Os Solos de Brasília e suas possibilidades de aproveitamento agrícola
Resumo
This article contains a series of observations concerning the area of what is now the Federal District and the capital of the whole country, especially as regards the determination of the main broad groups of soils. These studies have also proved useful as a contribution to geological mapping of the region in question, above all where the deep-reaching decomposition of the rocks makes it difficult to recognize the geological formations at first sight.
The strong folding of the strata has led to variety in the rocky outcrops, resulting in alternation of the bedrock for soil formation. Likewise erosion and transport of the weathered material have produced separation of the country rock from the corresponding soil.
Though the arrangement of the strata may have promoted deeper weathering in some areas, it may be admitted that in different stages of decomposition they are more closely conditioned to the ages of the various erosion cycles.
The rocks of major importance as regards the area of outcrop are, in decreasing order, slate, phyllite, quartzite, micaschist and limestone. A table inserted into the text shows the various kinds of rock and the corresponding decomposition.
A summary appreciation is made of the time, cllmate and vegetation factors of the area with a view to soil formation in the region of Brasília. Thus, the oldest soils are the latosols, predominating on the series of plateaux above the 3,900 foot line (1,200 metres). With these soils, intense leaching is responsible for the advanced stage of laterization. The other soils, related to the present erosion cycle, are represented by regosols, lithosols and alluvial soils.
The climate of the region has not changed much since the Upper Tertiary, for the laterization levels are nearly always related to the surface levelling in that period and the formation of laterite demands a climate similar to the present one.
There is a correlation between the capacity of the soil to retain water and the type of plant cover. Thus the cerrado, a type of vegetation intermediate between scrub and savannah, which predominates on the plateaux, occurs on latosoil areas, whereas the campo limpo, grassland with only scattered trees, is to be found chiefly on areas of shallow, impervious rego-lithosols.
The denser formations-forests and others-occur in areas where gravity accumulation of water takes place and insolation is not so prolonged producing less evaporation.
The principal broad groups of soils in the region af Brasília are represented by latosols, which .are the most common and cover the widest area, rego-lithsols, mediterranean soils and alluvial soils, all af them descrlbed by Feuer in 1954 with the exceptian of the mediterranean soils. In a few small areas af the region under survey, podsolic soils were observed.
Comments are made on each soil and altogether 15 profiles are given, accompanied by morphological descriptions and tables af analytical data.
The floadplain soils, in view of their importance for the development of farming, were the object of special attention and three types were distinguished; the organic anuvial soils, the slightly organic soils and the relatively undeveloped soils or recent lluviums with a low content of organic matter.
In the course of this survey the question of laterite and laterization, tackled by Feuer at an earlier date, has also been gone into.
The outcrop areas of anuvial soils (the three types) and mediterranean soils of calcareous origin are marked on an accompanying map drawn approximately to the scale of 1:220,000. They are the soils that offer the greatest possibilities af farm utilization. As to the accurrence of the rego-lithosoils, they are shown together with the mediterranean soils on insets to the scale of 1:100,000.
The latosoils are the least useful from an agricultural point of view, but with the appli cation of certain measures indicated in this study they could be made productive.
