Over the last 20 years, there has been an increased occurrence of harmful algal blooms, such as Pfiesteria and cyanobacteria, in fresh waters of the U.S.A. These blooms have been linked to greater inputs of phosphorus (P) and having impaired human health and drinking-water treatment, culminated in recent voluntary and regulatory guidelines for managing P-rich residuals (i.e., animal manures and biosolids) applied to agricultural land. Although there is a wealth of agronomic trials determining P applications that meet crop yield goals, environmental P-risk assessment of land-applied residuals has until recently been based on expert advice, unfounded assumptions, and in some cases uneducated guesses. Methods have been developed to determine the environmental activity or mobility of P in residuals, which range from measures of water extractable P, other factors influencing P mobility in soil (e.g., Al, Fe, Ca, pH), to site assessment of P loss risk (which will be presented). In spite of supporting field-based research (which will be shown), there is a reluctance of regulators to consider adopting guidelines that address all factors which researchers know control P loss. This is due to often overly complex guidelines, a desire for guidelines to be based on state-specific data rather than region or often, the other side of the country, but most notably, because clearly-defined, black and white regulations that are needed to be enforceable, cannot adequately represent the environmental uncertainties associated with the risk of P loss (i.e., climate, edaphic, and socio-economic variability). Although several states have met this challenge, there is still the hurdle of making regulations flexible enough to be able to modify guidelines in a timely manner to reflect new research and a better understanding of factors dominating the potential for P loss from land-applied residuals.