TILLAGE-INDUCED SPATIAL-DISTRIBUTION OF SURFACE CRUSTS ON A SANDY PALEUSTULT FROM TOGO

The spatial distribution of crusts in coarse-textured soils and the pr ocesses affecting it are poorly documented, despite the potential impa ct of crusts on water infiltration, This study addresses the influence of tillage-induced microrelief on the morphology and spatial distribu tion of surface crusts in an Oxic Paleustult from southern Togo (West Africa), Replicate 1-m(2) plots were exposed to 217 mm of natural rain fall during a 6-wk period, during which the surface topography was mea sured three times, Subsequently, 24 undisturbed crust samples were use d for micromorphological analysis. The crusts exhibited a range of mor phologies but were nevertheless adequately mapped and characterized ac cording to two main types. Type 1 crusts (approximate to runoff crusts ) showed several superposed clay bands, 100 to 500 mm thick, buried wi thin a micromass-depleted sand layer less than or equal to 12 mm thick , Type 2 crusts (approximate to erosion crusts) had an exposed clay ba nd a few tenths of a millimeter thick, The spatial distribution of cru sts at the time of sampling appeared better correlated with the initia l than with the final microtopography of the plots. These findings sug gest that crust distribution should be regarded as history dependent a nd that erosion and deposition processes largely governed the developm ent of the crusts, This latter aspect is in agreement with the recent crust genesis model of Valentin and Bresson, as is the fact that clay bands In our plots were laterally continuous at all observational scal es less than or equal to 0.1 m, Other mechanisms proposed in the liter ature for the development of clay bands did not seem able to account a dequately for the observed pattern.