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Two field experiments were conducted with subsurface trickle-irrigated romaine lettuce (Lactuca sativa L. cv. Parris Island Cos) during the 1990-92 winter growing seasons in southern Arizona. The objectives were to determine 1) yield and quality response to varying combinations of soil water tension (SWT) and N fertilizer, 2) seasonal patterns of N uptake, and 3) unutilized fertilizer N. During 1990-91, N rates were 35, 120, and 205 kgha -1 . During 1991-92, the experiment was factorial with N levels from 50 to 300 kgha -1 and target SWT levels of 7.0 and 4.0 kPa. Unutilized fertilizer N was the difference between fertilized and nonfertilized plots in total N inputs not harvested in the crop. When excessive irrigation was not applied (SWT between 6.5 and 7.4 kPa), 95% of the maximum crop yield and yield quality (head length and fresh mass) response occurred at N rates of 156 to 193 kgha -1 , with unutilized fertilizer N 74% of its total N uptake in the 38 days before harvest. Unutilized fertilizer N increased sharply when adequate N and water rates were exceeded. These results suggest that a target SWT of no wetter than 6.5 kPa is appropriate for subsurface trickle-irrigated romaine lettuce. its total N uptake during the 4 weeks before harvest. Large N applications, low N recov- ery, and large amounts of residual N may

Nitrogen and Water Rates for Subsurface Trickle-irrigated Romaine Lettuce