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Episode 16: Potassium, Calcium, and Magnesium in Aquaponics 

Show Notes

Today we’re talking about three macro-nutrients: potassium, calcium, and magnesium. These are the three most commonly deficient nutrients for aquaponic systems.

Potassium (2:05)

Potassium is a primary plant nutrient available to plants as an ion, impacting cellular metabolism, plant defense, plant signalling, and even seed and root development. Since most fish food do not have enough potassium in it, potassium is also a common deficiency. Symptoms are chlorosis and sometimes even necrosis.

To supplement potassium, we recommend potassium hydroxide (KOH), a base that adds potassium while it raises pH. If your system is pH neutral, then you have options like potassium sulfate (very strong) and kelp meal concentrate. (Make sure it’s the concentrate, not just the meal.) Foliar sprays of potassium chloride are also available. This foliar application will give you fast results, but be careful using this because it’s a salt. If you use a foliar spray, you really need to know what you’re doing. (Ideally, you won’t have to use a foliar spray at all.)

Calcium (7:31)

Calcium is important for use as a building material for cell walls and for some signalling. Calcium is also available to plants as an ion in solution. Calcium, however, is taken up passively by plants,, so if you have high humidity or high CO2, then you might see calcium deficiencies because the plant isn’t taking up enough water to get the calcium it needs. Because it’s not moved around much in the plant, it often occurs on new growth.

Calcium is even more important for aquaponics because it impacts fish and microbe health as well as plants.

You can supplement calcium with chelated products, which we recommend. If you need to use something else, you can also use calcium chloride foliar spray or bone-/oyster meal (not recommended as they are carbonates). We love hydrated lime, which contains both calcium and magnesium. If you’re in Australia, this might be slightly different from the hydrated lime than we get here in the US. 

Magnesium (12:51)

If you’re raising pH with hydrated lime, then you won’t see problems with magnesium deficiencies. If you have a neutral pH, however, you’ll want to use magnesium sulfate (epsom salt) to supplement magnesium.

Magnesium deficiencies usually show up as bronzing on plant leaves. (Not quite necrosis, but a brownish or reddish color on the leaves. This changes a bit from crop to crop.)

Do you need to worry about a sulfur toxicity? No. It takes much more sulfur to cause a toxicity than you could put in your system by supplementing other deficiencies. In fact, I’ve never seen a sulfate toxicity in any system.


Not only are all three of these deficient in systems, but they impact each other when they are in excess. This can be confusing, but if you’re having trouble controlling any one of these it might be that one of the others is impacting it. In this case, it might be worth it to have a university do a water test or tissue analysis for you.

If you want more in-depth information on nutrients/plant interactions, check out USU courses, which are online courses built help small farmers build their farms and/or businesses.



Thanks for joining us today! Stay tuned for more on nutrients in aquaponic systems.

View all our episodes here.

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