A method for measuring the development of surface cracks in soils: Application to crack development after lowland rice

Surface cracks develop in many clay soils as they dry. Since they are difficult to adequately sample in most volumetric samples, it is useful to measure them so that volumetric measurements of water content, for example, can be corrected. A method for measuring cracks in soils is described. A transect consisting of six linked semi-circles, each of 1 m diameter, is used to count intercepts with the cracks, which allows estimation of the length of cracks per unit area, L(A). The semi-circular shape avoids any bias due to preferred orientation of the cracks. To minimise the effort required to achieve the greatest accuracy, which depends on the number of cracks counted and not the transect length, 50 intercepts are counted per replicate transect after which any remaining intercepts with the current semi-circle are counted. In addition, crack depth and surface width are measured at a total of 60 intercepts over all replicate transects. This allows calculation of means for depth and width. Mean vertical cross-sectional area and the volume of crack per unit area can be calculated by assuming a rectangular, triangular or square root shape model for the crack cross-section. The depth functions of L(A) and V-V (crack volume per unit volume) can also be calculated, although the latter requires assumptions about crack shape. The method is illustrated by application to two soils puddled for rice during the first four weeks of drying after field drainage.