Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 62

Cerrado landscapes, these erosion surfaces have little effect on agriculture, since
the same erosion surface may encompass a range of altitudes, rock, and soil types.
However, the Gondwana surface is generally small in area and often composed of
quartzite ranges and plateaus ( Figure 3.2 ), where soils are too steep, unfertile, and
rocky for an arable land use. Thus, most agricultural lands in the Cerrado are located
on the South American, Velhas, and younger surfaces. This geomorphic setting
implies that topography can locally control soil properties in a predictable way: the
most extensive uplands are slightly dissected plateaus, where savannic formations
and occasional dry forests occur. On these flat or gently rolling surfaces, soils are
typically very well drained and very deep, and thus suitable for a large-scale agri-
culture and urban development. Drainageways along these surfaces comprise soils
that are usually higher in soil organic matter (SOM) and nutrients due to convergent
flow, and hydromorphic in ca. 20% of the area (Skorupa et al., in press), often poorly
drained, and mostly covered by riverine forests (Figure 3.2) or veredas (a vegeta-
tion consisting of wetland grasses and Mauritia spp. palm trees). The plateaus often
present sharp borders and steep slopes, where soils are shallow and gravelly, suitable
only for extensive grazing. Graphic models of typical topographies and landscapes
of the Cerrado are presented by Curi and Franzmeier (1984), Macedo and Bryant
(1987), and Marques et al. (2004).
Climate in neotropical savannas and the Cerrado is typically the Aw of the
Köppen classification, marked by a stark contrast between the wet and dry seasons
(García 2008). The data in Table 3.1 show mean annual temperature (MAT), mean
annual precipitation (MAP), and mean annual evaporation (MAE) for selected loca-
tions within the Cerrado. The MAT is negatively correlated with altitude and latitude
( r = -0.84 and -0.82, respectively), whereas MAP and MAE show a more complex
distribution. Frosts are very rare and occur only in the southernmost areas, never to
the point of freezing soil. MAP is always >1000 mm and exceeds total evaporation
in many areas. Since rains are generally concentrated between October and April
(Inmet 1992), they can be very intense and highly erosive, but short (~10 days) dry
spells are frequent during the growing season and can pose a severe risk to young
rainfed crops. The dry season occurs between May and September and is marked
by very low precipitation and relative humidity, leading to natural wildfires whose fre-
quency is increased by agricultural land use. The climatic conditions of the Cerrado
promote intensive weathering and leaching of bases from all parent rocks, resulting
in soils predominantly containing low-activity clays (kaolinite and Fe/Al oxides and
oxi-hydroxides). An important consequence of this is that most of the cation exchange
capacity (CEC) depends on humified SOM (Silva et al. 1994), which is also the case in
most savannas of West Africa (Carsky et al. 2000). However, climate also favors annual
cropping by allowing harvest during the dry season. Furthermore, almost all rivers and
streams maintain a baseflow even during the driest winters, favoring the cultivation of
two or more crops per year with supplemental irrigation of nearby areas.
Organisms were treated by Jenny (1941) separately as microorganisms, veg-
etation, and man. In this chapter, vegetation and soil fauna are emphasized due
to their strong effect on soil properties in the Cerrado region. Although the word
“Cerrado” is a Portuguese term for “closed” or “dense,” this vegetation is actually
a complex of plant formations varying from treeless grasslands ( Campo Limpo , or