Agriculture Reference
In-Depth Information
THE RATIONALE OF THE SYSTEM
The rationale of the Brazilian system follows the current understanding of soil genesis. The
knowledge of the Brazilian soil scientist regarding soil genesis under equatorial, tropical, and
subtropical conditions has grown, but even so, the soil scientists agree that using the effects of the
soil genesis expressed by soil morphology is basic for land suitability appraisal for agricultural and
non-agricultural purposes. Grouping soils in terms of morphogenetics attributes and properties is
a good mechanism for building, fence-like, around a track of land in order to get somewhat
homogenous and similar individual soil classes. The differences in relation to other international
systems arise because of the emphasis placed on speciÝc attributes from the point of view of use
and management. The system is still highly biased toward agricultural use of soils.
There are some fundamental changes that have been introduced since our previous version, and
also with respect to the international systems. Unlike other systems, we have preferred to evaluate
soils by using the degree of expression and relative importance of diagnostic horizons that are
readily identiÝed or inferred in the Ýeld. This follows the mental process of the Ýeld soil scientist
who looks for diagnostic horizons and, in general, an ease of recognition.
The orders have been set apart based on the presence or absence of diagnostic horizons,
attributes, and properties believed to be the expression(s) of predominant(s) soil-forming process(es)
or mechanism(s). The logic for distinguishing the fourteen orders will be sketched brieÞy.
The Ýrst order to be keyed out, the Organossolos, is not a mineral soil, but rather organic soils,
distinguished from the others by having a high content of organic matter at several decomposition
stages, and developed under environments saturated with water for long periods. Thereafter is
followed by mineral soil classes. The second order, the Neossolos, consists of mineral soils with
low horizonation, a minimal expression, or no diagnostic horizons. This is followed by an order
in which the soil shows intense internal stresses because of its special mineralogical composition:
the Vertissolos. The vertic properties are considered a special expression of the cambic horizon,
though cambic horizons are expressions of an initial stage of soil formation. The next soils keyed
out, the Espodossolos, are characterized by having a subsurface E horizon with a subsurface horizon
of illuvial accumulation of organic matter and aluminum, with or without iron. They are followed
by the Alissolos, with high content of aluminum, and Planossolos, with an abrupt textural change.
The Gleissolos is the next order which is saturated with water for long periods, and shows evidence
of reduction processes.
These are followed by groups of soils showing expressions of intensive weathering processes,
Nitossolos and Latossolos, unique to the tropics and extensive in Brazil. The Nitossolos show the
blocky structure of the B horizon with shiny ped faces that are either clay coatings or pressure
faces. In some respects, these are with properties transitional to the Latossolos, but they are ÑbetterÒ
soils than the Latossolos and, for this reason, we have made the deÝnition of the Latossolos more
restrictive than Soil Taxonomy. The latosolic B horizon must be more than 50 cm to be diagnostic
for the Latossolos. At the same time, we tried to ensure that the charge is because of the mineral
colloid, and not to organic matter. We have also included a factor (silt/clay ratio) that is an indicator
of the weathering intensity. This ensures that the low activity is less inÞuenced by the parent material
and is essentially due to intense weathering. The Latossolos are weathered, generally with low
fertility, excessively to moderately well drained, with deep solum generally thicker than 2 m. They
are most common on Þat to gentle slopes on old geomorphic surfaces, and very extensive in Brazil.
The Chernossolos keyed out next are characterized by thick organic-rich surface horizon, solum
high in base saturation and B horizon showing high-activity clay with expressive amounts of
calcium, with or without accumulation of calcium carbonate. The next group of soils, the Cambis-
solos, is characterized by a subsurface incipient B horizon showing higher chroma and redder hue
than the underlying horizon. These are followed by Plintossolos, a group of soil having plinthite
as predominant feature. The next group, the Luvissolos, comprises soils lacking chernozemic A
