A CASTING METHOD FOR THE 3-DIMENSIONAL ANALYSIS OF THE INTRAPRISM STRUCTURAL PORES IN VERTISOLS

The structural voids in vertisols contain easily available water for p lants and their volume can be calculated from the shrinkage curve. Acc ess by plants to that water depends also on the geometric arrangement of the pores so that the water can flow through them. We have devised a method for studying the structural porosity by casting the pores in resin. The intraprism pore space of wet soil clods is impregnated with a UV fluorescent polyester resin under vacuum. When this has set we u se the swelling properties of the clay to separate the clay matrix fro m the resin. A cast so obtained is the real three-dimensional solid re production of the structural porosity. This representation of the pore system is easier to study than results from computerized reconstituti on of the three-dimensional space from two-dimensional images of soil in thin sections. Channels, packing pores and planar voids can be obse rved directly in three dimensions as the method saves the integrity an d continuity of pores as small as 10 mu m in diameter. The geometry of the cast shapes agrees with the interpretation of shrinkage and moist ure characteristic curves. The method offers direct qualitative observ ation of pore organization and volume measurements of the intraprism s tructural porosity in vertisols.