SURFACE SEAL MICROMORPHOLOGY AS AFFECTED BY FLUIDIZED-BED COMBUSTION BOTTOM-ASH

Amendments that alter clay dispersion and aggregate disintegration als o change soil porosity and sealing. Soils selected for this study had increased (Mollisol, Oxisol, and Vertisol),decreased (Oxisol), or non- affected (Oxisol and Ultisol) water infiltration when fluidized bed co mbustor bottom-ash (FBCBA) was surface applied to decrease dispersion. Soil was sieved to pass 8-mm, packed into small erosion pans, prewett ed by capillarity, and subjected to 110 mm h(-1) simulated rain until steady state infiltration. Image analysis was used to quantify crust m orphology, porosity and characteristics of the seal and the unsealed s oil below it. A conspicuous feature was a structural crust at the surf ace with a continuous dense layer of lesser porosity and smaller pores than uncrusted soil. The seal showed no evidence of a ''washed-in'' z one of illuvial clay in the control treatment, although dispersed clay was observed in the percolating water for some soils. Where FBCBA was effective in increasing water infiltration, an increase in total poro sity attributed to planar pores was observed. A sandy loam Oxisol was the least prone to sealing. For this soil, a considerable amount of di spersed clay was observed in the control, while FBCBA rapidly floccula ted clay and formed an illuvial clay layer, lowering infiltration. A l ayer of eluvial silt and fine sand was observed at the surface of soil s where considerable dispersion occurred on non-treated soils. Differe nces in steady-state infiltration could not be explained by the variat ion in total porosity or pore shape. Spatial variability, pore continu ity, and expansion/contraction of clays obscure any relationship with an averaged infiltration even on a small plot.