"Horizontes cimentados em Argissolos e Espodossolos dos tabuleiros costeiros e em Neossolos Regolíticos e Planossolos da depressão sertaneja no Nordeste do Brasil" / "Cemented Horizons in ultisol and spodosols from the coastal tablelands and in inceptisols and alfisols from the "sertaneja" depression of Northeast Brazil"

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

The present study was carried out in cemented horizons in Ultisols and Spodosols from the coastal tablelands, and in Inceptisols and Alfisols from the sertaneja depression of Northeast Brazil. The emphasis was concentrated on the cemented horizons developed in smooth depressions in the coastal tablelands. The main objective was to identify the cementing agents and to characterize horizons considered to be duripan and fragipan, and to infer the pedogenic processes involved in their formation. The morphological, micromorphological, physical, chemical, and mineralogical features of the cemented horizons were described. The identification of the cementing agents was achieved with selective extractions of the amorphous phases using ammonium oxalate, Tiron and sodium pyrophosphate. Detailed studies were carried out with an electron microscope. In the coastal tablelands region, the results indicated that the principal cementing agents are aluminum compounds, found to be amorphous hydrated aluminosilicates, and secondarily, organo-metallic complexes. The significant contents of amorphous iron compounds associated with organo-metallic complexes were found to cement thin ferruginous layers (placic horizon). The overall geochemical mass balance indicated the accumulation of aluminum in the cemented horizons composed mainly of quartz and kaolinite. Weakly cemented horizons were classified as fragipans. The more strongly cemented horizons were separated into duric, ortstein and placic horizons, according to their principal cementing agents. The traditional use of the term duripan seems to be inappropriate in the case of horizons in which the principal cementing agents are aluminous, such as those occurring in this region. In the smooth depression domain, the main mechanisms of the formation of these horizons are moderate podzolization, clay translocation, and the development of hydromorphic conditions, although temporary. In contrast, the main cementing agents in the sertaneja depression region are silicon compounds, always accompanied by aluminum. Strongly cemented horizons developed in Inceptisols have the characteristics of duripan. The weakly cemented horizon thicker than 10 cm within an Alfisol profile was classified as fragipan. The geochemical mass balance showed no silica accumulation in the cemented horizons. The mineralogical composition of these horizons includes kaolinite, quartz, feldspar and small amounts of 2:1 clay minerals. The investigations indicated that the relative proportions of fine fractions and cementing agents were the main factors in determining the degree of cementation. However, it was only possible to study these proportions in more detail in the cemented horizons of the soils formed on the coastal tablelands.

ASSUNTO(S)

ortstein duripan cimentações pedogenéticas fragipan placic horizon duripã fragipã ortstein horizonte plácido duric horizon pedogenic cementation horizonte dúrico

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