If you add water to soup it will dilute it. For phosphorus it is a timing issue. Ohio farmers don't start fertilizing their fields until later in the spring. Dr. Weidel check the phosphorus levels from June thru September data if you have it. The data below shows trends but doesn't tell the story of our high water period.
https://aslopubs.onlinelibrary.wiley.com/doi/10.1002/lno.10055