The connectivity of pore space is an important topological property of soil. Together with the porosity and the pore-size distribution, it governs the transport of water, solutes and gases. This paper describes the Euler-Poincare characteristic (EPC) as an index of connectivity of soil pore space in three dimensions, and shows how the EPC can be estimated by analysing pairs of parallel images, so-called disectors, automatically. We investigated the connectivity of pores > 50 mu m within two horizons of a Terra fusca from serial surfaces through undisturbed soil impregnated with resin. A method is proposed for the segmentation of the grey level images into solid and void which is especially suitable for the detection of the pore edges. This results in identification of topologically distinct features. The 3D EPC was estimated from pairs of adjacent surfaces, the disector, using an algorithm to count the topological features by digital image processing. The EPC from 10 adjacent disectors within the two horizons are significantly different, and confirm qualitative observations of more intensely connected pore space in the upper Ah horizon. Additional results from 100 adjacent disectors in the lower AhBv horizon show the continuous evolution of the EPC estimates over a distance of 5 mm where two different scales of variation can be distinguished clearly: a small-scale variation which is attributed to methodological errors and a large scale variation reflecting the variability of the structure.
