NITROGEN AND WATER INTERACTIONS IN TRICKLE-IRRIGATED WATERMELON

Rising water costs and concern for groundwater contamination by NO3 an d other agricultural chemicals are forcing growers in arid regions to improve irrigation and N fertilization efficiency. Applying N and wate r through a subsurface drip irrigation system in conjunction with in-s eason monitoring techniques for assessing crop water and N status has the potential to greatly improve water and N use efficiency. The objec tives of this research were to: (i) investigate the interactive effect s of water and N applied through a subsurface trickle irrigation syste m on watermelon [Citrullus lanatus (Thumb.) Matsum and Nakai var. lana tus] fruit yield and on the potential for leaching losses of NO3, 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, hype rthermic Typic Natrargid) in southern Arizona. Levels of target soil w ater 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 shelved a pronounced positive wat er x N interaction in both years of the study. Predicted marketable yi elds were 90 Mg ha(-1) when mean soil water tension was 6 kPa and appl ied 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. Sched uling of irrigations using feedback from tensiometers proved very usef ul in providing optimum amounts of water while avoiding conditions tha t favor N loss through leaching or denitrification.