Bring wild rice and its water to a boil in a saucepan. Cover partly and simmer for 45 to 55 minutes until many grains split and become tender, then drain.
Mix ground chicken, ricotta, breadcrumbs, egg, 1/2 teaspoon salt and 1/4 teaspoon pepper gently. Refrigerate for 15 minutes, then shape into 24 equal small meatballs with damp hands; the chill is included in prep time.
Heat oil in a soup pot over medium heat and cook leeks for 6 minutes. Add celery root, broth, sage and remaining salt and pepper and simmer for 12 minutes.
Lower meatballs individually into the gently simmering broth. Cover partly and cook for 10 to 12 minutes without vigorous stirring until the largest meatballs reach 165°F and celery root is tender.
Fold in drained wild rice and lemon juice and warm for 2 minutes. Divide the broth and vegetables among 6 bowls with 4 meatballs per serving.