Espodossolos in Brazil: a review of the criteria and conceptualization of the spodic B horizons.

ABSTRACT - The absence of quantitative limits for the diagnostic attributes that define spodic B horizons in the Brazilian Soil Classification System (SiBCS) allows the inclusion of soils in the Espodossolos (the equivalent of Spodosols or Podzols) classes that contradict the classical concepts that define the podzolization process, by including horizons with low organic carbon (Corg) content, and with alkaline pH, high sum of bases and sodic or solodic characteristics. This study aimed to propose quantitative criteria and limits to identify spodic B horizons and to contribute to the hieraquical structure of SiBCS. Morphological, physical and chemical attributes were defined after analyzing a large set of spodic B horizons of profiles classified as Espodossolos in the SiBCS available in the literature. In total, 385 spodic B horizons were identified. From this total, they were identified as following: 93 as Bs(m), 127 as Bh(m) and 165 as Bhs(m). In terms of color, the main hues were 7.5YR and 10YR, while the value and the chroma were ≥4 in Bs(m), <4 in Bh(m) and a wide variation in Bhs(m). The means and medians of the Corg contents are 7.5 and 6.7 g kg-1 for Bs(m), 19.8 and 15.1 g kg-1 for Bh(m), and 19.0 and 14.5 g kg-1 for Bhs(m). When the limit of Corg content is established as ≥3.0 g kg-1, 90 % of the spodic horizons are included in this group, with 85 % of the horizons for Corg ≥4.0 g kg-1 and 81 % of the horizons for Corg ≥5.0 g kg-1. The pH(H2O) had 94 % of the spodic B horizons with values ≤5.9. Around 48 % of the spodic B horizons have Alo and Feo data and, by calculating the equation Alo + 0.5 Feo , around 47 % of the horizons have a value ≥0.25 %. The evaluation highlighted the potential for adopting the minimum Corg ≥5.0 g kg-1 and pH(H2O) ≤5.9 as quantitative limits for defining the spodic B horizon in the SiBCS. It is also suggested to apply a lower requirement for the Corg content in spodic horizons with very low clay content that is sand texture classes, using the values of Corg ≥3.0 or ≥4.0 g kg-1 as limits. Otherwise, the minimum Corg value proposed is 5.0 g kg-1 and the types of spodic B horizons could be differentiated using a second limit of Corg, distinguishing the Bs with a content of Corg between 5.0 and 15.0 g kg-1 and Bsh ≥15.0 g kg-1.