• Ferralsols represent a final stage of geochemical weathering processes involving the desappearance of easily weatherable primary minarals. • Small amounts of 2:1 phyllosilicates have been identified within Ferralsols, raising questions about their origin. • The presence of 2:1 phyllosilicates throughout the soil appears to result from the upward transport by termites from the bottom of the soil.. • Part of potassium present in these phyllosilicates is exchangeable but depletes rapidly once the soil is cultivated. In the humid tropics, Ferralsols can be considered as being the end of a geochemical sequence of weathering. Easily weatherable primary minerals should then have completely disappeared; consequently, their fine fraction which is largely dominated by low-activity clay minerals, and aluminum and iron sesquioxides. However, the occurrence of small amounts of 2:1 phyllosilicates has been recorded in many Ferralsols. Since the presence of these minerals in these intensively weathered soils has never been the subject of a comprehensive study, we conducted an analysis of the literature from the past sixty years. This has shown that, since the first studies, most of the 2:1 phyllosilicates recorded in Ferralsols have been identified solely by X-ray diffraction, thanks to a peak at approximately 1.4 nm that did not expand upon solvation with ethylene glycol and became a broad peak between 1.2 and 1.0 nm when heated to 300°C. These XRD features are similar to those observed for 2:1:1 phyllosilicates with incomplete Al-hydroxy interlayering. The name given to these minerals has evolved over the years as knowledge about phyllosilicates advanced. It was shown that these 2:1 phyllosilicates, often interpreted as hydroxy-Al interlayered minerals, were mainly present in the fine-silt and coarse-clay fraction. They were found in Ferralsols of the South-American, African and Asian intertropical zones. Although their possible presence in very small proportions has gradually been integrated into the definition of Ferralsols or equivalent soils according to the soil classification systems used, their origin remains debated. While some authors interpret them as resulting from neoformation processes within the soil or as being relict minerals that have resisted weathering processes, a growing number of authors interpret their presence, without calling into question the two previous hypotheses, as resulting from soil the mixing of by the by activity of social soil insects, in particular that of termites. It is hypothesized that material transported from the saprolite seeded the entire Ferralsol profile with 2:1 phyllosilicates, formed either through neogenesis or primary mineral weathering. These minerals remain observable today where soil fauna have redistributed them, and undergo aluminization and desilication consistent with the geochemical context to which they are exposed. Finally, studies on the availability of potassium potentially present in the interlayer space of the 2:1 phyllosilicates indicate that 5 to 30% of this potassium can be exchangeable when the Ferralsols considered are still under native vegetation. Once cultivated, this reserve of exchangeable potassium reserve is depleted within a few years, and subsequent annual inputs from the remaining potassium stock are very low.
Bruand et al. (Wed,) studied this question.