1) Potassium in aquaponics: commonly deficient and tricky to manage.
When it comes to interesting (and frustrating) nutrients in aquaponic systems, potassium takes the cake. Potassium is the most commonly deficient nutrient by far (9 out of 10 are a potassium deficiency). But that’s not all—potassium also has a complicated relationship with other nutrients in the system, which makes it tricky to manage.
Potassium is introduced to your system in the form of fish feed or supplements like potassium hydroxide or kelp meal concentrate. However, the potassium levels in most fish feeds aren’t adequate for both plants and fish.
This means that potassium is often deficient in aquaponic plants. This causes all sorts of problems.
2) Plants use potassium in 4 important ways
Potassium is a simple, soluble element, important for plants as well as animals. In fact, practically every living thing needs potassium to survive.
Plants use potassium in four very important ways:
Signaling
Because it’s simple, soluble, and charged, potassium is an important signaling ion. This means that plants use potassium to communicate with the different cells and parts of the plant, coordinating growth and reproduction and mounting defenses to diseases and pests. Thus, potassium-deficient plants are more susceptible to pests.
Water movement
Potassium is also a simple and effective ion to control the osmotic potential of plant cells. Basically, the plant uses tiny pumps to move potassium from one area to the next in order to “inflate” certain cells with water. Water moves from the cells with low potassium concentrations to the cells with higher concentrations. This allows the plant to keep its tissues full of water, which is important since plants are mostly made of water.
Stomata (gas exchange)
Potassium also allows the plant to do things like open tiny pores in the surface of the leaves (called stomata) to allow gas to enter and exit the plant.
Building proteins
Last but not least, potassium is important to building proteins, which are crucial to plant survival.
3) Potassium in fish feed is insufficient for both fish and plants
For fish, potassium is one of the top ten most important elements (by % of body weight). For this reason, fish feed usually contains quite a bit of potassium.
Unfortunately, the potassium in fish feed still isn’t enough for plants. Feed is formulated to be efficient, so it rarely includes more potassium than is necessary for the fish. This is because most fish feed isn’t made with aquaponics in mind, but aquaculture—where excess potassium would just go to waste. When traditional aquaculture feeds are used in aquaponics, there just isn’t enough potassium to go around.
Plants, especially flowering and fruiting plants, consume potassium at rates significantly higher than rates of input. Potassium is almost always deficient in mature aquaponic systems. So in order to have healthy plants, you’ll have to artificially bump up the amount of potassium in your system with supplementation.
4) Supplementing potassium in system solution
You can supplement potassium one of two ways:
- by applying it foliarly (spraying it on the plant leaves) using potassium chloride
- by adding potassium to your system solution using potassium hydroxide, kelp meal concentrate, or potassium sulfate
The most common (and often easiest) way is to supplement the solution. In some cases, supplementing potassium in the solution can be problematic. This is because the amount of potassium you add to your system isn’t always what is available to your plants.
Why wouldn’t it be available? Well, potassium can interact with other elements in ways that we don’t expect. Namely, it interacts with calcium and magnesium, both of which reduce the availability of potassium to plants. This means that plants can show potassium deficiency symptoms even if there is technically plenty of potassium in the system.
Calcium is the ringleader of the “let’s complicate potassium uptake” club. Systems with lots of calcium (maybe they use calcium carbonate to “buffer” pH or maybe the input water is hard with calcium-based carbonates) reduce the ability of the plants to take up potassium.
The same is also true of magnesium but to a lesser extent.
The key to potassium management is balancing Ca, Mg, and K
Balancing calcium and magnesium is key to supplementing potassium effectively. We must make sure that we are not supplementing one at the expense of the others.
There are two ways to maintain a balance:
- Adding proper levels of potassium, calcium, and magnesium based on your system’s needs, and
- applying one, or all of these nutrients foliarly (essentially bypassing much of the issues with trying to balance nutrients in solution).
5) Identifying potassium deficiencies
Potassium deficiencies can be difficult to diagnose, primarily because they resemble in many aspects, deficiencies in Calcium and Magnesium. There are some distinguishing features, however.
The marks of a potassium deficiency
Potassium deficiencies will show up initially as interveinal chlorosis (when the space between the plant veins yellow, but the veins stay green), starting in the older growth. This is because potassium is mobile in the plant, allowing the plant to reallocate what potassium it has from the old growth to the new, delicate growth.
If the potassium deficiency isn’t initially treated, it will get worse, culminating in browning, drying, and death of the edges of the leaves. In addition, you will find dead, necrotic spots on the leaves, as well as bronzing and cupping.
Each crop can show slightly different symptoms, and often, when the deficiency becomes extreme, it becomes impossible to differentiate from calcium deficiency.
O
ne telltale sign for many crops in aquaponic systems is the stunting of root growth. This means that plants show slowed or stunted growth and can be easily pulled from the media. The root system of pulled plants will be small, often browned, and lack lots of structure.
To help with diagnosis, please download the Nutrient Deficiency Key developed by Dr. Nate Storey. It isn’t foolproof, but it can give you a starting point for diagnosing potassium, calcium, magnesium and iron deficiencies, and help you to see the differences between them.
6) Correcting and managing deficiencies in the long-term
When it comes to managing nutrients with frequent deficiency, the best thing that a manager can do is to proactively solve the problem, instead of waiting for the problem to manifest. Managers can create a supplementation calendar to avoid deficiencies.
Depending on your actual pH range and your desired pH range, there are a number of ways to supplement potassium.
Low pH systems: potassium hydroxide (caustic lye)
“For low pH systems, the best potassium supplement is potassium hydroxide (caustic lye). This is a great way to supplement potassium while raising your pH. Potassium hydroxide is very strong base, meaning that is is an excellent way to raise pH. Potassium carbonate can also be used but is a carbonate, so it’s not appropriate unless your system trends to pH values around 6.0 consistently.”
For high pH or neutral systems: kelp meal concentrates or potassium sulfate
“For pH neutral or basic systems, you want to add potassium in a form that will not impact your pH. Your best options in this regard are kelp meal concentrates or potassium sulfate.
Kelp meal concentrates are available in a liquid or powdered form. If you want the most bang for your buck, buy kelp concentrate in powdered form. Typically, 1–2 dry ounces of powder will make 1 gallon of concentrate, significantly bringing down the cost of using kelp concentrate in your system. It should be noted that kelp meal concentrate also delivers other micronutrients that can be beneficial to your system health and can be applied foliarly for faster results.”
I’m ready to deal with potassium in aquaponics—but what about other aquaponic questions?
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How often would you spray the plants with kelp solution?
Hi Shawn, that depends on the dilution. You can spray pretty often if it’s dilute. If you you’re spraying a more concentrated solution, once a week is about all you want to spray.
Hi , i have a question
my aquaponics system have 40 – 80 ppm nitrate.
I would like to grow 5000 head lettuce .
So with this rate can i grow it .
So 40 – 80 ppm nitrate
equal to what ratio in NPK . I mean N ratio .
If this rate cannot what
should i add in to my system .
Thank you
Hi Wong, your nitrates are in the correct range for an aquaponic system. You’ll know if you’re getting too low- your plants will show a deficiency.
Hello, a good Humic Acid can help?
Thank you
Hi Ricardo, Humic acid can help with algae control (it turns the water black, blocking out sunlight) but doesn’t do much for potassium. For potassium supplements, you’ll be better off using potassium hydroxide, kelp meal concentrate, or potassium sulfate.
I run my system at a PH of 6.2 what do you suggest for my system? Potassium that is.
Thank you
John
What brand of kelp meal concentrate do you use? The only product I’ve found has only < 1 K.
Thanks
Hi Pat, we use Kelp4Less. Be sure to grab the concentrate though, not just the kelp!
What’s the best way to add potassium organically?
Hey Dan, kelp meal concentrate is the best way to do this. Be sure to get the concentrate or “extract”, not plain kelp meal.
hi
1. What are the recomended rate of N P K Ca Mg S in an aquaponic system for balance of nutrients and health of aquatic life, tilapia just now, later shrimps.
2. what are the maximum amounts of N P K Ca Mg S in an aquaponic system in therms of ppm?
3. in a more easy language, how many g/liter or oz/gal of what recommended nutrients can I add to an aquaponic system to max the ppms of N P K Ca Mg S
I went to the kelp4less webpage as you recommend them. they tell me to buy:
1. Azomite Powder
2. Fulvic Acid
3. Humic Acid
4. Kelp
5. SOP Powder
6. K Mag
what amounts of each one can I add to my aquaponic system to get to the recomended rates and ppms?
foliar?
Hi Homar, check out the aquaponic nutrients playlist – it describes how to supplement all of those nutrients. https://www.youtube.com/watch?v=Lgtue6tbbhc&list=PLAPahqrfGZZmLPh4vXzRDqWrXP7L9mnWT
hi. I have seen that video a lot of times! I have learn a lot from it. but, it still dont said how many grams or oz of each salt to add per gal of an aquaponic system
Excellent article. I use Potassium Bicarbonate in my aquaponics system. My pH tends to stay in the low 6 range, so this helps in raising the pH. I tend to add Potassium Bicarbonate to my system every 3-4 days.
Hi there
I’m using Gaia Green Kelp Meal powder (1-0-3 NPK) to supplement my system with potassium. I have no idea how much to add. Are there any general guidelines about how much to dilute into solution and then add to the system?
Cheers!
Jonathan
Hi Jonathan! There aren’t any rules of thumb for adding kelp because I don’t know what your potassium levels are currently. If you’re experiencing a deficiency it’s a little bit easier to feel it out by adding a little but until the deficiencies are taken care of. I will advise you to use kelp meal concentrate (sometimes called extract), not powder.
What about potassium nitrate
You could use potassium nitrate, but KOH or kelp meal would be better and less destabilizing.
Thank you so much Amy
How much per gallon
Whichever product you use, it should come with a dosing rate included.
Hello! If i want to apply potassium chloride foliarly, how much gallons/liters per gram should I use? Thanks!
Hi, can you apply potassium sulfate foliary, if not please explain why.
Hey Ramon, you can apply it foliarly, although you’ll want to use it sparingly that way and make sure to have great airflow.
We balance Ca, Mg, and K in a fairly large system (40,000gals) with unfiltered well water that is high in Ca and Mg. Would KOH still be an option if my pH is slowly and continuously dropping (currently 5.5-6.5) and I need to supplement high ppms of K to balance everything out? Also, what brands of KOH KCO do you use?
Can you tell me your recommended potassium, magnesium, and calcium parts per million and your ideal range for ph that won’t block any of those nutrients out? I’ve read the higher the ph the less available some of the nutrients. I do have equipment to test ppm. thank you.
I also along with many other would like an established ppm for all nutrients if possible. Thank you
In ppm, level of potassium in aquaponic.pls immediate answer from Nigeria
Hi Amos,
Potassium levels are variable depending on the levels of other nutrients in your system, especially calcium and magnesium.
Ppm per gallon pls
I have an idea. I have been thinking over it for sometime now but I’m not sure if it will work out. Here is it:
Instead of aquaponic or hydroponic, while dont we look for a source of protein, decompose it to produce ammonia eliminating the fish in the system. Supplementing ammonia to the system at a save level for only the plants and the bacteria from the decomposed organic matters like fly maggots for example. So we cut cost on fish feed. What do you guys think about my idea please. From Nigeria
And supplementing other nutrients needed in the system. Thanks
What i did is to check out the recommended level of nutrient in the soil in ppm and apply that to the system also then watch for deficiency of any kind.
I first want to say I Love what you are doing!
Q:what is your preferred mixing ratio for spraying potassium bicarbonate?
Louis—
The ratio should typically be listed on the product label, however, if it’s not, the rate is 5-20 grams/liter for 99% potassium bicarbonate.
Very pleased to find this source of info.
Can you supplement with mineral dust (rock dust)
Hi scott rumpel,
Yes, glacial or volcanic rock dust can be used. It is best to mix into the media if possible. Mixing into solution can lead to sediment which can potentially cause clogging in filters, pumps, emitters, etc.