There may be a couple of reasons you’re getting a stringiness from the asparagus in the soup. The asparagus may be old or the ends needed to be trimmed more. The bottom ¼-⅓ of a stalk of asparagus is tougher and stringier than the rest of the asparagus stalk. Finally, be sure to blend the soup throughly, and in batches if necessary, to make sure everything is nice and smooth.