Three field experiments using subsurface trickle irrigation with various rates of target soil water tension (SWT) and N rates were conducted in southern Arizona during 1990–93. The experiments were conducted with collard ( Brassica oleracea L. Acephela Group cv. Vates), mustard [ Brassica juncea (L.) Czerniak cv. Southern Giant], and spinach ( Spinacea oleracea L. cv. Indian Summer). The interactive effects of water and N treatments on crop yield, N uptake, and unutilized fertilizer N were studied. In general, excessive irrigation (SWT <5.6 kPa) resulted in lower yield and N uptake and higher unutilized fertilizer N. Optimum SWTs were 9 kPa for collard, 8 kPa for spinach, and 6 to 10 kPa for mustard.
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Thompson et al. (1995) studied this question.