Formação de pedons em litossequência cretáceo-quaternária na mesorregião oeste potiguar

Abstract

The understanding of soil pedogenesis influences in more appropriate management practices. The Potiguar West Meso-region encompasses one of the main poles producing Brazilian fruit crops, since it presents soils of good natural fertility, but with their morphological, physical, chemical and mineral properties, depending on the nature of the source material and the edafoclimities conditions of the region. Wherefore, the objective of this study was the pedon formation in the Cretaceous-Quaternary lithoses, in the West Potiguar Meso-region. Five soil profiles between the municipalities of Baraúna-RN and Mossoró-RN were selected for the study, which were identified and collected for physical, chemical and mineralogical characterization. The analyzes consisted of granulometry, clay dispersed in water and soil density; The conditions of pH and KCl, electrical conductivity, available phosphorus, exchangeable bases, some of exchangeable acids, extractable acidity, calcium carbonate, total organic carbon (TOC), total iron (Fe2O3) and micronutrients; the mineral fraction of the argil soil, were indentified by means of an X-ray diffractometer. The profiles were classified in 4th categorical level as: CAMBISSOLO HÁPLICO Tb eutrófico típico (P1); CAMBISSOLO HÁPLICO Ta eutrófico típico (P2); CAMBISSOLO HÁPLICO Ta eutrófico lépticofragmentário (P3); CAMBISSOLO HÁPLICO Carbonático saprolítico (P4); LATOSSOLO VERMELHO Distrófico argissólico (P5). The evaluated soil profiles presented a wide variation in the textural classes. With regard to grain size, it was observed that in most of the studied profiles the sand content decreased with depth as the clay content increased, except for P2. The ADA contents were high and behaved in increasing depth, only in P5, low levels were observed among the profiles. In relation to the soil density, the profiles presented similar values, where the larger ones were present in the superficial horizons and the smaller ones in the subsurface horizons. The chemical analysis showed a basic character in most of the analyzed profiles due to the high pH values in H2O. EC values were low in all profiles, ranging from 0.02 to 0.10 dS m-1. The behavior of the TOC was decreasing in depth for all profiles. In P5, as a function of the low pH, an aluminum saturation between 20 and 39% was observed. Base saturation was high in most of the profiles, only in D5 the dystrophic character was observed. In P4, in the Bi horizon, the highest levels of calcium carbonate (254.24 g kg-1) were observed, assigning the carbonate character. The iron oxides contents were increasing except in P4. For Fe and Mn micronutrients, high surface contents were observed, except for P4. The ferro-manganous concretions were observed in all profiles of Cambisols and presented in the homocentric form of variable sizes. The mineralogical composition of the diagnostic horizons in P1, P2, P3 and P4 presented coincident peaks of clays of type 2: 1, Caulinite, Ilite, Goethite and Hematite. Already in P5 there is a significant change of the mineralogical sequence, since the change of the original material only in this profile. The results allowed to conclude that ferro-manganous concretions were observed only in limestone profiles. The different materials of origin in the formation of the pedons are preponderant for understanding the pedogenetic processes and the identification of the minerals of the clay fraction confirms the cretaceo-quaternary lithosquence existing in the region.