Nitrogen and Water Interactions in Subsurface Trickle-Irrigated Leaf Lettuce: I. Plant Response

Rising water costs and concerns about groundwater contamination by NO 3 - are forcing growers in arid regions to adopt practices to improve water and N use efficiency. Subsurface trickle irrigation offers the ability to precisely deliver appropriate amounts of water and N to the crop root zone. The objectives of this research were to : (i) determine the optimum level (with respect to marketable yield) of soil water tension for subsurface trickle-irrigated leaf lettuce (Lactuca sativa L. cv. Waldmann's Green), (ii) determine N by water interactions on crop yield and quality, and (iii) determine seasonal patterns of crop N uptake. Three field experiments were conducted during the 1990 to 1993 winter growing seasons on a Casa Grande soil (fine-loamy, mixed, hyperthermic, Typic Natriagid [reclaimed]) in southern Arizona. Deficient to excessive N (35-300 kg ha - 1 ) and target soil water tension (SWT) treatments (12.0-4.0 kPa) were applied in factorial combinations each year. 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 ha -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 tension at 0.3-m depth should be maintained at 6 to 7 kPa for optimum yields of trickle-irrigated leaf lettuce.