Heat 3 tablespoons of the oil in a saucepan, add onion and bacon and cook over a low heat for about 3 minutes until translucent, stirring. Add kohlrabi, carrot, garlic, potatoes and about 8 cups of water; bring to boil. Reduce to a simmer, cover and cook for 1 hour. Discard bacon and add salt and pepper to taste. Pour into soup tureen, drizzle with remaining oil and serve immediately. Serves 6.