Spectral analysis of gilgai soil

Abstract

Spatial variation in gilgai soil mas studied on a transect across typical gilgai terrain of the Bland Plain of New South Wales. The soil was sampled at 4 m intervals over a distance of 1.5 km at Caragabal (33°50¿S, 147°40¿E). Soil morphology was recorded to 1 m and samples taken for laboratory determination of pH, electrical conductivity and chloride content from 0-10, 30-40 and 80-90 cm. Bulk density was determined for the 0-10 cm depth also. All soil properties, when graphed against their sampling positions, appear very erratic; but when filtered by a five-point moving average show recurrent short range variation, associated to some extent with the visible elements of the gilgai pattern. Block variance increases steadily with increasing block size to approximately 32 m for conductivity, pH at 80-90 cm, colour hue and chroma, depth to gypsum and the first principal component. Thereafter the rate of increase is less. Spectral densities were computed from correlograms for each property. Spectra for the properties above showed more or less strong peaks at 0.12 cycles, equivalent to a wavelength of 33.3 m. Gilgai evidently recurs sufficiently regularly on the transect for spectral analysis to reveal a periodicity, and the technique is judged to be sufficiently sensitive for analysing records made directly by data logger.