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Rising water costs and concern for groundwater contamination are encouraging growers to improve irrigation and fertilization efficiency. The objectives of this study were to determine water and fertilizer nitrogen (N) rates leading to optimum yield and harvest quality and to develop a plant tissue test to aid in melon nitrogen fertilization. In 1990, a field experiment consisting of a complete 3x3 factorial arrangement of optimum, sub- and super-optimum rates of urea ammonium- nitrate and water applied through a subsurface trickle irrigation system to cantaloupe, honeyloupe and watermelon was conducted at the Maricopa Agricultural Center. Petioles were sampled from the youngest mature leaf beginning at the early runner stage and then weekly until first harvest. Petiole nitrate concentrations were determined using a high pressure liquid ion chromatograph. Harvested melons were weighed and graded for marketability and soluble solids were determined. Petiole nitrate levels were highly responsive to N fertilizer treatments and accurately quantified visual observations of crop N status. Petiole nitrate results also indicated that later fertilizer split applications occurred after the point of maximum plant uptake. Tensiometer readings suggested that the highest rate of water application led to deep percolation and nitrate leaching where nitrogen fertilizer was excessive. Watermelon showed the clearest yield response to the water and nitrogen treatments. Honeyloupe responded well to high water but poorly to higher nitrogen application rates. Cantaloupe yields responded best to higher nitrogen and medium water levels.

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A field experiment using subsurface drip irrigation was conducted at the Maricopa Agricultural Center on a Casa Grande sandy loam soil to evaluate the effect of varying nitrogen (N) sources on the growth and yield of 'Sweetie' 82' sweet corn when applied with, and without four nitrification inhibitors (Nl). The NI treatments included nitrapyrin (N-Serve*), dicyandiamide (DCD), ammonium thiosulfate, and N-Hib Calcium™. Nitrogen was supplied as urea -ammonium nitrate (UAN -32). In addition, an all nitrate source plus a control which received no added N were used. Al! N treated plots received a total of 111 lbs. N acre in split applications at the V3, V6 and the V12 stages. All solutions were applied through buried, perforated PVC tubing to simulate application through the buried drip irrigation system. The inclusion of the nitrification inhibitors with UAN-32 had no significant effect on marketable ear yield, total N uptake or nitrogen use efficiency.

Two field studies were conducted during the 1993-1994 season to evaluate the response of lettuce to soil applied amendments. Treatments included gypsum, polymaleic acid (Spersal), and two acid products (N-phuric and Phos-phuric). Gypsum rates (0, 2240, and 4480 kg /ha) were the main plots and other soil amendments were subplot randomized within the mainplots. Overall, gypsum reduced early growth and vigor of lettuce. Gypsum had no effects on marketable yield and quality parameters in experiment 1, but the highest gypsum rate (4448 kg /ha) decreased marketable yield in experiment 2. There were no differences in plant stands due to subplot treatments. However, there were differences in earlygrowth and vigor. In experiment 1, both "N-phuric" and "Phos-phuric" increased early lettuce growth compared to the control. In experiment 2, Phos-phuric was superior to N-phuric. In experiment 1, the benefits of early growth and vigor to the "N-phuric" and "Phos-phuric" carried to harvest where marketable yield and quality were significantly increased. Spersal did not significantly effect yield and quality of lettuce in either experiment.

Lettuce produced in the desert typically shows large yield responses to N fertilization. However, concern about the potential threat of nitrate-N to ground water has prompted additional studies aimed at developing improved N management practices. Field experiments were conducted between 1992 and 1994 to evaluate the response of iceberg lettuce to N rate and N management practice. The use of controlled release N sources (CR19 were compared to a soluble N fertilizer applied preplant (PP), and a soluble N fertilizer applied in split-sidedress applications (SD). Rates of N fertilizer application ranged from 0 to 300 kg ha⁻¹. Lettuce generally showed significant responses to N rate and N management practice. However, response to management practice varied by site-season. When conditions for N losses were high, SD and CRN management strategies were superior. However, in other site seasons SD management sometimes resulted in inferior head quality and marketable yield when compared other management strategies. Data averaged over six -site seasons shows improved yield and quality to CRN management strategies compared to PP and SD strategies. Although the controlled-release fertilizers used in these experiments cost three times more the conventional soluble sources used, preliminary analysis shows the use of CRN strategies would sometimes be economically favorable.