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

Abstract

AbstractRising 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 Natriargid [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 × 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.