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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350 Soil Genesis and Classification
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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352 Soil Genesis and Classification
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, slash-and-burn management is not possible. The scenario of events that have taken place over a 30-year period in the cerrado of Brazil provides an overview of potential Oxisol use by humans. In 1965 there was almost no human habitation in the cerrado, and farmers attempting agriculture by slash-and-burn techniques were discouraged by negligible yields (Wright and Bennema 1965). Inability to sustain indigenous human populations is a characteristic of the least fertile
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Oxisols where the vegetation is so nutrient poor that large natural fauna are rare. When cattle were introduced to graze on native cerrado vegetation, their bones deteriorated from calcium and phosphorus deficiency unless supplemental mineral concentrates were supplied.
As research provided a clear understanding of the chemical limitations of Oxisols in the cerrado and in areas where economic stability enabled infrastructure necessary for commercial agriculture, an entirely different relationship of human interaction became possible. With initial applications of lime and phosphate, mixed as deeply as possible to maximize a favorable rooting depth, food crops could be successfully grown. Small amounts of zinc and copper were needed on some sites. The initial investment in rather massive amounts of phosphate fertilizer to overcome the fixation by iron and aluminum and lime to neutralize the acidity often exceeded the purchase price of the land and expense of clearing the cerrado vegetation. These initial applica-tions had residual effects and can be viewed as capital investments. In subsequent years, annual applications of nitrogen, phosphorus, and potassium fertilizer needed to replace nutrients exported in the harvested crop and lime to maintain pH values are no greater than on any other soil growing similar crops.
By 1992, with only 10 million hectares of the estimated 204 million hectares of Oxisol-dominated cerrado cultivated, 28% of the grain production in Brazil was from the cerrado area (Lopes 1996), and beef production was rapidly increasing on fertilized pastures. With stability of markets, it became economically feasible to make initial applications of phosphate fertilizer to overcome the natural acidity and phosphate fixation capacity of the most infertile Oxisols. Facilitated by the low CEC, calcium rather rapidly moves downward within Oxisol profiles, especially when applied as gypsum, and replaces the exchangeable aluminum acidity chemically enhancing a deeper root zone (Ritchey et al. 1980).
During 8 years of soil analysis-based fertilizer and lime management on forested sites of Xanthic Hapludox soils near Manaus, Brazil, yields on 1 hectare equaled yields from 24 hectares under shifting cultivation (Cravo and Smyth 1997). Lepsch et al. (1994) monitored the long-term effects of moderate to high input farming on 77 sites, paired to represent natural and farmed Perox, Ustox, and Ustox soils in Sao Paulo, Brazil, and found no significant decreases of organic carbon in the 020 cm depth. Slight decreases of organic carbon, significant at the 0.05% level were found at the 60100 cm depth in Acrudox and Acrustox soils. Significant increases in exchangeable Ca2+ and base saturation percentage with decreases in Al3+ and K+ were detected in the subsoil (60100 cm) of farmed sites in all the Udox and Ustox soils.
Many Oxisols afford advantages to mechanized commercial grain production that are not available on many other kinds of soil. Many Oxisol-dominated landscapes are nearly level thus facilitating the use of large equipment. Road construction is facilitated by physical stability and rapid permeability of the strongly granular structured low activity silicate clay and iron oxide. Low indigenous populations in the cerrado region facilitated acquisition of large management units for efficient mechanized agriculture. Oxisols with ustic and udic SMRs and isothermic and isohyperthermic STRs have
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reliable rains during at least one growing season each year followed by a period of dryness as grain crops mature. A dry, or drier, season each year decreases the risk of grain spoilage, decreases cost of drying harvested grain, and permits mature grain to more fully dry prior to harvest allowing for maximum efficiency of harvest equipment and marketing infrastructure.
Kellogg and Orvedal (1968) clearly saw the potential of Oxisols and prophesized that they constitute the largest reserve of uncultivated soils available for development to meet world food needs. With the development of infrastructure to market harvested crops and finance lime and fertilizer purchases, farmers with both technical and business ability are now utilizing thousands of hectares of Oxisols for soybean, wheat, corn, and coffee growing and beef production on improved pastures in the cerrado of Brazil. Taking advantage of the favorable soil structure and nearly level topography, they now utilize the largest farm equipment available and most modern technologies in their successful operations.
Classification of OxisolsOnly a limited range of Oxisols are represented in the United States, and the Soil Survey Staff (1975) recognized that additional information was needed to improve the classification of Oxisols. From 1977 to 1986, an international committee for improving the classification of Oxisols (ICOMOX) collected data and involved soil classification experts around the world in workshops and field trips. In 1987 ICOMOX proposed several changes in the classification of Oxisols that were adopted in Soil Taxonomy (1999). Five suborders using soil moisture regime criterion are recognized in the Oxisol order: Aquox, Torrox, Ustox, Perox, and Udox (Figure 16.2).
The most extensive Oxisols have ustic or udic soil moisture regimes, the Ustox and Udox, respectively. The Perox suborder was established to group those Oxisols that in normal years experience more precipitation than potential evapotranspiration every month of the year. With a perudic soil moisture regime, it is difficult to harvest crops that require drying. The lack of a dry season hampers slash-and-burn farming systems that require a dry period for the complete burning of biomass to release adequate amounts of nutrients and obtain good crop growth. In other soil orders, soils with a perudic soil moisture regime are grouped with udic soil moisture regime soils. Soils with aquic soil moisture conditions are present within most areas of Oxisols and classified in the Aquox suborder. (See Figure 16.3.) Limited areas of Oxisols with an aridic soil moisture regime have been identified in the Torrox suborder. The wide range of soil moisture conditions serves to illustrate the significance of parent mate-rial almost devoid of weatherable minerals in the genesis of Oxisols regardless of ambient climatic conditions.
Table 16.1 outlines the great groups of Oxisols. Particular note should be taken of the Eutr great groups. These are Oxisols, formed in sediments derived from mafic rocks that have more than 35% base saturation (CEC
7) in all horizons to a depth of
125 cm. Some Eutrustox may have nearly 100% base saturation throughout their
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356 Soil Genesis and Classification
Figure 16.3. Photo of a Fine, kaolinitic isohyperthermic Aeric Haplaquox profile in the Federal District of Brazil. Characterization data from this site is published on page 658 of Soil Taxonomy (Soil Survey Staff, 1999). For color detail, please see color plate section.
Figure 16.2. Diagram showing some relationships among suborders of Oxisols.
profiles (Moura et al. 1972). Although initial crop growth may be very good on these soils, the total quantity of essential bases is limited by low CEC and near absence of weatherable minerals. Sustained harvest of crops rapidly diminishes the limited quan-tity of essential bases, and productivity rapidly decreases unless the soil is fertilized.
The Acr great groups (Figure 16.4) identify Oxisols with an apparent ECEC of less than 1.50 cmol kg1 of clay in some subsoil horizon within 150 cm of the surface. Some horizons have no apparent ECEC, and some have a net positive charge. Although this may seem to be a disadvantage, research and farming practices have found low
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Table 16.1. Suborders and great groups in the Oxisols order
Suborder Great Groups
Aquox Acraquoxapparent ECEC less than 1.5 cmol kg1 clay and KCl pH of 5 or more within 150 cm of the surface
Plinthaquoxcontinuous plinthite within 125 cm of the soil surfaceEutraquox35% or higher base saturation (CEC
7) in all horizons within 125 cm
of the surfaceHaplaquoxother Aquox
Torrox Acrotorroxapparent ECEC less than 1.5 cmol kg1 clay and KCl pH of 5 or more within 150 cm of the surface
Eutrotorrox35% or higher base saturation (CEC7) in all horizons within 125 cm
of the surfaceHaplotorroxother Torrox
Ustox Sombriustoxsombric horizon within 150 cm of the surfaceAcrustoxapparent ECEC less than 1.5 cmol kg1 clay and KCl pH of 5 or more within
150 cm of the surfaceEutrustox35% or higher base saturation (CEC
7) in all horizons within 125 cm
of the surfaceKandiustoxmore than 40% clay in surface 18 cm and the upper boundary of a kandic
horizon within 150 cm of the surfaceHaplustoxother Ustox
Perox Sombriperoxsombric horizon within 150 cm of the surfaceAcroperoxapparent ECEC less than 1.5 cmol kg1 clay and KCl pH of 5 or more within
150 cm of the surfaceEutroperox35% or higher base saturation (CEC
7) in all horizons within 125 cm
of the surfaceKandiperoxmore than 40% clay in surface 18 cm and the upper boundary of a kandic
horizon within 150 cm of the surfaceHaploperoxother Perox
Udox Sombriudoxsombric horizon within 150 cm of the surfaceAcrudoxapparent ECEC less than 1.5 cmol kg1 clay and KCl pH of 5 or more within
150 cm of the surfaceEutrudox35% or higher base saturation (CEC
7) in all horizons within 125 cm
of the surfaceKandiudoxmore than 40% clay in surface 18 cm and the upper boundary of a kandic
horizon within 150 cm of the surface Hapludoxother Udox
ECEC in the subsoil to be an advantage because surface applied Ca2+ and other basic cations can rapidly move down in the soil. This encourages deeper rooting and there-fore a greater supply of moisture during rainless periods that may occur during the growing season.
The Kandi great groups include soils with surface horizons when mixed to 18 cm that contain more than 40% clay and significant increases of clay content in the subsoil, that is, a kandic horizon that contains less than 10% weatherable minerals. The rationale for including such soils in the Oxisol order is that the initial amount of
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phosphate fertilizer needed to overcome fixation is directly related to the amount of surface area, that is, clay content in the cultivated layer. Most soils now in these great groups were formerly classified as Ultisols but are spatially associated with Oxisols and have management requirements more closely related to Oxisols than to most Ultisols.
The mineralogy in Oxisols is very restricted by the order definition, and only a limited number of mineralogy families are used in Oxisols. (See Chapter 7.)
A reaction family, Allic, is used to identify Oxisols that have more than 2 cmols of KCl extractable Al per kg soil in a 30-cm layer above 150 cm. Oxisols in Allic families have more extractable aluminum than most Oxisols and therefore require greater amounts of lime to counteract aluminum toxicity. This is an economic disad-vantage when uncultivated Oxisols are first prepared for agricultural production.
PerspectiveOxisols are nonsandy mineral soils containing few weatherable minerals and low CEC. The most extensive areas of Oxisols are present in sediments that have been reworked during several cycles of erosion and deposition during which most silicate minerals, except quartz and 1:1 lattice clay, have been destroyed and oxides of iron and aluminum have been concentrated. Present soil moisture regimes range from aridic to perudic, suggesting that most Oxisols result because of the strongly weath-ered, nearly inert composition developed in the parent material prior to pedogenic
Figure 16.4. Photo of a very-fine, gibbsitic, isohyperthermic Typic Acrustox profile located in the state of Gois, Brazil. For color detail, please see color plate section.
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conditions at the present site. It can be reasoned that if the parent material consists of only quartz, kaolinite, and iron oxide, few pedogenic processes are possible, and a soil formed in such material will have Oxisol properties regardless of present climatic conditions at the site.
Except for the more fertile Oxisols, that is, the Eutr great groups, most support only sparse human populations subsisting with shifting cultivation techniques and low intensity grazing. With external sources of fertilizer, highly intensive plantation agriculture (mainly sugarcane, bananas, pineapples, and coffee) has long been suc-cessful in limited areas, and there can be little doubt that with continued improvement of infrastructure, many Oxisols can become some of the most productive soils in the world. The recent expansion of soybean, wheat, corn, coffee, and beef production on Ustox and Udox in central Brazil is an example of what can be done. Clearly intensive food crop production on even the most naturally infertile Oxisols, such as the Acr great groups, is no longer limited by lack of sustainable agronomic technology. However, lack of economic infrastructure such as roads, railroads, storage facilities, and readily available technical services such as soil testing, limits production within many regions of Oxisols (Wade et al. 1988; Lopes 1996).
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