2.4. Analysis of soil crack patterns
The soils in petri disheswere placed in a constant temperature incubator
at 25 °C for drying from submerged condition.When the soil water
content reached a steady condition, soil surface cracks were recorded by
a digital camera (SONY DSC-HX30). Thereafter a portion of the soils
were oven-dried at 40 °C, and ground b0.2 mm for measuring the SOC,
Fed/Ald, and Feo/Alo contents.
The images of soil crackswere firstly cropped along the interior edge
of petri dishes using Adobe Photoshop CS5. Tomeasure total cracks, the
crack area between interior edge of petri dish and soilwas also included
for the following analysis. The segmentation threshold was carefully
chosen based on visual observation to distinguish the cracks from
non-crack area. Due to irregularity of soil cracks, some cracks were
disconnected by unwanted white spots after the binarisation of image.
To connect the cracks, the binary images were dilated and eroded
using ImageJ 1.48v (Rasband, 2014). The filter median in ImageJ was
used to smooth the cracks. The binary images were converted to
skeleton images by skeletonization for measuring the crack length and
the number of junctions and end points. In order to quantitatively
identify the geometrical properties of the crack patterns, area density
(Dc), averaged width (AW), connectivity index (CI), and fractal dimension
(FD) were determined using ImageJ as follows: