Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 66

other properties: (1) deep weathering and effective rooting depth (always >1 m, often
>4 m); (2) low bulk densities (average 0. 9 g cm -3 , sometimes as low as 0.7 g cm -3 ),
especially in clayey topsoils with granular structure and large packing voids ( Figure
3.3 ), easing soil preparation (Araujo, M.A. et al. 2011); (3) perfect aeration and inter-
nal drainage in most cases, resulting from properties 1 and 2; and (4) general absence
of gravels and abrasive materials. On the other hand, the most severe soil-related
constraints of Cerrado Ustoxes have been technologically alleviated: (1) high acidity
and aluminum saturation, corrected by adequate liming; (2) low basic cation cor-
rected by liming in topsoil and by gypsum in subsoil; (3) strong sorption of phos-
phates minimized by localized fertilization, especially in no-till systems; (4) low N
levels enhanced by symbiotic nitrogen fixation by including legumes in rotation; and
(5) low micronutrient levels corrected by judicious liming and fertilization. There
are two other consequences of the low bulk density and granular structure that can
be mitigated but not fully corrected, which are low volumetric water holding capac-
ity of <10% between -6 and -100 kPa (Resck 1998) and strong susceptibility to gully
erosion resulting from low soil strength and weak cohesion of granular peds (Curi,
N., personal communication; Figure 3.3).
3.1.3 Q uartzipSaMMentS and S andy o xiSolS
These soils are primarily derived from sandstones, either in Cretaceous or recent
deposits, and to a lesser extent from Proterozoic quartzites. In the Brazilian System,
Quartzipsamments are classified as Neossolos Quartzarênicos (formerly Areias
Quartzosas ) and cover ~15% of the Cerrado (Adámoli et al. 1986). Almost all of
these soils occur on flat lands, are very deep, and are well drained, except for low-
lands with aquic moisture regimes. Clay contents are necessarily <15%, and their
color ranges from light gray to light yellow or red, depending on the proportion of
goethite and hematite concentrations. Although they are generally acidic and low in
basic cations, these soils have moderate fertility when derived from sandstone/basalt
strata (common in the Paraná basin). Unlike Ustoxes and Ustults, these soils do not
have strong P fixation. Therefore, these soils have been cultivated for annual crops
such as soybeans in many Cerrado areas. Nonetheless, arable land use causes severe
depletion of CEC and SOC. In western Bahia, 5 years of soybeans cultivation with
heavy harrows decreased SOC and CEC in Quartzipsamments by 73% and 61%,
respectively, whereas these losses were 45% and 29% in clayey Haplustoxes (Silva et
al. 1994). These severe losses are caused by several factors: (1) low antecedent SOC
contents in comparison with loamy and clayey soils (Zinn et al. 2005a); (2) occur-
rence of about half of the total SOC in the sand size fraction as particulate organic
matter (POM; Zinn et al. 2007a), which is more readily decomposable than humified
SOC; (3) none, low, or weak aggregation because these soils lack cementation by
clay, and thus, the few large aggregates are easily disrupted and release occluded
POM, which is rapidly oxidized; (4) the low clay fraction in these soils can have very
high SOC contents (Zinn et al. 2007a), but SOC sorption capacity is near satura-
tion and SOC can be lost even with low-intensity land use systems such as eucalypt
plantations (Zinn et al. 2002, 2011). On the other hand, some Quartzipsamments
under native Cerrado vegetation are so low in SOC (<0.7% at the 0-5 cm depth) that