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- Author or Editor: Noah J. Langenfeld x
Dissolved oxygen (DO) is critical for aerobic life in aquatic environments. Rapid and accurate measurements of DO are necessary to quantify the rate of oxygen uptake and maintain optimum conditions in root zones. DO meters are available across a price range of USD99 to more than USD1000. We compared three meters for stability, response time, and accuracy in freshwater [tap water, 0 g⋅L–1 sodium chloride (NaCl)] and saline water (simulated seawater, 35 g⋅L–1 NaCl) across multiple temperatures. The Yellow Springs, Inc. 550A (YSI) and Sper Scientific 850048 (Sper) meters were stable across a range of water temperatures (12–38 °C) and salinity. The Smart Sensor Roeam AR8210 drifted ±50% within minutes after calibration and was not evaluated further. In freshwater, the YSI meter was within 4% and the Sper meter was within 5% of the theoretical value at 12 and 22 °C. Meters were less accurate at 38 °C. The accuracy in saline water was similar to freshwater. Across temperature and salinity, the response time averaged 10 s for the YSI meter and 15 s for the Sper meter. We conclude that the YSI and Sper meters can provide rapid, stable, and accurate measurements of DO.
Copper (Cu) is typically adequate at 0.5 μM (0.03 ppm) in hydroponics and at 2 μM (0.125 ppm) in soilless media, but elevated levels can be used to inhibit pathogenic fungal growth. We studied the effect of elevated Cu on the growth of lettuce and tomato in peat-based media and deep-flow hydroponics. Lettuce growth in hydroponics was not hindered until a concentration greater than 4 μM (0.25 ppm) Cu was used, which is eight times greater than the adequate level. Tomato was more tolerant of elevated Cu, with no growth suppression up to 8 μM (0.5 ppm) in hydroponics. Organic matter tightly binds Cu, and bioavailability is thus determined by organic components in soilless media. We confirmed an adsorption capacity of 19 mg Cu per g of peat, which explains why there was no inhibition of lettuce or tomato growth up to 1000 μM (64 ppm) Cu in peat-based media. When chelated with ethylenediaminetetraacetic acid, Cu binding to organic matter was reduced and growth was decreased in lettuce but not tomato. Both species tolerated a 100-fold greater concentration of Cu in peat-based media than in deep-flow hydroponics. Elevated Cu in solution increased concentrations 20 times greater in root tissue than in leaves. These solution and tissue concentrations are greater than identified toxicity thresholds of pathogenic fungal and fungal-like organisms, and could thus be used to suppress root-borne fungal and fungal-like diseases.