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Do I actually have to measure for Potassium levels if I am deficient in K?

I have been told with some authority that the limiting factor in Aquaponics is Potassium (K).  What this means is that when plants grow, they remove all the K, and can't use the other nutrients.  This means that Nitrogen  builds up.  This is what I measure when I do the Nitrate test.   This suggests an approach for determinng the amount of K that needs to be added to zero the Nitrate level.  Since it is unlikely that all of the nitrate will be removed, I suggest the calculation only be performed when nitrate levels exceed some threshold such as 200.

I could probably get rid of all that nitrate if I just added the right amount of K,  but how much K do I have to add?

I looked at fertilizer tables and found that the K/N ratio is 1.2.  This means I have to add 1.2g of K for each gram of N that I have in excess.  I have a bottle of KCl (noSalt) that says that one quarter teaspoon contains 650 mg or 0.65 grams of K, th rest being the Cl.

I know my tank shows a Nitrate reading of 200 PPM, this coresponds to 200 mg/L for every liter in the tank.  The tank contains 170 gallons of water, or right at 650 liters.  This means I have 130 grams of nitrogen in the tank.  Since I need 1.2g of K for each gram of N, I need 156 grams of K.

I measured the mass of 1/4 tsp of nosalt and found it weighed 1.4g, so .65g of K is found in 1.4g NoSalt, this reduces to .464g of K per g of nosalt, and 156gK can be had with 336g of NoSalt.

If I resolve the above generalizing the volume of water in terms of Gallons,

The number of grams of NoSalt= VgN/100

Where Vg is the volume of he tank in gallons, N is the reading of Nitrate in PPM,

I have added my first 100g of noSalt to the tank tonigh.  Over the next few days I expect the Nitrate level to drop.  I will let you know.

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You may find that aquaponics is a bit more complex yet more simple than all this.

Your water chemistry can play a big role in the availability of potassium (if you have really hard water and lots of calcium then the potassium could be precipitating out as white stuff.)  My big system has this problem big time because 40% of it's media is calcium carbonate so my plants struggle with potassium deficiency and iron deficiency (due to pH lock out) and the nitrate levels in that system tend to stay high when the fish are eating well.

In large commercial systems they usually alternate the use of calcium hydroxide and potassium hydroxide to keep the pH up while providing a balance of calcium and potassium to the system.

And if you do not have enough plants in relation to the amount of feed the fish are getting, then you are likely to have an over abundance of nutrients (Nitrates) even if all the nutrients are balanced.  Amount of light is a limiting factor in some systems.

And sometimes it isn't just a single element that is limiting but there are lots of lesser trace elements which is why with new systems many people will give a few doses of seaweed extract to provide a good balance of those trace elements along with the potassium.  Once a system is more mature supplementing generally becomes less necessary as long as you are using a good quality fish feed and your source water isn't too extreme.



TCLynx said:

You may find that aquaponics is a bit more complex yet more simple than all this.

Your water chemistry can play a big role in the availability of potassium (if you have really hard water and lots of calcium then the potassium could be precipitating out as white stuff.)  My big system has this problem big time because 40% of it's media is calcium carbonate so my plants struggle with potassium deficiency and iron deficiency (due to pH lock out) and the nitrate levels in that system tend to stay high when the fish are eating well.

In large commercial systems they usually alternate the use of calcium hydroxide and potassium hydroxide to keep the pH up while providing a balance of calcium and potassium to the system.

And if you do not have enough plants in relation to the amount of feed the fish are getting, then you are likely to have an over abundance of nutrients (Nitrates) even if all the nutrients are balanced.  Amount of light is a limiting factor in some systems.

And sometimes it isn't just a single element that is limiting but there are lots of lesser trace elements which is why with new systems many people will give a few doses of seaweed extract to provide a good balance of those trace elements along with the potassium.  Once a system is more mature supplementing generally becomes less necessary as long as you are using a good quality fish feed and your source water isn't too extreme.

Regarding trace elements, I use something called flourish which is for aquariums, but maybe I should try seaweed extract.  I haven't noticed anything precipitating out, no white stuff.  When I began to add Potassium, everything perked up.  I have one heck of a lot of green right now.  It is a jungle, but I get your point.  I'll feel better when I start getting tomato blossums.
 
TCLynx said:

You may find that aquaponics is a bit more complex yet more simple than all this.

Your water chemistry can play a big role in the availability of potassium (if you have really hard water and lots of calcium then the potassium could be precipitating out as white stuff.)  My big system has this problem big time because 40% of it's media is calcium carbonate so my plants struggle with potassium deficiency and iron deficiency (due to pH lock out) and the nitrate levels in that system tend to stay high when the fish are eating well.

In large commercial systems they usually alternate the use of calcium hydroxide and potassium hydroxide to keep the pH up while providing a balance of calcium and potassium to the system.

And if you do not have enough plants in relation to the amount of feed the fish are getting, then you are likely to have an over abundance of nutrients (Nitrates) even if all the nutrients are balanced.  Amount of light is a limiting factor in some systems.

And sometimes it isn't just a single element that is limiting but there are lots of lesser trace elements which is why with new systems many people will give a few doses of seaweed extract to provide a good balance of those trace elements along with the potassium.  Once a system is more mature supplementing generally becomes less necessary as long as you are using a good quality fish feed and your source water isn't too extreme.

How could Potassium Chloride not be a salt? It is a salt.

I don't know where you got your "authoratative" advise from... but ignore it, and them... it's complete bullocks...

 

There is an interrelationship between potassium, calcium and phosphorus uptake in plants... but no real interaction between potassium and nitrates...

Dosing your system with potassium...wont lower your nitrates... well not directly anyway...

It may do so by promoting more growth in general... but plants primarily need potassium during the flowering/seeding/fruiting stages of growth... which are exactly the stages when high nitrogen is detremental... promoting leaf growth rather than flower/seed/fruit...

If there's a "limiting factor" in aquaponics... it's pH... in relation to trace element uptake... and Iron is the most often pH related deficiency...

Foliar feeding Maxicrop seaweed, or Maxicrop + Iron... will provide a level of potassium, and many other necessary trace elements.. in a naturally chelated form...

TCL has touched up the use of potassium hydroxide, or potassium bicarbonate (also good for powdery mildew) as a potassium source... you can also bury a banana in your grow bed as a source of potassium....

TCL also alludes to alternating between potassium & calcium hydroxide.... as not only provision of those elements... but as a buffer to raise pH... and because excessive dosing of potassium will begin to lock out calcium...

Potassium Chloride can be used as a source of potassium, but wont buffer pH... and excessive dosing will raise salinity levels.. as well as limiting calcium...

As I said, you really only need potassium at specific stages of plant growth... and potassium in itself...wont directly lower nitrate levels...

Your nitrate levels are certainly high... but not dangerous to fish in the short to medium term... but they certainly indicate that you should be planting a lot more vegetables to utilise them..

Perhaps you may need to add another grow bed... so you can add more vegetables... and use the nitrates up...

Thanks


 
RupertofOZ said:

I don't know where you got your "authoratative" advise from... but ignore it, and them... it's complete bullocks...

 

There is an interrelationship between potassium, calcium and phosphorus uptake in plants... but no real interaction between potassium and nitrates...

Dosing your system with potassium...wont lower your nitrates... well not directly anyway...

It may do so by promoting more growth in general... but plants primarily need potassium during the flowering/seeding/fruiting stages of growth... which are exactly the stages when high nitrogen is detremental... promoting leaf growth rather than flower/seed/fruit...

If there's a "limiting factor" in aquaponics... it's pH... in relation to trace element uptake... and Iron is the most often pH related deficiency...

Foliar feeding Maxicrop seaweed, or Maxicrop + Iron... will provide a level of potassium, and many other necessary trace elements.. in a naturally chelated form...

TCL has touched up the use of potassium hydroxide, or potassium bicarbonate (also good for powdery mildew) as a potassium source... you can also bury a banana in your grow bed as a source of potassium....

TCL also alludes to alternating between potassium & calcium hydroxide.... as not only provision of those elements... but as a buffer to raise pH... and because excessive dosing of potassium will begin to lock out calcium...

Potassium Chloride can be used as a source of potassium, but wont buffer pH... and excessive dosing will raise salinity levels.. as well as limiting calcium...

As I said, you really only need potassium at specific stages of plant growth... and potassium in itself...wont directly lower nitrate levels...

Your nitrate levels are certainly high... but not dangerous to fish in the short to medium term... but they certainly indicate that you should be planting a lot more vegetables to utilise them..

Perhaps you may need to add another grow bed... so you can add more vegetables... and use the nitrates up...

You are right.  It just isn't NaCl
 
Eric Warwick said:

How could Potassium Chloride not be a salt? It is a salt.

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