Electrophoretic mobility and clay mineralogy of some fragipan and non-fragipan soil horizons.

The physical properties of the horizons above and below the fragipan are different from those of the fragipan itself. Indeed, fragipans are characterized by massive, dense, and brittle consistence when moist, hard when dry, but their clods are completely disrupted when submerged in water. These physical characteristics have been related to the presence of clay and weak chemical binding, but there is no general agreement on the main process causing the development of this typical behaviour. We recently found that the low permeability and high bulk density of fragipans are linked to specific arrangements of particles: an open packing of the clay phase is associated to an extremely dense packing of silt and sand. This combination was not present in any other horizon of some Typic Fragiudalfs. Since the packing arrangement of soil colloids depends on the flocculation/dispersion behaviour of clay particles, we evaluated the electrophoretic mobility (EM) and the mineralogical composition of the water dispersible clay fractions of fragipans and non-fragipan horizons. The EM was determined in a pH range from 4 to 6.5, and the hydrodynamic diameter was recorded. The mineralogy of the clay fraction was assessed by XRD. The clay fraction was composed of a complex suite of minerals: vermiculite prevailed but, illite, chlorite, kaolinite and several interstratified minerals were also present. When clay was separated using only deionised water, the proportion of minerals varied. The EM data of non fragipan samples ranged between -1.2 and -3.3 μm cm s-1 V-1 as a function of pH, whereas fragipan samples had less negative values and varied from -0.7 to -2.3 μm cm s-1 V-1. In both cases, the permanent charge prevailed over the pH dependent, but the contribution of oxides to EM was greater in non fragipan. At the highest pH, when the electrostatic repulsion was higher, the hydrodynamic diameter was similar or slightly smaller in Bx than in non-Bx (0.7 μm and 0.7-0.9 μm, respectively). With decreasing pH, the changes in the electrophoretic mobility were accompanied by an increase in particle size in Bx horizons, up to 0.9-0.8 μm. At the lowest pH, the clay extracted from non-fragipans showed a smaller hydrodynamic diameter, suggesting repulsion within the clay-sized aggregate and confirming the role of oxides. As Bx horizons were richer in pedogenic Fe oxides than non-fragipans, our results indicate that the open packing of the clay phase typical of fragipans is the result of water stable aggregation among layer silicates and oxides, combined with a poorly aggregated water dispersible clay.