Critérios taxonômicos para horizonte B espódico do Sistema Brasileiro de Classificação de Solos: revisão e ampliação da nomenclatura e das definições

Abstract

According to the Brazilian Soil Classification System (SiBCS), spodic horizons are defined by the illuvial accumulation of humified organic matter combined with aluminum, and it may or may not contain iron. However, the lack of limits of values for the diagnostic attributes makes it difficult to identify the spodic horizon, and it is essential to review and amplify the taxonomic criteria. In this sense, the objective is to review the taxonomic criteria for the spodic horizon in the SiBCS. Based on bibliographical research, profiles of Spodosols were reclassified according to the current version of SiBCS, and the information of the morphological, physical and chemical attributes was registered in a spreadsheet. The spodic horizons are predominantly sandy, acidic, and with a low sum of bases and base saturation. The lack of limit values of the attributes in the SiBCS allows for the identification of horizons that present alkaline pH, high bases saturation and very low content of C org, as in the profiles of Pantanal region. Thus, the minimum values of organic carbon content (C org) ≥ 5 g kg-1 and pH ≤ 5,9 are suggested for the definition of the spodic horizons. The Bs (m) horizons have lower C org values, corroborating with the SiBCS definitions. However, the lack of criteria for Fe oxides accumulation in the Bs (m) and Bhs (m) horizons makes it impossible to propose a consistent classification for this criteria. In addition, the lack of data on the extraction of pedogenetic oxides in most the spodic horizons of the profiles in the database prevents the determination of quantitative criteria of these attributes that might be implemented in future versions of SiBCS. The color patterns are shown to be functional to separate the Bs horizons from the others because they present color values and chroma ≥ 4, and C org contents lower than 20 g kg-1. The spodic horizons with value and chroma < 4 and without morphological evidence of illuvial accumulation of amorphous iron can be separated according to C org content in: Bh - accumulation of C org < 20 g kg-1; B ̅ - prominent accumulation of iluvial organic matter with C org ≥ 20 g kg-1. In the soils with a hystic (H) surface horizon, the spodic horizons have relatively low C org, whereas in the surface horizons with high levels of C org such as A humic and A prominent, and in the environments with better drainage the content of C org in subsurface is superior in relation to profiles with other surface horizons types. In the profiles under hydromorphic conditions, the spodic horizon starts slightly closer to the surface. In the Coastal Tablelands profiles, because the highest clay content of the sediments that originated the soils, there is greatest accumulation of organic matter in the spodic horizon. On the other hand, in the Amazon region profiles, the fluvial environment favors the losses of organic matter from the system and the equatorial climate leads to intense cycling and the least accumulation of C org in the spodic horizon. In the High Mountain Ranges condition, the influence of the Fe and Al oxides was much higher when compared to the other environments, possibly due to the higher amounts of Al and Fe released directly from the rocks. The SiBCS taxonomic classification of Spodosols is based on subjective criteria, and less discriminating in the separation of the individuals evaluated. Meanwhile, the proposal of classification - 2nd categorical level: Ferrilúvicos, Humilúvicos and Hiper-Humilúvicos; 3rd categorical level: Dúricos, Arênicos, Espessarênicos, Órticos - allowed the equitable distribution of profiles within the proposed classes.