Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 65

3.1.2 o xiSolS
Oxisols are by far the dominant soil order in Brazil and in the Cerrado region, where
they cover about 46% of the total land area (Adámoli et al. 1986). The reasons for
such dominance are multifold: (1) high MATs and MAPs favoring extreme weather-
ing and intense leaching of bases; (2) Oxisols can develop from most parent rocks,
except quartzite and purely siliceous sandstone; (3) Oxisols typically develop on
flat or gently rolling uplands, which predominate in the Cerrado; and (4) signifi-
cant bioturbation by soil fauna, which homogenizes the entire soil profile. In the
Cerrado, Oxisols develop under ustic soil moisture regime, and most are classified
as Haplustoxes and Kandiustoxes. However, Cerrado Ustoxes are in fact highly vari-
able on the basis of principal soil-forming factors. According to the Brazilian Soil
Classification System (Embrapa 2006), Oxisols correspond to the order Latossolo (a
term often translated as Latosol ), in which all suborders are defined in terms of soil
color (e.g., dusky red, yellow-reddish, and yellow, in decreasing order of hematite
content). Thus, Ustoxes vary considerably in color as a consequence of different
proportions of the basic minerals (quartz, kaolinite, Fe/Al oxides), but other impor-
tant differences also occur simultaneously. When kaolinite is the dominant mineral
in the clay fraction, a face-to-face arrangement of clay domains is favored and the
structure tends to be blocky. Conversely, when goethite, hematite, or gibbsite pre-
dominates, a granular structure predominates (Ferreira et al. 1999). Oxisols with
blocky structure and high bulk density due to predominantly kaolinitic clay also
present much higher soil strength than granular, gibbsitic Oxisols with low bulk
density (Ajayi et al. 2009). Texture in Ustoxes is also highly variable, ranging from
a minimal clay content of 15% to as high as 85%. Silt content is generally <20% and
often <10%, except where siltstones are the parent material.
Although Ustoxes can develop from almost any parent material, the nature of
the rock has a clear effect on their properties. Ustoxes developed from granites and
gneisses typically present a yellow-reddish hue and sand (mostly quartz) contents
of ~40%, whereas those derived from basalts and other mafic rocks are redder and
with much less sand, which can be rich in magnetite (strongly attracted to a hand
magnet). Ustoxes derived from sandstones and shales differ strongly in texture
(respectively, coarse or fine), and their color is highly variable from dusky red (drier
climate, higher position on the landscape, or strong internal drainage) to a much
lighter yellow-reddish, where climate is wetter or on footslopes (Curi and Franzmeier
1984; Macedo and Bryant 1987). Many Ustoxes developed from sediments or basalt
have small contents of ironstones or lateritic fragments. As a general rule, Ustoxes
are acidic and low in plant nutrients, but when derived from mafic rocks, they can
be somewhat richer in P and other nutrients. Ustoxes derived from limestones and
carbonate-laden sediments can contain significant illite (clay mica) and traces of
interstratified 2:1 minerals (Zinn et al. 2007a). However, since these carbonatic sedi-
ments are very old, and weathering is advanced, most derived Ustoxes are acidic and
low in bases, but still more fertile than those developed from other rocks.
Ustoxes are primarily used for the production of grains, cattle, and forest plan-
tation in the Cerrado region. Their high suitability for agricultural land use is not
only due to their areal extent and favorable topography but also because of a set of