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Managing pH in Aquaponics

6/30/2019

5 Comments

 

pH is arguably the most finicky aspect of an aquaponics system to manage. This is due to the fact that pH is an intrinsic property of the water that is affected by the fish, nitrifiers, and all the water additions we add so it's hard to manage all the factors individually.  Without the proper mix your system pH may shoot up or down drastically leaving you clueless and frustrated. 

One of the best practices to help you avoid the violent pH swings is simply to have a buffer. Buffers work by resisting the swings of pH through the dissociation of, in this case, carbonate and bicarbonate molecules. In layman's terms potassium and calcium, at certain pH levels, will separate from their respective carbonate groups, which can soak up the free hydrogen atoms that correspond to the pH. So, in practice Calcium Carbonate will want to keep a system at a pH of around 9 while Potassium Bicarbonate will want to stabilize around 6 (this is called the pKa). 

This works wonderfully for the aquaponic grower since you'd generally like a ratio of 4:1 of potassium to calcium which will keep the pH around 6.8-7. After then you can add half as much magnesium as you did calcium and that should balance the biggest ionic nutrients in an aquaponic system. The important properties of each nutrient is not their concentration but their respective ratios. This is true because the same ionic channels in the plant roots are responsible for Calcium and potassium, for example. Optimally, you'd want the right ratios of each nutrient interacting with the cellular channels, and this all happens by chance so it is, in the end, just a game of chance. 
To give yourself a fighting chance start with a well balanced solution. What I'd recommend is to start with rain water or Reverse Osmosis water which would have a pH of 5.5-6. From there make a 4:1 mixture of potassium bicarbonate and calcium carbonate and slowly add little by little to move the pH to 7. From there you can add half as much magnesium as you did calcium (this means keeping track of how much you added to achieve the pH of 7 and add 1/8 that amount in magnesium sulfate). From then on, it's just a matter of maintaining the desired pH 6.8-7 with additions of potassium and calcium as it is natural for a healthy system to drop in pH over time. 
To further ensure that your buffer is set up, measure the kH of your water. This is a measure of the alkalinity which is affected by the amount of carbonates in your system. Our systems run between a kH of 5 and 10 and we have never experienced a wild pH swing, even in our small ten gallon systems which are more susceptible to erratic pH changes.

Finally, we like to keep the input factors that affect pH to a minimum as they can also affect other nutrient levels. For this reason we don't like adding phosphoric acid unless for some reason we want to  drop the pH just a little. If your pH is very high (above 8) we need to first reduce kH to 0 (we did this once when we used well water with a kH of 16 and pH of 8.5) with muriatic acid so you don't end up adding a ridiculous amounts of phosphorus to your system. To supplement phosphate we simply use a dual root zone method. Unfortunately this method is  more difficult for the traditional deep water cultures, but in media beds it is perfect. This is done by placing a soil-filled burlap sack an inch above the highest water level in a media bed which allows us to add in chelated iron, rock phosphate and other rhizosphere-amplifying goodies directly into the soil pot. The nutrients are not washed away by the water, don't affect pH and the plant is able to access them as needed. We highly recommend the dual root zone method especially for any fruiting/flowering plants.

In Summary:

Keep your kH between 4-10. 
To lower your pH if it's 8+ use Muriatic Acid
To lower your pH if it's below 8 use Phosphoric Acid
To raise your pH without affecting the kH use Potassium Silicate
To raise your pH and your kH use Potassium Bicarbonate or Calcium Carbonate

Cheers and Good Growing!
5 Comments
Marius
2/15/2020 12:26:05 pm

Hi there,
Great Article and great Blog. I thought this was some brilliant info exactly what I need to figure out how to manage the pH and kH drop of my systems once they mature. So i got some Calcium and Potassium Bicarbonate and tried it. I am not sure why but my pH actually goes up when adding PotBiCarb. kH was 0 and gH was 15 and pH was 5.5 when I started adding slowly. after adding PotBiCarb every second day slowly I am now at 3kH 15gH and pH7.4. I would like kH at 5ish but I dont want the pH to rise further. If I understood your post correctly PotBiCarb on its own should lower the pH not raise it? From my own research now I found that PotBiCarb actually settles at 10pH which would explain it but why are you saying it settles at 6pH? Or did I missunderstand? Would be delighted if you could explain what I am missing here ;-)

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Javier Kane link
8/29/2020 03:33:38 pm

Hey Marius, sorry that it took us so long to get back to you!

I've just updated this particular blog post to clear things up a bit more in summary. Potassium bicarb and calcium carb will raise your pH and kH. Since in a healthy aquaponic system your pH will always be dropping, these are the most commonly used compounds in the industry to regulate pH. Thanks for the read and good growing!

Reply
Marius Schudel
9/1/2020 12:53:56 am

Hi Javier, thanks a lot for yout time and for the reply better late than never ;-)
I have learnt a lot about kH and pH since my comment. It is also very system dependent and in the system/water I use I just cannot get my kH above 1 unless my pH goes way up to 8+. It depends on the water chemistry and many other factors.
I still strongly disagree though with your statement that potassium bicarbonate settles at 6 pH. I have done a lot of searching and all sources state 8.2ph+. I dont know where you get that from and would be delighted to find out. Because if you have a prodcut that increases kH and actually settles below 7pH that would be totally awesome and total gamechanger for my system and many others. With potbicarb since it settles above 8pH IMO I just cannot add enough to raise the kH without seriously affecting pH to go above 7

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4/21/2022 11:34:13 pm

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    Javier Kane

    Co-owner of Kodaponics and Aquaponic biochemist.

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