Induration of ferrallitic microaggregated soils (Northeastern Brazil).

Studies of soils in the northeastern region of Brazil showed the widespread occurrence of hard subsurface horizons that strongly restrict the penetration of plant roots, water and ploughing tools, and that consequently reduce the agricultural yields. The objective was to characterise the groundmass of these hard horizons and to identify the cementing agent responsible for the hardness. Samples were collected from friable subsoil horizons which are latosolic horizons, and from hard subsoil horizons which are either fragipã or duripã according to the Brazilian soil classification. The mechanical stability of the groundmass was measured. Thin-sections were prepared and examined in scanning electron microscopy using backscattered electrons. The thin sections were also used for microprobe analysis. The results showed a great stability of fragipã and duripã when they are shacked in water. This stability clearly indicated that they are indurated horizons and related to duripans. The results showed also that the groundmass of the indurated horizons differed from friable latosolic horizons by the continuity of the solid phase. The clayey material responsible for the continuity consisted mainly of kaolinite with small variation of the aluminium and iron contents. The difference of consistence between the non-indurated and indurated horizons would be related to an increase in the aluminium content and a decrease in the iron content. Thus, aluminium compounds would play a major role as cementing agent within the clayey material.