Blackwell Publishing Journal Backfiles 1879-2005
Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
In semi-arid tropics soil hardening may reduce both water infiltrability and biological activity, thus inducing the development of large almost bare areas. A sandy-loam soil with contrasting loose and underlying hard horizons was studied in the southern plain of the Chad basin. The fabrics of these horizons were studied using combined sieving and sedimentation techniques, mercury porosimetry and scanning electron microscopy. The horizons had similar particle size distributions of the skeleton grains. The hard horizon differs by a small increase in its fine clay (〈 0·2 μm) content. The hardness is closely related to a fabric with clay coatings on the skeleton grains and clay wall-shaped bridges linking the latter. This induces a strong continuity of the solid phase. This fabric requires a minimum of clay content (6%) to make the coatings and the wail-shaped bridges, and it can be 30% less porous than the loose horizon, without any change in the packing of the skeleton grains. These characteristics of the fabric of the hard horizon are like those of fragipans elsewhere. The continuity of the solid phase, from the microscopic to the macroscopic scale, as well as the absence of a network of cracks explains the considerable strength of the hard horizon, and consequently the difficulties for water infiltration, root penetration and tillage.
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