The effect of water-repellent soil-induced preferential flow on the spatial distribution of nutrients and salts in the soil profile of a commercial orchard

Treated wastewater (TWW) has gained recognition as an alternative source for freshwater irrigation and is steadily expanding worldwide, particularly under the current climate change. Beyond its many advantages, it has been found that prolonged use of TWW renders the soil water-repellent to certain degrees. The flow in these soils has been known to take place in preferential flow pathways (unstable flow). This lecture presents the results of a study performed in a commercial citrus orchard grown on sandy-loam soil in central Israel that has been irrigated with TWW. Electrical resistivity tomography (ERT) surveys revealed that water flow in the soil profile is occurring along preferential flow paths, leaving behind a considerably nonuniform water-content distribution. The preferential flow in the soil profile led to uneven distribution of salts and nutrients, with substantially high concentrations in the drier spots and lower concentrations in the wetter spots along the preferential flow paths. The chemical's pore concentration, which depends on the local soil water content, is higher than paste-measured concentrations and may even reach toxic values. This could partially explain the negative effect that prolonged TWW irrigation has on soil and trees. The relationship between water-repellent soils and the spatially nonuniform distribution of nutrients and salts in the root zone was verified in a consecutive in-situ study where soil water repellency was eliminated by surfactant application to the soil. Repeated ERT surveys and chemical concentration measurements in disturbed soil samples along transects revealed that the surfactant application diminished the preferential flow pathways and rendered the soil water and dissolved chemicals uniformly distributed. The preferential flow elimination and increased chemical distribution uniformity result in a yield increase compared to the surfactant-untreated soil. The different aspects of the results will be further presented and discussed.