banana production irrigated with treated effluent in the canary islands

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Reclaimed water is a non-conventional resource, traditionally not accounted for in the hydrological balance. An increase in wastewater reuse (currently 17 × 106 ...
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BANANA PRODUCTION IRRIGATED WITH TREATED EFFLUENT IN THE CANARY ISLANDS M. P. Palacios, D. Z. Haman, E. Del-Nero, A. Pardo, N. Pavon ABSTRACT. The Giant Cavendish variety of bananas was grown for two years under four different microirrigation (drip) water quality treatments. The treatments consisted of fresh groundwater (FW), fresh groundwater (70%) mixed with secondary effluent (30%) (FW+SE), desalinized secondary effluent (70%) mixed with secondary effluent (30%) (DSE+SE), and desalinized secondary effluent (DSE). The experimental design was a Randomized Complete Block (RCB) design with three replications and four treatments. The production was under greenhouse conditions, typical for the Gran Canaria, Canary Islands. Plants were grown following typical water and fertilizer application used by the best growers on the Island. Productivity of banana plants, estimated as mean bunch weight, was significantly affected by water quality. Plants irrigated exclusively with desalinated secondary effluent showed significantly lower yield than those irrigated using fresh water. Desalinized secondary effluent had most of the salts removed; however, this treatment had the highest levels of SAR. There was no significant difference in yield in the two higher salinity treatments and the 30% addition of secondary effluent did not negatively impact plant growth or banana yield as compared with the FW treatment. Keywords. Effluent irrigation, Bananas, Salinity, Hydraulic conductivity, Water quality, Desalinized effluent.

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he Canary Islands, located in the Atlantic Ocean at latitude 28°N, are classified as a semiarid region. Gran Canaria, the second in size and the most populated island (700,000 permanent residents not counting numerous tourists) receives 300 mm of rainfall per year (Allue-Andrade, 1990). The total use of water on the Island, including agriculture, is 130 × 106 m3/year. Only 11 × 106 m3 of the water comes from surface water sources. Most comes from groundwater, desalination, and wastewater reuse. Gran Canaria’s desalination plants provide 21 × 106 m3 of water per year. An additional 47 × 106 m3 come from rainfall recharged groundwater. Until now, nonrenewable groundwater resources were used to provide the remaining 51 × 106 m3 (Consejo Insular, 1995). This unsustainable situation resulted in salt intrusion into the aquifer and increased cost of groundwater due to the need for desalinization and deep drilling. Reclaimed water is a non-conventional resource, traditionally not accounted for in the hydrological balance. An increase in wastewater reuse (currently 17 × 106 m3/year) can partially alleviate Article was submitted for publication in August 1999; reviewed and approved for publication by the Soil & Water Division of ASAE in January 2000. Florida Agricultural Experiment Station Journal Series No. R- 07036. The mention of trade names or commercial products is solely for the information of the reader and does not constitute an endorsement or recommendation for use. The authors are Maria del Pino Palacios, Esteban Del-Nero, Amaya Pardo, University of Las Palmas de Gran Canaria, Islas Canarias, Spain; Dorota Z. Haman, ASAE Member, Department of Agronomy and Animal Production, Carretera Trasmontaña, Bañaderos, Arucas 35416, Gran Canaria, Spain (University of Florida, Gainesville, Florida); Ninoska Pavon, Bureau of Wastewater Reuse, Gran Canaria Island, Paseo Juan XXIII, 35011 Las Palmas de Gran Canaria, Islas Canarias, Spain. Corresponding author: Dr. Dorota Z. Haman, University of Florida, Department of Agricultural and Biological Engineering, 115 Rogers Hall, PO Box 110570, Gainesville, FL 32611-0570, phone: 352.392.8432, fax: 352.392.4092, e-mail: .

the pressure on non-renewable groundwater resources and help balance water extraction and recharge, resulting in a more sustainable system (Consejo Insular, 1995). Agricultural production, next to tourism, plays a very important role in Gran Canaria’s economy. Tomatoes and bananas are the two main crops produced on the Island. There are 2000 ha of banana plantations on Gran Canaria with an average water consumption of 9900 m3 ha–1/year (Consejería de Economía, 1989). Banana is sensitive to salt and demands high water quality for irrigation (Wardlaw, 1961). It is known that the salinity threshold for banana plants is about 1 dS m–1 (Israeli et al., 1986). Higher salinity levels result in yield reduction. Understanding plant response to the quality of irrigation water is critical to banana production especially since high quality water is limited on the Island. Palacios et al. (1998) demonstrated in preliminary results that wastewater reuse could be successfully applied in banana production in commercial orchards under proper management. The preliminary results showed also that wastewater salinity, one of the concerns of most growers, had no significant effect if a proper fertigation program was used. The quality of reclaimed wastewater is of concern to farmers, especially in banana production, since these plants are salt sensitive. According to Wardlaw (1961), the maximum concentration of soluble salts in soil water extract should not exceed 500 ppm. The municipal water used on the island has a relatively high salt content [approximately 1,200 ppm (Consorcio Insular de Aprovechamiento de Aguas Depuradas de Gran Canaria, 1998)]. This is usually higher than the salinity of good quality groundwater available for banana production. In addition, seawater intrusion into the pipes that carry wastewater to the treatment plants increases the salinity of the effluent. This phenomenon is typical for islands and coastal areas. As a result, farmers demand the use of advanced wastewater treatment, such as desalination, to

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improve water quality of the treated effluent. However, desalination of wastewater greatly increases the cost of treated effluent (approximately by $0.33 m–3 Argudo, 1998). It is known that water quality can have numerous effects on plants and soils. Various combinations of electrical conductivity (EC) and sodium adsorption ratio (SAR) in irrigation water and their effects on the soil hydraulic conductivity of soils with significant clay content have been widely demonstrated (Frenkel et al., 1978; Mc Intyre, 1979; Suarez et al., 1984; Bajwa et al., 1993). The change in hydraulic conductivity is also a function of water EC. For the same SAR, the hydraulic conductivity would be higher in a system with a higher salt concentration (Ayers and Wescot, 1985; Nakayama and Bucks, 1986; Oster and Rhoades, 1984). The effects of high salinity and SAR on plant growth, was investigated by Israeli et al. (1986). They found that under high salinity levels (4 to 5 dS m–1) with high levels of SAR water quality adversely affected banana yield. However, the medium and high salinity levels used in this study, were above the threshold recommended for commercial banana production. Plant growth can also be affected by nutritional problems due to nutrient excess, the imbalance or nonavailability of the nutrients applied, or problems related to specific phytotoxicities caused by metals (MartinezBarroso and Alvarez, 1997). Nutrient leaf content can also affect plant response against pests and diseases (Borges et al., 1991). Finally, large amounts of suspended solids in the wastewater, can result in decreased soil permeability or clogging problems of microirrigation emitters, and have an indirect effect on plant growth and yield (Nakayama and Bucks, 1986). The objectives of this research were to: (1) determine the best way to reuse wastewater in commercial banana production; and (2) quantify the impact of water quality on hydraulic conductivity of soil and plant production using the best management available in a commercial orchard.

MATERIAL AND METHODS A field experiment was conducted in Galdar, Gran Canaria Island. The variety of bananas used in the experiment was Giant Cavendish. The banana plantation used in the experiment was a commercial plantation established in October 1996. The data collected during the first two years are presented in this article. The production was under greenhouse conditions, typical for banana production on the Island. Plants were grown following typical water and fertilizer applications used by the best growers. The amount of fertilizer was applied according to the grower’s regular practices. Fertigation was adjusted depending on the quality of effluent, plant growth, and the amount of fertilizer applied by the grower in the other sections of the banana plantation. Water quality was checked and recorded at the water treatment plant. The range of BOD during the experiment varied from 2 to 140 ppm of O2 and suspended solids varied from 10 to 200 ppm. These variations were not related to the time of the year. The water in all treatments was filtered using media filters at the head of the irrigation 310

system. There was no injection of chlorine or acid into the irrigation system during the time of experiment. Filters were cleaned before each irrigation event. Uniformity tests were performed several times during each year and the water application uniformity was above 90% at all times. There were 1,080 (1.04 ha) total plants in the greenhouse with plants in 18 double rows spaced 1.75 m apart and 1.75 m between plants within the row, forming a triangle. Each double row was separated by a 5.5-m-wide access road (99 m long and 105 m wide). The plants were irrigated with drip irrigation (RAM, Netafim). Each double row was irrigated with three drip laterals with pressure compensating, 3.5 L h–1 emitters spaced 60 cm apart along the line. Irrigation water was applied three to five times each week depending on the season. The amount applied depended on the age of the plant. The application was slowly increased for six months until it reached 90 L/plant/week in the winter and 100 L/plant/week in the summer. Soil at the research site is a loamy sand. Standard classification is difficult due to the fact that the soil is transported to the sites from different parts of the island and deposited close to the coast for banana production. It is a slightly alkaline soil with about 1% organic matter, cation exchange from 30 to 35%, and two horizons (from different parts of the island): horizon A, from soil surface to about 25 cm in depth, and horizon B, from 25 cm to about 50 cm in depth and less fertile than the A horizon. The experimental design was a Randomized Complete Block (RCB) with three replications and four treatments. The blocks were used to account for soil and temperature variation within the greenhouse. The treatments consisted of different irrigation water qualities: fresh groundwater (FW); fresh groundwater (70%) mixed with secondary effluent (30%) (FW+SE); desalinized secondary effluent (70%) mixed with secondary effluent (30%) (DSE+SE); and desalinized secondary effluent (DSE). Unmixed secondary effluent (100%) was not used as a treatment in the greenhouse since its salinity was known to be too high for commercial banana production. For each treatment, water samples were collected monthly and analyzed in the laboratory using standard methods of water analysis. The parameters, pH, electrical conductivity (EC), suspended solids (SS), macro and micronutrients, chloride, sodium and HCO 3 , were measured. Sodium adsorption rate (SAR) was calculated for each sample using the following formula (Richards, 1954): SAR = Na/[(Ca + Mg)/2]1/2

(1)

For each treatment, nine replications of hydraulic saturated conductivity were measured in situ three times during the experiment—at the beginning of the experiment (Oct. 1996), a year later (Oct. 1997), and in the second year (Oct. 1998)—using a Guelph permeameter. The measurements were taken for both horizons A and B. Soil samples were also taken for each treatment and submitted for analysis to determine the salinity and ion concentration in the saturated extract following standard soil testing laboratory procedures. For each treatment, the third leaf of two plants was sampled for nutrient analyses according to established TRANSACTIONS OF THE ASAE

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procedures (Martin Prevel, 1984). Each leaf was tested for total N, total P, K, Ca, Mg, Fe, Mn, Zn, and Na. The bunch weight for each treatment was calculated using the mean weight of nine bunches per block. SAS program was used to analyzed the obtained data. We used Proc. GLM with Least Square Means (LSM) analysis to discriminate significant differences among treatments.

RESULTS AND DISCUSSION The effect of water quality on banana production, estimated as mean bunch weight (in kg), was significantly affected by water quality (table 1). In general, plants irrigated exclusively with desalinated secondary effluent had lighter bunches than those irrigated using fresh water. During the first year of the experiment there was a significant difference in yield between the FW and DSE water treatments. Although the differences among treatments during the second season were not statistically significant, there was the same trend in yield among treatments. In addition, the average production per plant from both harvests was significantly different for the FW and DSE treatments. The mean bunch weight for the FW treatment was 48.39 kg as compared to 44.92 kg for the DSE treatment. There was also a statistically significant weight increase of the average bunch from the first harvest to the second harvest due to the fact that the plantation was newly established and the plants typically have lower yield during the first year of production. The mean weight of the fruit in the second year was higher (52 to 55 kg) than that obtained from the average commercial plantations of the same variety irrigated with fresh groundwater, which ranged from 35 to 45 kg (Galán Saúco, 1992). Seasonal variations of EC and SAR of the water used in this study are presented in table 2. There were two groups with statistically significant differences in average salinity. Two treatments had lower average levels of salinity (FW and DSE) and two had higher average levels of salinity (FW+SE and DSE+SE). Also, the average levels of SAR were significantly different for FW as compared to DSE+SE and DSE treatments. The results show that there was no significant effect of salinity on the mean weight of bunch (fig. 1) during both years (two harvests) of this experiment for the given experimental conditions. However, ECs in all treatments were under the known threshold salinity level for banana

Table 2. Seasonal variations and average values of EC and SAR in all treatments during both years* Jan-March

April-June

July-Sep.

Oct-Dec.

Average

Treatment

EC EC (dS/m) SAR (dS/m) SAR

EC EC EC (dS/m) SAR (dS/m) SAR (dS/m) SAR

FW FW+SE DSE+SE DSE

0.598a 0.874b 0.875b 0.436c

0.716b 0.993a 0.959ab 0.753b

3.95a 5.90b 5.80b 5.90b

0.608a 0.769a 0.768a 0.644a

4.83a 4.98a 5.67a 6.54a

4.73a 6.00a 6.32a 6.07a

0.694b 0.934b 0.905b 0.462a

3.2a 5.7ab 6.1b 7.6b

0.650a 0.879b 0.876b 0.615a

4.34a 5.57ab 6.04b 6.22b

* The same letter means no significant differences (α = 0.05).

plants (1 dS m–1, Israeli et al., 1986). Figure 2 shows the relationship between SAR in the irrigation water and the weight of the mean bunch; lower SAR values resulted in higher bunch weights. For the same year and SAR values (DSE+SE and DSE for the second harvest), lower bunch weights were obtained with lower ECs in the irrigation water, showing the synergetic effect of SAR and salinity. Table 3 documents the change in saturated hydraulic conductivity (KS) of the soil A horizon (0 to 25 cm) over time as a function of irrigation water quality. There was no statistically significant difference in hydraulic conductivity among treatments at the beginning of the experiment, indicating relatively uniform soil conditions. However, at the end of two years, a significant decrease in conductivity in the A horizon was observed for all treatments, with the exception of FW. The decrease in hydraulic conductivity corresponded to the trend in decreased banana yield (fig. 3). A decrease in soil saturated hydraulic conductivity results in poor drainage conditions. Madramootoo and Jutras (1984) reported an adverse effect on banana yield

Figure 1–The relationship between salinity in the irrigation water and the mean weight of a bunch.

Table 1. Bunch mean weight per harvest (kg) and average yield (kg) for all treatments* Harvest I 1997 Treatments† FW FW+SE DSE+SE DSE

Mean 42.02 b 40.73 a,b 38.59 a,b 37.50 a

Harvest II 1998

Mean Weight

SE

Mean

SE

Mean

SE

1.76 1.89 2.03 1.85

54.76 c 53.17 c 53.14 c 52.35 c

2.30 1.90 1.95 1.85

48.39 b 46.95 a,b 45.86 a,b 44.92 a

1.45 1.34 1.41 1.30

* The same letter in the row or column means no significant differences, (α = 0.1). † FW = fresh groundwater; FW + SE = fresh groundwater mixed with secondary effluent (70% : 30%); DSE + SE = desalinized secondary effluent mixed with secondary effluent (70% : 30%); DSE = desalinized secondary effluent.

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Figure 2–The relationship between SAR in the irrigation water and the weight of the mean bunch. 311

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Table 3. Saturated hydraulic conductivity (KS) of soil in the horizon A (0-25 cm), for all treatments* KS (cm h–1) Horizon A

October 1996

October 1997

October 1998

FW FW+SE DSE+SE DSE Year

1.070522 β a 2.87504 δ a 3.467517 µ a 0.796324 π a 1.707407 Ω

0.977722 β b 1.95405 δ b 2.073241 µ b 1.528996 π b 1.379828 Ω

4.290491 β c 0.05732 λ d 0.058259 ϕ d 0.019508 φ d 0.129301 θ

* Latin letters designate differences among treatments for a given year and Greek letters designate differences among years within the treatments (significance level, α = 0.05).

caused by a high water table on a heavy clay soil. The excessive soil moisture may be localized under microirrigation; however, the results in figure 3 are consistent with Drew (1992, 1997) who studied the adverse effect of the oxygen deficiency in the root zone due to poor drainage on depression of growth and yield. According to his results, in root apical zones where O2 consumption rates are high, localized injury and death of roots can occur. The initial values of the KS in the horizon B (25 to 50 cm) were not the same for all treatments and the changes in hydraulic conductivity were not consistent at this depth during the experiment. It appears that two years of irrigation were not enough to change soil conditions in the B horizon. It can be expected that the change in KS at the depth of B horizon (25 to 50 cm) would have less impact on banana plant since the majority of banana roots are located at 20 to 30 cm depth (Galán Sauco, 1992). The yield response was consistent with EC and SAR limits published by Ayers and Wescot (1985). The DSE treatment in year 1 (SAR: 6.5, EC: 0.5 dS m–1, as shown in figs. 1 and 2) was at the limit of high risk interval for soil structure stability:, SAR > 6 and EC < 0.5, with the most effect on bunch weight (letter a in table 1). The DSE treatment in year 2 (SAR: 6.1, EC: 0.69 dS m–1, as shown again in figs. 1 and 2) was in the moderate risk interval, SAR > 6 and EC from 0.9 to 0.5, and the yield in that year did not show significant differences from other treatments. The average of the annual means of the nutrient contents and pH of the irrigation water for each treatment is presented in table 4. Since the quality of effluent changes with season and is usually poorest during summers, table 5 presents the same parameters for the summer season as the

two years average values. As expected, DSE+SE had higher nutrient contents than the other treatments. Even the FW had noticeably higher N, P, and K contents during the summer season, which suggests contamination due to excess nutrient applications and poor water management practices in agriculture and landscapes. Fertigation rates for each treatment were adjusted based on water analysis. Smaller variations in the nutrient content of the effluent water would be desirable to facilitate fertigation management programs in commercial orchards. At the irrigation rates used during the summer, the nutrient contents applied with the wastewater treatments were sometimes in excess of plant requirements. However, the nutrient levels of banana leaves showed no significant differences (except Mg) among all the treatments (table 6) and the nutrient contents were within the optimal ranges (except N). This implies that the excess nutrients applied in the wastewater did not negatively affect banana plants. Leaf concentration of other ions was studied for phytotoxicity. There was no sodium phytotoxicity since significantly higher contents were found in FW (table 7) than in other treatments, and FW consistently resulted in the highest yield. However, elevated levels of zinc were found in all treatments as compared with the FW treatment which may indicate future problems related to zinc phytotoxicity under prolonged irrigation with effluent. More studies are necessary to define phytotoxicity levels for this variety of bananas under the conditions used in this experiment, especially since wastewater carries significant amounts of that ion (table 8). It should be noted that DSE water had no detectable amounts of Zn, after the desalination treatment. In all treatments, the chloride levels measured in water were under the threshold of 600 ppm established for banana (Israeli, 1986). Based on soil pH and texture and irrigation water analysis performed at the wastewater treatment plant, boron and aluminum are not Table 4. Mean values for water pH and nutrient contents (mg L–1) for two years (significance level, α = 0.05) Treatment FW FW+SE DSE+SE DSE

pH

N

P

9.07 b 9.63 a 0.65 a 8.13 a 34.95 ab 6.94 b 8.04 ab 24.99 ab 8.52 b 7.44 a 47.39 b 5.49 b

K

Ca

10.40 a 21.07 ab 23.04 b 15.01 ab

Mg

20.02 ab 11.36 a 22.97 b 13.07 a 21.56 ab 12.10 a 13.2 a 8.58 a

SO4 21.22 a 45.98 ab 71.99 b 62.39 b

Table 5. Mean values for water pH and nutrient contents (mg L–1) for the summer for two years (significance level, α = 0.05) Treatment FW FW+SE DSE+SE DSE

pH

N

8.77 a 7.62 a 7.42 a 7.23 a

P

29.33 a 5.7 a 39.00 a 11.98 a 19.75 a 12.00 a 48.70 a 8.45 a

K

Ca

Mg

20.53 a 34.80 a 33.67 a 22.65 a

18.93 a 22.65 a 21.95 a 19.9 a

10.77 a 15.95 a 13.02 a 9.88 a

SO4 23.73 a 62.50 ab 69.90 b 70.47 b

Table 6. Nutrient contents in the third banana leaves in 1998 at flowering period, following the MEIR method (Martin-Prevel, 1984)* (significance level, α = 0.05)

Figure 3–The relationship among bunch weight, water quality, and KS. 312

Treatment

N (%)

P (%)

K (%)

Ca (%)

Mg (%)

Optimum* FW FW+SE DSE+SE DSE

2.7-3.6 2.06 a 1.99 a 1.64 a 1.83 a

0.18-0.27 0.29 a 0.28 a 0.27 a 0.27 a

3.5-5.4 3.77 a 3.47 a 3.76 a 3.78 a

0.25-1.2 1.03 a 1.08 a 1.08 a 0.84 a

0.27-0.6 0.52 b 0.32 a 0.47 b 0.34 a

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Table 7. Leaf contents of Na, Fe, Mn, and Zn (significance level, α = 0.05) Treatment Optimum FW FW+SE DSE+SE DSE Phytotoxicity

Na (%)

Fe (ppm)

Mn (ppm)

Zn (ppm)

0.46 b 0.39 ab 0.44 b 0.29 a

80-360* 366.33 a 371.00 a 393.33 a 347.00 a

200-2000* 181.66 a 174.50 a 161.33 a 166.83 a 4000*

20-50* 36.50 a 37.66 ab 39.50 b 37.66 ab

* Marchal (unpublished) cited by Martin-Prevel, 1984.

Table 8. Water contents of Na, Cl, Fe, Mn and Zn (significance level, α = 0,05) Treatment FW FW+SE DSE+SE DSE Phytotoxicity

Na (ppm)

Cl (ppm)

Fe (ppm)

Mn (ppm)

Zn (ppm)

97.95 a 135.47 bc 141.77 c 113.37 ab

138.26 a 169.48 a 135.71 a 131.55 a 600†

Nd* 0.022 a 0.067 a 0.045 a

Nd Nd Nd Nd

Nd 0.011 a 0.017 a Nd

* Not detectable. † Israeli, 1986.

expected to cause any phytotoxicity, and no symptoms of excessive levels of these ions were observed in the experiments.

CONCLUSIONS The results of this experiment show crop response to elevated SAR and not to the higher salinity levels of irrigation water. The yield results were statistically significant in the first year and showed the same trend in the second year. The lowest level of yield was observed in the treatment with the desalinized secondary effluent that had most of the salts removed (lower EC), however this treatment had the highest levels of SAR. The SAR values for all three effluent treatments were not statistically different; however, there was a statistical difference in hydraulic conductivity of the treatments with desalinized secondary effluent as compared the fresh water treatments. There was no significant difference in the yield in two treatments with higher salinity (FW+SE and DSE+SE) as compared with the FW treatment and the addition of 30% of secondary effluent to fresh water or desalinized effluent did not have a negative impact on banana yield. The results suggest that the use of desalinized secondary effluent may result in decreased soil hydraulic conductivity and consequently in lower banana yield. The elevated SAR in FW+SE and DSE+SE had less effect on saturated hydraulic conductivity of soil due to the higher EC, since SAR and EC are interdependent in their effect on KS. These results are important to banana growers due to the high cost of desalinization of the effluent that may not be necessary and in fact, may be harmful.

REFERENCES Allue-Andrade, J. L. 1990. Atlas Fitoclimático de España (Phytoclimatic Atlas of Spain). Madrid, Spain: MAPA, INIA.

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Argudo, C. 1998. Project LIFE — 92-E-005. Personal communication. Las Palmas de Gran Canarias, Canary Islands, Spain. Ayers, R. S., and D. W. Wescot. 1985. Water quality for agriculture. FAO Irrigation & Drainage Paper No. 29. Rome, Italy: FAO. Bajwa, M. S., O. P. Choudhary, and A. S. Josan. 1993. Effects of continuous irrigation with sodic and saline-sodic waters on soil properties and crop yield under cotton-wheat rotation in northwest India. Agric. Water Manage. 22(4): 345-356. Borges, A., M. Fernández, J. J. Bravo, J. F. Perz , and I. López. 1991. Enhanced resistance of banana plants (Dwarf Cavendish) to Fusarium Oxisporum sp. cubense by controlled Zn nutrition under field conditions. Banana Newsletter 14: 24-26. Consejería de Economía y Comercio. Gobierno de Canarias. 1989. Monografías Estadísticas. Agricultura, ganadería y pesca (Department of Economy and Commerce. Canary Islands Government. Statistic Yearbook: Agriculture, Livestock and Fisheries). Edificio Usos Multiples. Las Palmas de Gran Canaries, Canary Islands, Spain. Consejo Insular de Aguas de Gran Canaria. 1995. Las Aguas del 2000. Plan Hidrológico de Gran Canaria (Gran Canaria Island Water Bureau. Water for 2000—Hydrologic Plan of Gran Canaria Island). Paseo Juan XXIII, 35011 Las Palmas de Gran Canaria, Islas Canarias, Spain. Consorcio Insular de Aprovechamiento de Aguas Depuradas de Gran Canaria (Bureau of Wastewater Reuse, Gran Canaria Island). 1998. Personal communication. Paseo Juan XXIII, 35011 Las Palmas de Gran Canaria, Islas Canarias, Spain. Drew, M. C. 1992. Soil aeration and plant root metabolism. Soil Sci. 154: 259-268. _____. 1997. Oxygen deficiency and root metabolism: Injury and Acclimation under Hypoxia and Anoxia. Annu. Rev. Plant Physiol. Plant Mol. Biol. 48: 223-250. Frenkel, H., J. O. Goertzen, and J. D. Rhoades. 1978. Effects of clay type and content, exchangeable sodium percentage and electrolyte concentration on clay dispersion and soil hydraulic conductivity. Soil Sci. Soc. Am. J. 42: 32-39. Galán Saúco. 1992. Los frutales tropicales en los subtrópicos. II Plátano (Banano). Cultivares (Tropical fruits in subtropics. II Banana. Varieties): 35-42. Ed: Mundi Prensa. Madrid, Spain. ISBN 84-7114-395-X. Israeli, Y., E. Lahav, and N. Nameri. 1986. The effect of salinity and sodium adsorption ratio in the irrigation water, on growth and productivity of bananas under drip irrigation conditions. Fruits 41(5): 297-302. Madramootoo, C. A., and P. J. Jutras. 1984. Supplemental irrigation of bananas in St. Lucia. Agric. Water Manage. 9: 149-156. Martinez Barroso, M. C., and C. E. Alvarez. 1997. Toxicity symptoms and tolerance of strawberry to salinity in the irrigation water. Scientia Hort. 71: 177-188. Mc-Intyre, D. S. 1979. Exchangeable sodium, subplasticity and hydraulic conductivity of some Australian soils. Australian J. Soil Res. 17: 115-120. Martin-Prevel, P., 1984. Banana. In Plant Analysis as Guide to Nutrient Requirements of Temperate and Tropical Crops, eds. P. Martin-Prevel, J. Gagnard, and P. Gautier 40: 637-670. Tenerife, Spain: INIA. Nakayama, F. S., and D. A. Bucks. 1986. Trickle Irrigation for Crop Production: Design, Operation, and Management. Amsterdam, the Netherlands: Elsevier. Oster, J. D., and J. D. Rhoades. 1984. Water management for salinity and sodicity control. In Irrigation with Reclaimed Municipal Wastewater—A guidance Manual, eds. S. Pettygrove, andT. Asano. Davis, Calif.: Lewis Publishers, Inc. Palacios, M. P., E. Del-Nero, and J. M. Gil. 1998. Banana tree trickle irrigated with municipal wastewater in the Canary

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Islands: Effects on soil, plants and irrigation works. II In Proc. Int. Symp. Wastewater Treatment and Reuse, eds., L. Bonomo, and C. Nurizzo, 999-1003. Milano, Italy. Richards, L. A., ed. 1954. Diagnosis and Improvement of Saline and Alkali Soils. Agriculture Handbook No. 60. USDA. Washington, D.C.: U.S. GPO.

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Suarez, D. L., J. D. Rhoades, R. Lavado, and C. M. Grieve. 1984. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil Sci. Soc. Am. J. 48: 50-55. Wardlaw, C. W. 1961. Banana Diseases, 62-66. London, U.K. Longmans.

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