Variations in soil structure and reflectance during a controlled crusting experiment

The results of an experiment to show variations in the directional reflectance factor of a Luvisol during a controlled crusting experiment are described. Soil sampled in the field after tillage was sieved into free-draining trays, and exposed to artificial rainfall for differing periods of time, ranging from 5 to 60 min. The resulting samples demonstrated different stages in the development of the soil's structural crust. The topography of each dried sample was characterized over a 5×5 cm area using a laser profilometer, and digital surface models (DSMs) were subsequently analysed using variogram models. DSMs were also used to generate statistical measures of random roughness. Directional reflectance factors of each sample were characterized in the solar principal plane under clean skies using an ASD FieldSpec Pro spectroradiometer, using an 8° foreoptic attached to an A-frame device. Directional reflectance factors were analysed in relation to spatial statistical measures obtained from the laser profilometer data. The results demonstrate that changes in the sill variance of soil samples following crusting, and hence changes in soil structure, were best described by backscattered radiation measured at +30° in the visible and near-infrared (e.g. R 2 = 0.947 (658 nm)), and at +15° in the short-wave infrared (e.g. R 2 = 0.992 (1700 nm)). View zeniths are expressed from the nadir, and were relative to the solar zenith angle, which ranged from 80.76° to 74.55° during the measurement sequences. The results from these tests show great promise for broader-scale monitoring of soil condition, particularly when considered in the context of the new pointable remote sensing systems in operation, coupled with new-generation sensors with in-built directional capabilities.