Liquid and plastic limits of clayey, organic C-rich mountain soils: Role of organic matter and mineralogy

The soil liquid (LL) and plastic limit (PL), and their difference PI (plasticity index) are physical properties related with the soil mechanical behavior, determined on the < 0.425 mm soil fraction, and used in different fields of earth and soil sciences. They are known to depend on soil organic matter (SOM) and/or clay content, but these soil properties are generally determined on the < 2 mm fraction (fine earth). Furthermore, these dependencies can be affected by many qualitative aspects, such as the specific surface area (SSA) and the equivalent basal spacing (EBS) of clay minerals, and clay aggregation. The aims of our research were therefore i) to investigate the role of clay and SOM contents in a set of clay- and organic C-rich mountain soils, considering both the < 2 mm and < 0.425 mm fractions; ii) to assess the effect of the mineralogical composition (type of minerals, EBS and SSA) and clay aggregation in the < 0.425 mm fraction. The relationships observed between Atterberg limits and the clay and organic C amounts evidenced the importance of considering the < 0.425 mm soil fraction instead of the fine earth. The relative role of clay and organic C contents in the < 0.425 mm fraction was comparable for LL, while in the case of PL the most relevant variable was TOC. Therefore, in a first stage of increasing water content, the role of SOM is fundamental (i.e. in the semisolid state and plastic interval), then the interactions among clay particles dominate. No clear relationship was instead observed with the mineralogical composition, although SSA and EBS were both correlated positively with LL and PI. The aggregated clay amount was positively related with LL and PI, and EBS. This, together with the relationships with EBS and SSA, evidenced that a role of clay exists in determining the soil mechanical behavior, but it seems to be more related to the interactions among clay particles occurring in the soil, than to the simple mineralogical composition. The findings suggested that, while PL seems to be more controlled by the organic fraction, LL and PI are more influenced by SOM and clay minerals and their interactions.