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One of the troubles I have had with my aquaponics tank has been nitrogen buildup in the form of nitrates.   Plants need different nutrients to grow.  Limit one nutrient, and that limits the absorption of all of the others.  I guessed that the limiting nutrient might be either potassium, iron, or magnesium, not sure which would make the difference.  I am still not completely sure, but when I dosed with the 3, the plants went into a growth spurt.  It was like putting a green light on nitrogen use, and the nitrogen in the water dropped.  I saw this this morning, and I was very pleased. I think I will try to find a potassium test kit.  My new Nitrate level is 100ppm and has been cut in half, and I am feeding the fish what they will eat.


Another thing I found was a hydroponic model for tomato growth from University of Ohio.  I found a couple things I might be doing that is counterproductive.  For one, in the fruiting stage, soil temperature (read that tank temperature) should be around 72, not 78 like I had it, and pH should be around 6 and not 7.


Now these are real numbers with real reasons behind them.  So, last night with great sadness, I lowered the temp on my heating elements.  Even more sadly, my power use will go down.  The pain.  I can also relax and let nature take its course with pH.  My buffer costs will go down.


Will all this make a difference?  University of Ohio thinks so.  These guys probably have done scientific tests to back up their claims.


I am a retired engineer.  I like claims that have substance behind them, but I will also jump into something like Aquaponics knowing nothing about it - Frantic learning ensues.  Angst at not knowing rules my life, blood pumps, web searches happen, and finally - almost as an afterthought, the nitrogen level normalizes.




Now what?

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It's just as well we dont have a buzzer/siren like Stephen Fry in the QI program.... it would be going off so many times in relation to your post Marc... that we'd need ear plugs...

You need to be careful about "cause and effect"...


Firstly, the mechanism of ammonium and nitrate uptake/transportation in plants is well documented... and the same... regardless of what medium or method is used for plant production...

Early germination/root growth is primarily by direct absobtion of ammonium through the roots... later leaf growth and subsequent plant growth is via nitrate tranportation to the leaves and photosynthesis..

Bacteria do not, in themselves, affect pH.... (pH however can, and does directly affect bacterial growth)... the chemical processes that result from bacterial action are what affects pH...

And nitrification, by the very nature of the chemical processes... always trends towards acidicy.. I'm sorry, but the chemistry is of the nitrogen cycle is what it is...


The only reason that you see your pH "maintaining at around 7-8" ... is due to the carbonate hardness of your source water.... not your bacteria....

And the pH will stay that way, regardless of the bacterial nitrification pH trend downwards... until (unless intervened)... the carbonate buffering is depleted/exhausted...


Certainly, microbial active... enzyme production... has a direct affect on nutrient uptake... essentially naturally "chelating" and making "motile"...minerals to make them available to plants that might otherwise have a pH range that would inhibit the uptake of essential minerals...

But optimal uptake of minerals, especially trace elements.. occurs at lower pH ranges.. such as employed in hydroponics... and many mature aquaponics systems...

The "way plants feed in water"... is the same, by mechanism... but the efficiency of mineral uptakes is directly related to pH....

If your pH is not optimal... then all the plant, and microbial/enzymatic processes that a plant employs for survival... will still not result in optimal mineral uptake and growth...


LECA, hydroton, expande clay... is pH neutral... and inert.. it will not affect your pH...

Thank you, I was going to put correlation does not equal causation in caps.

RupertofOZ said:

It's just as well we dont have a buzzer/siren like Stephen Fry in the QI program.... it would be going off so many times in relation to your post Marc... that we'd need ear plugs...

It is possible that with the initial introduction of some leca or expanded clay product you might see some pH shift due to the dust on that particular product but in general once well rinsed those products should not affect pH long term.

There are pH variations from one expanded shale/slate product to another but again that is generally due to the dust on the product when you first get it and should settle down later with good rinsing before use.

And I have to agree with those that say the bacteria don't keep the pH up.  The chemical workings of the bacteria in our bio-filters will actually cause the pH to drop over time if there is no buffering present.

That buffering (usually calcium carbonate from hard source water) is something I've gained much experience with.  I'm not trying to collect more rain water to help me balance things out better.

From a few pages ago on the nitrogen cycle, pH and temperature both have an effect on NH3 and NH4 levels.  In both cases the higher the value the more concentration of NH3 will result.  A

Nate in our cold water system, vegetative plants are using far more NH4 than NO3.  That being said our NH4 level has been running .3mg/L in inffluent water.  Our pH and temps are low, so almost all of it (99%) is in NH4.  We also have low amounts of nitrification going on since we are flow through.  Our nitrates are .24ish mg/l.  We also do not have the ability (cost reasons) to amend.

Here's a fun bit of info:

1 gram of TAN yields 4.42 gram nitrate, and 5.93g carbon dioxide.  CO2 mixed with water= carbonic acid.  Alkalinity buffers against this pH drop.  However ever gram of TAN that is converted to NO3 consumes 7.14 grams of alkalinity along the way.  So long term you get the pH drop because your buffer is consumed, and the byproduct is acidic.  And that's just in the nitrification part of the system.  Remember fish also produce CO2, as do the microbes that are reducing the suspended solids.  The whole system is aimed at getting to a lower pH.

Rupert I await for fact checking.  Or

maybe you should add an algae scrubber + vermiponics this would allow you to control the Nitrogen problems also with vermiponics potassium is never a problem... the worms are cheap and very 'hardy' to look after...

No buzzer on the TAN figures Matthew....


But while young seedlings principally use ammonium during initial root development stage... subsequent leaf development and plant growth is via nitrate uptake and transportation to the leaves for photosynthesis...


With you "flow through" system... and your nitrates basically equivalent to your ammonium...


I think your plants are still principally uptaking the (constantly refreshed) base level of nitrates for growth... rather than the ammonium, which while, as with the nitrates, is always present... is always being flushed through the system...

Hi Stephen,

I would like you  to tell us what kind of vegetables you are growing and what is the growing stage at the time of your experiment? how about the results of adding potassium to the growing system? and what is the the final nitrate level you have tested?


I don't know if I would call worms cheap when people will pay only a few dollars for a pound of fish and they will pay $25-$35 for a pound of worms.

I found this interesting study regarding low pH and nitrification. It certainly explains why some low pH AP systems nitrify. 

Key Points

  • The biofilm must first be established before letting the pH drop naturally
  • Adequate dissolved oxygen levels need to be maintained: Note this study used gaseous oxygen to maintain a DO of 8mg/L. There is no discussion that the high DO could have influenced the outcome of the test. Normally a DO of 2mg/L is the minimum required by aerobic bacteria.
  • The alkalinity concentration needs to be high enough to provide an inorganic carbon source for the bacteria. 


High-Rate Nitrification at Low pH in Suspended- and Attached-Biomass Reactors

Sheldon Tarre and Michal Green*
Faculty of Civil and Environmental Engineering, Technion, Haifa, Israel
*Corresponding author. Mailing address: Faculty of Civil and Environmental Engineering, Technion, Haifa 32000, Israel. Phone: 972-4-829-3479. Fax: 972-4-829-5696.
Received May 11, 2004; Accepted July 4, 2004.

This article reports on high-rate nitrification at low pH in biofilm and suspended-biomass reactors by known chemolithotrophic bacteria. In the biofilm reactor, at low pH (4.3 ± 0.1) and low bulk ammonium concentrations (9.3 ± 3.3 mg · liter−1), a very high nitrification rate of 5.6 g of N oxidized · liter−1 · day−1 was achieved. The specific nitrification rate (0.55 g of N · g of biomass−1 · day−1) was similar to values reported for nitrifying reactors at optimal pH. In the suspended-biomass reactor, the average pH was significantly lower than that in the biofilm reactor (pH 3.8 ± 0.3), and values as low as pH 3.2 were found. In addition, measurements in the suspended-biomass reactor, using isotope-labeled ammonium (15N), showed that in spite of the very low pH, biomass growth occurred with a yield of 0.1 g of biomass · g of N oxidized−1. Fluorescence in situ hybridization using existing rRNA-targeted oligonucleotide probes showed that the nitrifying bacteria were from the monophyletic genusNitrosomonas, suggesting that autotrophic nitrification at low pH is more widespread than previously thought. The results presented in this paper clearly show that autotrophic nitrifying bacteria have the ability to nitrify at a high rate at low pH and in the presence of only a negligible free ammonia concentration, suggesting the presence of an efficient ammonium uptake system and the means to cope with low pH.

What does BSA stand for?

Nate Storey said:

Vlad, just saw that last part there- you're also correct when you say that this isn't the ideal for starting out!  You want to work into low pH systems slowly and only when you have really really high system BSA.

Hi Bob...I think by BSA, Nate meant Biological Surface Area...

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