Tableland soil of North-Eastern Brazil:characterization, genesis and classification

Abstract

The coastal low level tablelands of the State of Alagoas are located in the north-eastern region of Brazil between 8° 25' and 10° 20' south latitude, and from 35° 10’ to 37° 15' west longitude of Greenwich. These surfaces formed by Tertiary Sediments are flat or slightly undulated with slopes rarely higher than 4%. It enjoys a tropical climate with a dry summer and a cons­tant temperature throughout the year. The dominant vegeta­tion is formed by the so-called "Atlantic Forest", presen­ting also minor areas of "Cerrado". Nine profiles corresponding to the soil types present in, a) the well drained surfaces of the tablelands - which clas­sify as Oxisols, Ultisols and Oxisol-Ultisol intergrades -, and b) in poorly drained depressions - classified as Spodo-sols -, were selected for this work. The influence of the morphogenesis of the landscape and the topographical location on soil development is evident from the soils studied. The properties of the soils are strongly related to geomorphic processes. The soils were characterized in terms of their morphological, physico-chamical, mineralogical and micromorphological pro­perties. The sedimentological analysis of the sand fraction(63 - 2000ym) was also carried out. The mineralogy of the sand fraction is similar in all pro­files. The light minerals are dominant ( 91-98% ) and almost entirely composed of quartz grains and no more than traces of feldspars. In the heavy fraction the opaques are dominant, ( 84-92% ); among the transparent minerals zircon dominates, followed by tourmaline and rutile. The composition of the clay fraction is also very similar throughout the region, dominated by kaolinite and minoramounts of gibbsite and oxi-hydroxides of iron and aluminium. With the sole exception of profile 2, all the soils are developed from highly uniform parent materials. At the sametime the parent materials of the different soils are also mutually very similar from sedimentological point of view. " Selective erosion 11 of fine soil components as a conse­quence of run-off is throughout the plateaux the dominant morphogenetic process, which influences the physico-chemi­cal properties, the classification and the distribution of the soils in the landscape. The micromorphological analysis indicates similar features among the dominant soils of the area; and that the illuvia­tion of clay is present in all profiles, which however is not manifested as clay skins in the field as a result of poor structure development. The soils were classified according to Soil Taxonomy, FAO/UNESCO, the Brazilian System of Soil Classification and under the I.N.E.A.C. ( Zaire ) System.