In 1998 the Maryland Nutrient Management Law was passed requiring nitrogen (N) and phosphorus (P) based planning for nearly all commercial agricultural operations in the state. The law included a P site index as a primary tool for ranking field vulnerability for P losses to surface waters. At the time, an index that could accurately assess the risk across all of Maryland did not exist. The rigid implementation timeframe of the law required release of Maryland's P site index well before it could be fully validated. We will present a statistical method for determining the most influential factors in Maryland's P site index based on data collected from almost 4000 fields between 2001 and 2007. Interpretation of the results of the statistical analysis in the context of the best available science on the movement of P in the environment will be discussed. Furthermore, we will discuss the successes and short-comings of the rapidly deployed P index and how it will continue to change as new science and data becomes available.