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Guihong Bi and Carolyn F. Scagel

storage and result in poor growth and flower development during forcing. Foliar sprays of urea in the fall after terminal bud set can increase reserve N in deciduous plants without stimulating new growth late in the season ( Cheng et al., 2002 ; Sanchez

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Muqiu Zhao, Huaibao Zhao, Qianjin Du, and Yunfeng Shi

Urea has become one of the most commonly used N fertilizers in the world due to its high N content, stable nature, easy application, and other characteristics ( Subbarao et al., 2012 ). However, urea applied to the soil surface can be rapidly

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Guihong Bi, Carolyn F. Scagel, and Richard Harkess

. Foliar fertilization in the fall is considered an alternative to supplying N to the soil of woody perennial nursery plants. Spray application of urea to deciduous plant species after terminal bud set increases N storage without stimulating new growth or

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami, and Richard P. Regan

In woody plants, growth in the spring relies on remobilization of nitrogen (N) reserves before substantial root uptake occurs ( Cheng et al., 2001 ; Henry et al., 1992 ; Millard, 1996 ). Foliar fertilization with urea in the fall is a common

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Bernard A. L. Nicoulaud and Arnold J. Bloom

In short-term experiments (10 days), urea applied foliarly as the sole N source promoted growth of `T-5' tomato (Lycopersicon esculentum Mill.) seedlings. The optimum urea concentration in the spray solution was 0.2% (w/w), and the best application frequency was once a day. Higher urea concentrations suppressed growth, producing severe leaf damage. The growth observed with foliar urea was less than that observed when inorganic N was supplied to the nutrient solution. Tomato seedlings absorbed 75% of the foliar applied urea within 12 hours and 99% within 24 hours after application. Urea concentrations in the plant tissues increased rapidly after foliar application. The maximum concentration was obtained in shoots at 12 hours and in roots at 24 hours after application. After that, concentration in the tissue declined to its original value within 48 hours. Tissue ammonium concentrations also increased after foliar application of urea. Shoot and root ammonium concentrations reached a maximum after 12 hours and stayed constant for the remainder of the 48-hour observation period. In the long-term experiment (5 weeks), the growth obtained with daily foliar applications of urea as the sole N source was only 10% of that when mineral N was available in the nutrient solution. Ammonium concentrations in the tissues of urea-treated plants were higher than those of plants treated with mineral N in the nutrient solution. Although urea concentrations were initially higher in plants treated with mineral N, after 2 weeks urea concentrations declined in these plants and increased in the shoots of plants receiving foliar applications of urea. These results indicate 1) that urea applied foliarly can supply at least part of the N required to sustain growth; 2) that urea is absorbed and assimilated fast enough to alleviate N deprivation; and 3) that failure to promote rapid growth with foliar urea is probably due to phytotoxicity and not to N deprivation.

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R. Scott Johnson, Harry Andris, and Shanti Handley

Foliar urea sprays offer an alternative to soil applied fertilizers which could greatly reduce the potential for nitrate pollution of groundwater, The approach in the past has been to apply relatively small doses of urea in order to minimize leaf phytotoxicity. Our approach is to apply relatively large doses in the fall when leaf phytotoxicity is not a serious concern. Results on peach trees in the field indicated rapid uptake of foliar applied solutions of 4.3 to 8.8% urea (w/w) (2.0 to 4.0% N). About 80-90% of the urea was absorbed by the leaf within 24 hours. Leaf N levels suggest the majority of this urea was translocated from the leaf into the tree within 1 week despite damage to the leaf. There were no negative effects on flowering, fruit set and production in the following year as long as a very low biuret formulation of urea was used.

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Carol J. Lovatt

cultivars, a single winter prebloom foliar application of low-biuret urea (50–54 lb/acre, 46% N, ≤ 0.25% biuret), at about the initiation of floral organogenesis ( Lord and Eckard, 1987 ), increased flower number and total yield per tree and juice total

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Richard C. Rosecrance, Scott Johnson, and Steven A. Weinbaum

The ability of peach leaves to absorbed and translocated foliarly applied 15N-urea in mature peach (Prunus persica) trees was determined. Urea uptake experiments were conducted in June, October, and November 1995. Peach leaves absorbed ≈80% of the urea within 48 hr of application in all three experiments based on urea rinsed from leaf surfaces. Similarly, leaf 15N content reached a peak 48 hr after application. Translocation of 15N out of leaves, however, was more rapid in October then November. In October, 24% of the 15N remained in the leaves 2 weeks after application, while, in November, 80% stayed in the leaves and fell to the orchard floor. Thus, applying urea in mid November did not allow enough time for the N to be transported out of the leaves before leaf abscission. Timing of foliar urea application is critical to maximize N transport into perennial tissues of peach trees. 15Nurea resorption out of leaves and into perennial tree parts (roots, trunk, current year wood, etc.) is discussed.

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Francis Zvomuya and Carl J. Rosen

Polyolefin-coated fertilizers are slow-release fertilizers coated with thermoplastic resins that have a temperature-dependent nutrient release pattern. A field study was conducted on a Hubbard loamy sand during 1997 and 1998 at Becker, Minn., to evaluate the effect of polyolefin-coated urea (POCU) fertilizers (Meister, Chisso Co., Japan) on yield and quality of irrigated `Russet Burbank' potatoes (Solanum tuberosum L.). The coated fertilizers were POCU-50 and POCU-70, which release 80% of their N in 50 and 70 days, respectively, at 25 °C, and a 1 POCU-50: 1POCU-70 mixture. The study compared three soluble urea treatments (N at 0, 140, and 280 kg·ha-1) split-applied at planting, emergence, and hilling vs. the same N rates of coated urea fertilizers applied in a band at planting. In 1997, a season characterized by high leaching, total and large tuber (>168 g) yields were higher with coated urea sources than soluble urea at equivalent N rate, but the N sources gave similar yields in 1998 when leaching was minimal. In both years, doubling the rate of N as soluble or coated urea from 140 to 280 kg·ha-1 had no effect on total yield, but increased the marketable yield (tuber size). Yields were higher in 1998 compared to 1997 due to poorer tuber set in 1997. However, the percentage of large tubers was higher in 1997. Specific gravity increased slightly with N rate but did not differ with N source at equivalent N rate. Hollow heart incidence was similar among all treatments in 1997, but it increased with N rate and was similar among N sources in 1998.

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Bhaskar Bondada, Peter D. Petracek, and Jim Syvertsen

Recent interest in reducing nitrate levels in ground water has stimulated the re-examination of foliar application of urea on citrus trees. Because the cuticle is the primary barrier to foliar uptake, we examined the diffusion of 14C-urea through isolated citrus leaf cuticles. Cuticles were enzymatically isolated from leaves of the four youngest nodes (1 month to 1 year old) of pesticide-free grapefruit trees. The diffusion system consisted of a cuticle mounted on a receiver cell containing stirred buffer solution. Urea (1 μL) was pipetted onto the cuticular surface, and buffer solution was sampled periodically through the side portal of the receiver cell. The time course of urea diffusion was characterized by lag (time to initial penetration), quasi-linear (maximum penetration rate), and plateau (total penetration) phases. Apparent drying time was less than 30 min. Average lag time was about 10 min. The maximum penetration rate occurred about 40 min after droplet application and was about 2% of the amount applied per hour. Rewetting stimulated further penetration. The total penetration averaged about 35% and tended to decrease with leaf age. Dewaxing the second node cuticles by solvent extraction significantly increased maximum penetration rates (30% of the amount applied per hour) and total penetration (64%).