Soil properties influencing surface sealing of some sandy soils in the Sahel
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Degraded Sahelian Alfisols are prone to sealing because of low soil organic matter content and exposure of finer textured subsoil attributable to erosion. We characterized the occurrence of surface seals on a water-shed in Niger and related spatial distribution and structure of surface seals to selected soil properties. Study sites were located on six soil series of the Hamdallaye watershed (500 ha), with soil textures ranging from sandy loam to sand, classified as Psammentic Kandiustalf and Petroferric Kanhaplustult. All seals sampled in the watershed were 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. Clay 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 than 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 unsealed. pH values of sealed sites were 1.3 units lower than those of unsealed sites and were similar to those of the subsoil. Organic C contents of sealed sites and were low (1-2 g kg-1) at 0-50 mm depth and slightly higher at unsealed sites. Sealed sites had greater dry-stable and 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 related 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 cases, with no change in layer thickness thereafter. Soil with more than 15% (silt+clay) content formed a 2-layer structural seal, whereas coarser textured soils developed 4-layer structural seals. Seal structure of intact field samples and rainfall simulator-prepared samples differed in that many field samples did not have characteristic layers above the clay accumulation layer, which was attributed to erosion.
