UBC Theses and Dissertations
Structural stability and surface sealing as related to organic matter depletion of a shallow organic soil Bonsu, Mensah
A physically based model describing the mechanism of surface sealing of soil was evaluated in the context of aggregate stability. The intent of the model study was to better understand the effect of mixing fine-textured mineral subsoil with organic surface soil on structural stability and surface seal formation. The mixing results from tillage and harvesting operations, and management practices such as levelling. The index derived from the model showed that sealing of the shallow organic soil increased with an increase of mineral matter content. The mathematical formulation of the model was based on the principle of conservation of mass and Darcy's law for flow of water through a layered soil column. Assuming convective flow, it was shown theoretically that the rate of surface seal formation is proportional to the flux density of the filtrate, as assumed by Scheidegger (1974). In the model it was further assumed that the pore necks at the soil surface clog first before the seal develops. The assumption that convective flow alone was responsible for the movement of the suspension is likely incorrect for suspensions derived from medium or coarse textured soils, since sedimentation does influence the movement of larger particles. However, introducing a constant sedimentation parameter into the convective flow model did not improve the model. Therefore, it is likely a non-constant sedimentation parameter could improve the model considerably. The model showed that for sufficiently large times the flux density of a filtrate flowing through a soil column at a constant hydraulic head is proportional to inverse square root of time. Testing the model experimentally showed a good agreement between theory and experiment. A highly significant correlation between the soil stability factor derived from the model and aggregate stability suggests that the index is a soil structural attribute. The soil stability factor was exponentially related with aggregate stability and mineral matter content. However, whereas the relationship between the soil stability factor and aggregate stability gave a positive exponent, a negative exponent was obtained with mineral matter content. Further studies showed that structural stability and saturated hydraulic conductivity of the aggregate beds were positively and significantly correlated exponentially. However, saturated hydraulic conductivity and mineral matter content were negatively and significantly correlated exponentially. Collateral to the results of the model, the strong negative correlation between wet-sieved aggregate stability and mineral matter content confirmed the deleterious effect of mixing fine-textured mineral soil on the structure of the shallow organic soil. It was theorized that aggregates stabilized through clay-organic complexing are likely to be much stronger than aggregates stabilized through other mechanisms. This implies that whenever the mineral matter content is much higher than the organic matter content, the surplus mineral matter that does not interact with organic matter will be most dispersible. The high silt content of the mineral matter fraction is likely to be an important factor contributing to the decrease in structural stability with increasing mineral matter content. Once the clay and the organic colloids have interacted, the silt that remains is not capable of forming stable aggregates without colloids (Baver et al. 1972). From measurements of the air to water permeability ratio, the decrease in saturated hydraulic conductivity of the aggregate beds with increasing mineral matter content was attributed to slaking of the mineral matter fraction. However, it is possible for the soil with high mineral matter content to be stable if the mineral matter is allowed to be in contact with the organic matter for a long period of time.