An introduction to temperature and dissolved oxygen in Hydroponics:
In order to maximize plant growth hydroponically, it’s necessary to balance factors that are often interrelated. This is the case for the nutrient solution’s temperature and capacity for dissolved oxygen. This requires a nutrient solution that is neither too cold nor too warm. For most applications the desired temperature will fall roughly within 18-24 °C (54-75 °F). This article is intended to introduce the relationship between a nutrient solution’s temperature and capacity for dissolved oxygen (DO), while also preparing gardeners for more technically related discussions regarding the subject.
Water Chemistry:
To begin, it’s important to understand oxygen from the atmosphere can be absorbed by a volume of water. The practical extent to which this occurs can range anywhere from 0 parts per million (ppm) to 15 ppm dissolved oxygen, depending on water temperature, salinity and air pressure. For all intents and purposes, the gardener needs to only be concerned with the role of temperature. Simply, as water cools the potential maximum DO level increases and as water warms the maximum DO level decreases. The graph below illustrates the relationship between temperature and DO saturation while highlighting the approximate zone optimal for hydroponic gardening. If you wish to plot this graph yourself you may do so using the formula:
14.652 + 0.41022T + 0.0079910T2 + 0.000077774T3
This formula was taken from page 1.75 of the McGraw-Hill Handbook of Environmental Engineering Calculations 2nd Ed. The calculation performed here assumes standard air pressure and pure water. Units are in degrees Celsius.
Maximum possible DO concentrations for fresh water at sea level.
Plant Physiology:
Since plants require oxygen not only at the crown level (everything above ground) but also in the root zone, the availability of oxygen in a nutrient solution is a detail that should not be overlooked. Unfortunately, optimal growing conditions cannot be achieved simply by maintaining an icy cold nutrient solution, thereby increasing the maximum possible DO level. At any moment a growing plant performs a countless number of biochemical reactions. The “quickness” that these reactions can take place is dictated largely by the temperature of the surrounding environment. As a general rule, the rate of these biochemical reactions will double for every increase of 10 °C degrees. It would be more appropriate to say, in this context, that the rate of a plant’s biochemical reactions will be cut in half for every decrease of 10 °C. Don’t let any of this frustrate you. All that’s necessary is a simple compromise between temperatures that are too cold and too warm.
What several authorities recommend:
Dr. Resh, author of Hydroponic Food Production, states in his book that a nutrient solution should generally be maintained between 16-18 °C (60-65 °F). During a personal correspondence with Harley Smith, professional hydroponic consultant and research director of Hydrodynamics Intl., temperature of 20-22 °C (68-72 °F) were recommended. The now defunct Integral Hydroponics by G. Low recommended 22-23 °C (68-73 °F). George Van Patten recommends to the readers of Gardening Indoors that they maintain a nutrient solution of 16-24 °C (60-75 °C). Taken as a consensus from these and additional sources it can safely be stated that an ideal temperature is approxiamtely 18-24 °C (54-75 °F). These temperatures also apply while using aeroponic methods. Please note that this information should not serve as guidelines during seed germination. The above information is summarized below.
| Authority | Temp. |
| Dr. Resh | 16-18 °C (60-65 °F) |
| Harley Smith | 20-22 °C (68-72 °F) |
| George Van Patten | 16- 24 °C (60-75 °F) |
| G. Low | 22-23 °C (68-73 °F) |
Summary:
In conclusion, hydroponic gardeners should generally maintain their nutrient solution between 18-24 °C (54-75 °F), since plant productivity can become adversely affected when the nutrient solution is either excessively warm or cold. This applies only specifically during mature growth. Depending on the species in question, seedling germination is often favored given warmer temperatures of 20-30 °C (68-86 °F). Additionally, steps should be taken to ensure that adequate water aeration and turnover is provided using air stones and/or water pumps. While beyond the scope of this article, several methods and instruments exist for the purpose of testing the oxygen concentration of a nutrient solutions. Because this information has been presented as a general overview and introduction, details regarding specific complications involving plant pathology (disease), diminished nutrient uptake, etc. have been omitted.
References and Additional Reading:
C. Lee. Handbook of Environmental Engineering Calculations. p. 1.75 American Society of Civil Engineering Committee on Sanitary Engineering Research, 1960.
Fred Senese. How can I predict the oxygen solubility in water 2010. http://antoine.frostburg.edu/chem/senese/101/solutions/faq/predicting-DO.shtml (Accessed February 7, 2010)
Fred Senese. Does a 10°C temperature rise double reaction rates? 2010.http://antoine.frostburg.edu/chem/senese/101/kinetics/faq/temperature-and-reaction-rate.shtml (Accessed February 7, 2010)
Reyes, D. M, Stolzy, L. H., Labanauskas, C. K. Agron J. 1977. Temperature and Oxygen Effects in Soil on Nutrient Uptake in Jojoba seedlings. http://agron.scijournals.org/cgi/content/abstract/69/4/647 (Accessed February 7, 2010)
Selmer-Olsen, A.R., Gislerød, H.R. 1982 Effect of Root Temperature on Nutrient Uptake by the Chrysanthemum.http://www.actahort.org/books/126/126_48.htm (Accessed February 7, 2010)
Setter, T.L., Greenway, H. 1988. Growth Reductions of Rice at Low Temperature: Decreases in Nutrient Uptake and Development of Chlorosis. http://jxb.oxfordjournals.org/cgi/content/abstract/39/6/811?ck=nck (Accessed February 7, 2010)
Heirloom Seed. Germination tables from Heirloom Seeds – Know when to plant your vegetables.http://www.heirloomseeds.com/germination.html (Accessed February 7, 2010)