Letters From The AEssenseGrows Plant Science Team - Pathogens-Oxidizers

Aeroponic and Hydroponic cropping systems are exposed to comparably fewer pathogens than traditional field-based systems; however, the source of plant pathogens is not limited to soil – infected seeds and cuttings, water, and even staff, can all introduce pathogens to a growing environment. Furthermore, once introduced, if not properly controlled and suppressed, recirculating water systems can rapidly spread root-infecting pathogens between crops. The most effective way to avoid a systemic spread of plant pathogens is to maintain water cleanliness. Many water disinfection techniques are available- such as filtration, pasteurization, irradiation, and chemical treatment. While many of these processes require expensive equipment and foresight to build into a facility, chemical disinfection can be applied to nearly any existing system with minimal footprint or disruption.

Chemical oxidation is a common and inexpensive method of disinfecting water before and during use in hydroponic systems. Water is “cleaned” using a chemical oxidizing agent that reacts with organic matter present in the solution or pipes through which the solution is passing. When implemented correctly, small, unwanted organic matter such as waterborne pathogens are destroyed by the oxidation, while larger organisms like plants are left unharmed. Chemical oxidizing agents accomplish this by reacting with and damaging the structural integrity of the organic matter in the water – killing or disabling them.

Several chemical oxidizing agents exist - the most common of which include hydrogen peroxide (H₂0₂), hypochlorous acid (HOCl), ozone (O₃), and chlorine dioxide (ClO₂). Despite all being oxidizing agents, the process by which each chemical reacts with the organic matter in solution is different. More importantly, the effective concentrations at which each oxidizer effectively controls various pathogens differ substantially. A sure way to avoid over or underdosing, is to begin by following the product’s specified recommended dose rate.

One method to monitor the activity of a chemical oxidizer in solution, is to use an ORP sensor. ORP (Oxidation-Reduction Potential) is a measurement, in millivolts (mV), of a solution's capacity for electron transfer (oxidation or reduction). This capacity for electron transfer, measured by a reference electrode within an ORP sensor, can indicate the relative cleaning capacity of a chemical oxidizer that has been added to the solution. Generally speaking, solutions in modern hydroponic systems maintain a positive ORP value between 200-400 mv and tend to increase for a period of time after an oxidative cleaning agent is introduced.

The use of an ORP sensor in concert with a well-planned chemical oxidizer dosing regimen can provide additional information about the potential of the chemical cleaning agent in solution. However, due to the large variety of factors that can influence ORP readings, ORP values are abstract without context and should not alone be used to inform chemical dosing decisions. An ORP sensor is unable to discriminate the activity of one oxidizing agent from another and different chemical oxidizers cause different ORP value responses. Additionally, many external factors can also influence ORP; if the effect of these factors are not considered, misleading conclusion will be drawn – resulting in under or over overdosing solution, leading to poor pathogen control, nutrient lockouts, or phytotoxicity. In any IPM program, it is imperative that the grower understands the active and cumulative concentration of the chemical being applied.

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