1. In a heavy 6- to 8-quart casserole combine the split peas, pig's feet, salt pork and water. Bring to a boil over high heat, skimming off the foam and scum as they rise to the surface.
2. Reduce the heat to low, partially cover the pan, and simmer for 3 hours. Then add the potatoes, leeks, celery root and celery leaves, and simmer, partially covered, for 30 minutes.
3. With tongs or a slotted spoon, transfer the pig's feet and salt pork to a cutting board. Remove and discard the skin, gristle and bones from the pig's feet, then cut the meat and the salt pork into ½-inch dice.
4. Return the diced meats to the soup and add the sliced sausage, crumbled summer savory and a few grindings of black pepper.
5. Stirring constantly, bring the soup to a simmer over moderate heat and cook for a few minutes to heat the sausage through.
6. Taste for seasoning and serve at once from a heated tureen or in individual soup plates.
