IRON BANDS, FRAGIPANS AND DURIPANS IN THE NORTHEASTERN PLATEAUS OF BRAZIL - PROPERTIES AND GENESIS

Iron bands, fragipans and duripans are common in yellow low-activity c lay soils developed from the Barreiras Group in coastal plateaus of no rtheastern Brazil. Such indurated horizons are found in depressions of the plateaus where sugar cane growth is greatly reduced. Little resea rch has being done on the nature of their bonding agents and their gen esis. Research carried out in arid zones has frequently attributed dur ipan consistency to amorphous silica; however, duripans in northeast B razil occur under higher rainfall (1500-2000 mm). The objective of thi s work was to study the nature and distribution of soil features in a plateau to better understand the processes associated with soil harden ing. Two transects, across a small and a large depression, were invest igated using field soil characterisation, micromorphological technique s, and mineralogical and geochemical analyses. The formation of the in durated horizons studied is due to two Sequential processes: developme nt of aquic conditions and incipient podzolization. Such processes hav e affected the upper part of a thin (<0.6 m in small depressions) or t hick (>0.6 m in large depressions) compact clay horizon, showing horiz ontal plans likely inherited from the sediment. Onset of aquatic condi tions formed the first fragipans overlying iron bands. Later, aquic co nditions are combined with incipient podzolization to produce bleached loose horizons overlying duripans and iron bands. As the bonding agen ts in the duripans are organo-metallic complexes, these duripans appea r to be very different from those described in arid environments.