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The production of lettuce (Lactuca sativa L.) types other than crisphead (i.e., leaf, boston, bibb, and romaine) has recently increased due to expanding consumer demand. Fertilizer P recommendations for these lettuce types are largely based on soil-test calibrations for the crisphead type only. However, biomass production and morphological traits of the different lettuce types vary. Four field experiments were conducted to compare the relative efficiencies of these lettuce types to P fertilization. All lettuce types showed large yield and quality responses to P. Because environmental conditions affected yield potential, P rates required for optimal yield varied by lettuce type within experiments. However, the P rates required for optimal yield were similar over all experiments. Furthermore, the relationship between relative yield and soil-test P across all seasons showed a similar soil-test P level was required for maximum yield of all lettuce types. The results of this study show that soil-test-based fertilizer recommendations for crisphead lettuce may be adequate for all lettuce types

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Abstract

Studies were conducted during five cropping periods to evaluate the response of sweet corn (Zea mays L.) to N fertilizer source and rate on Histosols. Ammonium nitrate and ammonium sulfate were top-dressed at four rates (0, 20, 40, and 60 kg N/ha) to sweet corn plots at the 15-leaf (V15) stage. Nitrogen-15-depleted materials were used to assess the recovery of fertilizer N applied to these soils. From 13% to 62% of the N applied was accounted for in the aboveground plant (stover and ears). Recoveries of fertilizer N in the marketable ears ranged from 9% to 37%. The addition of N increased sweet corn yields in four of the five experiments, regardless of N source. Fertilizer N did not improve sweet corn yield in one experiment, but resulted in significant increases in sweet corn ear quality. These results suggest that under most conditions sweet corn yields on Histosols are maximized between 40 and 60 kg N/ha applied just before tasseling.

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Diagnosis and Recommendation Integrated System (DRIS) norms were derived for crisphead lettuce (Lactuca sativa L.) from field fertility experiments conducted over the past 20 years on mineral and organic soils in Florida. Preliminary testing indicates that DRIS diagnoses generally agree with diagnoses using the sufficiency range approach, with the advantage of predicting the degree of nutrient limitation. DRIS also appeared to correctly predict a response to K where sufficiency ranges currently used did not. Overall, DRIS appears to be a useful adjunct to the sufficiency range approach currently used to diagnose nutritional deficiencies in crisphead lettuce.

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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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Four field experiments were conducted during two production seasons to evaluate soil-test P fertilizer recommendations for celery (Apium graveolens var. dulce) produced on Histosols, which often are linked hydrologically to environmentally sensitive wetlands, and to evaluate band placement as a strategy for improving P fertilizer-use efficiency in celery in such areas. Phosphorus was applied (broadcast or banded) at 0,50, 100,150, and 200 kg P/ha. Broadcast P was surface-applied and disked into the soil ≈ 15 cm deep 1 day before planting. Banded P was applied 5 cm below the soil surface and 5 cm to the side of each celery row. Total above-ground mass, marketable trimmed yield of celery, and yield of the larger grade sizes increased with P rate in all experiments. Band P placement was not a viable strategy for improving P fertilizer-use efficiency for celery. However, our results indicate that previous soil-test-based P fertilizer recommendations for celery were too high for the cultivars grown currently, and improved P fertilizer-use efficiency can be obtained with revised soil-test calibrations.

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A study was conducted during four seasons to evaluate the nutrient requirements of `Valencia' oranges converted from flood to a pressurized spray irrigation system. The experiment was a 3×2×2 factorial with 3 N rates (0.34, 0.68, and 1.36 kg/tree/year), 2 P rates (0 and 0.11 kg/tree/year) and with and without added micronutrients (Fe, Zn, Mn, and Cu). There were no growth or yield responses to micronutrients. Phosphorus fertilization increased fruit yield, improved juice quality, and reduced peel thickness. There were trends for N to reduce juice quality and increase peel thickness when P fertilizer was not added. Tree growth increased by N fertilization only the first season after conversion. Fruit yield also increased by N but only when P was added. Leaf tissue N concentrations increased with time during the first two years within N treatments. These data suggest that the higher rates of N may only be needed initially after conversion as the tree roots adapt to the new irrigation system.

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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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Potassium is often considered the nutrient element most limiting to crop production on organic soils. On Histosols in southern Florida, K2SO4, rather than KCl, is often used for lettuce (Lactuca sativa L.) production to minimize the risk of salt injury. However, recent soil-test calibration research suggests that current K fertilizer recommendations for lettuce may be too high. Four field studies were conducted from 1989 to 1991 to evaluate the response of five lettuce types to K rate and source. The five lettuce types evaluated were leaf, bibb, boston, romaine (cos), and crisphead. Two sources of fertilizer K (K2SO4 and KCl) were evaluated at rates ranging from 0 to 600 kg K ha-1. Lettuce showed a minimal or no response to K fertilization. Potassium chloride had detrimental effects on lettuce only when applied at rates in excess of those required for optimal production. These studies showed that K fertilizer recommendations for lettuce produced on Histosols in Florida can be reduced. Furthermore, KCl, a more economical source, is suitable when the K is applied at appropriate rates.

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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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Five field experiments were conducted from 1986 through 1988 to evaluate the response of radish (Raphanus sativus L.) to rate and source of P (triple superphosphate and phosphoric acid) and to rate of K (KC1) on Histosols. Marketable radish root yields increased with P fertilization when the soil tested <13 mg P/dm3 using a test for water-soluble P. No significant differences were due to P source. Results of leaf tissue analysis suggested that the critical concentration of P in radish leaves was 0.45%. Radish did not respond to K fertilization in any of the five experiments, even though preliminary soil-test K levels ranged from 20 to 102 K/dm3. Histosols used for crop production in Florida rarely test below 20 mg K/dm3; thus, radish rarely would require supplementary K fertilization for optimal yield.

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