AGGREGATE SIZE AND SEAL PROPERTIES

The formation of a disrupted layer or a seal at soil surfaces exposed to the impact of raindrops is a common feature of many cultivated soil s, The objectives of this study were to investigate the dependence of a disrupted layer (which consists of broken and compacted aggregates) and of a seal (a thin dense layer of very low permeability) on aggrega te size in order to better understand the mechanisms involved in seal formation, The effects of aggregate size on seal permeability, thickne ss of the disrupted layer, and the relative rate of aggregate disinteg ration in two soils exposed to simulated rain was studied, Aggregates with sizes of <4, 4-9.5, and 9.5-12 mm from a grumusol (Typic Chromoxe rert) and a loess (Calcic Haploxeralf) were exposed to distilled water (DW) rain with a kinetic energy of 12.4 kJ m(-3). Thickness of the di srupted layer was estimated from microscope observations, Aggregate st ability of aggregates with sizes of 2-4, 4-9.5, and 9.5-12 mm was dete rmined by applying drops with kinetic energy of 3.1 kJ m(-3) to dry ag gregates placed on a sieve with a size opening of 0.3 or 0.8 mm, For g rumusol, aggregate size increase from 2-4 mm to 9.5-12 mm resulted in increases of (i) aggregate stability from 8 to 56%, (ii) thickness of the disrupted layer from 1.5 to 4.3 mm, and (iii) cumulative infiltrat ion from 29.8 to 47.8 mm, Similar results were observed for the loess, The final infiltration rate of the fully developed seal in both soils was low (<5 mm h(-1)) and tended to increase with an increase in aggr egate size despite the accompanied increase in the thickness of the di srupted layer, These observations suggest that (i) rate of seal format ion is determined by the rate of aggregate disintegration and (ii) thi ckness of the disrupted layer is not related to seal permeability, In soils exposed to rain, aggregate disintegration and formation of a dis rupted layer are fast processes, whereas physico-chemical dispersion o f clay is slow and determines seal permeability, The permeability of t he seal rather than that of the disrupted layer determines the equilib rium infiltration rate of the soil.