Thank you for all the answers you gave me. I have just one short question if I may, in reference to the system in my previous question (4m x 20m DFT tank).
If I shut the pump down at night and only run it during the day time, do you think it will be ok in terms of oxygenation? I am planning to construct an additional three grow tanks each with its own pump, and would like to save a bit on energy costs by turning the pumps off at night.
(The previous question was answered in my August column. It related to the set up and management of a 20,000 litre Deep Flow Technique (DFT) system, which held an extremely high 13 litres of nutrient solution per plant.)
Answer by RICK DONNAN
Certainly, you could make significant savings by shutting down your pumps over night. However, what are the risks involved?
Oxygen is needed by the plant roots for the critical process of respiration. This not only occurs in the plant roots, but also throughout the entire plant, and for 24 hours per day. Therefore, you don’t want to allow your nutrient solution to be depleted in oxygen overnight.
In your case, however, you have a huge volume of water per plant in your system, which means that the risk of you getting significant oxygen depletion overnight if you shut off the recirculation pumps is very low. Nevertheless, I would recommend that you do some pumping overnight in case there is stratification within your growing tank. Perhaps have four sessions of pumping for a half hour each, giving a total of two hours, which would turn over your tank once overnight. In particular, this should help maintain some water movement within the tank.
This is a good opportunity to go over the often misunderstood process of plant respiration.
Photosynthesis is a much more widely known process, where plant leaves take in energy from the sun and use it to convert carbon dioxide (CO2) from the air and water from the roots into sugars for the plant and release oxygen into the air.
This obviously happens when there is light shining upon the leaves. Other than when supplementary lighting is used, this only occurs during the day when the sun is shining. The amount of photosynthesis is influenced by the light intensity and duration, the CO2 concentration, and the instantaneous leaf temperature.
Respiration uses this energy to convert the sugars into the huge number of compounds found within plants: that is, proteins, celluloses, starches, vitamins, etc. In this process it takes in oxygen and releases carbon dioxide. There is no animal equivalent to photosynthesis, however, we do have a rough equivalent to plant respiration in that we similarly breathe in oxygen and exhale carbon dioxide. In animals, this is an active process: that is, our muscles are actively involved. In plants, the process is passive.
The important aspects of respiration that are misunderstood are:
• It occurs in all parts of the plant, including the roots.
• It occurs over the full 24 hour day.
• It is influenced by the long term plant temperature, rather than the instantaneous temperature, averaged over up to seven days.
Because photosynthesis is the dominant process during the day, sometimes growers assume there is no daylight respiration. Consequently, this assumption goes on to state that respiration only occurs during the night, or when the plant is in the dark. Both are incorrect.
There are similar misconceptions with nutrient uptake as there are with respiration. By this I mean that nutrient uptake is relatively constant over the full 24-hour day. In contrast, water uptake is largely for transpiration to keep the plant cool and is responding to the radiation load from the sun. Therefore, water uptake is very high during the day and very much lower during the night.
This has been interpreted by many growers that plants drink during the day and only feed at night. Major problems can arise if this approach is taken too far. I have met growers who irrigate with only water during the day and add some fertiliser late in the day because “the plants need to do their feeding overnight”. This extreme approach has many negative impacts: plants prefer relatively stable conditions in the root zone and this approach is inherently unstable. This also usually results in a relatively low average EC, which leads to poorer taste and shorter shelf life. Ω
September 2015 / Issue 159