This is part of a series on Aquaponic Water Testing.
Potassium is very important to plant growth. A Potassium deficiency will cause older leaves to appear chlorotic, and uneven ripening(if at all) in fruit. Unfortunately, fish require no dietary Potassium. Therefore most fish feed is not supplemented with Potassium.
How much Potassium?
Hoagland suggests a plant “ideal” of 235ppm. Another approach is to aim for a value 2/3 of your nitrate levels, or twice your phosphate levels. Excess Potassium may lead to deficiencies in Magnesium, Manganese, Zinc, and/or Iron.
Potassium Testing Methods
There are two methods of testing for Potassium; turbidity based tests, or using an Ion Specific Probe. I have the LaMotte Double Tube test kit, and it works, but is cumbersome. I have also used the YSI method and love the ease and rapidity of the test. It does have a low range (0-12ppm), I usually diluted my sample 10 to 1 which increases that range to 0-120ppm.
- LaMotte 3138-01 Double Tube (0, 6, 7, 10, 20, 30, 40, 50)
$65.20 / 100 = $0.65ea
I have and use this double tube setup. If my potassium is >50, I could re-run the test after diluting the sample with distilled water, but often don’t.
- Hanna HI96750 Portable Photometer (10-100; Resolution 1)
Meter: $176 / $196@Amazon
Reagent: $85 / 100 = $0.85ea
- LaMotte Smart3 + 3639-SC (0-10.0, MDL 0.8)
$59.65 / 100 = $0.60ea - YSI YPM189 (0-12; MDL 0.5)
$35.67 / 50 = $0.71ea OR $79.17 / 250 = $0.32ea - Horiba LAQUAtwin Potassium 39-3900ppm
Meter: $320@Amazon +
Sensor: $127@Amazon / 1500 = $0.08ea
Requires calibration before every use.
Potassium Supplementation
There are a number of chemicals we can use to supplement the Potassium in our Aquaponics systems. I use MonoPotassium Phosphate, Potassium Nitrate, Potassium Bicarbonate, and Potassium Silicate. I save my banana peels for my compost bins.
- Potassium BiCarbonate
- Potassium Hydroxide
- Potassium Nitrate
I love potassium nitrate. For every 3ppm Potassium added, you also get 1ppm Nitrate. - Potassium Sulfate
Potassium Sulfate works, but since plants don’t use as much sulfate as potassium, you could end up with sulfate issues. - Mono Potassium Phosphate (MKP)
- Potassium Silicate
Allowed in organic production as Sil-Matrix, an aqueous Potassium Silicate, as a “pesticide”. - Banana Peels
Not recommended. The number of banana peels needed means your growing media would be banana peels, not rocks.
Do you know of a different way of testing for Potassium in Aquaponics? Let me know in the comments and I’ll update this page.