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How to know if I have enough potassium / magnesium / iron / salt in my system.

My system seemed to be stabilized with PH running around 7 same as my top up water and amonia and nitrites at zero and nitrates just north of zero.

Plants were struggling though until I added some magnesium, potassium and iron so that is cool, however I don't know what are the right amounts, I'm still on the "toss 'er in and see what happens" shoot from the hip measuring plan. Ok well I do measure the potassium chloride one rock / tablet at a time but they vary in size.

I'm getting the hang of knowing some of the deficiencies - does anyone know what the signs are when you go too far with these nutrients?


Is there a simple, cheap, easy way to measure these things? I'm a little Dutch and a lot cheap


For the record, my system is a hybrid with one IBC growbed using reclaimed clay brick for media and a second IBC DWC with rafts on top and three 55 gallon barrels for fish tanks, one pump in the DWC bed pumps the water to the top barrel and it cascades barrel to barrel to barrel to grow bed to DWC and back up again - the grow bed is constant flood - yep one too many beers when I was thinking that one thru but it's all good

Any thoughts, ideas, brain waves are most appreciated.

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I think the simple answer for you is no.  Especially if you are cheap.  Standard protocol calls for being reactive at first and then figure out a regiment that works with your water.  I have not heard of to much K, Fe, however put to much Magnesium in and it will fall out of solution.

When you do your pH up addition switch between hydrated lime and KOH(potash).  That will keep you K and Ca in the right area.  I spray with iron rather than put it in solution, the results are quick.

Keep shooting from the hip bud.  When in doubt pour in some kelp.

Thanks Matthew,

Some of the bok choy are still showing a slight yellow edge on the older leaves, where as the new growth seems ok. Should this yellowing clear up when i hit the right levels, or should I just focus on the new growth?


I'm using potassium chloride - my understanding was it should not raise the PH - I am wrong about many things in this life my kids can provide a list if needed - I'm hoping I have that right though because my system seems to be running at 6.8 / 7.0 range just with my top up water, which is about 7.0. I have to add about 2 gallons per day. Does that make sense??

Hi Bart, Your understanding is correct. KCl will not significantly affect your pH. There are relatively simple (and cheap ways) ways to measure how much of what you are adding to your system. You need to know your systems total water volume, own at least a decent kitchen scale, and be prepared for a little bit of math. (Knowing, some of the atomic weights of these elements helps too, but is hardly necessary, since you can look that kind of stuff up easily).

If you want to, let us know how much total water is in your system and we can go through some examples. It's not that hard once you get the hang of it. And that way, when you shoot from the hip, you can at least somewhat take aim.

Iron is very unique among the plant essential elements in that it does not behave antagonistically toward any other of the metal cations. So too much of it wont effect the uptake of anything else at all. (The others must all co-exist in a balance with one an other). Also, Iron toxicity in many plants is almost unheard of (I mean, you have to be trying really, really fucking hard to screw that one up if you manage to). Plants will engage in "luxury consumption" of Fe with no deleterious effects. The only negative effect using too much Fe should have, will be on your pocketbook, since good Fe chelates are not cheap, as I'm sure you've noticed.

Bok Choy, like many other things in the brassica family (chinese cabbage, broccoli etc...) will definitely need a bit more Mg and K than you're typical leafy greens (meaning lettuce). Don't worry about that little yellowing along the edges (slight K def) at all. Just watch that it doesn't develop into yellow spots on the broad surface of the leaf (K def 'stage two'). Those yellow spots can then turn into necrotic lesions (we'll call it K def 'stage three')...But since you added some K hopefully that will take care of that. Again, if you give us your water volume, and the weight of how much you added, we can work out weather you added too much, too little etc...

In most cases too much (toxicities) are almost always worse than not enough (deficiencies). So either way, it pays to do the 30 seconds of rough math to make sure you're OK.

Most all base metal cations (not just Mg) will precipitate out of solution or otherwise become plant unavailable, depending on pH, (and temps to some extent)...particularly in our phosphate (due to all the the fish poo) rich AP waters. Keeping a pH below 7 helps to keep this from happening. 

Matts got a good trick up his sleeve with that Iron. Spraying Fe foliarly is more efficient (takes less product, and is faster acting...rectifies a deficient situation much quicker) than keeping it in solution at the roots. But I like to keep my leaves free of any excess water because of the potential for fungal pathogens...that and I'm pretty lazy, so 0.5 to  ppm additions every 3 to 4 weeks does it for me. (2 to 3ppm in solution is the recommendation though. I've been getting away with a bit less).

Vlad - If my math is correct, that's a rather small amount of Fe. For a 300-gallon system, 3 ppm is less than a teaspoon. Correct?

Vlad Jovanovic said:

2 to 3ppm in solution is the recommendation though. I've been getting away with a bit less

Well, 3ppm is 3ppm whether in one liter or one thousand liters. As to the volume/weight of product it depends on what percent is actually Iron. No Iron chelate product is 100% Fe

The chelate I happen to use is 6% actual Fe. Others use a 12% product. So I would have to use double the amount of 'teaspoons' in the same amount of water to achieve a 3ppm solution with my product.

So if I add "one teaspoon" full of product, and only 6% of that teaspoon is actually Iron I need to add 96% more.

KCl is only 52.45% potassium by weight, so you have to take those type of things into account.

MgSO4-7H2O (Epsom salt) is only about 10% (9.8% actually) Mg by weight...and so forth. So you cant just add the total weight of the compound and expect to get the ppm of Mg you were after (in this case you would just multiply that by 10).

Make sense? 

Good point. My solution is 5%. So 3 ppm for 300 gallon comes out to about 14 teaspoons or 2.3 ounces. Makes sense. Thanks for the clarification. 

Vlad Jovanovic said:

depends on what percent is actually Iron. 

Thanks for the feedback Vlad, and yeah the price of Iron rubs me the wrong way but if I understand correctly the need to add it to the system should reduce over time as the system matures but for now I'll suck it up


Ok so here are the stats we're working with.

350 gallons of water in the system

I've added about 90g of KCI, 50g Epsom Salt, and i'm going to call it 15g of iron so far and the iron i'm using is 7% as per the package. 

The PH has been south of 7 for about 4 to 5 weeks now, prior to that it had been running high, around 8 - I used muritatic acid in my topup water to pull it down but have let it be since getting it south of 7 - thank you to both yourself and Jon for your help understanding how to manage the PH issue.

I'm adding about six litres of water per day, half is evaporation, half is the wife steeling for her potted plants - it's city water which comes from both Lake Erie and Huron, London has a pipline to each so I have no way of knowing which I'm getting not that it would matter but I'm thinking the toxicity levels would be less in Huron.


Any direction you can provide me would be much appreciated.



Firstly, let’s start by saying that milligrams per litre (mg/ L)  is exactly the same thing as parts per million (ppm) since there are 1000 mg’s in a gram. So it is much, much easier to start by converting your system water volume from US Gallons to litres. (Since we all have 10 fingers and toes this makes things simpler than “teaspoons, ounces, quarts blablabla)…Just multiply your gallons by 3.785 (or round to 3.8)

I’m going to due this first part the very looong way so that people who are’nt mathematically minded can easily follow along (those that are will get it anyways and can cut out the extra steps)…

Ok, so in liters that’s 1324.89, but we’ll just call it 1300. So 1.3 kg is 1ppt

0.65kg is 500ppm ...

so 0.325kg is 250ppm.

For KCl 52.45% of that is K (we’ll just call it half, the other half are fish benificial chloride ions). So, if you added 325 grams of KCl that’s 250ppm of KCl which is 125ppm (half) of potassium.

I don’t know what you are growing exactly what all you are growing (other than bok choy) but you could halve that 325 of KCl grams if you want (since I assume you aren’t growing toms, cukes, squash etc…this time of year). So around 165 grams of KCl total (125ppm KCl, is say 65ppm K) would be fine. Since you've already added 90 grams you can add say 75 grams more if you want. That should be plenty for now.

If your intent was to use a 2:1 ratio of K to Mg then just redo this next part, but I’d suggest something more along the line of 3:1  K:Mg. So we'll divide by 3

If 165 grams divided by 3 (to get our 3:1 ratio) is 55 grams (or 41.6ppm of epsom salt) and since only 10% of that is actually Mg we’d multiply that 55 grams by 10. And round to 500 grams. That would give us our desired 40ppm of Mg in solution. Your water may already contain 'significant' amounts of Mg...So again you can use less if you want.

Now for our 2-3ppm of Iron…Since we already know that 55 grams of whatever in 1300 litres of water gives us 41.6ppm of that ‘whatever’…and we are shooting for 2-3ppm…Just divide 41.6 by a number that gives us about 2.5. In this case 17. So 41.6 divided by 17 is 2.447ppm. Close enough for handgrenades to call it 2.5ppm. So we take our 55 grams and divide that by 17 and get 3.23 grams of Fe. If we had 100% iron that would be it, but since our Fe-chelate is only 7%. For the lazy like me, we’ll call that 10% and say that 30grams of that Fe product will give us close to 3ppm of iron.

Honestly though, I’d cut that recommendation in half. My bok choy, lettuces, Chinese cabbage, chard, cilantro have been, and are doing fine with no signs of Fe deficiencies with far less. 

For K and Mg I used pretty standard and very common on the mid-lowish end of the scale from well respected folks like: (Asher, C.J. and Edwards, D.G., l978,pp. 13–28 in A.R. Ferguson, B.L. Bialaski, and J.B.Ferguson (Eds.), Proceedings 8th International Colloquium, Plant Analysis and Fertilizer Problems. Information Series No. 134. New Zealand Department of Scientific and Industrial Research, Wellington, New Zealand ; Larsen, J.E., 1979, in Proceedings of the First Annual Conference on Hydroponics:The Soilless Alternative, Hydroponic Society of America, Brentwood, CA). blablabla...

I would add these things as a "one time shot in the arm" for a new system just while it builds up a store of some of these plant essential elements (with the possible exception of Fe...that you might continue to add once a month or so for a while...and see how things go now that your pH is under control (and I'm assuming your using a quality fish feed). If you notice you need to add K once or twice a year, then do so. Try to make the timing additions coincide with your cultivars (say when your toms start flowering would be a good time to add K if you were already going to etc...

Hope this stuff made sense.

Hope some of this helps


Makes perfect sense, I'm going to add the suggested amounts as per your calculations and will monitor from there.

How much higher should the levels be to support cucs & toms?

Thanks again for the detailed explanation.

Vlad maybe I am missing something but you mentioned a cheap and easy way to measure these things and I don't see that part. I would love to know some cheap was of getting at these levels.

But the fact is Bart most people really don't need these levels once you figure out what how to identify all of the deficiencies. I do a little more the science route, and minus a spec I have no good way of looking at the cation concentrations.

And listen Vlad, he reads. :P

Yeah, it looks like you missed it Matthew... Bart, by expending a few calories of mental energy now know the ppm value in solution vs. the weight of the compounds he is adding...the plants tell you the rest of the story...

I do agree that 'figuring out/identifying' all of the deficiencies look like is important. In the last almost 20 years now, that I've been tinkering with soil-less culture systems I've spent a good deal of time creating certain deficient conditions for particular cultivars so that I can witness firsthand what those look like. This has helped me immensely...Just as much as the "reading" has. By reading up on threshold amounts of plant essential element 'X' below which visual signs of deficiency begin to manifest for a particular thing you are growing, (assuming all else is in order) you can easily gauge where your system is at.

And this is OK...but really, by the time a deficiency makes itself manifest and you go about correcting it (some are not even correctable, causing 'permanent' damage to plants) you have already lost weeks and weeks of productivity. This is a really weak strategy for dealing with things. Knowing ahead of time, the nutrient strengths and weaknesses of your growing system (whether, aquaponics, pee-ponics, vermiponics, some other form of bio-ponics etc...) As well as the needs of the cultivar you are growing allows to to pre-emptively stay above minimum threshold levels. Without spending hundreds, or even thousands of dollars on sensors and equipment.

I've seen dozens and dozens of photos and posts here where people are having troubles with say cucumbers. "Gee, I have enough nitrates, and I'm adding Iron...but my leaves are still turning yellow between the veins"...and such. This is because cucurbits are total Mg hogs (they actually need more Mg than they do K! ...Which is quite rare for most common food crops. Intervenal chlorosis of OLDER leaves is always indicative to Mg def, intervenal chlorosis of NEW leaves is always Iron def...and so yes it is important to know what these defs look like as well as how they make themselves manifest. As well as knowing what to expect ahead of time. Or, "My toms are HUGE but wont fruit...or the fruit wont ripen (indicative of not enough K)...

We can take advantage of some of these cultivars and their specific needs like the "canary in the mine". For instance, I like to keep an oregano plant handy in a system because it will show signs of Fe deficiency much earlier than many other things I grow in that same system. So when I see a deficiency of Fe beginning to show in the oregano, I know damn well that I am approaching that systems minimum threshold target amount for Fe in solution and that it's time to add Fe. This is both very cheap and very easy. Saves me on wasting expensive Fe-chelates as well as spending money on iron titration tests or god forbid, in-line Fe sensors...etc...etc...etc...The same can be done (canaries in the mine) with some other plant essential anyone...for next to free. 

"Getting to these levels" of plant essential elements with salt compounds is still way cheaper than as opposed to through fish and fish food (and all the other things that fish would entail...oxygen, electricity time etc)...

This year I am finding that extracting/reclaiming some of plant essential elements from my own urine is proving to be cheapest of all...Reclaiming biological phosphates, calcium, potassium etc...through very simple and safe process'. It's very cheap, very sustainable, and quite a fun actually. Though I admit it might not be qualified as "easy" for someone without at least a basic understanding of simple organic chemistry pH, acid/base reactions blablabla...Not 'rocket science' difficult or anything mind you. Very low-tech/DIY is in only the head where you need to have the most "resources" to fall back on.

I'm trying to keep things, free and cheap and sustainable, but it will take a considerable amount of learning on the part of the operator to keeps things that way. Sorry for rambling off topic sorta...

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