Now that's interesting! I never would have thought to try a tomato broth with potatoes.
The way my mother taught me to make potato soup was fairly simple. It isn't quite from scratch, but it gets the job done.
-6-7 red potatoes, peeled and diced
-1 can cream of herb
-3-4 cups vegetable or chicken broth
-1 bunch green onions, sliced
First, boil the potatoes. Cook through and drain. Mash half the potatoes. Combine the mashed potatoes, diced potatoes, cream of herb, and the broth. Some people like milk, I prefer broth as it adds more flavor. Go slowly until the soup reaches the desired consistency. Garnish with green onion.
Some good additions: carrot sticks, bacon, and shallots.