| The Fragile Tropics of Latin America: Sustainable Management of Changing Environments (1995) |
|Part 4 : The semi-arid north-east|
|White sand soils in north-east Brazil|
4 White sand on the Conde upland, Paraíba
The Conde upland, part of the higher tabuleiro surface, with an altitude of about 110 metres, is located about 15 km south of João Pessoa (fig. 11.4). Most of it has been deforested and converted to farming or left as a low thicket.
The results of field survey on this upland were reported by Matsumoto and Watanabe (1986). This report and subsequent results undergird much of what follows. The distribution of white sand on the Conde upland (fig. 11.5) shows a distinctive pattern: (1) the white sand alternates horizontally with brown-coloured sandy soil (red yellow podzolic soil), here called simply "brown sand;" (2) the two kinds of sand reveal a clear-cut boundary; (3) the white sand occupies a slightly depressed portion of the upland; (4) a dissecting valley follows the downslope of the white sand zone; (5) springs are found near the head of the dissecting valleys.
Richards (1952) suggested that tropical rain-forest rivers flowing out from the white sand area, such as the Rio Negro in Amazonia, always carry "black water." The spring water of the Conde upland
Table 11.1 Chemical properties of spring water of the Conde upland, at W-1 - W- 4 in figure 11.5
|Location no.||Water temperature(°C)||pH||Electrical conductivity(µS/cm)|
|W-4||26 0||4.20||38 4|
Compared to the grain size of brown sand, the white sand is slightly coarser in mean diameter and better in sorting measure, i.e. the mean diameter in phi units for the white and brown sands is 1.467 (0.36 mm) and 1.664 (0.32 mm), respectively; the sorting measure (standard deviation in phi units) is 0.786 and 1.108, respectively (fig. 11.6). These characteristics are explained by the paucity of silt and clay fractions in the white sand.
Concerning chemical properties (fig. 11.7), the hydrogen ion concentration of soil solution is low (pH = 4.0 to 5.0) in both soils, and the concentration of elements such as sodium (Na), calcium (Ca), potassium (K), and magnesium (Mg) in soil solution is generally poor. Iron (Fe) and aluminum (Al) concentrations in soil solution appear as mere traces in the case of white sand, being high in the upper portion of the brown sand. The content of carbon (C) is low in white sand and moderate in brown sand; that of nitrogen (N) is low in both sands.