The current USGS projections suggest that nonrenewable resource of phosphorus (P) mineral rock will run out in another 50 years in much of the world.  Florida is a leading producer of P fertilizers over the world.  However, in about 25 years, Florida’s P reserve will be depleted. The farmers will have to import P fertilizers. Heavy reliance on the imported P fertilizers could lead to narrow profitability for farmers. Therefore, improving P use efficiency is imperative for crop production. This study was conducted in 2010 on a Quincy fine sand (mixed, mesic, Xeric Torripsamments) in Benton county, WA, using potato (variety, Umatilla Russet) with four replications under center-pivot irrigation. The objectives of the study were to (1) assess the effects of nitrogen (N) rates on tuber yield; (2) compare marginal yields with different N rates; (3) quantify the effects of N rates on P-use efficiency. Treatments included no N control (CK), (A) 112 kg N ha^-1 each as controlled release fertilizer (CRF, polyon coated urea,) and urea; (B) 168 kg N ha^-1 as CRF and 56 kg N ha^-1 as urea; (C) 224 kg N ha^-1 as CRF; (D) 336 kg N ha^-1 as urea; (E) 168 kg N ha^-1 each as CRF and urea; (F) 252 kg N ha^-1 as CRF and 84 kg N ha^-1 as urea. N was applied as pre-plant broadcast except in D, which received 224 kg N ha^-1 as five in-season fertigations at 2 weeks interval began 4 weeks after seedling emergence. All treatments received pre-plant broadcast of uniform P and K rates at 33 and 214 kg ha^-1, respectively. The results indicated that tuber yields ranged from 7.2±1.0 to 75.0±2.3 Mg ha^-1. The tuber yields across the different N treatments were 8 to 9 fold greater than that of CK. This indicates the yields increased 56±4.1 to 68±2.3 Mg ha^-1. The marginal yield of A, B, or C was 282±6.9, 249±18.2, and 264±17.1 kg tubers kg^-1 N ha^-1. However, that of D, E, and F was only 48±28.8, 76±20.9, or 55±29.5 kg tubers kg^-1 N ha^-1 for the extra amount of N, respectively. A, B, and C had 2.3 to 4.9 fold greater marginal yield than D, E, and F because the latter three had 50% extra N but had only approximately 10% increase in yield.  Application of optimal N rate increased P-use efficiency by 9 or 10 fold. These data allow us to conclude that keeping all nutrients in harmony is extremely important for optimizing P use efficiency. Over fertilization exacerbates the thin marginal profitability and may also cause water quality and other environmental problems. Under the condition of this study, 224 kg N ha^-1 rate is adequate for optimal potato production.