When substituting celery seed in soups, the general rule is to use it sparingly because of its strong, concentrated flavor. As a guideline, use about 1/4 to 1/2 teaspoon of celery seed for every cup of soup or as a replacement for 1 stalk of fresh celery.
Celery seed has an earthy, slightly bitter taste that closely resembles the flavor of fresh celery, making it a great substitute when you don’t have celery stalks on hand. However, because of its intensity, adding too much can overwhelm the dish. Start with a small amount, taste, and adjust as needed.
Additionally, if you want to mimic the texture of celery, consider pairing celery seed with chopped carrots, onions, or another crunchy vegetable.