High pH, low alkalinity pond water used for overhead irrigation does not affect plant growth of select flowering shrubs

Authors: LEBUDE, ANTHONY - North Carolina State University; Owen Jr, James - Jim; HOLMES, C - North Carolina State University

Submitted to: Journal of Environmental Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2020
Publication Date: 3/19/2021
Citation: LeBude, A.V., Owen Jr, J.S., Holmes, C. 2021. High pH, low alkalinity pond water used for overhead irrigation does not affect plant growth of select flowering shrubs. Journal of Environmental Horticulture. 39(1):22–32. https://doi.org/10.24266/0738-2898-39.1.22.
DOI: https://doi.org/10.24266/0738-2898-39.1.22

Interpretive Summary: Pond surface water can exceed the best management practices (BMP) range for pH of 5.8 to 7.0 outlined for growing container ornamental crops in season. Elevated pH (>7.0) in ponds used for source water to irrigate is common among producers across the southeastern U.S. Remediation of pond water to properly improve quality within BMP guidelines consists of screens or finer filtration devices (e.g., sand or disc filters), monitoring pH and total alkalinity to determine the correct injection rate, then injecting an acid (e.g., sulfuric) followed by proper resonance time to thoroughly mix chemicals with water. Afterwards, within the irrigation system monitoring equipment is used either in-line, which can automatically inform and adjust the upstream injection rate, or monitored manually at the point of trajectory (sprinkler head) using portable devices or litmus tests to ensure pH remains within recommended BMP ranges. This study suggests that irrigating container plants overhead with high pH, low alkalinity (<100 ppm total alkalinity) source water from ponds may not affect growth enough to warrant investment in an injection system simply to lower pH to meet BMPs. The range for pH stated in BMPs is still important for producers treating water with sanitizing chemicals to reduce microorganisms. If sanitizing chemicals are used, many systems benefit from reducing pH prior to chemical injection and therefore might feasibly justify an acid injection system.

Technical Abstract: In southeast U.S., pH of source water from ponds used for overhead-irrigating container crops can exceed the range (pH 5.8-7.0) for best management practices. Artificially maintaining this pH range is not common among producers using surface water for irrigation, nor is it known whether this would affect growth. Therefore, the objective was to test whether this source water affects growth of five flowering shrubs in nurseries in eastern North Carolina. Pond water at six nurseries with a pH range of 4.9-8.1 (control) was injected before irrigation with sulfuric acid (lower) or potassium bicarbonate (raise) onsite to maintain a pH of 5.8-6.2 (treatment). Ambient photosynthesis and stomatal conductance was measured in July, August, and September on leaves of forsythia (Forsythia x intermedia 'Mindor‘ ShowOff®) during irrigation runtime mini-experiments at three nurseries. For mini-experiments, pre- and post-treatment physiology was measured for plants receiving 0 (handwatered), 30, or 60 minutes of treated or nontreated overhead irrigation. Dry weight of all shrubs and gas exchange of forsythia was not affected by high pH, low alkalinity (<100 ppm) irrigation water. Southeastern producers using this source water for overhead irrigation may not need to adopt a system that reduces pH to improve growth.