Degraded Sahelian Alfisols are prone to sealing because of low soil or
ganic matter content and exposure of finer textured subsoil attributab
le to erosion, We characterized the occurrence of surface seals on a w
atershed in Niger and related spatial distribution and structure of su
rface seals to selected soil properties. Study sites were located on s
ix soil series of the Hamdallaye watershed (500 ha), with soil texture
s ranging from sandy loam to sand, classified as Psammentic Kandiustal
f and Petroferric Kanhaplustult. All seals sampled in the watershed we
re structural seals and were morphologically similar, with a 0.1 to 1.
0 mm thick continuous plasmic clay layer within 4 mm of the surface. C
lay content averaged 12.1% in the crust samples (0-3 mm depth), which
was five times greater than the clay content of unsealed sites at 0-50
mm depth (2.5%), Sand content was significantly lower in the sealed t
han in the unsealed 0 to 50-mm soil layer. Of the 12 sites sampled in
the Hamdallaye watershed, all those with more than 5% clay at 0-50 mm
depth were sealed, whereas all sites with less than 5% clay were unsea
led. pH values of sealed sites were 1.3 units lower than those of unse
aled sites and were similar to those of the subsoil, Organic C content
s of sealed sites and were low (1-2 g kg(-1)) at 0-50 mm depth and sli
ghtly higher at unsealed sites, Sealed sites had greater dry-stable an
d greater wet-stable aggregation than unsealed sites because of higher
clay content; however, aggregation was too weak to withstand raindrop
impact, Extractable Fe and Al contents of the six soil series were re
lated to clay content, which was likely the controlling factor of seal
formation. With simulated rainfall of 90 mm h(-1), the same six soils
formed a seal during the initial 30-minute rainfall event in most cas
es, with no change in layer thickness thereafter, Soil with more than
15% (silt+clay) content formed a 2-layer structural seal, whereas coar
ser textured soils developed 4-layer structural seals, Seal structure
of intact field samples and rainfall simulator-prepared samples differ
ed in that many field samples did not have characteristic layers above
the clay accumulation layer, which was attributed to erosion.