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A.K. Alva

Nitrate pollution of surfacial aquifer is fairly widespread in deep sandy soil areas of Central Florida. Since citrus is a predominant crop in this area, despite lack of conclusive evidence suggesting citrus fertilization as the source of nitrate pollution, investigations are in progress to develop Best Management Practice (BMP) recommendations for N fertilization of citrus in an effort to improve N use by the trees and to minimize potential nitrate leaching. Our ongoing studies on both young and mature trees have demonstrated that the use of improved fertilizer formulations and programmed application schedules have facilitated to decrease the rate of N application considerably without any adverse impact on tree growth and/or fruit production while minimizing nitrate leaching below the rootzone. Our approach involves developing BMP recommendations on the basis of iudicious irrigation management and generating database on N removal by the fruits, annual N contribution to the trees by mineralization of organic N, and N losses including leaching, denitrification, etc.

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A.K. Alva and T.A. Obreza

Citrus trees planted in alkaline soils typically show iron (Fe) deficiency chlorosis. Currently, Fe-EDDHA (ethylenediiminobis-2-hydroxyphenyl acetic acid) chelate is the most effective source of Fe for high pH soils. Iron humate (FeH), a by-product of the drinking water decolorization process, was compared with Fe-EDDHA for Fe deficiency correction on nonbearing `Ambersweet' orange and `Ruby Red' grapefruit Citrus paradisi Macf., and bearing `Hamlin' orange Citrus sinensis and `Flame' grapefruit trees, all on Swingle citrumelo rootstock, planted on high pH (>7.6) soils. Iron humate was applied under the tree canopy in spring at rates from 2 to 200 g Fe (nonbearing trees), or 22 to 352 g Fe (bearing trees) per tree per year. Application of FeH to nonbearing trees decreased twig dieback rating and increased flush growth, flush color rating, tree size, and leaf Fe concentration. Addition of urea or ammonium nitrate to FeH did not increase Fe availability. Iron amendments (22 g Fe per tree per year) increased fruit yield after the 1st year of application. Further increases in the rate of Fe, from 22 to 352 g Fe per tree per year as FeH, did not significantly increase tree growth, fruit yield, or fruit quality. This study demonstrated that FeH was an effective Fe source for citrus trees planted on alkaline soils.

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A.K. Alva and D.P.H. Tucker

Recent trends in increased nitrate contamination of groundwater in parts of Central Florida have prompted this study to evaluate the potential for reducing the rates of nitrate (N) fertilizer to citrus. `Pineapple' orange trees on Swingle citrumelo rootstock growing on Candler fine sand were selected for the study. Fertilizer blends were applied using N as: (1) soluble ammonium nitrate and calcium nitrate in 4 applications per year and (2) controlled-release sources (Meister, Osmocote, Escote in one application per year and IBDU and Neutralene in 2 applications per year). Nitrogen rates varied 18-154, 36-308, and 57-454 g/tree/year during the first, second and third year after planting, respectively. Tree growth and leaf mineral concentrations (during the first 3 years) and fruit size and juice quality (second and third years) were not adversely affected by application of reduced N rates. Fruit yield (third year) was lower in the trees which received ≤ 114 g N/tree/year than in those which received higher rates of N.

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Y.C. Li, P.J. Stoffella, A.K. Alva, D.V. Calvert and D.A. Graetz

Compost amendment to agricultural soils has been shown to either reduce disease incidence, conserve soil moisture, control weeds or improve soil fertility. Application of compost can range from 5 to 250 Mt·ha–1 (N content up to 4%). Large application of compost with high N and P levels may result in excessive leaching of nitrate, ammonium, and phosphate into groundwater. It could be a serious concern on the east coast of Florida with its high annual rainfall and shallow water table. In this study, five composts (sugarcane filtercake, biosolids, and mixtures of municipal solid wastes and biosolids) were collected from different facilities throughout Florida. Composts were applied on a surface of 15-cm sandy soil columns at the rate of 100 Mt·ha–1 on the surface basis and leached with deionized water by 300 ml·d–1 for 5 days (equivalent to 34 cm rainfall). The concentrations of NO3-N, NH4-N, and PO4-P in leachates reached as high as 246, 29, and 142 mg·L–1, respectively. The amount of N and P leached following 5-day leaching events accounted for 3.3% to 15.8% of total N and 0.2% to 2.8% of total P as inorganic forms.

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Y.C. Li, A.K. Alva, D.V. Calvert and M. Zhang

It is generally believed that the interception of rain by the citrus tree canopy can substantially decrease the throughfall under the canopy as compared to that along the dripline or outside the canopy (incident rainfall). Therefore, the position of placement of soil-applied agrichemicals in relation to the tree canopy may be an important consideration to minimize their leaching during rain events. In this study, the distributions of rainfall under the tree canopies of three citrus cultivars, `Marsh' grapefruit (Citrus paradisi Macf.), `Hamlin' orange (Citrus sinensis L. Osbeck), and `Temple' orange (Citrus hybrid), were evaluated at four directions (north, south, east, west), two positions (dripline and under the canopy), and stem flow. There was not a significant canopy effect on rainfall amounts from stem flow or dripline, compared with outside canopy, for any citrus cultivar or storm event. However, throughfall varied significantly among the four cardinal directions under the canopy of all three citrus cultivars and was highly related to the wind direction. Among the three citrus cultivars evaluated in this study, throughfall, stem flow, and canopy interception accounted for 89.5% to 92.7%, 0.5% to 4.7%, and 5.8% to 9.3% of the incident rainfall, respectively.

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Z.L. He, D.V. Calvert, A.K. Alva and Y.C. Li

Fertilization is among the most important factors influencing fruit quality of citrus. Effects of Individual element such as N, P, or K on fruit quality have been well-documented. Much less has been done on the interactions of N, P, and K in relation to citrus fruit quality. A field experiment was conducted from 1994 to 1999 in a commercial grove on a Riviera fine sand (Loamy, siliceous, hyperthermic Arenic Glossaqualf) to investigate the effects of fertilizer rates and sources on fruit quality of 26-year-old `White Marsh' grapefruit trees (Citrus paradisi Macfad.) on Sour Orange rootstock (Citrus aurantium Lush). Fertilizer was applied as water-soluble dry granular broadcast (three applications/year) at N rates of 0, 56, 112, 168, 224, and 336 kg/ha per year using a N;P:K blend (1.0:0.17:1.0). There was a quadratic relationship between fruit weight or peel thickness and fertilizer rates. Fruit weight per piece increased with fertilizer rates from 0 to 168 kg N/ha per year, but decreased from 168 to 336 kg N/ha per year. Fruit size was small at zero or low fertilizer rates due to nutrient deficiencies. Large fruit sizes of `White Marsh' grapefruit in the sandy soil were achieved at fertilizer rate around 168 kg N/ha per year. Increasing fertilizer application rates higher than 168 kg/ha per year greatly increased the number of fruit per tree, but decreased the size of fruit. Peel thickness, which is related to the fruit size, declines at higher fertilizer rates. Increase in fertilizer rate from 0 to 336 kg N/ha per year increased solids content and fruit acid concentration of the grapefruit. Fertilization rate effect on fruit Brix concentration was more complicated. Brix concentration was not affected by increasing fertilizer rates from 0 to 168 kg N/ha-per year, but was increased at higher fertilization rates (168 to 336 kg N/ha per year). As a result, the Brix/acid ratio was, in general, decreased by increasing fertilizer rates.

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Y.C. Li, A.K. Alva, D.V. Calvert and M. Zhang

Rapid and accurate diagnosis of plant nutrient deficiency is critical for growers to use fertilizer economically and to minimize environmental concern. The objective of this study was to evaluate the use of leaf chlorophyll meter (SPAD-502) to predict nitrogen status and yield response of grapefruit trees. The study includes two irrigation treatments, three fertilizer sources, and four fertilizer rates. No significant differences in SPAD readings and leaf N were found between two irrigation treatments. Correlations between SPAD readings and leaf N were higher for the spring flush (r = 0.61) than that for summer flush (r = 0.49). There were poor relations between SPAD meter readings and the extractable chlorophyll content in the spring flush on a dry-weight basis (r = 0.25). Higher correlations between SPAD meter readings and N rates or fruit yields than between leaf N concentrations and N rates or fruit yield.

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Y.C. Li, A.K. Alva, D.V. Calvert and M. Zhang

The objective of this study was to evaluate the use of a leaf chlorophyll meter (SPAD-502) to predict nitrogen status and yield response of grapefruit trees. The study included two irrigation treatments, three sources, and four rates of fertilizer [dry soluble granular fertilizer broadcast or through fertigation with N rates either 50, 100, 150 lb/acre/yr (56, 112, or 168 kg.ha-1.yr-1)], and controlled-release fertilizer with N rates either 25, 50, 100 lb/acre. Irrigation treatments did not affect SPAD readings and leaf N concentrations. Correlations between SPAD readings and leaf N were greater for the spring flush (r 2 = 0.61) than for the summer flush (r 2 = 0.49). High correlations of SPAD meter readings with either fruit yields or leaf N suggests that SPAD meter reading is an acceptable index of N status and fruit production of the trees. SPAD readings can be made rapidly without destructive sampling of the leaves. This study demonstrates that the SPAD meter can be used to evaluate the tree N status and improve a N fertilization program for grapefruit trees.

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Y.C. Li, A.K. Alva, D.V. Calvert, R.M. Sonoda, R.R. Pelosi and D.J. Banks

The objective of this study was to evaluate the role of chloride (Cl-) and other anions in soil water on the incidence and severity of Rio Grande Gummosis (RGG) in grapefruit (Citrus paradisi MacFadyen) trees. White `Marsh' grapefruit trees on sour orange (C. aurantium Lush.) rootstock were grown on two-row raised beds. Due to differential rates of K application (as KCl), the trees received various rates of Cl- ranging from 0 to 156 kg·ha-1 per year. Soil water was sampled using suction lysimeters at 120- and 180-cm depths. The severity of RGG was evaluated 18 months following the application of various rates of KCl. The concentration of Cl- in soil water samples varied from 16.2 to 617 mg·L-1, with mean concentrations across all treatments of 160.7 and 188.4 mg·L-1 for 120- and 180-cm depth water samples, respectively. The concentrations of Cl- and other anions (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{PO}_{4}^{3-}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{SO}_{4}^{2-}\) \end{document}) in soil water at either 120- or 180-cm depth were poorly related to the incidence or severity of RGG (R = -0.10 to -0.28). Chloride concentrations in the soil water ranged from 20 to 617 mg·L-1 near the RGG-free trees, and from 16.7 to 310 mg·L-1 near the trees that were affected by RGG. This suggests that under the conditions of this study, neither the incidence nor the severity of RGG was influenced by high concentrations of Cl- in the soil water.