ARIZONA is one of four important citrus-producing

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1 Published online August 3, 2006 Response of Microsprinkler-Irrigated Navel Oranges to Fertigated Nitrogen Rate and Frequency Ayako Kusakabe, Scott A. White, James L. Walworth, Glenn C. Wright, and Thomas L. Thompson* ABSTRACT Microsprinklers allow precise control of irrigation water applications and offer the potential for higher efficiency of water and fertilizer use compared with flood irrigation. A field experiment was conducted during in central Arizona (AZ) to evaluate effects of various N rates and fertigation frequencies on fruit yield and quality, leaf N concentration, and residual soil N of Newhall navel oranges (Citrus sinensis) on Carrizo citrange (Porcirus trifoliata x Citrus sinensis) rootstock (planted in 1997) grown in a Gilman (coarse-loamy, mixed, superactive, calcareous, hyperthermic Typic Torrifluvents) fine sandy loam. The experiment included nonfertilized control plots and factorial combinations of three fertigation frequencies (27, 9, and 3 applications annually) and three N rates (68, 136, and 204 g N tree 21 yr 21 ). Maximum yields occurred at N rates of 105 to 153 g N tree 21 yr 21 for the fourth to the sixth growing seasons. The yield-maximizing N rates were equivalent to 17 to 34% of currently recommended N rates for citrus grown in AZ. Fruit and juice quality did not show significant response to N rate or fertigation frequency. Leaf N concentrations at yield-maximizing N rates were above the critical leaf tissue N range of 25 to 27 mg g 21, indicating that this range may be too low for these Newhall navel orange trees. During all three seasons, higher residual soil NO 3 concentrations resulted from the highest N rate. Our results suggest that optimum N rates for microsprinklerirrigated Newhall navel oranges in AZ are lower than currently recommended N rates. ARIZONA is one of four important citrus-producing states in the USA with more than ha of commercial citrus production (National Agricultural Statistics Service, 2002). Southwestern citrus production depends greatly on inputs of irrigation water and N fertilizer to obtain optimum tree growth, and fruit yield and quality. Escalating water costs and declining water availability are prompting growers to adopt alternative production practices, such as microsprinkler irrigation in place of conventional surface flood irrigation. Microsprinklers offer the potential for higher application efficiency of water compared with flood irrigation (Smajstrla et al., 1991), make it possible to use high frequency fertigation with small doses of liquid fertilizers, and allow precise control of irrigation water and watersoluble nutrient applications (Boman and Obreza, 2002). Thus, nutrient losses can be reduced when microsprinkler irrigation and fertigation are properly managed (Paramasivam et al., 2000). A. Kusakabe, S.A. White, J.L. Walworth, and T.L. Thompson, Dep. of Soil, Water and Environmental Sci., Univ. of Arizona, Tucson, AZ 85721; G. Wright, University of Arizona, Yuma Agricultural Center, 6425 W. 8th Street, Yuma, AZ Received 14 Oct *Corresponding author (thompson@ag.arizona.edu). Published in Soil Sci. Soc. Am. J. 70: (2006). Nutrient Management & Soil & Plant Analysis doi: /sssaj ª Soil Science Society of America 677 S. Segoe Rd., Madison, WI USA 1623 Existing N fertilization guidelines for young citrus trees grown in AZ call for 0 230, , , and g N tree 21 yr 21 for trees in their first, secondthird, fourth-fifth, and sixth plus years after planting, respectively (Doerge et al., 1991). Recommended N rates in Florida are , , and g N tree 21 yr 21 for first, second, and third years after planting, respectively (Tucker et al., 1995). Recommended N rates for navel oranges grown in Florida, after the fourth year from planting, are kg N ha 21. Previous research with newly planted Newhall navel orange trees in central AZ showed that N uptake efficiencies in microsprinkler-irrigated trees receiving 25 and 33% of the maximum recommended N rates for the first and the second growing seasons were,6 and,25% of the N applied with microsprinkler fertigation, respectively (Weinert et al., 2002). We concluded that N application rates currently recommended for young, non-bearing flood-irrigated navel oranges can be substantially reduced for microsprinkler-irrigated and fertigated citrus during the first 2 yr after planting (Weinert et al., 2002). Alva and Paramasivam (1998) studied the response of microsprinkler-irrigated mature Hamlin orange trees to fertigation, dry granular, or controlled-release N. They reported that N fertilizer required with microsprinkler fertigation was 58% of that reported in earlier literature, due to improved irrigation and fertilization with microsprinklers. Syvertsen and Smith (1995) reported that N application rates could be reduced by 22 to 69% for young Redblush grapefruit trees on Sour Orange rootstock that received microsprinkler fertigation on a fine sand soil in central Florida compared with recommended N rates. Thus, there is evidence that existing N fertilization guidelines should be adjusted for microsprinkler irrigation systems. There is also a lack of good information on the response of young fruit-bearing citrus to microsprinkler N fertigation in the desert Southwest. Therefore, additional research is needed to refine existing N fertilization guidelines for use with microsprinkler systems. More than 80% of citrus tree roots are located within the top 30 cm of the soil profile (Paramasivam et al., 2000). Inefficient N fertilizer practices or low N uptake by trees may result in high residual soil NO 3 and substantial leaching losses (Syvertsen and Smith, 1995). Excessive irrigation or heavy rainfall may cause NO 3 leaching through soil profiles. Due to the limited root zone of citrus trees, it may be important to increase fertigation frequency to minimize NO 3 leaching. Studies on N fertigation frequency of microsprinkler-irrigated citrus have been reported in Florida (Marler and Davies, Abbreviations: AZ, Arizona; TA, titratable acidity; TSS, total soluble solids.

2 1624 SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER OCTOBER ; Willis et al., 1991; Tucker et al., 1995; Boman, 1996; Alva and Paramasivam, 1998; Alva et al., 1998). Evidence suggests that lower N fertilizer rates and increased fertigation frequency may enhance nutrient-use efficiency and tree productivity while minimizing NO 3 leaching (Alva and Paramasivam, 1998; Alva et al., 1998). However, little research has been performed in the desert Southwest (Weinert et al., 2002). Therefore, optimum fertigation frequency should also be evaluated with microsprinkler irrigation. Modified fertilization guidelines for microsprinkler irrigation systems will help to improve nutrient-use efficiency. This in turn may enhance growth, fruit yield and quality, and economics of production, and minimize NO 3 N losses from the root zone to ground water. The objective of this field experiment was to determine effects of N application rate and fertigation frequency on fruit yield, fruit and juice quality, leaf N concentration, and residual soil N of microsprinkler-irrigated Newhall navel orange trees during the third through sixth growing seasons. MATERIALS AND METHODS Site Characteristics and Treatments Newhall navel oranges budded onto Carrizo citrange rootstock were grown on 3 by 6 m centers at the University of Arizona Citrus Agricultural Center, Waddell, AZ. The field is mapped as the Gilman series. This soil was formed on an alluvial fan and has a layer of coarse sand and pebbles approximately 0.9 m beneath the soil surface. The soil was fallowed for 9 yr after an application of manure in To decrease concentrations of residual NO 3 N, sudangrass (Sorghum sundaneses L.) was planted in summer 1996 and grown for about 5 mo before planting the navel orange trees in January Sudangrass was cut four times, and the aboveground biomass was removed. Soil samples collected before initiation of the experiments in 1997 showed NO 3 N concentrations, 4 mg kg 21 in the surface 0.9 m. A field study was initiated when the trees were planted in March During March 1997 through December 1998, the trees received factorial combinations of various fertigation frequencies (3, 9, and 27 N applications annually) and fertilizer N rates (0, 45, 90, and 136 g N tree 21 yr 21 ). Results of this experiment are described in Weinert et al. (2002). The N fertilizer rates were increased slightly for the current experiment to account for increased growth of the trees. The experiment consisted of factorial combinations of three fertigation frequencies (27, 9, or 3 applications per growing season) and three N rates (68, 136, or 204 g N tree 21 yr 21 ), arranged in a randomized complete block design with five replications. A nonfertilized control treatment was also included. Each plot contained two trees. All N was applied as urea ammonium nitrate solution (UAN ) through the irrigation system. The N fertilizers were applied to each treatment by using a Dosatron fertilizer injector (Dosatron Products, Sunnyvale, FL). The irrigation lines were flushed for at least 10 min after fertigation. Fertigation was applied from 7 Apr. to 10 Sep. 1999, 27 Mar. to 25 Sep. 2000, 26 Mar. to 24 Sep. 2001, and 21 Jan. to 22 Jul Preplant soil tests indicated that all other essential plant nutrients were present in adequate quantities. Calendar years 1999, 2000, 2001, and 2002 represented the third, fourth, fifth, and sixth growing seasons after planting. Irrigation Management All trees received identical irrigation management. One pressure-compensating, 300-degree microsprinkler (Maxijet, Dundee, FL) was located 5 cm from the north side of the trunk of each tree. The trunks were protected from wetting and sunburn with waxed paper collars. The emitters were equipped with a deflector that limited the wetted radius to 0.6 m. The flow rate was 38 L h 21 at a pressure of 240 kpa, which wetted 8% of the orchard floor. Main and branch irrigation lines were constructed from 12.5 mm (ID) black polyethylene pipe, and the lines were arranged to apply the appropriate treatment to each tree. Irrigation was applied when soil water tension, measured by tensiometers with cups placed at the 30-cm depth, reached 30 kpa. Irrigation amounts were intended to replace evapotranspiration, plus a 5 to 10% leaching fraction. The Arizona Meteorological Network (AZMET) station was located within 100 m of the study site. Average annual rainfall and reference crop evapotranspiration (ET o ) at the site are 243 and 2058 mm, respectively. During the third, fourth, fifth, and sixth growing seasons, annual precipitation amounts were 62, 249, 170, and 81 mm. Total ET o was 2053, 2002, 1959, and 2088 mm. Total irrigation water applications (relative to the wetted area) were 530 mm during 1999, 1490 mm during 2000, 1800 mm during 2001, and 3350 mm during Irrigation was applied every 4 to 12 d during the 4 yr of this study. Leaf Sampling and Analysis Tree nutritional status was evaluated by using leaf N concentrations. Thirty spring-flush leaves were randomly collected from each plot in September of each year. Leaf tissue was rinsed with distilled water, dried at 658C, and ground to pass a 0.2-mm sieve. The ground leaf tissue was analyzed for total N by a Kjeldahl digestion method modified to recover NO 3 N followed by steam distillation and titration (Bremner and Mulvaney, 1982). Fruit Yield, Sampling, and Analysis No fruit was harvested during 1999 or 2000, but fruit was harvested on 31 Jan. 2001, 17 Jan. 2002, and 12 Dec Trees were manually strip-picked for the harvest. Harvested fruit from each plot was collected in boxes and weighed, and all fruit was passed through an automated electronic fruit sizer (Aweta-Autoline, Reedley, CA). The fruit sizer measured weight, color, exterior quality, diameter, fruit grade (USDA, 1999), and fruit packout (size categories; Anonymous, 2004). Interior fruit quality was determined on eight fruit randomly selected from each plot. Fruit quality attributes measured were fruit weight, juice weight and volume, rind thickness, and fruit diameter. Juice quality attributes measured were ph, total soluble solids (TSS), and titratable acidity (TA). Each fruit to be analyzed for quality was sliced in half and juiced with a commercial electric juicer (Sunkist, Sherman Oaks, CA). The juice was strained to remove pulp and weighed. The TA was measured by titrating 25 ml of juice with M NaOH to a ph of 8, and TSS was measured with a handheld refractometer (Wardowski, 1990). Soil Sampling and Analysis Soil samples were collected following the growing seasons in December 1999, January 2001, and January Two soil cores, 4 cm in diameter, were collected per plot using a hydraulic sampling rig. Samples were collected under the tree

3 KUSAKABE ET AL.: NITROGEN RATE AND MICROSPRINKLER-IRRIGATED ORANGES 1625 Table 1. Analysis of variance summary for fruit and juice quality measurements among fertilized treatments. Source df % Fancy % No.1 % No. 2 Fruit weight Rind thickness brix % acid Year 2 *** ** ** ** ** NS ** *** ** *** *** ** NS NS *** N rate 2 NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS Frequency 2 NS NS NS NS NS NS NS NS NS * NS NS NS NS ** Year 3 N rate 4 NS NS NS NS NS NS NS NS NS NS NS NS NS NS * Year 3 frequency 4 NS NS NS NS NS NS NS NS NS NS NS NS NS NS ** N rate 3 frequency 4 NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS Year 3 N rate 3 frequency 8 NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS Rep 4 NS NS NS NS NS * NS NS NS NS NS NS NS NS NS Error 100 * Significant differences at p ** Significant differences at p *** Significant differences at p NS, not significant. canopy to a depth of 1.2 m, divided into 0.30 m increments, mixed thoroughly, and composited for analysis. Soil samples were oven dried at 658C and ground to pass a 2-mm sieve. Ammonium and NO 3 N were measured in 1 M KCl extracts by using steam distillation and titration (Keeney and Nelson, 1982). Data Analysis Analysis of variance (ANOVA) was performed on fruit yield, fruit quality, juice quality, and leaf N during the experiment by using the GLM procedure in SAS (SAS Institute, 1999), and mean separations were accomplished by using Duncan s Multiple Range Test. Orthogonal contrasts (unfertilized vs. fertilized) of response variables was also accomplished with SAS GLM. When there were no significant (p, 0.05) fertigation frequency or frequency 3 N rate interactions on yield (as during the fourth and the fifth growing seasons), quadratic response equations were developed for yield as a function of N rates. When significant fertigation frequency or interaction effects existed (as during the sixth growing season), separate quadratic response equations (yield vs. N rate) were developed for each fertigation frequency. Regression analysis was accomplished by using the REG procedure in SAS. RESULTS AND DISCUSSION Fruit Yield and Quality No fruit was harvested during the third growing season (1999). In general, fruit yield, fruit quality, and juice quality were significantly affected only by year (Table 1). During the fifth growing season, percentage Fancy, percentage No. 2, fruit weight, and rind thickness were significantly higher than during the fourth and sixth seasons (Table 2). Alva and Paramasivam (1998) reported that production of larger fruits was directly correlated with the reduction of fruit yield on mature Hamlin orange trees in Florida. However, our results did not follow similar trends. The fruit were sweeter (higher8brix) and less acidic during the fourth season than during the other two seasons. Koo (1980) reported that when fertilizers were applied via fertigation, the TA in juice was low, and the TSS/TA ratio was higher compared with dry fertilizer applications. Our results indicated relatively higher TSS, lower TA, and higher TSS/TA ratio compared with those of mature Valencia orange trees under trickle irrigation reported by Koo (1984). Nevertheless, variations in N rates and fertigation frequency did not significantly affect fruit and juice quality during the three harvests. Orthogonal contrast comparisons (Table 3) revealed that, during each season, some fruit and juice quality measurements were significantly different between fertilized and unfertilized treatments. During the first two harvests (January 2001 and January 2002), maximum predicted yield from quadratic response equations was 10 kg tree 21 in the fourth, and 19 kg tree 21 in the fifth growing season (Fig. 1). During the sixth growing season only (December 2002), yield was significantly (p, 0.05) affected by fertigation frequency. During this season, trees receiving 27 fertigations per growing season had the highest yields among three fertigation frequencies (Fig. 2). Yields in treatments receiving 3 and 9 fertigations per growing season were lower than those receiving 27 fertigation events per season. The reason for this response is unclear. Maximum predicted yield with 27 fertigation events per growing season was 30 kg tree 21 (Fig. 2). The maximum predicted yields occurred at N rates of 114 g N tree 21 yr 21 for the fourth, 105 g N tree 21 yr 21 for the fifth, and 153 g N tree 21 yr 21 for the sixth growing season (27 fertigations only) (Fig. 1 and 2). During this experiment, considerably less N fertilizer was applied to the trees under microsprinkler irrigation com- Table 2. Average values for selected fruit quality and juice quality measurements among treatments receiving N fertilizer. Growing season fancy No. 1 No. 2 Fruit weight Rind thickness brix Acid % g mm % 4th th th LSD NS NS 0.7 LSD 5 Least significant difference (p ) for pairwise comparison of means within columns, according to Duncan s Multiple Range Test; NS, not significant.

4 1626 SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER OCTOBER 2006 Table 3. Significance of orthogonal contrast comparisons (control vs. fertilized) for fruit quality. Growing season Fancy No. 1 No. 2 Fruit weight pared with current AZ recommendations, which are 110 to 450 g N tree 21 yr 21 for the fourth and the fifth growing seasons, and 450 to 910 g N tree 21 yr 21 for trees more than 5 yr after planting (Doerge et al., 1991). Thus, the yield-maximizing N rates in this experiment were equivalent to 17 to 34% of currently recommended N rates. One reason could be the relatively low yields, and thus N removal, in this experiment. Based on average citrus fruit N content (from Doerge et al., 1991), N removal in harvested fruit at the maximum predicted yield was 18, 35, and 52 g tree 21 during seasons 4 through 6. However, our finding that yield-optimizing N rates with microsprinkler irrigation were lower than currently recommended rates agrees with previous research from Florida (Alva and Paramasivam, 1998; Syvertsen and Smith, 1995) and AZ (Weinert et al., 2002) that showed that N rates currently recommended for floodirrigated citrus can be reduced for microsprinklerirrigated citrus. Leaf Nitrogen Concentrations Rind thickness brix Acid th NS NS NS NS NS NS NS NS * NS NS NS NS NS * 5th NS NS NS NS NS NS NS * NS NS * NS NS NS NS 6th NS NS NS NS * NS NS NS NS NS NS NS NS NS NS * Significant differences at p NS, not significant. During the fourth, fifth, and sixth growing seasons, leaf N concentrations were significantly affected (p, 0.05, p, 0.05, and p, 0.01, respectively) by N rate. There were no significant effects of fertigation frequency or frequency 3 N rate interactions on leaf N concentration during the 4 yr of this experiment, except during the third growing season when leaf N concentrations were significantly affected (p, 0.01) by fertigation frequency. During this season, leaf N concentrations in all treatments were above 25 mg g 21, which is the generally accepted critical leaf tissue N concentration (Fig. 3; Tucker et al., 1995; Sauls, 2002; Kallsen, 2003). This suggests that all trees contained adequate N for optimum growth. Leaf N concentration in the unfertilized trees was similar to that of the fertilized trees. The high N status of the unfertilized trees may be explained by remobilization of the tree N reserves for tree growth (Weinbaum and Van Kessel, 1998; Weinert et al., 2002). During the fourth growing season, leaf N concentrations were above 25 mg g 21 in all fertilized and unfertilized trees (Fig. 3). Leaf N concentration at the yield-maximizing N rate was 27.8 mg g 21. During the fifth growing season, leaf N ranged from 24.3 mg g 21 in unfertilized controls to 31.5 mg g 21 at the highest N rate. The unfertilized control trees had the lowest N concentration among treatments, indicating the lower N status of the trees. This is not surprising, because the unfertilized trees had not been fertilized for the 5 yr since planting. Leaf N concentration at the yieldmaximizing N rate was 27.9 mg g 21. During the sixth growing season, leaf N concentrations in all treatments were again above 25 mg g 21. Leaf N concentration at the yield-maximizing N rate with 27 fertigation events per growing season was 30.0 mg g 21. All fertilized trees were adequately supplied with N for all 4 yr, according to accepted leaf tissue concentration standards. Throughout the experiment, leaf N concentrations at yieldmaximizing N rates were above the critical leaf tissue N rate of 25 to 27 mg g 21 (Tucker et al., 1995; Sauls, 2002; Kallsen, 2003) (Fig. 3). These findings may indicate that the commonly used critical leaf N concentration range of 25 to 27 mg g 21 was too low for these trees. Fig. 1. Fruit yield of Newhall navel orange trees in response to N fertilizer rates during the fourth and the fifth growing seasons. The x-intercepts denote maximum predicted yield. Fig. 2. Fruit yield of Newhall navel orange trees in response to N fertilizer rates among three fertigation frequencies (27, 9, or 3 fertigations per growing season) during the sixth growing season. The x-intercept denotes maximum predicted yield.

5 KUSAKABE ET AL.: NITROGEN RATE AND MICROSPRINKLER-IRRIGATED ORANGES 1627 Fig. 3. Effect of N fertilizer rates on leaf N concentration of Newhall navel orange trees during the third through the sixth growing seasons. The x-intercepts denote N rate at the maximum predicted yield. Residual Soil Nitrate Residual soil NO 3 distributions showed important differences among treatments (Fig. 4). In general, when the N fertilizer rate was #136 g N tree 21 yr 21,little residual NO 3 was present (Fig. 4). In contrast, application of 204 g N tree 21 yr 21 usually resulted in substantial amounts of residual NO 3 across the three fertigation frequencies. Residual soil NO 3 in unfertilized controls was,9 mg kg 21 throughout the soil profile during the three seasons. During the fourth growing season, residual soil NO 3 was lower than during the other two seasons (Fig. 4), with soil NO 3, 7mgkg 21.Duringthisseason, 280 mm of rain fell between the first N fertigation and soil sampling, and 178 mm fell between the last N fertigation and soil sampling. This likely resulted in substantial NO 3 leaching below the sampling depth. However, as in the previous and subsequent seasons, NO 3 concentrations were highest at the high N application rate (Fig. 4). During all three seasons, fertigation frequency did not significantly affect residual soil NO 3. Willis et al. (1991) studied the response of 1-yr-old microsprinkler-irrigated Hamlin orange trees to fertigation frequency, and reported that fertigation only five times per year resulted in higher soil NO 3 in the top 15 cm 1 wk after fertilization compared with weekly or monthly fertigation frequency. Alva et al. (1998) also found that increased fertigation frequency minimized NO 3 movement under microsprinkler-irrigated mature Valencia orange trees. In our experiment, however, it was N rate, rather than fertigation frequency, that affected concentrations of residual soil NO 3. Fig. 4. Soil residual NO 3 N concentrations among control plots and three fertigation frequencies (27, 9, 3 fertigation events per growing season) for Newhall navel orange trees during the third through fifth growing seasons. Error bars, where shown, indicate significant differences (p, 0.05) according to Duncan s Multiple Range Test.

6 1628 SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER OCTOBER 2006 CONCLUSIONS We evaluated the response of 3- to 6-yr-old microsprinkler-irrigated Newhall navel orange trees to various N rates and fertigation frequencies. The maximum fruit yield of the trees occurred at N rates of 113 g N tree 21 yr 21 for the fourth, 105 g N tree 21 yr 21 for the fifth, and 153 g N tree 21 yr 21 for the sixth growing season. These yield-maximizing N rates corresponded to 17 to 34% of currently recommended N rates. Infrequent fertigation adversely affected yield response to N fertilizer during only one of the three harvests. During all three harvests, fruit and juice quality, and fruit packout were not significantly affected by N rate or fertigation frequency. Leaf N concentrations at yield-maximizing N rates were consistently above the critical leaf tissue N rate of 25 to 27 mg g 21. Thus, the application of N at rates well below the currently recommended rates did not compromise the N status of these trees. But, our findings also suggest that the recommended critical leaf N concentration range of 25 to 27 mg g 21 may be too low for these trees. Higher residual soil NO 3 concentrations resulted from the highest N rate. Our results suggest that optimum N rates for microsprinkler-irrigated Newhall navel oranges in Arizona are lower than currently recommended N rates developed for flood irrigation. REFERENCES Alva, A.K., and S. Paramasivam Nitrogen management for high yield and quality of citrus in sandy soils. Soil Sci. Soc. Am. J. 62: Alva, A.K., S. Paramasivam, and W.D. Graham Impact of nitrogen management practices on nutritional status and yield of Valencia orange trees and groundwater nitrate. J. Environ. Qual. 27: Anonymous Citrus packing handbook. Sunkist Growers, Inc. Sherman, Oaks, CA. Boman, B.J Fertigation versus conventional fertilization of flatwoods grapefruit. Fert. Res. 44: Boman, B.J., and T.A. Obreza Fertigation nutrient sources and application considerations for citrus. University of Florida Coop. Ext. Serv. UF/IFAS Bulletin. Circular Bremner, J.M., and C.S. Mulvaney Nitrogen-Total. p In A.L. Page et al. (ed.) Methods of soil analysis. Part II. Agron. Monogr. No. 9. ASA and SSSA, Madison, WI. Doerge, T.A., R.L. Roth, and B.R. Gardner Nitrogen fertilizer management in Arizona, University of Arizona. College of Agric., Rep. no Kallsen, C Fall leaf tissue samples important for maintaining citrus growth, fruit quality and yields. University of California Cooperative Extension. Available online at edu/pub/subtropics%20fall%2003.pdf (verified 24 Apr. 2006). Keeney, D.R., and D.W. Nelson Nitrogen-inorganic forms. p In A.L. Page et al. (ed.) Methods of Soil Analysis. Part 2. 2nd ed. Agronomy No. 9. ASA and SSSA. Madison, WI. Koo, R.C.J Results of citrus fertigation studies. Proc. Fla. State Hort. Soc. 93: Koo, R.C.J Effect of trickle irrigation and fertigation on fruit production and juice quality of Valencia orange. Proc. Fla. State Hort. Soc. 97:8 10. Marler, T.E., and F.S. Davies Microsprinkler irrigation and growth of young Hamlin orange trees. J. Am. Soc. Hortic. Sci. 115: National Agricultural Statistics Service Census of Agriculture, Available online at (verified 1 May 2006). Paramasivam, S., A.K. Alva, and A. Fares An evaluation of soil water status using tensiometers in a sandy soil profile under citrus production. Soil Sci. 165: SAS Institute SAS user s guide. SAS Inst. Cary, N.C. Sauls, J.W Texas citrus and subtropical fruits: Nutrition and fertilization. Available online at citrus/nutrition/l2288.htm (verified 28 Apr. 2006). Smajstrla, A.G., B.J. Boman, G.A. Clark, D.Z. Haman, D.S. Harrison, F.T. Izuno, D.J. Pitts, and F.S. Azsueta Efficiencies of Florida agricultural irrigation systems. University of Florida Coop. Ext. Serv. UF/IFAS Bulletin. BUL247. Syvertsen, J.P., and M.L. Smith Nitrogen leaching, N uptake efficiency and water use from citrus trees fertilized at three N rates. Proc. Fla. State Hort. Soc. 108: Tucker, D.P., A.K. Alva, L.K. Jackson, and T.A. Wheaton Nutrition of Florida citrus trees. University of Florida Coop. Ext. Serv. UF/IFAS Bulletin. SP169. USDA United States standards for grades of oranges (California and Arizona). USDA Agricultural Marketing Service. Available online at (verified 28 Apr. 2006). Wardowski, W.F Citrus quality control assessment methodology. Citrus Research and Education Center, Univ. of Florida. Lake Alfred, FL. Weinbaum, S., and C. Van Kessel Quantitative estimates of uptake and internal cycling of 14N-labeled fertilizer in mature walnut trees. Tree Physiol. 18: Weinert, T.L., T.L. Thompson, and S.A. White Nitrogen fertigation of young navel oranges: Growth, N status, and uptake of fertilizer N. HortScience 37: Willis, L.E., F.S. Davies, and D.A. Graetz Fertigation and growth of young Hamlin orange trees in Florida. HortScience 26: