If I had to use only two herbs for the rest of my life, I’d choose thyme and parsley. There is hardly a soup that thyme doesn’t improve by blending subtly and enhancing the flavor of the other ingredients.
Because thyme releases its flavor slowly into a surrounding liquid, it should be added to a soup near the beginning of cooking—usually in the bouquet garni. When composing a bouquet garni, I use about 5 sprigs of fresh thyme (or one teaspoon dried leaves) for 2 quarts of soup.
I prefer fresh thyme to dried, but if you can’t find it fresh, it is still worthwhile to use dried thyme. The best dried thyme is sold with the leaves still on the branches, but this can be hard to find, so you may have to settle on the little jars of thyme leaves. Don’t bother buying ground thyme.