Microstructural development in volcanic ash soils from South Chile

Abstract

Streaming potential measurements and amplitude sweep tests were performed on a range of Andosols from South Chile to find out about aggregation strength and mechanisms, as well as thixotropy and microstructural stability. Different stages in soil development and types of land use (pasture vs. natural forest) were considered and compared. Younger, alu-andic Andosols along with Al-humus-complexes contained higher levels of volcanic glass, indicated by their low pH values (pHH2O<5), and high ratios of sodium pyrophosphate extractable Al (Alp) to acid ammonium oxalate extractable Al (Alo) (Alp/Alo>0.5). Alu-andic Andosols were also found to have (super)hydrophobicity in addition to typically high contents of amorphous iron (ferrihydrite). On the contrary, in well-developed, sil-andic Andosols (Alp/Alo<0.5), allophane contents increased, accompanied by higher pH values (pHH2O>5 and <7). Based on rheological data, the gel–sol–gel transformation (thixotropy) was better defined in sil-andic Andosols. Integral z, a dimensionless rheological parameter that represents quasi-elasticity was used to quantify stiffness degradation, and to identify single stages of thixotropy in allophanic Andosols. Scanning electron micrographs revealed that mineralogical components were predominated by volcanic glass (e.g. vesicular) and amorphous iron oxides in addition to fungal hyphae in younger acidic Andosols, while these characteristics were absent in well-developed Andosols. Here weathered minerals e.g. biotite, as well as halloysite, (proto)imogolite and allophanes were identified, indicating desilication and a shift from alu-andic to sil-andic Andosols. Zeta potentials derived from conducted particle charge density (PCD) measurements supported the assumptions that (i) Al-humus-complexes promoted aggregation in partially superhydrophobic topsoils of alu-andic Andosols, especially at ungrazed sites (or 1 year under pasture), and that (ii) thixotropic behaviour is related to allophanic, sil-andic Andosols, and better pronounced at sites which had been under pasture for 50 years.