complexing organic compounds, and possibly by migration of Si and Al and their later precipitation in the B horizon (neoformation), probably does play a major role in the formation of some argillic horizons and natric horizons (Thorp et al. 1957, 1959). Fine clays move more readily than do coarse clays, so the fine clay-to-total clay ratios are typically higher in the B horizon (0.60.8) than in the A and E horizons (0.30.6 [Isbell 1980]). This distinguishes
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argillans from rapidly formed alluvial coatings, called gleyans by Brammer (1971), in certain Fluvents. Scarcity of argillans in the upper B horizon (Figure 8.1C) is considered to be a result of their destruction at a faster rate than that of their formation. Shrink-swell cycles (by both freeze-thaw and wet-dry processes) (Nettleton et al. 1969), soil creep, and biologic mixing (Vepraskas and Wilding 1983) are more intense in the upper horizons where argillans are few, and where they are largely embedded inside peds (Figure 8.1C). Interfingering of albic material into the argillic or natric horizon, and in extreme cases the formation of a glossic horizon, are examples of the destruction of the upper part of argillic or natric horizons (Ranney and Beatty 1969), and disruption of not only argillans, but of the entire clay-enriched soil fabric. Grainy ped coatings observed in the upper part of the argillic horizon of degrading Udolls (Arnold and Riecken 1964) may represent a first stage in this process.
Deposition of clay, often with sesquioxides and organic matter, in the argillic horizon may be brought about by (1) depletion of percolating waters via sorption by peds, (2) swelling shut of voids and consequent slowing of percolating water, (3)sieve action by clogging of fine pores, and (4) flocculation of the negatively charged clay by positively charged iron oxides in the Bt horizon or by calcium in the higher-base-saturation lower solum. The clay may have originated in eluvial (A or E) horizons, may have formed by crystallization of soluble constituents produced by weathering elsewhere in the pedon, or may have been produced locally by weathering in the B horizon. Conditions for significant formation of argillans in Alfisols may berelatively rare, with intensely rainy periods following prolonged droughts being especially con-ducive. The laminar structure of some argillans, visible in thin sections, reflects epi-sodic deposition of mineral and organic fines.
Considering a ped of the argillic horizon as a pedologic unit cell, we may expect that the microclimate is different in the interior from that on the ped surface (Heil and Buntley 1965; Buntley and Westin 1965). An argillan acts as a barrier to penetration of water, nutrients, and growing roots as well as to the movement of soil fauna into and out of the ped (Khalifa and Buol 1969).
The evolution of pedons of Paleustalfs in southeastern Australia (Walker and Chittleborough 1986) illustrates the formation of A and E horizons that are largely composed of silt and sand in the inorganic fractions and B horizons that have become more bimodal in texture as clay has been added to the original material. With progressive blocking of voids by illuvial clay, seasonal waterlogging has become more frequent in the upper solum. Weathering, a process that began at or before time zero, has become predominant over translocation, although this interpretation is complicated by evidence of eolian addition to soils of the region (Walker et al. 1988) and by possible stratification of the initial geologic material.
Munks (1993) study of a sequence of soils formed from 2000 to 200,000 year-old alluvium in Californias Central Valley also suggests that a number of mechanisms are responsible for clay accumulation in the Alfisols (Figure 8.4). Haploxeralfs form in latest-Pleistocene to early Holocene deposits by illuviation and in situ mineral weathering. Plugging of voids in deeper horizons causes clay to accumulate above the
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zones of slower permeability (in the initial stages, the argillic horizons grow from the bottom up). The slower permeability also favors the in situ weathering of primary minerals directly to clays...