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Coping with a Suspected Potassium Deficiency

I have an aquaponics install that is currently "fishless" in that I am in the final phases of getting it to cycle ammonia into nitrogen.  After transplanting a bunch of sickly plants started during cold weather, I was encouraged when even sickly bean plants recovered and grew thick, deep emerald green leaves.

Soon, though, I noted the same spotty yellowy mottling was coming back.  I thought this was fungus and following the advice of a local hydroponic store sprayed a $14.00 fruit smoothy fungicide all over.

This did not correct the problem.

My wife Gemey did some research and we dialed back the frequency of the flood/fill cycle from 15 minutes every hour to 15 every 90 minutes.  No improvement after several days.

More research yielded the theory that we are deficient in potassium.  Murry Hallam acknowledged that AQP systems would be potassium poor and that he supplemented his tanks.  I posted images of the plant leaves - this is affecting tomatoes, squash, beans and peppers.  The strawberries and herbs so far are doing OK.

I added 200ml of Sweet and Heavy "Bloom" fertilizer. (Earth Juice) which has a slug of potassium.  24 hours later, my wife thinks the plants look happier.

I observe that new leaves - a few days old, are wonderful and healthy looking.  It's older leaves that are curling and looking burnt.

I burned about half a bushel of hardwood (Oak and maple) twigs and added a cup of ash to a gallon of water and let it sit for 10 hours.  I strained the water after it settled into a spray bottle to make a potassium foliar spray.  I sprayed two plants that looked like they were about to go bad, and a plant that was otherwise healthy to see if this may act as an emergency fix to halt the problem, given that organic fertilizers take a few days to become available to plants.

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Comment by Tim McNabb on June 28, 2011 at 9:43pm
Update - Adding potassium-rich Sweet and Heavy and a shot of blackstrap molasses every few days has had its effect.  Halted the leaf curl and "burned edges".
Comment by Tim McNabb on June 15, 2011 at 4:11pm

Red Sea has a Potassium Test Kit - (link) - Check out the video on the site.  It works out to be about a dollar per test.

The Blackstrap Molasses I slugged into the system does not appear to have killed anything :), however the water now looks like cloudy beer.  I suspect it is for the same reasons - micro-herd is having a party.  I may in the future try daily doses of 50ML rather than a big slug.

Today Nitrites were non-existent, Nitrates were about 80PPM, Ammonia less than .05 PPM.

Comment by TCLynx on June 15, 2011 at 3:58pm
I recommend patience
Comment by TCLynx on June 15, 2011 at 3:57pm
Might be able to use a soil test kit but I don't know how accurate it would be nor how to correlate the reading to anything useful.
Comment by Robert B Winebrinner on June 15, 2011 at 3:56pm
Has anyone found a potassium home test kit?  I have had Ca and Mg test kits for my old coral aquariums, but I haven't found a way to test for K in the water??
Comment by Tim McNabb on June 14, 2011 at 8:21pm

I chickened out and added 200ML of unsulferated blackstrap molasses ("Slow As" brand) sporting 14% potassium.  It also has carbs which some of the micro-herd like and it did not have much to change PH or phosphorus.

The plants do not look any worse, and some of the leaves that were not sick are still not sick.  I am going to designate some "index plants" and mark the leaves somehow (maybe use a paper punch to make a hole) and monitor them day by day.  What I am doing is kind of random and I am not doing a good job of maintaining records.

Comment by Tim McNabb on June 13, 2011 at 5:32pm
I checked the ash-water PH, and it was in the 7-8 range.
Comment by TCLynx on June 13, 2011 at 5:14pm
Beware with the ash water that it can affect pH.
Comment by Tim McNabb on June 13, 2011 at 12:13pm

It's been 72 hours since slugging the system with Sweet and Heavy Bloom with its 4% potassium.  So far I am not seeing any new damage to leaves that are older and the new growth seems to be doing just fine.


I am going to try a shot of the equivalent of 100 ML of this formula with my ash-water.  I'll have to use over a liter, as the ash-water is only .375% potassium by volume.

Comment by Tim McNabb on June 12, 2011 at 8:05pm

I am using a seaweed extract from General Hydroponics called BioWeed - It sure does not seem to have much potassium.  I slugged the system with another 100ml of Sweet and Heavy Bloom which has a good deal of potassium by volume.  However, I wanted to avoid over-treating with nitrates since eventually the fish will be providing the raw material (pee and poo) for nitrogen.


I have calculated that my 1 cup hardwood ash/1 gallon water recipe is .375% potassium (K) by volume. Some chemicals can reach "supersaturation" and you simply cannot get more of the chemical into solution. I know that water soluble K can be as high as 4%, since the the bloom fertilizer is.  Therefore I suppose I can put more ash into the mix to get it to a higher volume of K into the solution.

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