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Field Evaluation of Recycled Water for Avocado Irrigation


California’s mild coastal regions produce 90% of avocados grown in the United States. However, drought and urbanization are placing a strain on water supplies for agriculture. Avocados are a thirsty crop (an acre of avocado orchards consumes 4 acre-feet/year), and are particularly sensitive to salt. Recycled water has been proposed for irrigation, however it is higher in salt content than freshwater sources. Results from this study identify potential problems that may arise from avocado irrigation with recycled water and strategies for mitigating such issues. Trees irrigated with potable water at rates of 14, 12, and 9 gallons per hour were compared with trees irrigated with recycled water at the same application rates. Effects of recycled water on soil quality, tree health, fruit yield, and fruit quality were assessed and provide a framework for future implementation of recycled water irrigation in avocado orchards. Findings showed that recycled water impaired tree growth and health. Leaves were also deficient in micronutrients such as iron and zinc, while elevated in macronutrients (nitrogen, phosphorus and potassium). Poor tree health and suboptimal leaf nutrition reduced yield and fruit quality among trees irrigated with recycled water. Fruit rot occurred more frequently and with greater severity in trees irrigated with recycled water; these fruits also had reduced post-harvest shelf-life. Soil clay content also affected tree health. Trees irrigated with potable water at 9 and 14 gallons/hour showed higher rates of saltburn damage than expected; these trees were situated on heavy clay soils, which would explain abnormally high leaf damage for trees irrigated with potable water. Clay reduces soil aeration and leaching; reduced oxygen availability to avocado roots encourages chloride uptake and chloride damage. However, these trees still produced a copious crop. The study demonstrates that improvements in treated wastewater quality are needed prior to application of recycled water to avocado orchards, and that recycled water irrigation necessitates non-traditional strategies for orchard management.

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