High Application Rates of Reclaimed Water Benefit Citrus Tree Growth and Fruit Production

Conversion of wastewater to reclaimed water for crop irrigation conserves water and is an effective way to handle a growing urban problem: the disposal of wastewater. Water Conserv II is a large reclaimed water project developed by Orlando and Orange County, Fla., that presently irrigates ≈1900 ha of citrus. The project includes a research component to evaluate the response of citrus to irrigation using reclaimed water. Citrus trees in an experimental planting responded well to very high application rates of reclaimed water. Irrigation treatments included annual applications of 400 mm of well water, and 400, 1250, and 2500 mm of reclaimed water. The 2500-mm rate is excessive, and since disposal was of interest, this rate was used to determine if citrus could tolerate such high rates of irrigation. The effects of these treatments were compared on ‘Hamlin’ orange [Citrus sinensis (L.) Osb.] and ‘Orlando’ tangelo (C. paradisi Macf. x C. reticulata Blanco) combined with four rootstocks: Carrizo citrange [Citrus sinensis (L.) Osb. x Poncirus trifoliata (L.) Raf.], Cleopatra mandarin (C. reticulata Blanco), sour orange (C. aurantium L.), and Swingle citrumelo (C. paradisi x P. trifoliata). Growth and fruit production were greatest at the highest irrigation rate. Concentration of soluble solids in the juice was usually lowered by the highest irrigation rate, but total soluble solids per hectare were 15.5% higher compared to the 400-mm rate, due to the greater fruit production. While fruit soluble solids were usually lowered by higher irrigation, the reduction in fruit soluble solids observed on three of the rootstocks did not occur in trees on Carrizo citrange. Fruit peel color score was lower but juice color score was higher at the highest irrigation rate. Crop efficiency (fruit production per unit of canopy volume) was usually lower at the 2500mm rate and declined as trees grew older. Weed cover increased with increasing irrigation rate, but was controllable. Irrigation with high rates of reclaimed water provided a satisfactory disposal method for treated effluent, benefited growth and production of citrus, and eliminated the need for other sources of irrigation water. Reclaimed water, once believed to be a disposal problem in Florida, is now considered to be one way to meet irrigation demands. Sewage wastewater or effluent is often viewed as a disposal problem, but it can be a source of reclaimed water for irrigation. This approach creates an alternative disposal method for a waste treatment facility, may benefit agriculture as a source of irrigation water, and reduces the demand for use of surface or ground water for irrigation. Potential disadvantages of using reclaimed water for agricultural irrigation include real or perceived concerns about water quality, such as viruses, diseases, heavy metals, and the safety of crops for human consumption (Parsons and Wheaton, 1996; Parsons et al., 1995). Other disadvantages include the possible requirement to use more water than is needed for irrigation or to use it during rainy periods when it is not needed. 30.7 mm/week, but application rates >30.7 mm/week decreased yields in poorly drained Spodosols. In another study, irrigation rate did not affect young grapefruit tree growth (Maurer and Davies, 1993). In order to deal with wastewater disposal, the City of Orlando and Orange County began a project called Water Conserv II. The goal of this project was to dispose of urban wastewater by converting it to reclaimed water and using the water for citrus, ornamental, and golf course irrigation. Since the city and county wanted to dispose of the wastewater, there was interest in determining if large volumes of this water could be applied to citrus groves without causing damage to trees. Contracts were set up with growers who agreed to accept either 625 or 1250 mm of reclaimed water per year. Water Conserv II presently irrigates ≈1900 ha of citrus and has become one of the largest reclaimed water agricultural irrigation projects of its type in the United States. It is the first project in Florida to be permitted to irrigate crops for human consumption with reclaimed water (McMahon et al., 1989). The objective of this research was to determine the effects of irrigation with reclaimed water on two citrus cultivars and four rootstocks. Treatments included commonly used irrigation rates as well as excessively high application rates to determine the capacity of a citrus grove to act as a disposal site for reclaimed water. Materials and Methods Experiment site. The experiment was located near the Water Conserv II distribution center west of Orlando, Fla. (lat. 28°28 ́20”N, long. 81°38 ́50”W, elevation 64 m). This is an area where citrus has been traditionally grown, but it is subject to periodic damaging freezes. The soil is an excessively drained Candler fine sand (hyperthermic, uncoated Typic Quartzipsamments). It has very low waterholding capacity (available water <0.06 cm·cm with <1% organic matter and <3% clay. The majority of citrus tree roots are in the top 90 cm, but some roots can extend to depths below 1.5 m (Menocal-Barberena, 1999). Reclaimed water. The reclaimed water used in this study was provided through the Water Conserv II project. This water is normal sewage effluent collected from the City of Orlando and surrounding communities and treated with advanced secondary treatment with highlevel disinfection, coagulation, filtration, and chlorination. Water quality standards were established in negotiations between Water Conserv II, Univ. of Florida researchers, and growers to ensure that water would be suitable for citrus irrigation. This reclaimed water meets drinking water standards for a number of elements (Table 1), has no color or odor, and is very low in heavy metals. While this water is suitable for irrigation (Ayers and Westcot, 1976), it was not known how trees would respond to the high application rates that the city desired for disposal. Experimental design. ‘Hamlin’ sweet orange and ‘Orlando’ tangelo trees in combinaIn some arid regions where freshwater supplies are limited, irrigation with reclaimed water is already commonly practiced (Feigin et al., 1991). In humid regions such as Florida, wastewater had been considered to be a disposal problem, but has recently become regarded as a potential source of reclaimed water for irrigation. Reclaimed water has been used for irrigation of a number of crops, including forage crops, turfgrass, golf courses, and woody perennials for biomass production (Pettygrove and Asano, 1985). It has been used successfully for irrigation of fruit trees (Basiouny, 1982) and has increased trunk diameter growth, fruit number, and yield of apples (Neilsen et al., 1989). In an observational survey where irrigation application rates were unknown, no adverse effects of reclaimed water irrigation were seen in Florida citrus groves (Zekri and Koo, 1993). In Spain, Lapena et al. (1996) found that irrigation of citrus trees with reclaimed water was not harmful. Maurer et al. (1995) showed that grapefruit yield in Florida increased at reclaimed water irrigation rates of 23.1 and 6909, p. 1273-1277 12/19/01, 1:07 PM 1273 HORTSCIENCE, VOL. 36(7), DECEMBER 2001 1274 SOIL MANAGEMENT, FERTILIZATION, & IRRIGATION tion with four rootstocks were obtained from commercial nurseries and planted at a 3.0 × 6.1 m spacing in Fall 1987. They were planted in a split-split plot arrangement with four replications of 24-tree plots (six trees in four adjacent rows). Irrigation treatments consisted of annual applications of well or reclaimed water at 400 mm (W400, RW400), and reclaimed water at 1250 and 2500 mm (RW1250, RW2500). Each irrigation main plot was subdivided into plots with the two scion cultivars, which were further divided into plots assigned to the four rootstocks (Table 2). All data were analyzed using the SAS– GLM split plot model of the analysis of variance. Annual fruit quality measurements for the 400and 2500-mm treatments were averaged over the 5-year period from 1993 through 1997. This time period was chosen because trees were representative of mature trees in Florida. Cultural practices and measurements. Trees were irrigated with 360° microsprinklers that delivered ≈57 L·h and covered ≈80% of the land area. The differential irrigation treatments were begun on 1 Oct. 1989. The 400mm rate is near the recommended rate and is commonly used for Florida citrus. The 1250mm rate is the maximum amount growers on the Water Conserv II project were expected to use, and 2500 mm is a very high rate that could be potentially damaging to citrus. From 1989 to 1992, all treatments were irrigated every second or third day regardless of rainfall. Beginning in 1993, scheduling of the W400 and RW400 treatments was changed to maintain soil-available water above one-third depletion in the spring and above two-thirds depletion during the rest of the year. Depending on rainfall, annual rates for the 400-mm treatments varied from 400 to 550 mm. Evapotranspiration (ET) was estimated from data collected by a weather station at the site. Rainfall was measured, daily ET was calculated, and a water budget system was used to determine irrigation. Trees were irrigated every 2 to 3 d in the spring and every 4 to 10 d from July to January. The 1250-mm treatment (Table 2) was discontinued in 1993 to allow for another study. The 2500-mm rate continued to be irrigated every second day. Annual rainfall ranged from 983 to 1790 mm. Complete fertilizer was applied to all treatments at the recommended rate of N at 224 kg·ha per year in split applications (Tucker et al., 1995). Because N fertilizer was applied in adequate amounts, the nutritional value of reclaimed water was not considered as a factor in this study. Because differences in weed growth among irrigation treatments were observed, two independent observers visually estimated weed cover of the herbicide strips in the summer and their estimate of percent weed cover was averaged. Tree growth was initially determined by measuring trunk diameter annually and later by calculating canopy volume. Trunk diameter measurements were made ≈10 cm above the bud union on a smooth section of the trunk. Canopy volume was calculated from measurements of canopy height and diameters (Wheaton et al., 1995). Yield was measured in the autumn when fruit were mature by harvesting two representative trees in each plot. Crop efficiency was estimated by dividing the weight of the fruit per tree by the canopy volume. Fruit samples were collected at the time of harvest each year to determine the treatment effects on fruit and juice quality. Juice was extracted using standardized automatic extraction machinery and analyzed for soluble solids and acid concentration with computerized test equipment found in Florida processing plants. External fruit color was measured with a HunterLab Color Difference Meter (HunterLab, Fairfax, Va.) and reported as the a/b ratio, which is negative for green fruit, close to zero for yellow fruit, and increases in positive value with improving orange color (Wheaton and Stewart, 1973). Juice color was determined using a citrus colorimeter.