The Iowa phosphorus (P) index is an assessment tool that was developed to assess the risk of P loss from fields to water resources. It provides a risk rating that also can be used to prioritize fields or field zones for manure or fertilizer P application and for implementing improved soil conservation practices. Scientists have been proposing such a tool since the early 1990s, and it has evolved from a simple subjective tool to a more complete objective tool without becoming a complex model of P flow. The need for a P index has its origin in two main issues. One is that P accumulation in many soils in excess of amounts needed by crops has increased P losses from fields and has resulted in poor water quality in many streams and lakes. Water quality is impaired through a process known as eutrophication, which occurs when nutrient levels in water (mainly P) are high and stimulate excessive algae growth. Excessive algae growth reduces water oxygen levels and creates ecological imbalances that result in reduced populations of desirable fish species as well as reduced drinking and recreational value of lakes and streams. The second issue is that P loss from fields cannot be appropriately assessed or predicted only from knowledge of soil-test P, manure or fertilizer P applied, and method of P application used. Although these P source factors are important, how P can move off fields through various transport mechanisms is as important. Therefore, a new tool that integrates P source and transport factors was needed to estimate risk of P loss from fields. The P index also provides information useful to decide among several soil conservation and manure or fertilizer P management practices that can maintain a low risk of P loss or can reduce it.