MICROMORPHOLOGICAL AND MINERALOGICAL COMPONENTS OF SURFACE SEALING INLOESS SOILS FROM DIFFERENT GEOGRAPHIC REGIONS

Mineralogy of clay sized particles and rainstorm characteristics are a mong the major factors that determine the nature of soil sealing. This research was designed to further our knowledge about the role of clay mineralogy and rainstorm characteristics on crust formation. Four loe ss soils from different geographic locations (Canada, USA, China, and Belgium) were used in this study. Preliminary studies were carried out on Grenada Bt (USA), Lishi (China), and Bierbeek Bt (Belgium) horizon s. Detailed infiltration and microscopic studies were performed only o n the Bm1 horizon of the Saskatchewan soil, Canada and the Ap horizon of the Grenada soil, Mississippi, USA. Two levels of rainstorm were ap plied (similar to 40 and 80 mm h(-1)). While smectite was the major co mponent in the Saskatchewan Bm1 horizon, vermiculite and illite were f ound to be the dominant clay minerals in the Bt horizon of the Grenada soil. Formation of the washed layer together with thin lamellar crust coincided with the preferential movement of clay sized particles thro ugh runoff in both the Saskatchewan Bm1 and Grenada Ap horizons. The h igher initial infiltration rate with higher rainfall intensity in both soils were likely due to higher matric suction, but rapid reduction i n infiltration in this treatment was due to rapid aggregate destructio n and a dense packing of the fundamental soil particles. No washed-in zone could be observed in the Bm1 horizon of the Saskatchewan soil in both rainstorm intensities. However, drastic reduction in infiltration , especially with high rainstorm intensity (after 7-8 min), was attrib uted mainly to the swelling of smectite in this soil material. Washed- in materials were visible in the Ap horizon of Grenada under high rain storm intensity. Due to the presence of iron oxides and clay coatings, aggregates were stable and no sealing could be observed in the Grenad a Bt and Bierbeek Bt materials. The degree of seal development in the Lishi soils was low. This was due to low clay and high carbonate conte nts. This study shows clearly that higher and continuous rainstorm int ensity causes rapid seal development and as a result more erosion woul d occur, especially in the soils with high smectite content.