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John D. Lea-Cox, James P. Syvertsen, and Donald A. Graetz

15Nitrogen uptake, allocation, and leaching losses from soil were quantified during spring, for 4-year-old bearing `Redblush' grapefruit (Citrus × paradisi Macf.) trees on rootstocks that impart contrasting growth rates. Nine trees on either the fast-growing `Volkamer' lemon (VL) (C. volkameriana Ten & Pasq.) or nine on the slower-growing sour orange (SO) (C. aurantium L.) rootstocks were established in drainage lysimeters filled with Candler fine sand and fertilized with 30 split applications of N, totaling 76, 140, or 336 g·year-1 per tree. A single application of double-labeled ammonium nitrate (15NH 15 4NO3, 20% enriched) was applied at each rate to replicate trees, in late April. Leaves, fibrous roots, soil, and leachates were intensively sampled from each treatment over the next 29 days, to determine the fate of the 15NH 15 4NO3 application. Newly developing spring leaves and fruit formed dominant competitive sinks for 15N, accounting for between 40% and 70% of the total 15N taken up by the various treatments. Large fruit loads intercepted up to 20% of this 15N, at the expense of spring flush development, to the detriment of overall tree N status in low-N trees. Nitrogen supply at less than the currently recommended yearly rate of 380 g/tree exceeded the requirements of 4-year-old grapefruit trees on SO rootstock; however, larger trees on VL rootstock took up the majority of 15N from this rate over the 29-day period. Nitrogen-use efficiency declined with increasing N rate, irrespective of rootstock. The residual amounts of 15N remaining in the soil profile under SO trees after this time represented a significant N leaching potential from these sandy soils. Therefore, under these conditions, present N recommendations appear adequate for rootstocks that impart relatively fast growth rates to Citrus trees, but seem excessive for trees on slower-growing rootstock species.

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Darren L. Haver and Ursula K. Schuch

The objectives of this study were to determine 1) the minimum controlled-release fertilizer (CRF) rate and the lowest constant medium moisture required to produce the highest quality plants and 2) if this production system affected quality of these plants under two postproduction light levels. Two New Guinea impatiens (Impatiens sp. hybrids) `Illusion' and `Blazon' (Lasting Impressions Series) differing in salt tolerance were grown for 42 days with a CRF at three rates (3.3, 6.6, or 9.9 g/pot) and two medium moisture levels (low or high) without leaching. The high moisture level (tension setpoints of 1 to 3 kPa) and 6.6 g of CRF/pot produced optimum biomass. Low medium moisture (tension setpoints of 4 to 6 kPa) reduced leaf area, leaf number, leaf N content, root, stem, and leaf dry masses as CRF rate increased from low to high for `Illusion'. Similar results in `Blazon' were observed as CRF rates increased from 3.3 to 6.6 g. Biomass decreased no further at the high rate of 9.9 g/pot. Biomass increased in both cultivars under high medium moisture when CRF rates increased from 3.3 to 6.6 g. Biomass of `Illusion' decreased at 9.9 g/pot, although no symptoms of salt sensitivity were observed (i.e., leaf tip burn). `Blazon' maintained a similar biomass when amended with 9.9 or 6.6 g CRF/pot, although electrical conductivity (EC) in the medium was 5.9 dS·m-1 in the upper half and 4.1 dS·m-1 in the lower half of the medium at the end of production. Growth of `Illusion' responded more favorably to postproduction light levels that were similar to those of production regardless of treatment imposed during production. Similar biomass responses occurred for `Blazon' regardless of the postproduction light level.

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Sanliang Gu, Lailiang Cheng, and Leslie H. Fuchigami

`Early Girl' tomato plants (Lycopersicon esculentum Mill.) were grown in a medium containing peatmoss and perlite (60%:40% by volume). The medium was drenched with 0% or 5% GLK-8924 antitranspirant. Half of the plants were flushed daily with 250 mL water (leaching), and the other half were subirrigated by capillarity. The solution osmotic potential of the medium was reduced significantly by 5% GLK 8924 treatment, then recovered gradually to the control level after 3 days with leaching or 10 days without leaching. Leaf stomatal conductance, transpiration rate, and plant growth were depressed by the antitranspirant application, and the depression was alleviated by leaching. Neither antitranspirant GLK-8924 treatment nor leaching influenced leaf abscisic acid (ABA) content. The effect of the antitranspirant on leaf gas exchange and plant growth was highly related to the reduction in the solution osmotic potential of the medium, but not to leaf ABA content. Younger leaves had higher stomatal conductance and transpiration rate but lower ABA content than older leaves in general.

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Jyotsna Sharma and William R. Graves

Attributes of Leitneria floridana Chapman have been recognized, but this North American shrub remains rare in commerce, and little information on propagation is available. We studied germination of seeds collected from several disjunct populations of L. floridana in 2002 and 2003. In 2002, ≤5% germination occurred when ripe drupes from Missouri and Florida were sown soon after collection. Effects of GA3 (750 mg·L-1 for 24 hours) were assessed on stored drupes leached with water and on seeds excised from stored drupes. Germination percentages were 21 and 32 for leached drupes and excised seeds from Florida, respectively, but ≤5% germination occurred among germplasm from Missouri and among untreated drupes from both provenances. Viability of ungerminated seeds among treatments ranged from 0% to 7%. In 2003, fleshy, apparently unripe drupes from Texas, which were scarified with H2SO4 and then treated with 1000 mg·L-1 GA3 showed 48% germination (germination value = 3.9). Up to 29% germination (germination value = 2.7) occurred when seeds were excised from unripe drupes from Arkansas and Missouri and then were treated for 24 hours with 750 or 1000 mg·L-1 GA3. We conclude that provenance, developmental stage of drupes when collected, storage, and pregermination treatments influence viability and germination of seeds of L. floridana. Barriers to germination may be avoided by collecting drupes when they are green and fleshy.

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Lincoln Zotarelli, Johannes Scholberg, Michael Dukes, Hannah Snyder, Eric Simonne, and Michael Munoz-Carpena

On sandy soils, potential N contamination of groundwater resources associated with intensively managed vegetables may hamper the sustainability of these systems. The objective of this study was to evaluate the interaction between irrigation system design/scheduling and N fertilization rates on zucchini production and potential N leaching. Zucchini was planted during Fall 2005 using three N fertilizer rates (73, 145, 217 kg/ha) and four different irrigation approaches. Irrigation scheduling included surface-applied drip irrigation and fertigation: SUR1 (141 mm applied) and SUR2 (266 mm) using irrigation control system (QIC) that allowed time-based irrigation (up to five events per day) and a threshold setting of 13% and 15% volumetric water content (VWC), respectively; Subsurface drip irrigation (SDI) using a QIC setting of 10% VWC (116 mm) combined with surface applied fertigation; and a control treatment with irrigation applied once daily (424 mm). Leacheate volumes were measured by drainage lysimeters. Nitrate leaching increased with irrigation rate and N rate and measured values ranged from 4 to 42 kg N/ha. Use of SDI greatly reduced nitrate leaching compared to other treatments. SDI and SUR1 treatments had no effect on yields (29 Mg/ha). However, SDI had a 15% and 479% higher water use efficiency (WUE) compared to SUR1 and the fixed irrigation duration treatment. Application of N in excess of intermediate N-rate (standard recommendation) did not increase yield but yield was reduced at the lowest N-rate. It is concluded that combining sensor-based SDI with surface applied fertigation resulted similar or higher yields while it reduced both water use and potential N leaching because of improved nutrient retention in the active root zone.

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Sidat Yaffa, Bharat P. Singh, Upendra M. Sainju, and K.C. Reddy

Sustainable practices are needed in vegetable production to maintain yield and to reduce the potential for soil erosion and N leaching. We examined the effects of tillage [no-till (NT), chisel plowing (CP), and moldboard plowing (MP)], cover cropping [hairy vetch (Vicia villosa Roth) vs. winter weeds], N fertilization (0, 90, and 180 kg·ha-1 N), and date of sampling on tomato (Lycopersicon esculentum Mill.) yield, N uptake, and soil inorganic N in a Norfolk sandy loam in Fort Valley, Ga. for 2 years. Yield was greater with CP and MP than with NT in 1996 and was greater with 90 and 180 than with 0 kg·ha-1 N in 1996 and 1997. Similarly, aboveground tomato biomass (dry weight of stems + leaves + fruits) and N uptake were greater with CP and MP than with NT from 40 to 118 days after transplanting (DAT) in 1996; greater with hairy vetch than with winter weeds at 82 DAT in 1997; and greater with 90 or 180 than with 0 kg·ha-1 N at 97 DAT in 1996 and at 82 DAT in 1997. Soil inorganic N was greater with NT or CP than with MP at 0- to 10-cm depth at 0 and 30 DAT in 1996; greater with hairy vetch than with winter weeds at 0- to 10-cm and at 10- to 30-cm at 0 DAT in 1996 and 1997, respectively; and greater with 90 or 180 than with 0 kg·ha-1 N from 30 to 116 DAT in 1996 and 1997. Levels of soil inorganic N and tomato N uptake indicated that N release from cover crop residues was synchronized with N need by tomato, and that N fertilization should be done within 8 weeks of transplanting. Similar tomato yield, biomass, and N uptake with CP vs. MP and with 90 vs. 180 kg·ha-1 N suggests that minimum tillage, such as CP, and 90 kg·ha-1 N can better sustain tomato yield and reduce potentials for soil erosion and N leaching than can conventional tillage, such as MP, and 180 kg·ha-1 N, respectively. Because of increased vegetative cover in the winter, followed by increased mulch and soil N in the summer, hairy vetch can reduce the potential for soil erosion and the amount of N fertilization required for tomato better than can winter weeds.

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George Hochmuth, Terril Nell, J. Bryan Unruh, Laurie Trenholm, and Jerry Sartain

turfgrass lands had more leaching losses than fertilized grasslands. The authors emphasized that changing from agricultural land to urban grasslands would have a benefit of reducing N losses. In a study of urbanization impacts on water quality in small

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Christine M. Worthington and Chad M. Hutchinson

The St. Johns River has been identified by the state of Florida as a priority water body in need of restoration. Best Management Practices were evaluated for potato (Solanum tuberosum L. `Atlantic') production in the Tri-County Agricultural Area to reduce nitrate run-off from about 9,300 ha in production. Objectives of this study were 1) determine the influence of soluble and controlled release fertilizer (CRF) and timing of leaching irrigation on nitrate leaching and 2) compare yield and quality of the potato crop fertilized with either a soluble or controlled release nitrogen fertilizer in a seepage irrigated production system. The experiment was a split-split plot with four replications. Main plots were irrigation events (0, 2, 4, 8, and 12 weeks after planting, (WAP)), nitrogen source and rates included (ammonium nitrate (AN) 224 kg·ha–1 or controlled release fertilizer (CRF) 196 kg·ha–1). About 7.6 cm of water was applied at each irrigation event and surface water runoff collected. CRF decreased NO3-N loading by an average of 35%, 28%, and 32% compared to AN fertilizer during the 2, 8, and 12 WAP irrigation events, respectively, compared to AN. Plants in CRF treatments had significantly higher total and marketable tuber yields (30 and 25 t·ha–1) compared to plants in AN treatments (27 and 23 t·ha–1), respectively. Plants in the CRF treatments also had significantly higher total and marketable yields in 2005 (28 and 23 t·ha–1) compared to plants in AN treatments (25 and 21 t·ha–1), respectively. CRF was an effective alternative to conventional soluble forms of fertilizer maintaining yields and protecting natural resources from nonpoint source pollution.

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Richard J. McAvoy

`Rose Grenadine' and `Buckaroo' garden chrysanthemums [Dendranthema ×grandiflorum (Ramat.) Kitamura] were produced in 15-cm pots in the greenhouse and fertilized with either 550 or 1000 ml of a 15 mol·m-3 N solution at each irrigation. The nutrient solution applied to half the pots contained a wetting agent (WA), and the remaining pots received no WA. Core samples were removed at 15-cm increments to a depth of 90 cm from the soil beneath the pots. The average leaching fraction (LF) from pots receiving a WA was 0.29 but was 0.26 from pots receiving no WA. However, WA did not affect the leachate NO3-N concentration or the total NO3-N deposited on the soil beneath; these were most influenced by LF. After week 2, NO3-N concentration in the upper 15 cm soil layer was 3.4 times higher with a high LF than with a low LF (30 and 8.8 g·m-3 respectively). At week 10, the NO3-N concentration in the 30 to 45 cm soil layer averaged 71.9 g·m-3 under the high LF and 35.5 g·m-3 under the low LF. Total N and NO3-N in the potting medium was higher in the low LF pots than the high LF pots, while NO3-N was higher in the medium of pots irrigated without WA than with WA. Final plant shoot mass was higher in pots irrigated to a high LF or without WA than in pots irrigated to a low LF or with WA.

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Timothy K. Broschat

Release rates at 21 °C were determined in sand columns for 12 commercially available soluble and controlled-release Mg fertilizers. Lutz Mg spikes, K2SO4, MgSO4, MgSO4·H2O, and MgSO4·7H2O released their Mg within 2 to 3 weeks. Within the first 6 weeks, MgO·MgSO4 released its soluble Mg fraction, but little release occurred thereafter. Dolomite and MgO released <5% of their Mg over 2 years while MagAmp released <20% of its Mg. Florikan 1N-0P-26K-4Mg types 100 and 180 exhibited typical controlled-release fertilizer characteristics, with most of their Mg release occurring during the first 15 weeks.