Chlorine from calcium hypochlorite, sodium hypochlorite, or Cl gas sources has complex chemistry in irrigation water. Once added to water, Cl is converted to hypochlorite (OCl–) and hypochlorous acid (HOCl), which along with dissolved Cl gas are collectively termed free Cl. The balance between hypochlorous acid and hypochlorite is pH-dependent, whereby hypochlorous acid predominates at solution pH below 7.5 or hypochlorite ions above pH 7.5 (Morris, 1966). Water pH also influences the sanitizing strength of a Cl solution, because HOCl is estimated at up to 80× more effective as a biocide than OCl– (White, 1992). The effect of pH on control of waterborne pathogens in a Cl solution was illustrated by Lang et al. (2008), whereby control of Pythium aphanidermatum and P. dissotocum zoospores was achieved at either 2.0 mg·L−1 Cl at pH 8.1 or 0.5 mg·L−1 Cl at a pH of 6.3.
There is extensive research on effects of free Cl on waterborne plant pathogens. Control of Pythium and Phytophthora zoospores has been reported with 2 mg·L−1 of free Cl (Cayanan et al., 2009; Hong and Richardson, 2004; Hong et al., 2003; Lang et al., 2008). Chlorine oxidizes and chlorinates living tissue and organic compounds, resulting in damage to membranes, enzymes, and nucleic acids of microorganisms (Stewart and Olson, 1996). Increasing concentration of Cl, lowering solution pH, increasing ORP, and increasing contact time are factors that improve pathogen control (Lang et al., 2008).
Oxidative strength of a Cl solution can be measured in millivolts (mV) using an ORP meter. A positive correlation was found between ORP and control of coliform bacteria by chlorination of wastewater (Yu et al., 2008). Coliforms and pathogenic bacteria were rapidly controlled in post-harvest wash water if ORP was maintained between 650 and 700 mV (Suslow, 2004). Lang et al. (2008) found that Pythium aphanidermatum and P. dissotocum zoospores were killed within 0.25 to 0.5 min when ORP was above 780 mV in chlorinated water.
In the presence of organic and inorganic N including ammonia, a range of equilibria reactions occur to form “complexed” chlorinated molecules such as chloramines in equations [1 to 3] adapted from the U.S. Environmental Protection Agency (EPA, 1999). The concentration of both free Cl and complexed Cl together make up the concentration of “total” Cl.
In municipal water treatment facilities, a dose of one part of inorganic N to every three parts of free Cl reportedly results in 99% conversion of free Cl to chloramines after 0.2 s at pH 7 or 147 s at pH 4 (White, 1992). In horticulture irrigation, water-soluble fertilizer containing ammonium, nitrate, and/or urea N is often supplied in irrigation between 100 and 200 mg·L−1 total N and then treated with free Cl concentrations between 1 and 2 mg·L−1 Cl. With such a high N to Cl ratio, the majority of free Cl would therefore be expected to rapidly convert to chlorinated N forms. Chlorine also reacts with urea, although the chain of reactions is more complex and slow-acting than chlorination of ammonia (Blatchley and Cheng, 2010).
Chloramines are considered weaker sanitizers than free Cl from hypochlorous acid (White, 1992) because of the longer contact time that chloramines require to control human pathogens compared with an equal concentration of free Cl. Control of 99% of Escherichia coli with hypochlorous acid at 1 mg·L−1 Cl required a 1-min contact time, whereas with combined Cl forms including NH2Cl and NHCl2 at 1 mg·L−1 Cl required more than 100 min (Akin et al., 1982). However, the greater stability of chloramines in the presence of organic compounds compared with hypochlorous acid may increase penetration of chloramines into biofilm, resulting is greater inactivation of biofilm bacteria (LeChevallier et al., 1988). The American Water Works Association (AWWA, 1991) provided guidelines for residual concentration of between 0.5 to 1 mg·L−1 chloramine for disinfection of groundwater (White, 1992) such as from wells or irrigation catchment areas. Based on experience with control of human pathogens in water supply, irrigation of edible crops with complexed Cl is likely to require longer contact times and/or higher total Cl application concentrations compared with free Cl for control of human pathogens. However, data are not available for plant pathogens.
There are limited research data on the residual level of free or total Cl in the presence of nutrient solutions despite the common practice of dual injection of Cl and water-soluble fertilizer and the potential impact on pathogen control. The research objective of this study was to determine the free and total Cl and ORP responses over time when nutrient solutions were blended with sodium hypochlorite.
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