Rheological Parameters as Affected by Water Tension in Subtropical Soils

Abstract

Rheological parameters have been used to study the interaction between particles and the structural strength of soils subjected to mechanical stresses, in which soil composition and water content most strongly affect soil resistance to deformation. Our objective was to evaluate the effect of water tension on rheological parameters of soils with different mineralogical, physical, and chemical composition. Surface and subsurface horizons of four Oxisols, two Ultisols, one Alfisol, and one Vertisol were physically and chemically characterized; their rheological parameters were obtained from amplitude sweep tests under oscillatory shear on disturbed soil samples that were saturated and subjected to water tension of 1, 3, 6, and 10 kPa. In these samples, the rheological parameters linear viscoelastic deformation limit (γ L ), maximum shear stress (τ max ), and integral z were determined. By simple regression analysis of the rheological parameters as a function of soil water tension, we observed increased mechanical strength with increasing water tension up to at least 6 kPa, primarily due to increased capillary forces in the soil. However, increased elasticity assessed by γ L was not as expressive as the increase in structural rigidity assessed by τ max and integral z. Elastic deformation of the soil (γ L ) increases with the increase in the number of bonds among particles, which depend on the clay, total carbon, expansive clay mineral, and cation contents; however, maximum shear resistance (τ max ) and structural stiffness (integral z) mainly increase with clay, kaolinite, and oxide content by increasing the strength of interparticle bonds. A decrease in mechanical strength occurs for water tension of 10 kPa (the lowest water content evaluated) in sandy horizons or in horizons with a high proportion of resistant microaggregates (pseudosand), when associated with low bulk density, due to fewer points of contact between soil particles and therefore lower capillary force.

Microstructural strength of four subtropical soils evaluated by rheometry: properties, difficulties and opportunities

“…Thus, the results of the amplitude sweep test confirm previous research (Markgraf et al, 2006;Markgraf and Horn, 2007;Markgraf et al, 2011;Holthusen et al, 2012a, c;Pértile et al, 2016) in proving that rheological properties are influenced by soil texture, mineralogy and carbon content.…”

“…In certain soils (as the Oxisol 1, see Table 2 and Figure 4), this transition is not achieved within the chosen range of the amplitude sweep test (0.0001 to 100 % of deformation) (cf. Markgraf et al, 2011;Baumgarten et al, 2012;Pértile et al, 2016). As a consequence of their different characteristics and determination, the different rheological properties can indicate different directions of microstructural strength (stabilization vs. destabilization) within the same soil.…”

“…These investigations have contributed to the understanding of the interaction processes on the particle-toparticle scale, mechanisms for aggregation, strengthening and microstructural stability of different soils, and have evaluated the effects of water content, texture, mineralogy, concentration of salts, organic matter content and soil fertility management on these processes (Markgraf et al, 2006;Markgraf and Horn, 2007;Pértile et al, 2016). However, hitherto, this methodology has rarely been applied to subtropical/south Brazilian soils (Markgraf et al, 2006;Markgraf and Horn, 2007;Pértile et al, 2016), although these soils are quite different from the relatively well investigated European (temperate) soils.…”

“…However, hitherto, this methodology has rarely been applied to subtropical/south Brazilian soils (Markgraf et al, 2006;Markgraf and Horn, 2007;Pértile et al, 2016), although these soils are quite different from the relatively well investigated European (temperate) soils.…”

Microstructural strength of tidal soils – a rheometric approach to develop pedotransfer functions

“…Knowledge of the processes at the microscale has been made possible in recent years by rheological studies of natural soils (e.g. Ghezzehei and Or, 2001;Holthusen, 2010;Markgraf and Horn, 2009;Pértile et al, 2016;Torrance, 1999). Key drivers of rheological behavior identified in these studies are soil organic matter and varying electrolyte concentrations (e.g.…”