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.
Oxisols: Low Activity Soils
Oxisols are sandy loam or finer-textured mineral soils with less than 10% weatherable minerals in the 50- to 200-micron sand fraction and low apparent cation exchange capacity (CEC) clay. If, after mixing, the surface 18 cm contains less than 40% clay, the clay content either does not increase with depth or increases so gradually that no kandic horizon is present and the upper boundary of an oxic horizon is present within 150 cm (60 in.) of the mineral soil surface, or if, after mixing, the surface 18 cm contains 40% or more clay either the upper boundary of an oxic horizon is present within 150 cm of the soil surface or the upper boundary a kandic horizon containing less than 10% weatherable minerals in the 50- to 200-micron sand is present within 150 cm (60 in.) of the surface.
Oxisol profiles have low bulk density, generally between 1.0 and 1.3 g cm3. Most horizons have strong fine and very fine granular structure. Macro pores formed between the strong granular peds provide for rapid hydraulic conductivity, much greater than normally predicted from clay content. Silt content is low and plant- available water-holding capacity is often between 0.05 and 0.15 cm cm3 of horizon thickness (Buol and Eswaran 2000).
Oxisols include many of the soils previously called Laterites, Lateritic, or Latosols (Baldwin et al. 1938; Thorp and Baldwin 1940; Thorp and Smith 1949; Varghese and Byju, 1993). However, not all soils previously classed as Latosols or Lateritic (Reddish Brown Lateritic, Yellowish Brown Lateritic, etc.) can be placed in the Oxisol order. Most Ferralsols (FAO 1988) are Oxisols, but some have a kandic horizon but less than40% clay in the surface 18 cm and classify as Ultisols or Alfisols. Many soils commonly called red tropical or subtropical fail the requirements of less than 10% weatherable minerals and low CEC of the clay fraction as defined by the oxic horizon and classify as Inceptisols.
Although almost all Oxisols occur between the Tropic of Cancer and the Tropic of Capricorn, it is important to recognize that many other soils are also present in that part of the world. Van Wambeke (1991) estimates that only 22.5% of the soils in the tropics are Oxisols. Oxisols are rare to absent in many tropical countries.
SettingMost Oxisols have isothermic or isohyperthermic soil temperature regimes although there are some Oxisols with hyperthermic, thermic, or isomesic soil temperature regimes (Opdecamp and Sottiaux 1983). Oxisols are known to occur in all soil
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moisture regimes, udic (53.0%), ustic (31.6%), perudic (11.82%), aquic (3.26%), and aridic (0.003%) (Soil Survey Staff 1999). The most extensive contiguous areas of Oxisols are in Africa and South America related to mid- to late-Tertiary geomorphic surfaces composed of material that has been subjected to surface-related weathering, eroded, transported, and re-deposited many times before coming to rest in its present location. The material within which Oxisols form on such surfaces is an accumulation of soil material composed almost entirely of minerals resistant to weathering. Some of the deposits may have stone lines of quartz and other resistant minerals, oxide-cemented gravel or a mixture of both. (See Figure 3.1.) Most often identification of individual layers is not possible because of mineral and particle size similarity. Geologically reworked and transported materials that contain appreciable quantities of weatherable minerals, such as glacially derived deposits, usually are not suitable for Oxisol formation regardless of climatic conditions (Jongen 1960; Ollier 1959; Ruhe 1956a).
Oxisols also form when easily weatherable materials, such as basic and ultra basic rocks, are exposed to warm humid conditions on stable surfaces of inactive volcanic islands and other geologically old volcanic areas (Buurman and Soepraptohardjo 1980; Beinroth 1982). These occurrences are generally of limited spatial extent and shown only on large-scale soil maps. A few Oxisols in the humid tropics and subtropics form on very stable geomorphic surfaces by slow transformation of the acid igneous rock into soil material with low activity clay over long geologic time periods.
One of the most extensive areas of Oxisols is the Sur Americana surface in central Brazil where ancient rocks were exposed to several episodes of weathering and erosion especially during the warm humid climates of Cretaceous and early Cenozoic time (Orme 2007). Most of that area has an ustic moisture regime and is vegetated with grasses and dwarf woody species commonly referred to as cerrado. On the same geomorphic surfaces, Quartzipsamments occupy extensive areas where the parent materials are too sandy to qualify as oxic horizons that must be sandy loam or finer texture (Lepsch and Buol 1988). It is common to have Oxisols on old fluvial terraces, pediments and other high lying erosion surfaces with Inceptisols, Ultisols, Alfisols, or even Mollisols on adjacent side slopes if mafic (basic) rock is exposed to soil formation (Figure 16.1). In Sierra Leone, Oxisols are present on young alluvial floodplains where sediment nearly devoid of weatherable minerals is being deposited (Odell et al. 1974).
Several studies in the upper Amazon basin indicate that Oxisols are not present in that area as had been anticipated from genetic considerations (Sanchez and Buol 1974). Although observations are sparse in these areas, it appears that Oxisols are confined to the lower Amazon basin and formed in material transported from the Guyana and Brazilian shields (Camargo et al. 1981; IBGE 2007). In the western portion of the Amazon basin, the sediments are derived from the Andean Mountains, contain more weatherable minerals, and Ultisols predominate with some Alfisols and Mollisols present in more basic sediments.
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Oxisols support a rather wide range of vegetation including tropical rain forest, scrub and thorn forest, semideciduous forest, and grassland savanna (Mohr and van Baren 1954; UNESCO 1961; Van Wambeke 1991).
Pedogenic ProcessesDesilication and concentration of aluminum and iron oxides are the major processes affecting the mineral components of well-drained Oxisols. Silicon loss is to be expected in the surface of almost all soils as rainwater infiltrates and moves through the surface layer. As infiltrated water moves downward in the soil, silicates are dissolved. The amount of silicon removed depends upon the residence time of the water around the silicate mineral, the type of silicate mineral, and the amount of surface area exposed (Wilding et al. 1977). Sand-sized quartz particles persist because of their small amount of surface area per unit weight. The loss of silicon results in almost complete decompo-sition of weatherable silicate minerals and 2:1 clay minerals, except the aluminum interlayer 2:1 and 2:2 intergradational (hydroxy interlayered minerals [HIM]) minerals. Kaolinite, and in some soils gibbsite and iron oxides, dominate the clay fractions of most Oxisols. Iron released from iron-bearing silicates by desilication accumulates as oxides. The citrate-dithionite extractable iron, often with considerable Al substitution, accounts for almost all of the iron in Oxisols (SMSS 1986; Fontes and Weed 1991a, 1991b).
Much desilication apparently takes place in the weathering crust near or at the rock surface in the initial material (Cady 1951). Although possibly active in present
A = OxisolsA = Oxisols (sandier than A)B = Alfisols and UltisolsC = Mollisols and InceptisolsD = Ultisols and AlfisolsE = Ultisols
Parent material legend
Recent colluvial deposits (Holocene),yellowish sandy loamSandy loam surficial noncalcareous redmassive deposits (Post-cretaceous)Gravel bedSubarkosic sandstone with calcareouscement (Bauru formation; Cretaceous)
Figure 16.1. Block diagram showing distributions of soils and parent materials in the vicinity of Echapora on the Occidental Plateau, So Paulo, Brazil. (Lepsch et al. 1977a)
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Oxisol profiles, extensive desilication appears to have taken place during repeated cycles of weathering, erosion, and transport from soil profile to profile over vast expanses of time (Orme 2007). Once formed, the inert nature of oxic soil material precludes many other pedogenic processes. When oxic soil material is transported and deposited on younger geomorphic surfaces, it retains its properties unless mixed with weatherable minerals from other sediment sources, loess, or volcanic activity. However, significant increases in CEC and bulk density have been measured in soils formed in oxic materials on the lower portion of slopes. This has been attributed to silicon enrichment by lateral subsurface water flow during brief periods of saturation without chemical reduction (Moniz and Buol 1982; Moniz et al. 1982).
In Oxisols that have, or in the past had, a fluctuating water table relatively near the soil surface, there is often some localized iron concentration (absolute accumu-lation of DHoore 1954) to form the red-and-gray mottled redoximorphic features and/or plinthite. Plinthite-like material has been designated as laterite, first by Buchanan (1807), with later studies also using the term lateritic iron oxide crust (Marbut 1930; Alexander and Cady 1962; Du Preez 1949; Maignien 1959; Prescott and Pendleton 1952; Sivarajasingham et al. 1962). If large amounts of plinthite have accumulated, it tends to form a continuous phase in the soil. If the solum above the plinthite is eroded, the plinthite is subjected to repeated wetting and drying and becomes indurate nodules and concretions of petroferric material. In some cases a petroferric contact is formed. Plinthite and petroferric formation is of minor extent in Oxisols and is also present in Ultisols and other soils. Petroferric material is often a rather prominent feature where iron-rich materials are exposed on eroding escarpments. Such exposures have often led the casual observers to overestimate the extent and significance of plinthite and petroferric material in Oxisol dominated landscapes. Petroferric material may subsequently erode from the escarpments andbe deposited as ironstone gravel stone lines in adjacent alluvial fans (Buol and Eswaran 2000).
Melanization and humification take place in all Oxisols. These processes are especially prominent and significant in those Oxisols with high rainfall. The high amount of biomass resulting from year-round warm temperatures also humifies and mineralizes rapidly, but a high content of organic carbon is maintained (Bennema 1974; Sanchez and Buol 1975). The organic carbon content of Oxisols is indirectly proportional to soil temperature (DHoore 1968). Mean organic carbon content to a depth of 1 meter in Oxisols has been found to be slightly greater than in Mollisols (Eswaran et al. 1993). Soil organic carbon contents in Oxisols have been found to have a positive linear relationship with silt plus clay contents (Lepsch et al. 1994; Zinn et al. 2007). In general, Oxisols are not as dark in color as other soils with similar organic carbon contents. It is probable that the redness of the iron oxides in Oxisol epipedons tends to mask the blackness of the organic carbon. Indeed, it is difficult if not impossible to judge the organic carbon content of Oxisols by color.
Gleization is an active process in Aquox that are saturated with water and reduced at some period or periods of time during most years. The gleization process often
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produces subsoil horizons with gray and red redoximorphic features that resemble plinthite but do not harden with repeated wetting and drying. Iron oxide content is low in Aquox with gray subsoil colors (SMSS 1986).
Pedoturbation appears extensive in some Oxisols. Faunal pedoturbation by insects and animals often appears to have disturbed the entire upper solum (Nye 1955; Watson 1962). The most active fauna in this process are termites that build numerous mounds using subsoil material. It should be noted that termite activity is not limited to Oxisols. The size and shape of termite mounds varies greatly throughout the world, but in the central plateau of Brazil, they characteristically are about 1 meter high and 50 cm in diameter. Termite mounds formed from poorly drained, gray-colored material clearly contrast from mounds formed from well-drained, red-colored material clearly locating the boundaries to be drawn during soil mapping.
Uses of OxisolsRelative to most other soils, most Oxisols are chemically infertile. Their degree of infertility is reflected in the native vegetation they support. Significantly greater contents of exchangeable Ca2+, Mg2+, and K+ as well as contents of extractable P, Zn, Cu, and Mn are present in surface horizons of soils with greater density and size of woody vegetation in the cerrado of Brazil (Lopes and Cox 1977a, 1977b). Even the more highly base saturated Oxisols that support dense forests have low total quanti-ties of calcium, magnesium, and potassium relative to most other soils. Phosphorus reacts with the aluminum and iron oxides present in the surface horizons of most Oxisols to such a degree that although total quantity may be rather great, it is very slowly available to plants.
Fast-growing crop plants seldom sustain satisfactory growth on most unfertilized Oxisols. Due to very low nutrient reserves in weatherable minerals, phosphorus retention by oxides and low CEC of Oxisols, practically all of the nutrients in the natu-ral ecosystems are within living or dead plant tissue. On the more fertile, perhaps best stated least infertile, Oxisols forest stands slowly accumulate plant essential nutrients in their biomass, and when the forest is cut and burned, enough nutrients are rapidly released from their organic biomass to allow subsistence farmers to harvest one or two food crops. The site is then abandoned and a natural succession of woody vegetation reestablishes because it is able to grow with a slower rate of nutrient uptake than crop plants. After several years the biomass of the natural vegetation acquires enough essen-tial nutrients that subsistence farmers can repeat the slash-and-burn process.
On the least fertile Oxisols that naturally support only savanna grasses and sparse, stunted woody vegetation...