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Automatic feedback control of precision irrigation scheduling allows the determination of optimum soil water potential for crops, close determination of N fertilizer requirements, and other measures of crop development and yield responses. Soil water potential (SWP) was measured with granular matrix sensors (GMS; Watermark Soil Moisture Sensors Model 200SS, Irrometer Co., Riverside, Calif., USA) at 0.2-m depth below onion and potato rows. Sensors were calibrated to SWP. The GMS were connected to a datalogger (CR10X datalogger, Campbell Scientific, Logan, Utah, USA) via five multiplexers (AM416 multiplexer, Campbell Scientific). The datalogger was programmed to read the GMS in each irrigation treatment zone 4 to 8 times a day and irrigate each zone individually as necessary. Irrigations were controlled by the datalogger signaling a controller (SDM-CD16AC controller, Campbell Scientific) connected to solenoid valves for each zone. The pressure in the drip lines was maintained at 69 kPa by a pressure regulator in each zone. The amount of water applied in each zone was recorded by a water meter installed between the solenoid valve and the drip tape. The irrigation criteria for onion (Allium cepa) and potato (Solanum tuberosum) were determined for subsurface drip irrigation (SDI). N fertilizer requirements may be reduced due to reduced N leaching under SDI.