Influence of organic matter on rheological properties of soil

Recently rheological methods have been used to investigate the mechanical properties of soil micro-structure. Rheological techniques provide a number of quantitative physically based measurements and offer a better understanding of how soil micro-structure behaves when subject to stress. Here a rotational rheometer with a parallel-plate measuring device was used to explore the viscoelastic properties of soil as a function of soil organic carbon (SOC). We used samples from the long-term experimental site at Broadbalk, Rothamsted, UK. Data from amplitude sweep tests (AST) with controlled shear deformation are reported as well as mineralogical data. In general, water content, and SOC content, in addition to soil texture, clay mineralogy, carbonate, and cations all have an effect on stiffness, structural stability, and shear behavior. Storage modulus G′ and loss modulus G″, yield point, the linear viscoelastic range (LVE), loss factor tan δ, and integral z characterize micro-structural properties of soil on the particle-to-particle scale. Long-term applications of farmyard manure (FYM) lead to a significant increase in soil organic carbon (SOC), in comparison with the application of ammonium nitrate in form of mineral NPK fertilizer and bare fallow. For wet soils, increased SOC provided greater resistance to deformation and improved soil elastic properties allowing greater deformation before a yield point was reached. In drier soils however, (− 6 and −15 kPa) this relationship was less clear.

Research highlights
► Effect of SOC on soil rheological properties most apparent in wet soil.
► Soils with higher SOC can deform to greater extent before failure.
► Rheometry appropriate tool for semi-quantifying viscoelasticity.