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R.L. Roth, B.R. Gardner, and C.A. Sanchez

A study was conducted during four seasons to evaluate the performance of mature `Valencia' oranges converted to pressurized irrigation systems. Trickle, bubbler, spray, and sprinkler systems were all compared to the traditional flood-border irrigation. During the second year after conversion, trees irrigated by flood grew significantly more than trees irrigated by any of the pressurized systems. However, there were no differences in tree growth during the third and forth year, suggesting that the trees adapted to the new irrigation systems. Effects of irrigation treatments on leaf concentrations of N, P, Fe, Zn, Mn, and Cu were minimal. There were significant differences in orange yields among the irrigation treatments within years. However, average or total yields over the four year period did not vary by irrigation treatments. Similarly, there were no consistent differences in fruit or juice quality. Overall, results from this study indicate the mature citrus can be converted to pressurized irrigation systems with minimal effects on fruit yield and quality.

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M. Wilcox, C.A. Sanchez, and T.M. Blackmer

Several studies in the midwestem United States have shown that chlorophyll meter readings (Minolta SPAD 502) are useful in determining the N status of corn (Zea Mays L.), and show promise as a tool for the efficient N management of corn. Studies were conducted to evaluate the potential of the `chlorophyll meter for evaluating N deficiencies in lettuce (Lactuca sativa L.). Data for chlorophyll meter readings, midrib nitrate-N, lettuce growth rate, and marketable lettuce yield were collected in five N fertility experiments in 1993 and 1994. Chlorophyll meter readings not only varied among lettuce types (butter, cos, leaf, crisphead), but also among cultivars of the crisphead type. Chlorophyll meter readings were generally poorly correlated to midrib nitrate-N levels and marketable lettuce yield. Lettuce leaves have more color variation than corn leaves, and perhaps this variation in relation to the small sensor size on the SPAD 502 confounded readings. The observation that subtle N deficiencies in lettuce are usually manifested in growth rate reduction rather than abrupt color changes may also limit the usefulness of the chlorophyll meter for lettuce.

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Charles A. Sanchez, Abraham Galadima, and Jeffrey C. Silvertooth

Vegetable and fruit crops produced in the desert southwestern United States generally do not respond to K fertilization. Even when pre-plant soil test K levels are low and crop K accumulations are high, responses are infrequent. We have performed a number of evaluations aimed at understanding why crops produced in this region fail to respond to K fertilization. First, data show the potential for substantial K inputs through irrigation. For example, Colorado River water, which is widely used for irrigation in this region, contains ≈5 ppm K, resulting in potential K inputs of 30 to 60 kg K/ha. Second, many of the soils used for crop production have a clay content and mineralogy making a response to K unlikely. Studies evaluating the kinetics of K release from the mineral fraction of soils in the region has shown that many soils used for crop production have a high capacity to replenish K to the soil solution and exchange sites following crop uptake. Finally, the observation that Na can partially substitute for the K requirement of many fast-growing leafy vegetables may also be a contributing factor for the infrequent K fertilizer responses for these commodities.

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C.A. Sanchez, R.L. Roth, and B.R. Gardner

Six field studies were conducted from 1980-88 to evaluate the response of cabbage (Brassica oleracea L., Capitata group) to sprinkler irrigation and sprinkler-applied N fertilizer on a coarse-textured soil. The plots were irrigated using a modified self-moving lateral sprinkler irrigation system that applied five levels of water and five levels of N (liquid NH4NO3) in specified combinations of central composite rotatable design. Cabbage yields were significantly increased by water and N applications in all experiments. The N rates predicted for maximum yield exceeded typical cabbage N fertilizer recommendations. However, the above-average plant populations used in these studies resulted in above-average yields and plant N accumulation. Deficit and excess irrigation produced negative results. Generally, cabbage production was optimized and N losses to the environment were minimized when crops were irrigated for evapotranspiration (ET) replacement. However, even when irrigated for ET replacement, these data demonstrate the potential for N leaching at high N rates, presumably as a result of rainfall.

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R.T. Nagata, C.A. Sanchez, and F.J. Coale

Four field experiments were conducted during 1988 to 1990 to evaluate the response to fertilizer P of six crisphead lettuce (Latuca sativa L.) cultivars grown on Histosols. There were season × cultivar interactions for total mass produced, marketable yield, and P uptake by lettuce. A significant yield response to fertilizer P was demonstrated during all four seasons. The performance of individual cultivars within a given season led to cultivar × P rate interactions for marketable yield. However, there were no significant P rate × cultivar interactions for total mass produced, P uptake, and marketable yield during the one season when growing conditions were near ideal. Calculated critical soil-test P values for both eastern and western lettuce types produced in Florida were all within error currently associated with P fertilizer recommendations for lettuce produced in Florida. Therefore, we conclude that no immediate change in P fertilizer recommendations is required for the new western-type lettuce cultivars produced on Histosols in Florida.

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C.A. Sanchez, S. Swanson, and P.S. Porter

Five field experiments were conducted from 1986 to 1989 to compare broadcast and band P fertilization of crisphead lettuce (Lactuca sativa L.) on Histosols. Rates of P were 0, 50, 100, 200, and 300 kg P/ha applied broadcast or banded. Broadcast P was surface-applied and disked into the soil 1 day before bedding and planting. Banded P was placed in strips 8 cm wide, 5 cm below the lettuce seeds at planting. Lettuce yields were significantly(P < 0.01) increased by P rate in all experiments. However, significant rate-by -placement interactions indicated that response of lettuce to P varied by placement. Lettuce yields were generally optimized with a band P rate one-third of that required with broadcast placement. Analysis of soil samples collected in the lettuce bed after fertilization indicated that banded P increased available P in the lettuce root zone compared to broadcast fertilization. Lettuce leaf P concentration increased with P rate and generally was greater when P was banded. The critical concentration of P in lettuce leaf tissue at the six- to eight-leaf stage was 0.37%. Banding P fertilizer did not reduce the availability of other essential nutrients, as indicated by tissue analysis.

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C.M. Tankou, B. Schaffer, S.K. O'Hair, and C.A. Sanchez

The effects of applied N and shading duration on net gas exchange and growth of two southern Florida cassava (Manihot esculenta Crantz.) cultivars grown in containers were determined. Both cultivars responded similarly to shading and N with respect to the measured variables. There were no interactions between shading duration and N application rate for any of the variables measured. Tissue dry weights, total leaf N and chlorophyll concentrations, net CO2 assimilation (A), transpiration (E), water-use efficiency (WUE), and stomatal conductance (gs) were quadratically related to the concentration of N applied to the soil. The optimum N application rate for maximum growth of both cultivars was 60 mg/plant per day. Increased shading duration reduced A, E, gs, WUE, storage root number, and weight and increased the shoot : root ratio.

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A. Galadima, C.A. Sanchez, J. Palumbo, B. Tickes, M. Matheron, and M. McGiffen

Experiments were conducted during 1998–99 seasons to evaluate the potential for organic vegetable production in the low desert of the southwestern United States. The experimental design included three summer management options [fallow, cowpea (Vigna sinensis), and sudangrass (Sorghum vulgare)] in factorial combination with alternative production systems, which included organic and conventional systems. The crops cultivated were iceberg lettuce (Lactuca sativa L) during the fall–winter period and melons (Cucumis melo Reticulatus Group) during the spring. The organic plots were managed with strict adherence to California Certified Organic Farmers (CCOF) guidelines. Summer cover crop management seemed to influence the early growth and N uptake of lettuce, but had no final effect on yield and quality. The organic production system resulted in lower yields and inferior product quality compared to the conventional system. Generally, disease and weeds were not limiting factors, although labor costs for weed control would be slightly higher in organic plots. Insects, primarily aphids (various types) and thrips (Frankliniella Occidentalis Perancle), and fertility, primarily N, were factors limiting yield and quality in organic systems. Control of whiteflies (Bemisia argentifoli) was the limiting factor for melons. Studies during 1999–2000 are focused on overcoming the challenges of the insect and fertility management in organic systems.

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C.A. Sanchez, T.M. Blackmer, G.E. Meyers, and J.S. Schepers

Lettuce (Lactuca Sativa L.) produced in the low desert typically shows large yield responses to N fertilization. Concern about the potential threat of nitrate-N to ground water prompted the state of Arizona to pass legislation aimed at implementing improved N management practices. Nitrogen management guidelines recommended by the University of Arizona for lettuce suggest a preplant application based on a soil nitrate-N test and subsequent sidedress applications based on plant tissue monitoring. However, growers have some anxiety that close adherence to recommendations resulting from an average plant sample may compromise crop uniformity. Aerial photographs have the potential to detect differences in N status in any portion of the field. This study evaluated digital computer analysis of aerial photographs as a tool for evaluating the N status of lettuce. The digitized photographs appeared to detect deficiencies not apparent to the human eye. There were good correlations (R2 0.83 to 0.99) between Gray-scale ratio and N status, suggesting digital analysis of aerial photographs has potential for diagnosing N deficiencies in lettuce.

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C.A. Sanchez, R.L. Roth, B.R. Gardner, and Harry Ayer

Field studies were conducted to develop water and N response surface models for broccoli and cauliflower (Brassica oleracea L., Botrytis Group) produced in the low desert of the southwestern United States and to estimate profit maximizing combinations of water and N over a range of realistic price situations. Marketable broccoli and cauliflower yields were increased by water and N inputs in all experiments. Generalized response equations indicate maximum broccoli yields with 43 cm of water and N at 267 kg·ha–1 and maximum cauliflower yields with 65 cm of water and N at 338 kg·ha–1. Least-cost combinations of water and N changed with the costs of these inputs for yield levels below the economic maximum. However, profit maximizing N and water rates changed little regardless of input or crop prices investigated.