Abstract
The Apodi Plateau, located in the northeastern part of the state of Ceara in Brazil, has limestone as its parent material, with significant variation in the soils formed in this region, as well as the presence of iron nodules, having been identified. The iron oxides have been used as indicators for evaluating the weathering processes of some soils, with the nodules being sources of information about pedogenic processes. Seeking to evaluate the influence of the karstic parent material and of the iron nodules on soil genesis in the Apodi Plateau, the morphological, physical and chemical properties of five profiles were evaluated, as well as of the iron nodules present in some horizons. The profiles were classified as RED-YELLOW ARGISOL Abruptic eutrophic plinthosol (P1), Tb HAPLIC CAMBISOL Eutrophic latosol (P2), RED-YELLOW ARGISOL Abruptic eutrophic plinthosol (P3), Tb HAPLIC CAMBISOL Eutrophic latosol (P4) and RED ARGISOL Eutrophic nitosol (P5). Changes in chemical, physical and morphological characteristics were observed at depth in some profiles, suggesting variation in the characteristics of the parent material. Nodules of class C1 (0.053 to 0.25 mm) and C2 (0.25 to 2.00 mm) were concentrated on the surface, alternating with a sub-surface concentration of class C3 (2.00 to 4.76 mm), C4 (4.76 to 7.90 mm) and C5 (7.90 to 19.1 mm). Profile P5, considered as more evolved, showed a predominance of classes C1, C2 and C3 in the surface horizons, indicating degradation of the larger-diameter nodules with the advancing weathering process.
Highlights
Iron oxides can be used as pedogenic indicators as they make possible a better understanding of the evolution of the processes of weathering, being sensitive to variations in the environment, mainly to changes in the redox conditions of the soil, and so allow us to infer changes in the past forms of landscapes (COELHO; VIDAL-TORRADO, 2003; INDA-JUNIOR; KÄMPF, 2003).The segregation and accumulation of iron can take place in many ways, being generally related to mottling, nodules, concretions and petroplinthite, among other classifications (CONSTANTINI; PRIORI, 2007)
Colour is an important aspect of soils derived from limestone, and is used as one of the defining elements of the classification ‘Terra rossa’, employed for soils which have their origins in calcareous materials, with a clayey texture and reddish colours of varying hues ranging from 5YR to 10 R, and which are found in the Mediterranean region (AYDINALP; FITZPATRICK, 2009), being related to soils influenced by such iron oxides as hematite or goethite
The soils have depths exceeding 150 cm and display well-developed A and B horizons, indicating an evolving pedogenesis; with the exception of profile P3 there was no contact with the parent material of these soils
Summary
Iron oxides can be used as pedogenic indicators as they make possible a better understanding of the evolution of the processes of weathering, being sensitive to variations in the environment, mainly to changes in the redox conditions of the soil, and so allow us to infer changes in the past forms of landscapes (COELHO; VIDAL-TORRADO, 2003; INDA-JUNIOR; KÄMPF, 2003).The segregation and accumulation of iron can take place in many ways, being generally related to mottling, nodules, concretions and petroplinthite, among other classifications (CONSTANTINI; PRIORI, 2007). The formation of iron accumulated through nodules is influenced by seasonal variations in the water table (TAN et al, 2006), where materials having diffuse internal structures are formed by the continuous supply of iron. These materials can provide information about very common pedogenic processes in the soils of tropical and subtropical regions (CONSTANTINI et al, 2006; HUANG et al, 2008). Colour is an important aspect of soils derived from limestone, and is used as one of the defining elements of the classification ‘Terra rossa’, employed for soils which have their origins in calcareous materials, with a clayey texture and reddish colours of varying hues ranging from 5YR to 10 R, and which are found in the Mediterranean region (AYDINALP; FITZPATRICK, 2009), being related to soils influenced by such iron oxides as hematite or goethite. Soils with similar characteristics, developed in other areas and originating from calcareous materials have been documented, under semi-arid climates (KHORMALI et al, 2003)
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