You can use whatever root vegetables you can find, just be sure to use a variety.
Arrange vegetables, mushrooms, and kombu in the bottom of a large pot. Tuck ginger slices between vegetables. Pour dashi on top (enough so there is 1-2" of liquid in the pot) and season with a little sea salt or soy sauce. Bring to a boil, then reduce heat, cover and cook until vegetables are tender, about 25-30 minutes.
Serve with extra soy sauce on the side if you wish.
Other vegetables that you might want to try are carrots, burdock, parsnips, cabbage, leeks, or lotus root.