Proposal for taxonomic revision of thick and highly weathered sandy soils in the Brazilian soil classification system

Highly weathered thick (> 150 cm) tropical soils with clay contents between 80 g kg− 1 and 150 g kg− 1 and low amount of silt (< 100 g kg− 1) occur predominantly on flat geomorphic surfaces, usually on summits. Due to these grain-size characteristics, such soils have weak or massive structure and are considered poorly developed in some classification systems (e.g. Soil Taxonomy of the USDA and WRB of the IUSS Working Group). On the other hand, according to the former French system, these soils are highly developed, comprising the tropical Ferrallitic soils. Inspired by the Entisol Order, which comprises the Soil Taxonomy from USA, the Brazilian System of Soil Classification creates the Neossolos Order and the following Sub orders: Quartzarênicos (Quartzipsamments), Regolíticos (Psamments), Fluvicos (Fluvents), and Litólicos (Lithic). Based on a literature review, we propose to assign Neossolos Quartzarênicos (Arenosols) the status of highly developed soils, due to their large thickness, absence or low to minimal contents (≤ 5%) of wheatherable minerals, absolute prevalence of kaolinite (≥ 70%) in clay size fraction, high Ki index (SiO₂/Al₂O₃), and low values of base saturation and silt/clay ratios (≤ 1%). Our proposal is to exclude clay content and structure as the criteria for soil classification, and to choose an adequate representative subsurface horizon, instead of the C horizon. Thus, it will be possible to assume this soil as a Latossolo Arenoso (Arenic Ferralsol) or to classify it under a new Order in the Brazilian classification system, Arenossolos (Arenosols), replacing the current category of the Neossolos.