Humic Substances and the Viscosity of Soil Pastes

The influence of the water content of soil pastes on their viscosity is studied using vibrational viscometry, and the variable viscosity is explained by the structural organization of soil humic substances (HSs). The viscosity of pastes prepared from sod-podzolic soil increases to 4000-4200 mPa s with a decreasing water content of <36% and further remains almost the same. This contradicts the current views on the viscosity of suspensions, which should continuously increase with decreasing water content. This situation is explained by the fact that, in addition to water soil particles (aggregates and microaggregates), the soil paste contains one more component of supramolecular formations (SFs) from soil HSs, which could separate from soil gels and transit to disperse conditions. According to the literature data, the HS SFs are present in soils as fractal clusters (F-clusters) of particles-molecules. Because of the high stability of F-clusters, we can suggest the presence of soil particles in pastes sliding along F-clusters, whereas the change in the thickness of a sliding F-cluster layer has a minor effect on the viscosity of soil pastes. Our data on the viscosity of soil pastes confirm the significance of F-clusters for the formation of soil properties.