Mapping spatial distribution of soil properties using electrical resistivity on a long term sugarcane trial in South Africa

International audience; In experimental trials the spatial distribution of soil properties is a key factor to evaluate and determine the credibility of the research outcomes. To evaluate the spatial distribution of soil properties at different depths in the present study, the electrical resistivity method was selected because of its non-destructive nature and high spatial resolution. The investigation was conducted on BT1 (Burning versus mulching) the world's oldest sugarcane trial (established 25 October 1939) located on the premises of the South African Sugarcane Research Institute (SASRI). The experimental area, covering 7200 m(2), is divided in 32 plots with three crop residue retention treatments. These treatments are either fertilized and not fertilized. The RM15 resistance-meter equipped with a multiplexer was used to collect geophysical data. The latter was used to generate apparent electrical resistivity maps corresponding to three approximate depth ranges: (0-0.5 m, 0-1.0 m and 0-2.0 m). The spatial resolution of the shallowest map (0-0.5 m), was 0.5 m x 1.0 m. Bulk density and soil water content, for the 0-0.1 m depths, were also determined from samples collected on a grid of 4 m x 5 m. All data collected were georeferenced using a spatial resolution of 4 m x 1 m with the Digital Elevation Model (DEM). The results of this survey showed the effectiveness of geophysical mapping to reveal detailed information regarding soil spatial variability and to understand and establish the relationships between soil fertility and soil physical properties. Detailed analysis of these results led to the identification of two different soil types on the trial site: a Mollic Cambisol in the upper slope of the trial with a depth of < 1.0 m and a deeper Mollic Nitisol in the lower slope. Additionally, close to the southern edge of the site, higher values of electrical resistivity revealed a layer of colluvial deposits, that has not been considered in the past. The relationships between electrical resistivity and investigated soil properties (soil water content, bulk density, aggregate stability and fertilizer) initially seemed paradoxical. Although there is still a need for further investigations, the current study decently establishes the rather unexpected relationships. The most striking finding was the high resistivity in fertilized plots associated with the application of potassium chloride fertilizer which led to a decreased soil aggregate stability.