| Summary: | Arizona is ranked third in the nation in fresh vegetable production, however, overfertilization and overirrigation have been common practices used to maintain adequate yields and crop quality. Concern for groundwater contamination, water availability and costs have made it necessary to increase water and nitrogen (N) use efficiency. The objectives of this research were: 1. optimize water and N inputs applied through a subsurface trickle irrigation system to leaf lettuce, spinach and collards. 2. evaluate water and N effects on midrib nitrate (NO₃-N) concentration as a tool for monitoring plant N status throughout the growing season. A field experiment was conducted during fall-winter 1992-93 at the University of Arizona Maricopa Agricultural Center, Maricopa, AZ. A factorial design with three target soil water potentials and four N levels was used to determine response surfaces for yield, net return, and water and N use efficiency. Tensiometers were used as an aid to schedule irrigations, and midrib NO₃-N concentrations at critical growth stages were used to monitor N status. Spinach and collard midrib N03-N concentrations showed a high response to N rates throughout the growing season. Leaf lettuce midrib NO₃-N was responsive to treatments at the beginning and end of its growing season. Some yield and efficiency parameters were significantly affected by the irrigation treatments with an effect generally negative. Nitrogen had tremendous influence on them, perhaps due to the wet seasonal conditions which may have promoted nitrate leaching and denitrification. For leaf lettuce, maximum fresh weight was estimated to be 60861 kg ha⁻¹ at -4.3 kPa soil water potential and 367 kg N ha⁻¹. Maximum net return was estimated to be $18,089 ha⁻¹ when average soil water potential was -6.0 kPa and applied N was 355 kg N ha⁻¹. Estimated maximum nitrogen use efficiency (NUE) was 0.8 at -10.3 kPa soil water potential, and 2 kg N ha⁻¹. It was not possible to determine a maximum value for applied water use efficiency (A WUE). For spinach, maximum yield was estimated to be 33948 kg ha⁻¹ at -8.0 kPa soil water potential and 400 kg N ha⁻¹. Estimated net return was $18,573 ha⁻¹ when soil water potential and applied N rate were -8.2 kPa and 398 kg N ha⁻¹ respectively. For NUE, the estimated maximum was 0.7 at -11.2 kPa and -42 kg N ha⁻¹. It was not possible to determine a maximum for A WUE. For collards, 49466 kg ha-1 was the estimated maximum yield when soil water potential was -9.0 kPa and applied N was 396 kg N ha⁻¹. Maximum net return was estimated to be $15,561 ha⁻¹ at -9.3 kPa and 394 kg N ha⁻¹. It was not possible to determine maximums for A WUE or NUE. The relationship between midrib NO₃-N concentrations and chlorophyll meter readings was poor, and prediction of plant NO₃-N status was not as reliable as expected.
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