Rising water costs and concern for groundwater contamination by NO 3 and other agricultural chemicals are forcing growers in arid regions to improve irrigation and N fertilization efficiency. Applying N and water through a subsurface drip irrigation system in conjunction with in‐season monitoring techniques for assessing crop water and N status has the potential to greatly improve water and N use efficiency. The objectives of this research were to: (i) investigate the interactive effects of water and N applied through a subsurface trickle irrigation system on watermelon [ Citrullus lanatus (Thumb.) Matsum and Nakai var. lanatus ] fruit yield and on the potential for leaching losses of NO 3 , and (ii) determine the optimum range of soil water tension for subsurface trickle‐irrigated watermelon. Field experiments were conducted during 1990 and 1991 on a reclaimed Casa Grande soil (fine‐loamy, mixed, hyperthermic Typic Natrargid) in southern Arizona. Levels of target soil water tensions and fertilizer N were arranged in a factorial design to determine fruit yield response surfaces. Tensiometers at 0.3‐ and 0.6‐m depth were used to monitor soil water tension throughout the growing season. Marketable watermelon yield showed a pronounced positive water × N interaction in both years of the study. Predicted marketable yields were 90 Mg ha −1 when mean soil water tension was 6 kPa and applied N was between 200 and 270 kg ha −1 in 1990 and a predicted yield maximum of 102 Mg ha −1 at 7.2 kPa and 336 kg N ha −1 in 1991. Scheduling of irrigations using feedback from tensiometers proved very useful in providing optimum amounts of water while avoiding conditions that favor N loss through leaching or denitrification.
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Pier et al. (1995) studied this question.