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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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K.G. Weis, S.M. Southwick, J.T. Yeager, M.E. Rupert, R.E. Moran, J.A. Grant, and W.W. Coates

In continuing trials (1995-current), we have used a variety of treatments to overcome inadequate chilling, coordinate bloom, improve leaf out and cropping, and advance/coordinate maturity in sweet cherry, cv. Bing. Treatments have included hydrogen cyanamide (HCN, Dormex) and various surfactants or dormant oils combined with calcium ammonium nitrate (CAN17). Chill hour accumulation, (required chilling for `Bing' = 850 to 880 chill hours) has varied greatly in each dormant season from 392 (Hollister, 1995-1996) to adequate, depending both on the season and location (central valley vs. coastal valley). In 1998, 4% HCN advanced budbreak significantly compared to any other treatment, although other chemical treatments also were more advanced than the untreated control. Dormex advanced completion of bloom 11% to 40% more than other treatments, although other dormancy-replacing chemicals were at least 16% more advanced in petal fall than the untreated control. Dormex contributed to slightly elevated truss bud death, as did 2% Armobreak + 25% CAN17. In 1998, fruit set was improved by 2% Armobreak + 25% CAN17 (79%) compared to the untreated control (50%); all other treatments statistically equaled the control. Fruit set was not improved by Dormex, although bloom was advanced by a few days in this treatment. As fruit set was increased by treatments, rowsize decreased (as did fruit weight), as expected, but no treatment resulted in unacceptable size. In 1997, fruit set was also improved by 2% Armobreak + 25% CAN17; however, fruit set was so low overall in that year that no real impact was found. In 1997 and 1998, 4% HCN advanced fruit maturity compared to other treatments, with darker, softer, larger fruit at commercial harvest. These and additional results will be presented.

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Yin-Tung Wang

skeletons for ammonium assimilation ( Arnozis et al., 1988 ) and high O 2 consumption ( Matsumoto and Tamura, 1981 ), resulting in low sugar concentration in the roots and poor plant growth compared with NO 3 -N-fed plants. This may have resulted in smaller

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Alexander X. Niemiera, Linda L. Taylor, and Jacob H. Shreckhise

) substrate solution EC, pH, ammonium-nitrogen (NH 4 -N), and NO 3 -N. Solutions were extracted from substrates using the saturated media extract method (SME; Warncke, 1986 ) in which a predetermined volume (amount needed to saturate substrate) of deionized

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Timothy K. Broschat

Downy jasmines [Jasminum multiflorum (Burm. f.) Andr.] and areca palms [Dypsis lutescens (H. Wendl.) Beentje & J. Dransf.] were grown in containers filled with a fine sand soil (SS) or with a pine bark-based potting substrate (PS). Each of these substrates was amended with 0%, 10%, or 20% clinoptilolitic zeolite (CZ) by volume. Plants were fertilized monthly with a water-nonsoluble 20N-4.3P-16.6K granular fertilizer. Downy jasmines were larger and had darker color in CZ-amended PS and were larger in CZ-amended SS than in nonamended SS or PS. Areca palms, which tend to be limited by K in SS had better color and larger size when the SS was amended with CZ. In PS, where K is seldom limiting, areca palms did not respond to CZ amendment of the PS. Both ammonium (NH4)-N and potassium (K) were retained against leaching by CZ, but some of the NH4-N adsorbed to CZ was subject to nitrification, either before or after its release into the soil solution. Some phosphate (PO4)-P was also retained by CZ.

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A. Jeremy Bishko, Paul R. Fisher, and William R. Argo

Medium-pH above 6.4 is a common cause of micronutrient deficiency for container-grown plants, and technologies are required to correct an excessively high medium-pH. The objective was to quantify the dose response from application of several acidic materials that have been recommended for lowering medium-pH in soilless media. A 70% peat/30% perlite (by volume) medium was mixed with a preplant nutrient charge, a wetting agent and 1.5, 1.8, 2.1, or 2.4 kg·m-3 of a dolomitic hydrated lime resulting in four starting pH levels ranging from 6.4 to 7.6. Aluminum sulfate (17% Al) at 1.8-28.8 g·L-1, flowable elemental sulfur (52% S) at 3.55-56.8 mL·L-1, ferrous sulfate (20.8% Fe) at 1.8-28.8 g·L-1, Seplex-L organic acid at 0.32-5.12 mL·L-1, sulfuric acid (93%) at 0.08-2.56 mL·L-1, 21.1N-3.1P-5.8K water-soluble fertilizer at 50-400 mg·L-1 N (potential acidity 780 g CaCO3 equivalents/kg), and a deionized water control were applied at 60 mL per 126-cm3 container with minimal leaching as a single drench (except repeat sulfuric acid applications at 0.08 or 0.16 mL·L-1 and 21.1N-3.1P-5.8K treatments that were applied about every 3 days). Medium-pH and electrical conductivity (EC) were tested over 28 days using the saturated medium extract method using deionized water as the extractant. One day after application, aluminum sulfate, ferrous sulfate, and sulfuric acid lowered pH by up to 3 pH units at the highest concentrations and medium-pH remained fairly stable for the following 27 days. Flowable sulfur lowered pH gradually over the course of the experiment by up to 3.3 pH units, with no difference across the wide range in concentrations. Organic acid had minimal impact on medium-pH, and 21.1N-3.1P-5.8K did not lower medium-pH despite the fact that all nitrogen was supplied in the ammonium and urea form. At recommended concentrations, chemicals tested raised medium-EC, but not above acceptable levels for plant growth. The highest rates of aluminum and ferrous sulfates, and sulfuric acid, however, increased medium-EC by 2.0 dS·m-1 on day 1. Medium-pH-responses to acid-reaction chemicals would be expected to vary in commercial practices depending on additional factors such as lime type and incorporation rate, water alkalinity, media components, and plant interactions.

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Raul I. Cabrera

Seven nursery grade (8-9 month duration), polymer-coated, controlled-release fertilizers (CRF) were topdressed or incorporated into a 2 peat: 1 vermiculite: 1 sand (by volume) medium to yield the same amount of N per container. The pots (0.5 L) were uniformly irrigated with DI water every week to produce a target leaching fraction of 25%. Leachate N contents (ammonium plus nitrate), employed as indicators of N release, allowed for comparison of CRF performance as a function of temperature changes over a season. Two distinct N leaching (i.e., release) patterns were observed over the 180-day experimental period. The fertilizers Osmocote 18-6-12FS (Fast Start: OSM-FS), Prokote Plus 20-3-10 (PROK), Osmocote 24-4-8HN (High N: OSM-HN) and Polyon 25-4-12 (POLY) exhibited a N leaching pattern that closely followed changes in average daily ambient temperatures (Tavg) over the season. This relationship was curvilinear, with N leaching rates per pot (NLR) being highly responsive to Tavg changes between 20 and 25 °C. Temperatures above 25 °C produced an average maximum NLR of 1.27 mg·d-1 for these fertilizers. OSM-FS, PROK, and OSM-HN had the highest cumulative N losses over the experimental period. In contrast, the CRF group formed by Nutricote 18-6-8 (270: NUTR), Woodace 20-4-12 (WDC), and Osmocote 18-6-12 (OSM) showed a more stable N leaching pattern over a wider range of temperatures, with rates about 30% to 40% lower than those in the temperature-responsive CRF, and averaging a maximum NLR of 0.79 mg·d-1 for Tavg >25 °C. NUTR and WDC had the lowest cumulative N losses over the season. Soluble salt readings paralleled N leaching for each CRF, indicating similar leaching patterns for other nutrients. Incorporation produced significantly higher cumulative N losses than topdressing, but without effect on the actual N leaching pattern over the season. Regardless of the N formulation in the CRF, over 85% of the N recovered in the leachates was in the nitrate form.

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Ryan W. Dickson and Paul R. Fisher

.52 P, 2.99 K, 5.00 Ca, and 1.00 Mg. Sulfate increased with NH 4 + :NO 3 – ratio because NH 4 + -N was supplied from ammonium sulfate, and S concentration was 2.38, 3.80, and 7.06 mEq·L –1 for the 0:100, 20:80, and 40:60 solutions, respectively

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P.P. David, A.A. Trotman, D.G. Mortley, D. Douglas, and J. Seminara

A study was initiated in the greenhouse to examine the effects of five \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}:\mathrm{NO}_{3}^{-}\) \end{document} ratios on sweetpotato growth. Plants were grown from vine cuttings of 15-cm length, planted in 0.15 x 0.15 x 1.2-m growth channels using a closed nutrient film technique system. Nutrient was supplied in a modified half-strength Hoagland's solution with a 1:2:4 N:K ratio. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}:\mathrm{NO}_{3}^{-}\) \end{document} ratios investigated were 100:0, 0:100, 40:60, 60:40, and a control that consisted of a modified half-Hoagland solution with an N:K ratio of 1:2:4 and an \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}:\mathrm{NO}_{3}^{-}\) \end{document} of 1:7. Treatments were initiated 30 days after planting (DAP). Sequential plant harvest began 30 DAP and continued at 30-day intervals until final harvest at 150 DAP. Results showed a linear increase in fresh storage root fresh weight until 90 DAP for all treatments. However, from 60 DAP until the end of the growing season, plants grown in a 100% \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\) \end{document} solution consistently produced significantly less storage roots than in all other treatments. While all other treatments showed a decrease in storage root fresh weight after 90 DAP, plants grown in 100% \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document} and the control solution continued to increase linearly in storage root production. Storage root dry weight throughout the growing season followed similar trends to that of storage root fresh weight. Data suggest that a nutrient solution containing NO 3as its sole nitrogen source may be adequate for sweetpotato growth. This would make it possible for utilizing a one-way pH control method for nutrient solution.

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Efren C. Celaya and Brenda S. Smith

A field experiment was conducted on broccoli (Taki Marathon variety) at California Polytechnic State University, San Luis Obispo to evaluate three rates of AN-20 for weed control and crop phytotoxicity. The rates were: low-40 gal./A, standard-60 gal./A, high-80 gal./A, and untreated control. Each treatment was applied to the base of the broccoli plant to avoid crop injury. Each treatment had four replications. Data collected included hoe time/plot and percent visual control. Broccoli was not injured at any rate of AN-20. It was noticed that the older weeds, greater than five-leaf stage, managed to pull through, so size of weed is crucial. On a cost-per-acre basis, the money saved on the high rate is half that of the low rate and one third that of the control. Weed control was not adequately controlled at the standard and low rates. An economic analysis was conducted, and it was found there was a savings as less labor was required to hoe the field when AN-20 had been applied.