Preheat the oven to 180°C (350°F/Gas 4). In a bowl, combine the potatoes with the oil and season with salt. In a deep roasting tray, about 35 cm x 25 cm (14 x 10 inches), place half of the soil in an even layer to cover the base of the tray. Place a damp tea towel over the soil and then a piece of hessian sack on top. Place the potatoes on the hessian sack, then cover with the other piece of hessian and tea towel, making sure they are sealed from the soil. Place the remaining soil on top and tightly cover with foil. Bake for 3 hours, then reduce the oven temperature to 100°C (200°F/ Gas ½) and bake for a further 4 hours (or up to 5). The texture of the potatoes should be soft and creamy.