Part of the P applied to soils is intended to build up Dizocilpine P fertility in soils that have agronomically sub-optimal P concentrations. In Ireland, 20 kg P ha−1 yr−1 is advised for index 1 soils and 10 kg P ha−1 yr−1 for index 2 for build up (Coulter and Lalor, 2008). Accounting for the estimated area of index 1 and 2 soils on these farms and their associated potential P build-up requirements to achieve optimal soil P concentrations (index 3), an “Optimal P Balance” was estimated. This Optimal P Balance indicates how the actual annual farm P imports compare to the annual imports that would be required to build the levels of soil P fertility on the farm to the agronomic optimal level (index 3), with negative values indicating that more P imports would be required. Dairy farm-gate Optimal P Balances ranged from −7.2 to 5.1 kg P ha−1 with a mean of −1.7 kg P ha−1 (Table 3). An eco-efficiency metric was calculated as the surplus P per ha/t milk solids (MS) produced and ranged from −0.06 to 0.45 with a mean of 0.11 kg surplus P ha−1 t MS−1 yr−1.