A system for measuring soil properties on-the-go using a narrow profile sensor unit is provided on an implement for traversing a field. The sensor unit includes a front disk/coulter arranged to open a slot in the soil, a runner assembly arranged to follow behind the front disk/coulter for sliding contact with the soil in the slot, and a rotating disk/spoked wheel arranged to follow behind the runner assembly to close the slot. The front disk or coulter serves as a first electrode of an electrode array, the runner assembly has second and third electrodes attached thereto, and the rotating disk/spoked wheel serves as a fourth electrode. The electrode array can be used to measure soil electrical conductivity at multiple depths and to measure soil moisture. An optical window and pH sensor can also be incorporated into the runner assembly to measure soil reflectance and soil pH.