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Carl J. Rosen, Deborah L. Allan, and James J. Luby

The effects of pH and N form on growth and nutrition of blueberry (Vaccinium corymbosum L. × V. angustifolium Ait. cv. Northblue) and cranberry (V. macrocarpon Ait. cv. Searles) were tested in separate greenhouse hydroponic experiments. A factorial treatment arrangement of two pH levels (4.5 and 6.5) and three N forms (NO3-N, NH4-N, and NH4-N/NO3-N) was used for each clone. Blueberry shoot growth and final dry weight were greatest at pH 4.5, regardless of N form. In contrast, cranberry fresh weight accumulation and final dry weight were higher with NH4-N/NO3-N or NH4-N than with NO3-N alone. Cranberry plants receiving NO3-N alone accumulated low levels of tissue N and grew relatively poorly at both pH levels. Differences in N response by these two species may be due partially to the environments in which they were selected. Soil from the site where `Northblue' blueberry was selected contained relatively high NO3-N and low NH4-N levels; soil from commercial `Searles' cranberry bogs had relatively low NO3-N and high NH4-N levels. Both species accumulated relatively high levels of root Fe, regardless of pH or N form. Levels of Fe in the root were as much as 100 times higher than in the shoot. Based on X-ray microanalysis of cranberry roots, most of the Fe appeared to be precipitated on the root surface as iron phosphate. Concentrations of Mn in shoots and roots depended on N form and pH. In general, root Mn was highest at pH 6.5 and apparently was precipitated with Fe.

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W.M. Randle

To test the effects of high nitrogen (N) fertilization levels on onion quality and bulb flavor, `Granex 33' onions (Allium cepa L.) were greenhouse grown in hydroponic solution culture with increasing N concentrations. Nitrogen was adjusted in the solutions with NH4NO3 and increased incrementally from 0.22 g·L-1 to 0.97 g·L-1 over five treatments. Plants were harvested at maturity and subjected to quality, flavor, and mineral analysis. As solution N increased, bulb fresh weight and bulb firmness decreased linearly. Gross flavor intensity, as measured by enzymatically developed pyruvic acid (EPY) increased linearly for N concentrations between 0.22 and 0.78 g·L-1, but EPY was reduced slightly in bulbs grown at the highest N level (0.97 g·L-1). Soluble solids content was unaffected by solution N concentration. Solution N had an affect on flavor quality. Methyl cysteine sulfoxide, which gives rise to cabbage (Brassica L. sp.) and fresh onion flavors upon eating, generally increased in concentration as solution N increased. 1-Propenyl cysteine sulfoxide, which imparts heat, mouth burn, pungency, and raw onion flavors increased between the two lowest N concentrations, and then decreased as solution N increased. Propyl cysteine sulfoxide, which imparts fresh onion and sulfur flavors upon eating, generally increased with increasing solution N concentration. Several minerals were also affected by solution N concentration. Total bulb N and NO3 - increased linearly while B, Ca, and Mg decreased linearly. Total bulb S and K increased and then decreased quadratically in response to increasing solution N. Nitrogen fertility can have a pronounced affect on onion flavor and as a consequence, needs to be considered when growing onions for specific flavor quality and nutritional attributes.

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Gregory D. Goins, Neil C. Yorio, and Raymond M. Wheeler

The National Aeronautics and Space Administration (NASA) has been conducting controlled environment research with potatoes (Solanum tuberosum L.) in recirculating nutrient film technique (NFT)-hydroponic systems as a human life support component during long-duration spaceflight. Standard nutrient solution management approaches include constant pH regulation with nitric acid (HNO3) and daily adjustment of electrical conductivity (EC) equivalent to half-strength modified Hoagland's solution, where nitrate (NO3-) is the sole nitrogen (N) source. Although tuber yields have been excellent with such an approach, N use efficiency indices are expected to be low relative to tuber biomass production. Furthermore, the high amount of N used in NFT-hydroponics, typically results in high inedible biomass, which conflicts with the need to minimize system mass, volume, and expenditure of resources for long-duration missions. More effective strategies of N fertilization need to be developed to more closely match N supply with demand of the crop. Hence, the primary objective of this study was to identify the optimal N management regime and plant N requirement to achieve high yields and to avoid inefficient use of N and excess inedible biomass production. In separate 84-day cropping experiments, three N management protocols were tested. Treatments which decreased NO3 --N supply indirectly through lowering nutrient solution EC (Expt. I), or disabling pH control, and/or supplying NH4 +-N (Expt. III) did not significantly benefit tuber yield, but did influence N use efficiency indices. When supplied with an external 7.5 mm NO-3 --N for the first 42 days after planting (DAP), lowered to 1.0 mm NO3 -N during the final 42 days (Expt. II), plants were able to achieve yields on par with plants which received constant 7.5 mm NO3 --N (control). By abruptly decreasing N supply at tuber initiation in Expt. II, less N was taken up and accumulated by plants compared to those which received high constant N (control). However, proportionately more plant accumulated N was used (N use efficiency) to produce tuber biomass when N supply was abruptly lowered at tuber initiation in Expt. II. Hence, a hydroponic nutrient solution N management system may be modified to elicit greater plant N-use while maintaining overall high tuber yield as opposed to achieving high tuber yields through excess N supply and shoot growth.

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Harvey J. Lang and George C. Elliott

Autotrophic nitrifying organisms were enumerated in soilless potting media using the most probable number (MPN) technique. Populations of NH4 + and NO2 - oxidizing organisms varied widely between two soilless media—Metro-Mix 220 and 350. Estimates for NH4 + oxidizing organisms ranged from 0.7 to 7.8 × 105 organisms/cm3, while NO2 - oxidizers ranged from 1.3 to 9.5 × 105 organisms/cm3. Population numbers were similar to those typically reported in soils. There was a significant effect of medium type, NH4 + N : NO3 - N fertilizer ratio, and planting on MPN counts of both groups of organisms, with significant interaction between several of the factors. Estimates of NH4 + oxidizers were not linearly correlated with NH4 + oxidizing activity, implying low counting efficiency, heterotrophic nitrification, or rate-limiting substrate NH4 + level. In a separate study, a soilless potting medium was inoculated with pure cultures of either Nitrosomonas europaea or Nitrobacter agilis. Rates of NH4 + and NO2 - oxidation increased, respectively, as inoculum volume increased. Inoculation with nitrifying bacteria may help in the overall management of N in the rhizosphere and be feasible alternatives for the prevention of either NH4 + or NO2 - phytotoxicity with fertilizers containing urea or NH4 +.

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Anwar G. Ali and Carol J. Lovatt

The objective of this study was to test whether a single winter prebloom foliar application of low-biuret urea would increase the yield of 30-year-old `Washington' navel orange trees [Citrus sinensis (L.) Osbeck] on Troyer citrange rootstock [C. sinensis `Washington' × Poncirus trifoliata (L.) Raf.]. All trees received a winter (November to January) soil application of urea (0.5 kg N/tree). Trees were maintained under irrigation or irrigation was withheld from 1 Oct. to 1 Mar. To determine the optimal time for foliar urea application, trees in both irrigation main plots received one application of low-biuret urea in mid-November, mid-December, mid-January, or mid-February applied at a rate of 0.16 kg N/tree. There was a set of control trees that only received the soil application of urea. Trees receiving foliar-applied urea in mid-January or mid-February, independent of irrigation treatment, had significantly greater yield and fruit number per tree each year than the control trees for 3 consecutive years. The number of fruit with diameters of 6.1 to 8.0 cm increased significantly as yield increased (r 2 = 0.88). Withholding irrigation from 1 Oct. to 1 Mar. had a negative impact on yield. Annual winter application of low-biuret urea to the foliage did not significantly increase leaf total N at the end of 3 years.

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Donald J. Merhaut and Rebecca L. Darnell

Nitrogen uptake and N and C partitioning were evaluated in `Sharpblue' southern highbush blueberries fertilized with different N forms. Plants were grown in acid-washed silica sand and fertilized with a modified Hoagland's solution supplemented with 5.0 mm N as NH4 + or NO3 -. Nutrient solution pH was adjusted to 3.0 and 6.5 for the NO3 - and NH4 +-treated plants, respectively. After 12 months of growth, plants were dual labeled with 14CO2 and 10% enriched 15N-N as either NaNO3 or (NH4)2SO4 and harvested 12 hours after labeling. Fertilization with NO3 --N increased leaf, stem, and root dry weights compared to NH4 + fertilization. Total 15N uptake did not differ between N fertilization treatments, thus whole plant and root 15N concentrations were greater in NH4 +-fertilized vs. NO3 --fertilized plants. Fertilization with NO3 --N increased C partitioning to new shoots compared to NH4 +-fertilized plants. However, C partitioning to other plant parts was not affected by N form. Although NO3 - uptake in blueberry appears to be restricted relative to NH4 + uptake, this limitation does not inhibit vegetative growth. Additionally, there appears to be adequate available carbohydrate to support concurrent vegetative growth and N assimilation, regardless of N form.

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Catherine S.M. Ku and David R. Hershey

Poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch `V-14 Glory') grown as single-pinched plants and received constant fertigation of Hoagland solution with N at 210 mg·L-1 of 100% NO3-N or 60% NO3-N : 40% NH4-N; P at 7.8 and 23 mg·L-1; and leaching fractions (LFs) of 0, 0.2, or 0.4. The P at 23 mg·L-1 used in this study was about half the P concentration typically provided from a 20N-4.4P-16.6K fertilizer at 200 mg·L-1 N fertigation. The total P applied via fertigation ranged from 51 mg at the 0 LF to 360 mg at the 0.4 LF. The leachate P concentration ranged from 0.2 to 46 mg·L-1. With P at 7.8 mg·L-1, the percentage of total P recovered in the leachate was 6% to 7%. At 23 mg·L-1 P fertigation, however, the total P recovered in the leachate with 60% NO3-N treatment was 2-times greater than with 100% NO3-N treatment. This result is attributed to a lower substrate pH, which resulted from NH4-N uptake and nitrification processes with 60% NO3-N fertigation. The P concentration in the recently matured leaves with 7.8 mg·L-1 P fertigation was in the normal range of 0.3% to 0.6%. Fertigation P can be reduced by up to 80% and still be sufficient for producing quality poinsettias. Reducing the fertigation P concentration is beneficial because it reduces P leaching, reduces fertilizer costs, and reduces luxury consumption.

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Oswaldo A. Rubio, Patrick H. Brown, and Steven A. Weinbaum

Leaf N concentrations (% dry wt) appear relatively insensitive to high levels of applied fertilizer N (Weinbaum et al, HortTechnology 1992). This insensitivity may be attributable to growth dilation, lack of additional tree N uptake, a finite capacity of leaves to accumulate additional N or our inhability to resolve a limited increment. Our objective was to asses the relative accumulation of mobile forms of N (NO3, NH4 and amino acids) relative to a total N over a range of fertilizer N application rates in 3 year old, field-grown “Fantasia” nectarine trees. Between 0 and 136 Kg N/Ha/Yr we observed a linear relationship between N supply and all N fractions. Above 136 Kg N/Ha/Yr leaf concentrations of amino acids and total N remined constant, but NO3 and NH4 accumulation continued. These results suggest that leaf concentration of NO3 and NH4 are more sensitive indicators of soil N availability and tree N uptake than was total leaf N concentration.

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P.L. Preusch and T.J. Tworkoski

Composted poultry litter (CPL) may be applied as a mulch in fruit orchards to manage waste and to provide a slow-release nutrient source and weed control. With proper management, poultry manure and bedding (litter) can prevent environmental degradation, such as hypoxia in aquatic communities. Peach (Prunus persica L. `Sunhigh') plots all received preemergence herbicides in May and then the following treatments in June 1998: commercial fertilizer (N at 15 g·m-2), low rate CPL (N at 15 g·m-2 as CPL at 2.9 kg·m-2), high rate CPL (N at 62 g·m-2 as CPL at 11.6 kg·m-2), and no fertilizer or mulch control. Weeds were completely controlled by mulch and herbicide during 1998 but not during 1999. By Sept. 1999, the high rate of CPL had only 27% weed cover compared with 86% for the commercial fertilizer-treated plots. Soil N was highest (NH4-N and NO3-N at 16.4 and 18.6 mg·kg-1 soil, respectively) in plots treated with commercial fertilizer, 6 weeks after treatment (WAT). Soil N did not differ among the two CPL treatments and the control at any time. At the high rate of CPL, there was NH4-N and NO3-N at 3.2 and 0.7 mg·kg-1 soil, respectively, at 6 WAT. Water-extractable P (WEP) in the soil did not differ among the CPL and commercial fertilizer treatments at 6 WAT (P at §14 mg·kg-1 soil). However, at 47 WAT, plots with the high rate of CPL had significantly higher WEP, with P at 30 mg·kg-1 soil vs. 14 mg·kg-1 soil in plots treated with commercial fertilizer. High applications of CPL could elevate P in surface runoff to levels that cause environmental degradation. In general, Mehlich 1-extractable P (MEP) did not differ among the CPL- and fertilizer-treated plots (averaging P at 45 mg·kg-1 soil). MEP was lowest in control plots (averaging P at 21 mg·kg-1 soil). Results indicate that CPL could be used as a weed suppressant without adversely affecting N release to the environment; however, P concentration in soil water may be problematic.

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Julie P. Newman, Joseph P. Albano, Donald J. Merhaut, and Eugene K. Blythe

Release characteristics of four different polymer-coated fertilizers (Multicote, Nutricote, Osmocote, and Polyon) were studied over a 47-week period in a simulated outdoor, containerized plant production system. The 2.4-L containers, filled with high-fertility, neutral-pH substrate, were placed on benches outdoors to simulate the environmental conditions often used for sun-tolerant, woody perennials grown in the southwestern United States. Container leachates were collected weekly and monitored for electrical conductivity, pH, and concentrations of NH4 +N, NO3 N, total P, and total K. Concentrations of most nutrients in leachates were relatively high, but fluctuated frequently during the first third of the study period, and then gradually decreased and stabilized during the last 27 weeks. Osmocote often resulted in greater NH4 + and total inorganic N concentrations in leachates than other fertilizers during weeks 1 through 5, whereas Multicote produced higher NH4 + in leachates than most of the other fertilizer types during weeks 9 through 12. Overall, total P concentrations were greater with Multicote during a third of the experimental period, especially when compared with Osmocote and Polyon. Differences were also observed among treatments for leachate concentrations of K, with Polyon and Multicote fertilizers producing greater K concentrations in leachates compared with Osmocote during several weeks throughout the experimental period. Leachate concentrations of NO3 N and P from all fertilizer types were usually high, especially from week 5 through week 30.