Marginal Yield and Nitrogen Balancing Vs. Phosphorus Use Efficiency of Potato Under Center Pivot Irrigation.

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.