The balancing act – nutrients and pH

Signs of Iron Deficiency in the Mizuna

We’ve started to notice our plants getting lighter green and veins appearing on the leaves – we’ve got a nutrient deficiency on our hands… Working with an aquaponic system – particularly a young system, is a balancing act. The fish produce waste, bacteria break this down and then plants absorb the nitrates, and macro /micro nutrients. The ease at which nutrients can be absorbed by the plants is affected by the pH of the water. In our system the pH has been a little bit high for the nutrients to be in a form the plants can readily absorb. Bristol has very hard water – pH 7.4 in our tanks.  We’re using peat pellets to try to slightly acidify the system. Seems to be working – pH readings today were down to pH7.2 today.

You can see from the chart above (from here) that the way in which different nutrients are absorbed is affected by the pH.  Most nutrients are absorbed at pH 6.5 to 7. and most strains of Tilapia prefer a pH of 7 to 8. So we will aim in our system to maintain a pH of around 7 – in the hope that this ensures the plants can access all the nutrients they need, and that the fish are happy too.


7 thoughts on “The balancing act – nutrients and pH

  1. Harry Curtis says:

    Hi. You should not have to buffer the water down as this will happen naturally over time and if anything you may find you have to buffer the PH up over time with potassium hydroxide alternated with calcium hydroxide. This will add Potassium and calcium which both lack in an aquaponic system. Also every few weeks it is important to add chelated iron to the system as this is a common deficiency; hence the yellowing of the leaves especiallly interveinal chlorosis. Hope this helps. Feel free to get in touch.
    Harry Curtis

    • alicemariearcher says:

      Hi Harry – thanks so much for the advice! Learning as we go over here!!! Should be easy to lift pH back up mind – given our incoming water is at 7.4 – 7.6…

      • Harry Curtis says:

        Although you may still need to apply the base additions as the amount of top up water should be pretty negligible compared to the rate a system can fluctuate in PH. The best thing to do is constantly monitor the PH and add these additions as they will also add much needed nutrients which are lacking in aquaponic systems. The PH 7.4-7.6 is not unusualy high and is perfect for the fish but the plants begin to suffer around 7 and up but to compromise the fish and plant requirement it is important to aim for around 7 or just below. Be careful it does not drop below 6 as the fish will suffer.
        Also, do you have hard or soft water there?

  2. Sue Everett says:

    Peat pellets? Those of us working in conservation have been fighting for 30 years to stop peat-extraction. We haven’t succeeded yet. Peatland is a major global carbon store, it needs to be left in the ground – peat, in whatever form, should not be used for any project that claims to have sustainability credentials. Hope you can find a benign alternative. How about vinegar? Otherwise a really fascinating and ground breaking project and I’d like to try it on a domestic scale. What are the fish being fed on? (Could be another sustainability issue) I would like to find a use for garden snails …..

    • alicemariearcher says:

      Hmm I hear we could try Almond leaves…? Ha yes snails might work – or hens will eat them. We are experimenting with Black Soldier Flies at the moment as a potential feedstock…

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