Low pH from grow media?

[John]

It seems like something's in my grow media that is lower pH like crazy. 

My grow media is hydroton. I rinsed off the hydroton and tried starting a simple ebb and flow aquaponics system with 1 grow bed and 1 fish tank, but the pH started off below 6.0.

I tried correcting the system with baked egg shells and baking soda, but that only corrected the pH for a couple days. It reverts back to below pH 6.0 only after 3 or so days. Figuring there is a problem with the grow media, I took out around 1 pint of hydroton, rinsed them thoroughly 3-4 times with a lot of water, swishing it around with my hands. After letting them rest in fresh tap water over night, the pH goes back to below 6.0.

I'm measuring with the API freshwater kit. I have tested the tap water by measuring it straight from faucet (pH 7.0, no ammonia / nitrites / nitrates), and then testing the tap water again with aeration over 2 days with the same results (pH 7.0, no ammonia / nitrites / nitrates).

After trying to rinse a small amount of rocks so many times resulting in below pH 6.0, I'm not sure what the problem is, or how to fix it. I am considering using a hydrogen peroxide or chlorine wash, and trying a series of rinses.

[Harold Sukhbir]

Hi Jon,

Strange indeed! I assume you haven't started cycling as yet because of the PH. Usually hydroton comes neutral PH, and if anything it should even add buffers to raise PH in water depending on batch/manufacturing process. I personally have no experience whatever with this type of media, however the Peroxide rinse/soak sounds about right(say for 24hrs). I would also add 50/50 mix of lime/potash everyday for a week or as long as the media takes to stabilize as it eventually will.

[John]

I tried cycling, but wasn't really getting much in results. Ammonia would spike, but I didn't get the nitrite spike for several weeks. No nitrites + extremely low pH leads me to think I need to start over.

Lime / potash, do you just put it into the grow media, and let the ebb/flow take care of things? I'm wary of doing anything without really knowing what the cause of the low pH is.

Harold Sukhbir said:

Hi Jon,

Strange indeed! I assume you haven't started cycling as yet because of the PH. Usually hydroton comes neutral PH, and if anything it should even add buffers to raise PH in water depending on batch/manufacturing process. I personally have no experience whatever with this type of media, however the Peroxide rinse/soak sounds about right(say for 24hrs). I would also add 50/50 mix of lime/potash everyday for a week or as long as the media takes to stabilize as it eventually will.

[Harold Sukhbir]

Hi Jon,

For cycling you need a higher PH than 6. Bacteria won't establish themselves at 6 or below. In established AP systems(over a few months) with a constantly falling PH, both lime and potash are used as a buffer and is considered a norm in AP. From your post, the natural assumption for your falling PH, seems to be the media. If something in the media is causing this(acting like an acid to drop the PH) it will eventually deplete itself over time. By buffering with the lime/potash anywhere up to PH 8, you can at least start cycling in the meantime, and hopefully by the end of that period the media may correct/balance itself. If  you solve the problem with the media, in this way, you will still eventually have to buffer your AP with same or similar products anyway

[John]

Just as a post mortem, I kept adding lime and checking pH over a couple weeks. It did the trick, now I'm almost done nitrogen cycling, and seedlings seem fine. I'll be ready to add fish next week!

[Harold Sukhbir]

Hi Jon,

Good news I see!

For now you can dose with lime but start looking to source the potassium hydroxide. Lime will build in the system and eventually become saturated causing a "lockout" of other minerals like iron for instance. You will see these iron deficiencies appearing in the plants and by that time dosing with iron won't help any because the cause would be the calcium carbonate locking out the iron. That is why we dose 50/50 i mentioned in the previous post. Good luck

[John]

Hello Harold,

Thanks so much for recommending potassium hydroxide. I'm not sure where to source it though. A lot of what I see are lye flakes for making soap. Is this ok for my system?

[Vlad Jovanovic]

There are two types of common substances called 'lye'... sodium hydroxide (NaOH) and potassium hydroxide (KOH)...Most (but not all) lye sold for soap making is sodium hydroxide (NaOH) and not appropriate for AP...So just check the label.

If you happen to heat your home with wood, you can easily make your own potassium hydroxide (KOH)...Or if you for whatever reason have access to ashes from hardwood trees...

[Harold Sukhbir]

Hi John,

So you're looking for Caustic Potash or Potash Lye or Potassium Hydroxide. Where I live we have many Agricultural shops where we can get this product. Hydroponic shops also stock it. I found this on product on Amazon its food grade as well. 

http://www.amazon.com/Grade-Potassium-Hydroxide-Devil-Potash/dp/B00...