One of the most effective ways to reduce phosphorus surplus on dairy farms is to reduce excess phosphorus in diets. In research trials, excessive mineral P supplements fed to dairy cows were well correlated with high amounts of water soluble P in feces (R²=0.95). The findings led to the development of a technique to use fecal P as an indicator of excessive P feeding. The technique was evaluated on commercial dairies in the northeast US. The relationship between dietary P and water soluble P of feces deteriorated (R²=0.46) as compared to the research trials, mainly due to variations in pH and Ca content of feces on the farms. Using a dilute acid to replace water as extractant helped overcome the impact (R²=0.69), and has the potential to serve as an indicator of excessive P feeding. Many dairy farms import most of their grains while producing forages on farm. With typical P feeding levels, this importation results in accumulation of P in farm soils. If dietary P could be reduced by about 33% without impairing production, farm P balance would be greatly improved. Animal nutritionists need to be aware of farm P balance issues to know when to avoid use of byproducts high in P and unnecessary mineral supplements.