Evolução de uma topossequência nas condições de semiaridez de Irauçuba-CE

Abstract

The municipality of Irauçuba is located in the northern sector of the state of Ceará, between the latitudes of 3o 35’and 4o 44’ S and the longitudes 39o 38’and 39o 63’W. Its physiographic characteristics are directly related with the semiarid climate, which influences the genesis and development of soils and its physical and chemical characteristics deriving from hydric deficiencies from the aridity conditions. The objective of this work was to comprehend the occurrence of ferruginous nodules in the interior of a soil of which characteristics differ from the soils whose mechanical pedogenesis predominate in the area. The results of this research, con- ducted by using Structural Analysis of Pedological Cover, evidenced the existence of a toposequence in which the soils of its upstream sector were orignated by processes of pedogenesis which favoured the formation of ferrouginous nodules in the interior of their matrixes and that did not occur with the soils of the downstream sector, where the pedogenesis was mechanical. The occurrence of these nodules may indicate.that, in the past, more humid climatic conditions acted in the studied area. The geometry of the downstream sector of the toposequence suggests that it was originated under these conditions of greater humidity, with ferruginous nodules. The characteristics of inferior horizons indicate that they were formed in situ under the conditions of actual semiarid climate, being covered by superficial layers originated from the allochthon material deriving from the superficial erosion of the upstream sector, a process that happened under the conditions of necessary hydric defficiency that did not favour the formation of nodules.