AGGREGATE SIZE AND SEAL PROPERTIES1

Abstract

The formation of a disrupted layer or a seal at soil surfaces exposed to the impact ofraindrops is a common feature ofmany cultivated soils. The objectives of this study were to investigate the dependence ofa disrupted layer (which consists of broken and compacted aggregates) and of a seal (a thin dense layer of very low permeability) on aggregate size in order to better understand the mechanisms involved in seal formation. The effects of aggregate size on seal permeability, thickness of the disrupted layer, and the relative rate of aggregate disintegration 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 Chromoxerert) 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 disrupted layer was estimated from microscope observations. Aggregate stability of aggregates with sizes of 2-4, 4-9.5, and 9.5-12 mm was determined by applying drops with kinetic energy of 3.1 kJ m -3 to dry aggregates placed on a sieve with a size opening of 0.3 or 0.8 mm. For grumusol, 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 infiltration 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 aggregate size despite the accompanied increase in the thickness of the disrupted layer. These observations suggest that (i) rate of seal formation is determined by the rate ofaggregate disintegration and (ii) thickness ofthe disrupted layer is not related to seal permeability. In soils exposed to rain, aggregate disintegration and formation of a disrupted layer are fast processes, whereas physico-chemical dispersion of clay is slow and determines seal permeability. The permeability of the seal rather than that of the disrupted layer determines the equilibrium infiltration rate of the soil.