Phosphorus (P) is a key mineral in dairy cattle rations and farm nutrient management. In the dairy cow, P is essential for bone growth and repair, nerve function muscle contraction and acid-base buffering in blood and the rumen. Phosphorus is also a key element for energy metabolism at the cellular level. In crops, P is an essential element in photosynthesis and stimulates root development. Phosphorus is needed for energy transfer and is component of DNA which is required for plant growth and reproduction. Management of P in agricultural, urban and natural ecosystems has become an increasingly important topic because increasing P levels in aquatic environments has led to ecosystem imbalances including harmful algal blooms. The goals of P management on the dairy farm are to provide adequate P in the ration to support milk production, reproduction and health while minimizing excretion of manure P which can be lost to the environment.