Potential of rheometry in detecting cohesive soils in Brazil as an additional tool to morphological field description and tensile resistance quantification

Abstract

Although an experienced pedologist quickly identifies soils with a cohesive character (similar to hardsetting soils) directly in the field, advances in its correct diagnosis are needed, since the description requires wet and dry soil situations and relies upon qualitative aspects. Taxonomic systems usually consider metrics for other diagnostic attributes (clay fraction activity, abrupt textural change, etc.), but not for the cohesive character, although the latter is environmentally important and is referred to in soil taxonomy systems. Research is especially required at the microscale, where adhesive and cohesive forces are manifested as particle interactions. The aim of this study was to determine whether rheological properties obtained in amplitude sweep tests can serve as indicators of hardsetting or cohesive soils to improve the understanding of underlying processes in such soils. We analyzed soil samples from horizons identified as either cohesive or non-cohesive from six Brazilian soils (Typic Paleudalf, Ultic Hapludalf, Typic Hapludalf, Typic Paleustult, Kandiudox and Hapludox), separated by depth in top, middle and bottom layers. As rheological properties we determined: strain at the end of the linear viscoelastic range (LVR) (γLVR), shear stress at the end of the LVR (τLVR), strain at the yield point (YP) (γYP), storage and loss modulus at the YP (G′G″YP), maximum shear stress (τmax), strain at the maximum shear stress (γ τ max), and integral z (Iz). Cohesion only affected γLVR in the base position, and had a significant effect on τmax and γ τ max in the top and middle positions, respectively. Three soil materials did not reach the yield point. The Iz was either higher or lower in cohesive soils compared to non-cohesive soils. Where the Iz was higher in non-cohesive soils, this seems to be linked to a small dilatancy as a consequence of the high friction between soil particles. Also, τmax and Iz showed significant interactions of soil type and cohesion by means of a two-way ANOVA. Soil type had a greater effect on rheological properties, and the Typic Paleudalf had generally higher values. Parallely determined iron, organic carbon and clay contents, bulk density and normal force at the beginning of the test had the greatest impact on rheology. Tensile strength, a property evaluated on the mesoscale, had the closest relationship with γτ max and τmax. In conclusion, rheometry can support common analyses to identify the cohesive character, however, for an independent evaluation based upon most sensitive rheological attributes, further testing is needed, with a greater diversity of soils and at different water contents. The possibility to promptly identify cohesive character by quantitative means is essential for effective measures for the mitigation of the mechanical impediments of cohesive soils for agriculture. With the existence of a single parameter to distinguish cohesiveness and its magnitude, different soil management strategies can be easily tested and compared to obtain greater soil workability and, therefore, improved fertility.