Phosphorus loss from agricultural land is mostly from erosion. However some phosphorus can be in solution and may be lost in runoff. Over time, with heavy applications of phosphorus fertilizer or manure, phosphorus can move down in the soil profile.
This regional publication that focuses on phosphorus explains the science and gives some management options in more detail:
Agricultural Phosphorus Management and Water Quality Protection in the Midwest: A Heartland Regional Water Coordination Initiative Publication* RP187 (1.86 MB; 24 pages)
An Excel spreadsheet is available to help determine the risk of phosphorus movement from a field getting to a stream. Free download of software: Nebraska P Index (.xls file)
Also available is a companion circular that explains how to use the software:
The Nebraska Phosphorus Index (2007): Background and Users Guide* Extension Circular 195 (513 KB; 6 pages)
*Document in pdf format. Download the free Acrobat Reader.