The fractal dimension of pore distribution patterns in variously-compacted soil

Abstract

Pore-size distribution pattern significantly alters many soil properties affecting water movement and root growth. The distribution is largely influenced by soil compaction but information on how to describe this effect is very limited. In this study we used the fractal dimension to characterize pore distribution patterns in variously-compacted soil. The soil used was an Orthic Luvisol (Lublin Region, Poland). The various soil compaction was obtained by wheel traffic treatments: unwheeled (L); moderately compacted, 3 tractor passes (MC); strongly compacted, 8 tractor passes (SC). Pore distribution patterns of all pores (>0.3 mm) and water-conducting pores were analyzed with an image analyzer and the two-dimensional fractal dimension was estimated. All pores were analyzed on the drawings obtained from the polished surfaces of soil blocks 8×9×2 cm. To analyze the water-conducting pores, soil cores were taken in cylinders of length 20 cm and diameter 21.5 cm from the plots on which methylene blue solution was applied. The pores were analyzed on horizontal cuts at 2 cm depth intervals. Mean values of fractal dimensions for all pores ( D p2) in the horizontal plane of surface soil in L, MC and SC were 1.69, 1.42 and 1.35, respectively. In the vertical plane, the corresponding values were 1.48, 1.35 and 1.29. In L the fractal dimension reflected pores of different size ranging from a few tenths of a millimetre to a dozen or so millimetres with rather smooth walls. In MC the contribution of large pores decreased whereas that of medium-sized pores considerably increased forming net-like patterns. However in SC, the largest area was of massive structure with longitudinal cracks and scarce and unevenly-distributed larger pores. The D p2 was linearly correlated with areal porosity ( R=0.965) and the arithmetic mean of the areas of pores ( R=0.914). Mean values of fractal dimensions for the blue staining patterns ( D s2) in the plough layer ranged from 1.06 to 1.12 in L and MC whereas it decreased to 0.94 in SC. The wide range of D s2 values for 2 cm layers of upper soil in L reflected the high variability of pore structures in this treatment. In the subsoil, the D s2 varied from 1.03 to 1.09 and reflected mostly the distribution pattern of earthworm channels. The values of fractal dimensions for roots ( D r2) reflected different branching and root growth in variously-compacted soil. This study showed that fractal analysis provides a relevant quantification of the changes of pore and root structure in relation to soil compaction.