Micromorphological study of surface seals developed under simulated rainfall

Abstract

Soil surface seals were formed on microplots of homogeneous soil in the laboratory using simulated rainfall. The soil horizons studied varied in texture and organic matter content. The surface sealing was quantified by monitoring changes in infiltration rate, splash detachment and near-surface shear strength with time. The porosity of the resulting surface seals and unsealed soil material was studied in the laboratory using image analysis of thin sections. Various sealing indices demonstrated that the A horizons of the soil sealed to a greater extent than the corresponding Bt horizons. In general, as sealing increased, soil loss decreased from the microplots. Total porosity of the soil surface seal did not explain differences in infiltration rate, splash detachment or strength well; however, the percentage of planar pores was more related to the final infiltration rate. Different morphological types of surface seals could be found in the soils that were representative of various sealing processes. Even though some soils had a considerable amount of water-dispersible clay, the downward translocation of clay was a minor process. A more important process was the physical breakdown of aggregates and the accompanied compaction of the surface resulting in a zone with a lower percentage of planar pores. The primary particle size of the soil was a poor indicator of the soils' susceptibility to surface sealing, whereas, the water dispersible particle size, particularly the silt size distribution, was a good indicator of its sealing behavior.