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 usefu l to measure them so that volumetric measurements of water content, fo r 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 allow s estimation of the length of cracks per unit area, L(A). The semi-cir cular shape avoids any bias due to preferred orientation of the cracks . To minimise the effort required to achieve the greatest accuracy, wh ich depends on the number of cracks counted and not the transect lengt h, 50 intercepts are counted per replicate transect after which any re maining intercepts with the current semi-circle are counted. In additi on, crack depth and surface width are measured at a total of 60 interc epts over all replicate transects. This allows calculation of means fo r depth and width. Mean vertical cross-sectional area and the volume o f crack per unit area can be calculated by assuming a rectangular, tri angular or square root shape model for the crack cross-section. The de pth functions of L(A) and V-V (crack volume per unit volume) can also be calculated, although the latter requires assumptions about crack sh ape. The method is illustrated by application to two soils puddled for rice during the first four weeks of drying after field drainage.