Mineralogy and genesis of Planosols under a semi-arid climate, Borborema Plateau, NE Brazil

Abstract

Planosols, such as those found on the Borborema Plateau, NE Brazil, form an important class of moderately weathered soils that support subsistence agriculture in underdeveloped regions of Africa, Asia, and South America. Despite its socioeconomic importance, the genesis of this soil class has not yet been fully elucidated. This study was conducted with the aim of improving knowledge about the genesis of Planosols under a semi-arid climate. Three Planosol profiles on the Borborema Plateau in NE Brazil were collected, classified, and described morphologically, and the main physical and chemical attributes, the pedogenetic forms of iron oxides, and the mineralogical compositions of the sand, silt, and clay fractions were determined. The results indicate that: (a) parent materials richer in feldspar and biotite favor the formation of Planosols with a higher clay content; (b) the dissolution of primary minerals and the formation of clay minerals (known as the argillation process) are fundamental for the genesis of Bt horizons; (c) depth-drainage deficiency leads to hydromorphism, an associated gray color, and the formation of mottles; and (d) monosialitization and bisialitization processes (kaolinite formation and the formation of 2:1 clay minerals, respectively) prevail in Planosols located within landscape positions that have good and poor drainage, respectively. In addition, the present study shows that Planosols under a semi-arid climate can have a clay fraction consisting predominantly of kaolinite, and mineralogical alteration pathways similar to those described in soils under an analogous climate and geology in Northwest Africa; this refutes classical theories that soils with 2:1 clay minerals prevail in warmer and drier regions. This study thus provides greater knowledge about the attributes of Planosols in dry and poor regions and can be used as a reference in studies on the sustainable use and management of such soils.