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I am currently growing lettuce in a hydroponics system that I'm developing. The system has continuous pH monitoring and I recently noticed that when the lights go off at night the pH seems to increase. Is there some biological effect that would account for the pH increase? The pH seems to go back down when the lights come back on. The lights go out at around midnight and come back on at around 6am.

The first plot below shows recently logged data from the system. The second plot includes additional days before Dec 6-7 where the lights were on 24/7. Note that the sharp changes in pH/EC/Temp are caused by adding (cold) water and adjusting the nutrients. The big negative spikes in EC happen when the water level goes below the EC probe.

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EDIT: Added another plot to show longer time frame. Only time it doesn't really show up is when the nutrients/pH/water were adjusted just before the lights went off on Dec 12 at midnight.

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  • I don't think there is enough information to make an informed conclusion. First, there is enough inconsistency with the shown data that you should look at several days more to discern a pattern. Does this mist 24x7 or just when the light is on? (that one is important). What is up with the temperature? Why so variable? Is your pH within your range? I'm not sure you can expect a dead flat pH line. – Tim Nevins Dec 7 '18 at 13:31
  • The misting runs 24/7. The temperature changes because the system is in the basement in the vicinity of the furnace and the house temperature settings change at various points in the day. I don't expect a dead flat pH line at all. I was just wondering about it's apparent light dependency. I only recently started turning off the lights at night to see if it reduces tip burn. – crj11 Dec 7 '18 at 13:38
  • Cheap hand held pH meters are sensitive to electromagnetic fields. I learned this by trying to titrate something on a stir plate. When you turn the lamps off, you change the electromagnetic environment. – Wayfaring Stranger Dec 7 '18 at 19:05
  • @crj11 Maybe I am seeing things but it would seem that as your temperature increases the pH increases but it is lagging behind a bit. – Rob Dec 13 '18 at 18:31
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Pretty cool system you have there, it looks very sophisticated! You say that the lights go off at midnight and back on at 6 am. You can indeed see a bit of rise in pH during that time, but I see more fluctuations, what happened for example at 18h on 5 December? Overall the pH stays somewhere around 6-6.5 so I would not worry about it.

But your question is why does it increase when the lights go off. My guess is that it has to do with uptake of nutrients. During day time, the plants take up nutrients, such as K+, Ca2+ and Mg2+, which are all positive charged ions (cations). The roots compensate by releasing H+ ions, to keep the positive and negative ions in equilibrium, this causes the pH to decrease during day time. At night the uptake is stopped, and the pH will increase. You can find more about this here. I hope it helps!

  • Thanks. One clarification, if the uptake of positive ions stops, shoudn't the pH just stabilize versus go up? Does that mean that the uptake of NO3- and HPO4- are increasing when the lights go off. – crj11 Dec 7 '18 at 12:39
  • Yeah it is only one clarification, probably many more factors are involved. Temperature fluctuations might also cause different solubility of gasses (like CO2) or ions. – benn Dec 7 '18 at 14:15
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During the light periods CO2 is consumed by photosynthesis : Then it increases in the dark. I had this same situation in a salt water aquarium although I did not have your impressive monitoring system . I used standard pH indicator solution ( bromthymol blue ). In the morning pH would be 8.3 , at the end of the day pH would be around 8.8 . All water parameters were good. Filters ran 24/7. The animals continuously produced CO2 and I believe plants produce some CO2 in the dark. I don't know exactly what is happening in your system . I had a lot of algae ; enough was produced in the 75 gal tank to supply a couple pet shops.

  • OP has an increase of pH 6 to 6.5 during night time. You describe an increase of pH during day time (8.3 in the morning and 8.8 at the end of the day). Are we talking about the same here? – benn Dec 8 '18 at 8:31
  • Apparently not, I didn't look close enough at the charts. – blacksmith37 Dec 8 '18 at 21:25
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There's definitely not enough data to work with here, and it's almost all within the margin of error of non-laboratory-quality meters. I suspect the answer is almost exclusively related to the way you are measuring pH, and not any actual change in the acidity of the solution. 6.0 to 6.5 much too large of a change to be accounted for in nutrient uptake in a short period of time, assuming the measurement precision is to be trusted.

Temperature does affect pH measurements (but not acidity) due to the way pH is measured, although typically in the opposite direction of your observations. However, your measurement instrument is almost certainly also affected by temperature, as well as electromagnetic interference.

Also does this setup have an air pump, and is that pump on 24/7 or is it also turned off when the lights are turned off? You can certainly account for some difference in pH if oxygen injection differs over time.

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