NITROGEN AND WATER INTERACTIONS IN SUBSURFACE TRICKLE-IRRIGATED LEAF LETTUCE .1. PLANT-RESPONSE

Rising water costs and concerns about groundwater contamination by NO3 - are forcing growers in arid regions to adopt practices to improve wa ter and N use efficiency. Subsurface trickle irrigation offers the abi lity to precisely deliver appropriate amounts of water and N to the cr op root zone. The objectives of this research were to: (i) determine t he optimum level (with respect to marketable yield) of soil water tens ion for subsurface trickle-irrigated leaf lettuce (Lactuca sativa L. c v. Waldmann's Green), (ii) determine N by water interactions on crop y ield and quality, and (iii) determine seasonal patterns of crop N upta ke. Three field experiments were conducted during the 1990 to 1993 win ter growing seasons on a Casa Grande soil (fine-loamy, mixed, hyperthe rmic, Typic Natriargid [reclaimed]) in southern Arizona. Deficient to excessive N (35-300 kg ha(-1)) and target soil water tension (SWT) tre atments (12.0-4.0 kPa) were applied in factorial combinations each yea r. Maximum marketable leaf lettuce yields observed were 54.0 Mg ha(-1) . Nitrogen rates associated with maximum yields ranged from 120 to 300 kg ha(-1), and depended on average SWT. Excessive irrigation (5.3 kPa average SWT) resulted in lower yields, head weight, and head length. Marketable yields showed a pronounced water x N interaction, but yield quality (head length and weight) did not. Maximum N flux was 4.3 kg h a(-1) d(-1), and an average of 75% of total N accumulation occurred in the 38 d prior to harvest. Results suggest that the soil water tensio n at 0.3-m depth should be maintained at 6 to 7 kPa for optimum yields of trickle-irrigated leaf lettuce.