Backyard Aquaponics
A place to discuss aquaponics

Hi everyone.
Have my system up and running but seem to have a deficiency that keeps effecting my plants even though I am supplimenting.

I have 100L system, and At the moment I have been adding 50g of sulphate of potash, and 7g of epsom salt once every 2 weeks
But deficiency continues to develop.

Ph fluctuates between 6.8 and 7.2, water temp is around 18 degrees c, and nitrates consistent around 10ppm

I think it must be either a magnesium or potassium deficiency, bit am having trouble figuring which, and don't want to dose the wrong one and cause even more lockout of the other.

Any help much appreciated.
Photos attached.












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Have you also been adding Calcium?... Too much Potassium and/or Magnesium can lock out Calcium, and while some of the plants do look like they may have a Potassium deficiency, some are also showing symptoms of calcium deficiency IMO.

Why have you been supplementing, what was the deciding factor in that, was it obvious deficiency symptoms or just as a matter of course?

Most plants, especially fruiting plants, will struggle to take up Calcium, Magnesium and Potassium at root zone temps below 20C, and no amount of supplementing will rectify the issue, but could cause other issues.

Greenhouse temp gets up to 35 degrees
Low as 5 during night.

Started supplimenting k as noticed intervenal chlorosis in older leaves, which went away, but then came back, so decided a regular dosing made sense. started also adding MG as wanted to keep these two balanced.

Have not been adding calcium to date.


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This photo shows how bad the intervenal chlorosis got on a broccoli plant before I pulled it out.


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There's multiple deficiencies & heat stress.
K is obvious & I think Nitrogen.You say 10ppm,but how many fish do you have and what are you feeding them?
Don't put several things to the root zone.Foliar spray.In the future I would foliar spray the epsom salt,but the K should be foliar sprayed now to get it to the plants faster.I don't know if you can foliar spray SoP .

Sulphate of potash & Magnesium sulphate are both salts,something to avoid really in an aquaponic system.

You also have some greedy heavy fruiting plants in there,I think Potassium is going to be your main concern.

And how are you measuring the Nitrates?

Measuring nitrates with an api testers kit.

Have 25 goldfish, looking to double that shortly. Currently Feeding them a floating fish pellet I got from the guy who sold me some of the fish who also does aqyaponics, unsure of the brand.

I think nitrates were a problem previously, but are up around 10ppm now that I have added more fish and switched to the new food. (About 2 weeks ago)

Foliar sprayed some epsom salts today.
Could follow with sulfate of potash tomorrow (Google says you can apply foliarly?)
Or alternatively I have some seasol I could apply foliarly instead if you think the salts will become a problem.

I agree that regular potassium additions will probably be necissary with the crops I've got in there. What's the best way to manage this without risking overdoing it and locking out other nutrients?


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It's possible the Nitrates are being locked out due to everything else being out of balance.
Foliar spray the Mag & only add the K to the grow bed in small amounts every week.And for the future it would be better to use
Potassium Bicarbonate rather than some sort of Sulphate.
Remember your water has Calcium in it.

I have ordered a full spectrum test, so will share those results here for future reference if people want.


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Hey everyone,

Got my test back, and here are the results.

What I'm seeing here is a need for more iron
And phosphorous, and maybe a bit of boron.

Very much welcome tho opinions of those more knowledgeable than me though,
And a list of target ranges for all the minerals broken down would be amazing if it exists.

pH pH Units
7.4
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-
-
-
pHc* pH Units
8.3
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-
-
-
Electrical Conductivity (EC) mS/m
225
-
-
-
-
Approx Total Dissolved Salts g/m3
1,510
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-
-
-
Sample Temperature* °C
20.0
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-
-
-
Sodium Absorption Ratio (SAR)* (mmol/L)0.5
14.1
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-
-
-
Adjusted Sodium Absorption Ratio*
15.3
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-
-
-
Cations
Total Calcium g/m3
46
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-
-
-
Total Copper g/m3
0.090
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-
-
-
Total Iron g/m3
0.92
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-
-
-
Total Magnesium g/m3
10.6
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-
-
-
Total Manganese g/m3
0.00081
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-
-
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Total Potassium g/m3
23
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-
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-
Total Sodium g/m3
410
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-
-
-
Total Zinc g/m3
0.40
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-
-
-
Sum of Cations meq/L
22
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-
-
-
Anions
Total Alkalinity g/m3 as CaCO3
35
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-
-
-
Hydroxide Alkalinity g/m3 as CaCO3
< 1.0
-
-
-
-
Carbonate g/m3 at 25°C
< 1.0
-
-
-
-
Bicarbonate g/m3 at 25°C
43
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-
-
-
Free Carbon Dioxide g/m3 at 25°C
2.7
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-
-
-
Chloride g/m3
590
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-
-
-
Sulphate g/m3
146
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-
-
-
Nitrate-N g/m3
2.8
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-
-
-
Nitrate g/m3
12.3
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-
-
-
Sum of Anions meq/L
20
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-
-
-
Langelier Saturation Index
-0.9
-
-
-
-
Total Boron g/m3
0.057
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-
-
-
Total Phosphorus g/m3
0.103
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I think iron and phosphorus deficiencies are consistent with your initial photos. The test results really won't tell you what is actually available to the plants although they do give you a good idea of what is there. For example; A pH of 7.4 may mean that the iron in the system is not completely available even though your test results show the total iron in the system and it may look adequate (of course you are seeing a shortage but I'm just giving a hypothetical example). Phosphorus may be locked up for other reasons ... and so on.

Looking back at the first pictures, you have necrotic tips on older leaves (a sign of potassium deficiency) and yellowing in some of the younger leaves (a sign of iron deficiency, usually the interveinal chlorosis is more obvious than in this case). Both of these are very common in Aquaponics and pretty much what you'd expect. You probably didn't need to add Epsom Salts since Magnesium deficiency doesn't show up that often. Adding it didn't hurt anything and might have done some good but it doesn't need to be a regular addition for the vast majority of people doing AP.

You might have had a nitrogen deficiency based on some of the early photos but hopefully with the new feed you've taken care of that