Agriculture Reference
In-Depth Information
INTRODUCTION
(IBGE, 1987), has a wide variety of soils. The
soil moisture regime (Soil Survey Staff, 1999) ranges from perudic to aridic, and the soil temperature
regime ranges from isohyperthermic to thermic. The geologic history indicates rocks ranging in
age from Pre-Cambrian to recent volcanic. Geomorphologically, a signiÝcant part of the country
is occupied by extensive peneplains. Much of the country is also part of the catchment of the
Amazon River, which has helped to shape the landscape, particularly in the north part of the country.
The volcanic Andes mountains aligned north-south on the western part of the continent have active
volcanoes, and pyroclastics have been added to many of the soils of the country. All of these factors
provide the complexity of conditions that has resulted in the array of soils.
The extensive planation surfaces are the sites of highly weathered soils, and as Brazil has the
largest contiguous extent of such soils in the world, international soil classiÝcation systems have
relied on Brazilian contributions toward the evaluation and classiÝcation of such soils. The soils
are called Latosols, a term introduced in the United States in 1949 (Cline, 1975). With the advent
of modern classiÝcation systems, they are now referred to as Oxisols in Soil Taxonomy, and
Ferralsols in the legend of the FAO-UNESCO (1974) Soil Map of the World. In the recent World
Reference Base for Soil ClassiÝcation (Nachtergaele et al., 2000), they are still referred to as
Ferralsols. Transitions to and from these weathered soils are also extensive, and thus the country
presents a natural laboratory for the study of such soils, particularly soils in their pristine environ-
ment.
Systematic surveys of Brazilian soils began in the decade of 1950, and with them the use of
soil classiÝcation criteria and principles developed in the United States by Baldwin et al. (1938),
Thorp and Smith (1949), and Cline (1949). Bennema and Camargo (1964) reviewed the existing
Brazilian soil survey data, and proposed a partial classiÝcation of Brazilian soils titled ÑSecond
Partial Draft of the Brazilian Soil ClassiÝcation.Ò Twelve soil classes were recognized at the highest
categorical level. The classes were deÝned based on the presence or absence of diagnostic horizons,
most of them equivalent to those of the 7
Brazil, with a land area of about 8,511,940 km
2
Approximation (Soil Survey Staff, 1960). An attempt
was made to develop the lower categories of the well-drained Latosol class on the basis of clay
activity, base saturation, Al extracted by KCl, type of A horizon, color, texture, amount of total
Fe
th
ratio. The nonhydromorphic soils with textural
B horizons were split into two classes at the highest level. One class was made up of soils with
clay activity of less than 24 me/100 g of clay and either aluminum saturation
O
, silica/alumina ratio, and alumina/total Fe
O
2
3
2
3
>
50% or base saturation
<
35% (approximately equivalent to Ultisols). The other classes were differentiated by having a
clay activity of
>
24 me/100 g of clay and either aluminum saturation
<
50% or base saturation
>
35% (approximately equivalent to AlÝsols). In this scheme, the diagnostic clay activity criteria
(cation exchange capacity, CEC, by NH
OAC/pH 7) does not include the CEC of the organic matter.
The Brazilian soil survey program was accelerated in 1965 in order to make an inventory of
Brazilian soils. A number of soil surveys at several levels have been completed and interpreted for
agricultural purposes. During this soil survey program, fundamental concepts and criteria of other
systems (Soil Survey Staff, 1975; FAO-UNESCO, 1974) were incorporated and adapted for devel-
oping the Brazilian soil classiÝcation approaches, along with several soil Ýeld special investigations
and technical meetings. This goal was reached with the publication of the Brazilian Soil Map at
scale 1:5,000,000 in 1981 (Servio Nacional de Levantamento e Conservao de Solos, 1981).
During the course of subsequent soil surveys, criteria and attributes for characterization of soil
classes were reÝned to cope with soil characteristics under varied environments. Phases of soil
classes were also devised to highlight limitations for land use, constraints for speciÝc uses, and
ecological relationship concerning potential land use.
On a Ýeld trip for special soil investigation and correlation, driving on a newly open gravel
road of about 800 km, from Porto Velho to Manaus in the Amazon region, a predominant occurrence
of soils with plinthite was observed. For this reason, it received a nickname of Plinthic Route by
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