Difference Between ORP VS DO

Difference Between ORP VS DO


How DO & ORP Effect Hydroponic Nutrient Solutions


The difference between ORP VS DO is important to understand for assessing and managing the “health” of hydroponic nutrient solutions and water quality for use in cultivating high value crops. 


DO (dissolved oxygen) in simplest terms is a value indicative of the ability of a water supply to support healthy life; or the ability to support “aerobic” conditions.


ORP (oxidation reduction potential) is a value associated with how “hygenic” a water source or nutrient solution is.  


While DO and ORP may initially sound like they are the same or similar, there are some important differences and distinctions worth understanding.  Additionally, ORP and DO play a close relationship to one another, affecting the value of each–and the overall health and performance of a hydroponic nutrient solution.


Monitoring DO & ORP


For practical purposes and accuracy, DO and ORP may be monitored  using electronic devices or tested for periodically via weekly laboratory testing.  The ability to monitor and control these important cropping values can improve yields, crop quality and is key at avoiding costly issues, ie water borne plant pathogens.


Measuring DO, Hydroponics


Dissolved Oxygen levels for hydroponics and aquaponics applications are typically provided as PPM (parts per million) or as a % percentage oxygen.  This value naturally occurs in healthy water supplies from 4 to 6 PPM.  It is possible to increase these levels, especially in hydroponic systems like RDWC (recirculating deep water culture) and HPAC (high pressure aeroponic cloners). Values upwards of 12 ppm are achievable in such systems.  Higher levels may be obtained with specialized technology that is typically reserved for larger scale growers due to cost and scalability. Using specialized O2 diffusion systems can allow DO to remain at elevated levels for some time after treatment.  Using conventional methods, ie aeration and nutrient circulation, levels may drop quickly once aeration and circulation are halted, even temporarily. 


Hydroponic crops grown with elevated DO levels produce bigger yields and grow faster.  Additionally, they are able to use water and nutrients more efficiently and thrive at lower nutrient concentrations (provided they are replenished or topped up).  The result is reduced fertilizer use, better cropping and a reduced environmental footprint.



Additionally, the nutrient solution temperature plays a major  role in how much DO may occur for crops.  There is a very direct relationship between temperature and DO levels, with higher temperatures decreasing DO potential. *Henry’s Law* Chillers for cooling hydroponics systems help to keep DO levels optimal and may also decrease grow room air cooling costs as roots are kept at optimal temperatures more efficiently with liquid thermodynamic principles.


Measuring ORP, Hydroponics


For both hydroponics and drinking water applications, Oxidation Reduction Potential (ORP) is typically denoted as mV (millivolts).  However, the optimal value for each differs.  For sanitary purposes where drinking water is concerned, 650 mV (millivolts). Alternatively, for cropping purposes and optimal value is 300-400 mV. Most hydroponics growers filter water from municipal or well water sources prior to storage or applying to crops–this process reduces ORP values as chlorine is removed from the tap water supply (dechlorination). Chloride (Cl-) strongly increases ORP levels; largely why it is added to water supplies.


Unfortunately for both crops and growers the chlorine that occurs in water supplies is not good for plants; that’s why it’s removed.  However Hypochlorous Acid (HOCl – used in food handling) is a form of chlorine that will not harm plants when used as directed and does a great job of boosting and maintaining ORP levels.  Additionally it keeps systems, growing media and surfaces free of biofilms and nutrient salt build up.


How DO & ORP Work Together for Hydroponics


To recap: DO is the ability of the water or nutrient solution for healthy plant life and ORP is representative of the hygienic quality of the water or nutrient solution. Here’s a real life example of how they may interact.


It is the hottest part of the year and the grow room air handling system cannot keep air temperatures at optimal–additionally, the chiller for the hydroponic system was sized to the bare minimum to help save on capital outlay.  The nutrient solution temperature rises to over 80 Degree Fahrenheit during the lighting cycle. 


DO levels decrease at the roots while the entire plant is stressed from high temperatures. Bacteria occurring in the system once kept in check begin to flourish, creating disease pressures on the crop. This is all unseen to the grower who does not measure or manage DO–what is seen is the ORP levels decrease steadily.  Hypochlorous Acid is applied at higher frequencies and concentrations to maintain a base value of ~400 mV

The unknown grower has narrowly escaped crop infection, ie pathogen levels didn’t proliferate to disease causing levels because an optimal ORP was maintained even while DO fell below normal healthy levels. 


In conclusion, understanding the differences and interactions with DO VS ORP is an  important and sometimes less explored area relating to crop management, especially for hydroponics applications. Learning more and actively monitoring and controlling these essential cropping values can help protect your growing investment and yield healthier rewards.