Roswall, Lauren TPhosphorus (P) is an essential nutrient for sustaining plant, animal, and human health, but can be a pollutant in water bodies. In intensive animal agriculture regions globally, long-term manure application has led to an abundance of P in soils. Exploitation of finite reserves of P is concerning for the future of agriculture and water quality. This research investigates how better understanding the most labile pool of P in soils and manures, i.e., water-extractable P (WEP), can lead to better quantification and risk-assessment of P loss. High P soils were extracted using varying methodology to improve long-term P loss risk assessment. Poultry litter and products were added to a soil to understand how WEP-based application (WEP) affects the loss of dissolved P forms in runoff. We conclude that an accurate quantification of WEP in both soils and manures is needed to improve manure management and mitigate P loss to waterways.enThesisSoil sciences