Soil pore characteristics assessed from X-ray micro-CT derived images and correlations to soil friability Lars J. Munkholm a, b, ⁎, Richard J. Heck b , Bill Deen c a Aarhus University, Department of Agroecology, P.O. Box 50, DK-8830, Tjele, Denmark b University of Guelph, School of Environmental Sciences, Guelph, ON, Canada N1G 2W1 c University of Guelph, Department of Plant Agriculture, Guelph, ON, Canada N1G 2W1 abstract article info Article history: Received 27 December 2010 Received in revised form 9 February 2012 Accepted 17 February 2012 Available online 31 March 2012 Keywords: Soil friability X-ray CT scanning Drop-shatter test Soil pore characteristics X-ray computed tomography (CT) scanning technology has, in recent decades, been shown to be a very pow- erful technique to visualize and quantify soil structure. The objective of this project was to quantify soil pore characteristics, on undisturbed ﬁeld moist soil, using a high resolution X-ray CT scanner and link then these results to soil friability assessed using the drop shatter method. Minimally disturbed soil cores were taken from selected treatments in a long-term rotation and tillage treatment experiment located on a silt loam at the Elora Research Station near Elora, Ontario, Canada. Soil cores varied in porosity and pore characteristics. A drop shatter test was used as a reference procedure to quantify soil friability. The top 40 mm of the 80 mm high soil samples were scanned using a X-ray micro-CT scanner. The selected region of interest (36 × 36 × 36 mm) was reconstructed with a voxel size of 60 μm. Estimated surface area, produced from the drop-shatter test, varied between 0.2 and 1.62 m 2 kg -1 , and an average of 0.79 m 2 kg -1 . Total and air- ﬁlled porosity was determined on the soil cores using traditional methods. Total porosity ranged from 41 to 60 m 3 100 m -3 , and an average of 49 m 3 100 m -3 . The air-ﬁlled porosity, at sampling/testing, ranged be- tween 5 and 32 m 3 100 m -3 , with an average of 15 m 3 100 m -3 . The porosity determined from CT imagery ranged between 1 and 31 m 3 100 m -3 , with an average of 4.5 m 3 100 m -3 . The number of branches, junc- tions and end points averaged 298, 117 and 198 per cm 3 , respectively. We found signiﬁcant and strong cor- relations between the soil pore characteristics assessed on the whole soil cores and the characteristics of the air-ﬁlled pores determined using high-resolution X-ray computer tomography (CT). Our study conﬁrmed a signiﬁcant correlation between soil friability, expressed by surface area produced by standardized drop- shatter, and soil pore characteristics. The strongest correlations were found with porosity, surface area and number of junctions per cm 3 . © 2012 Elsevier B.V. All rights reserved. 1. Introduction X-ray computed tomography (CT) scanning technology has, in recent decades, been shown to be a very powerful technique to vi- sualize and quantify soil structure, as reviewed by Taina et al. (2008). The application was, in the early years after adoption in soil science, concentrated on describing macro-structures such as biopores (Capowiez et al., 2003; Gregory et al., 2003), tree roots (Pierret et al., 1999) and dense layers (Lipiec and Hatano, 2003). With the development of high-resolution micro-CT scanning over the last decade, the technique has also been applied to other as- pects of soil micromorphology. It has especially been used to quan- tify soil pore space characteristics on images with voxel size of b 50 μm(Elliot and Heck, 2007; Peth et al., 2010; Quinton et al., 2009; Schluter et al., 2011). Soil pore characteristics are important for a large range of essential soil functions such as colloid, water and gas transport, habitat for soil organisms as well as soil mechanical properties such as soil friability. The latter is a key soil physical property, yielding valuable informa- tion on the ease of producing a favorable seed- and rooting bed dur- ing tillage operations (Munkholm, 2011). Soil friability is related to brittle fracture of soil as described by Braunack et al. (1979) and Dexter and Watts (2000). Brittle fracture results from the progressive development of fracture planes, resulting in a crack opening and a sudden loss in strength (Hatibu and Hettiaratchi, 1993). The propaga- tion of cracks in an unconﬁned stressed soil depends on the frequency and the morphology (connectivity, orientation) of the air-ﬁlled pores as well as the strength at the crack tips as stressed by Hallett et al. (1995a,b). A strong link between soil fragmentation/soil friability and air-ﬁlled soil pore space was conﬁrmed by Munkholm et al. (2002), and this suggest a strong correlation between soil friability and pore characteristics derived from 3D images of soil structure. Geoderma 181–182 (2012) 22–29 Abbreviations: BD, Bulk density; CT, computed tomography; ε a , Air-ﬁlled porosity; ε T , Total porosity; ε CT , Air-ﬁlled porosity derived from CT imagery; MWD, Mean weight diameter; ROI, Region of interest; SA CT , Surface area of CT derived air-ﬁlled pore space; SA DS , Surface area generated by drop shatter test. ⁎ Corresponding author. Tel.: + 45 87157727. E-mail address: lars.munkholm@agrsci.dk (L.J. Munkholm). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2012.02.024 Contents lists available at SciVerse ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma