The Nitrogen response

[Stephen Eugene Robinson]

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.

 

Sigh

 

Now what?

[Vlad Jovanovic]

The thing is, no two systems (and no two operators) are exactly alike. Period. The water you use, the food you feed your fish, flow rates, the different microbes in these systems (and not just nitrobacter and nitrosomona, come to think of it, even the particular strains N.bacter and N.somona that you happen to have), the amount of oxygen, your height above sea level, the difference in you night time and day time temperatures, stocking densities, amount of algae present, what you want to get out of your system...the list goes on and on...

Yes, to me personally a pH of 7 or 6.8 would make me rather happy. Tomatoes do like a lower pH, and there are a ton of reasons why tomatoes wont blossom or set fruit. Light/dark cycles, not enough difference between night time and day time temps the list goes on, much of which is pH unrelated...  

I am a big believer in not trying to fight  the particulars of a given AP system, let that particular system show you where its most happy at. If you can keep a pH in the mid to upper 6 range, I think you'll be ridin' high (as far as nutrient lock out, plant productivity etc)...Also, it would seem that an AP system needs some time to mature. six months to a year is not that long for a system to sort of stabilize, build up nutrients, build up a web of different organisms, (a lot of which we can't see) etc...

Now I love to micro manage my hydro systems. Boy oh boy don't get me started. Sterilizing everything after each harvest, neurotically checking everything, adding, balancing re-balancing, but I just dont look at an AP system in the same way. Its more of a  natural system that I would like to guide along every now and then, intervening when necessary. Creating a mini eco-system.

There are people on this forum that cannot get their pH below 7.6-7.4 no matter what they do, then you have the gentleman you mentioned who swears he runs his at 5.5. For someone who has super hard water and a buffering media, a pH 5.5 (or even 6.8) would be ridiculous to shoot for. And if you bacteria are ticking away and you system seems fine at 5.5 (though this seems a bit difficult to me) and your fish seem alright, then it would be equally ludicrous to constantly buffer up to 7.6 and try to hold it there, would it not? There are folks who don't add any  supplements (except buffering), then there are folks who add all sorts of stuff, from worm castings to a host of nutrients. Some folks wont even think about heavy feeders like tomatoes being able to do well in the first few months after cycling, while others want and expect round red tomatoes right away...It really depends...hence some of the 'conflicting' opinions. 

It is probably prudent and wise that you intervene when your pH goes down to around 6, (and no, I think tomatoes would do great at an pH of under 6 in an AP system, keeping the rest of the system happy there might be a bit of work) since you like to tinker, you could buffer more often but not buffer so much. It's your system. Or if you are worried about Potassium already, buffer with Potassium Bi-Carbonate half the time and Calcium Hydroxide the other half. At that pH you would hardly have any lock-out problems to worry about. Or heck try to run your system at 5.5 and let us know how you do it. Depending on you and your particular system, it could well work out. Though I could not in good conscience advise you to do so. Still, you shouldn't let me or anyone else for that matter keep you from trying it if that is something you want to do. 

I think if you look at the "Rules of Thumb" section on this site you will find your generally accepted definitions of "good" parameters for all those thing that you mentioned.

[RupertofOZ]

Just to clarify again... if I'm the "gentleman" you're referring to Vlad...

I don't deliberatley run my systems at pH 5.5 ... I suggested that was related to recent hydroponics research...

I do however consistantly run between 6.0 - 6.6 ... and may dip below 6.0 at times before I buffer back...

 

The suggestion of alternating your buffer between Potassium and Calcium buffers is certainly wise and advisable... as too much Potassium can inhibit Calcium uptake, and even Nitrates (to a much lesser degree)...

The later, and the need for application of a Calcium buffer... apart from benefit to plant and fish . is a requirement of the nitrification process...

[Vlad Jovanovic]

..."In short, it appears people just keep their fingers crossed, not sure of what works and what does not.  This may be the state of this art"....

I just wanted to address this since its kind of a bummer that you feel that way. I think if you read a lot of the older threads from people who've been at this a while you will see patterns. Particular conditions causing particular problems and so forth as well as a well an established history of what is generally "good". Here, and on the Aussie forums as well. I more get the impression that if you respect certain basic guidelines or principals most folks get the same results. You will also see a lot of people making the same mistakes (usually because they are acting on the same mistaken presumptions or they are impatient, i.e lowering your pH to at or below neutral while cycling is a common one. And I can understand why. You here somewhere that 7 is a good pH for an AP system, so you latch onto that number without understanding the difference in what goes on in a running system as opposed to a cycling system... Why would you bother since the bacteria don't mind that your well water is 7.8 or whatever. Your actually better off with higher pH during cycling (the bacteria dig it). And since the nitrification process is going to bring it down all by itself anyway, why fight that needless battle? Patience and time is all that was required...Even though there are probably already a hundred posts addressing this very thing, people still seem to do this..?

Anyways...Anything that could possibly come up, in all likelihood already has. Use the "Search" function on this site. You'll see that anything you can think of has probably already been addressed, much of which in great detail by experienced and knowledgeable people. And you'll probably see certain patterns emerge of what is generally "good". It's really not as chaotic as it appears. People get away with different things, with different systems, it is good to strive to recognize the how's and why's...

Stephen Eugene Robinson said:

OK Great.  I take from you you recommend pH 7.  You hear different things from different people on this forum, and that's a fact.  It is not possible to know for certain anything in this environment, and to be sure, I don't, but I do look for answers, and when I believe I see a cause and effect, I try to latch onto it.  One fella on this forum swears he runs at 5.5 and occasionally buffers back up, and thinks I'm nuts for trying to control pH.  I have heard this more than once. 

 

In short, it appears people just keep their fingers crossed, not sure of what works and what does not.  This may be the state of this art. 

 

In my experience, I have seen pH trend downward to around 6.  I don't know where it would go after this since I have intervened.  Tomatos seem to like a lower pH, and fish seem not to mind.  Wherein is the difficulty?  Is it just the dirty word Hydroponics?  The only thing the Univ. of Ohio is saying is that tomatos like that pH.  Is it possible that they would like a pH of 6 under hydroponics, and do poorly with it under aquaponics?  Or maybe it is the bio filter.  I didn't think the bio filter did poorly at this pH level.  Maybe I should double check.

 

Maybe what is needed on this forum is to attempt to define what "good" means.  Good would mean pH, Nitrogen level, Potassium level, and maybe a few other variables.  If we could come up with what good was, then some of these comments might actually be useful.

 

So, I propose that you folks who think you have a well running Aquaponic system, what parameters do you think are useful, and what are your current values for those parameters?  If we were to answer these questions, it might just constitute the closest thing to guidance that can be given to someone starting out.

 

This foum then might actually develop some value beyond just air movement.

 

What say?

 

I would be happy to help coordinate and collect any information offered and synthesize a result.
 
Vlad Jovanovic said:

Yes, but the University of Ohio has a hydroponic research facility not an aquaponic one. 'Dems a little like the proverbial apples and oranges. In hydro, you only have the plants to cater to, so things like a pH of 5.5 or 6 pose no problem whatsoever. In aquaponics however...

Though you can operate your system at a pH of 6, I don't know that you'd want to. It seems like you'd have to have a really keen grasp on the workings of your system and be super vigilant with other aspects of water quality testing and control. So as not to end up crashing your bacterial colony, for instance. In which case what the University of Ohio says wont make a lick of difference to you. 

The different phases of plant growth, including growth spurts, are a well documented phenomenon. Are you sure that you weren't maybe witnessing this? If your plants are going through an explosive growth phase, (and there a few in most of their life cycles) this would explain your drop in nitrates. At a pH of 7 you shouldn't be having too much trouble with iron/calcium, magnesium lock-out.

Potassium is generally a concern during the fruiting stages. Again, in hydro you can limit nitrogen (as well as phosphorous) after flowering and bump up the potassium. But how do you go about limiting nitrogen in aquaponics? People grow great looking tomatoes with aquaponics under parameters that would make a hydro-researcher scream "that is not possible".

There seem to be a lot of things in aquaponics that defy conventional hydroponic wisdom. Many of these things have yet to be studied and explained scientifically (probably because there is as of yet, no aquaponic "industry" to finance such serious studies). Hopefully someday we'll know  exactly why something that just wont fly in hydroponics, works great in AP. 

Seriously, before you contemplate running your system at a pH of 6, ask around, it seems like anything but a "relaxing letting nature take its course" sort of deal. I believe Nate Storey successfully runs his system at a low (6.2 - 6.7) pH and it doesn't appear to be a laidback option for someone just starting out. 

[RupertofOZ]

Vlad Jovanovic said:

..."In short, it appears people just keep their fingers crossed, not sure of what works and what does not.  This may be the state of this art"....

I just wanted to address this since its kind of a bummer that you feel that way. I think if you read a lot of the older threads from people who've been at this a while you will see patterns. Particular conditions causing particular problems and so forth as well as a well an established history of what is generally "good". Here, and on the Aussie forums as well. I more get the impression that if you respect certain basic guidelines or principals most folks get the same results. You will also see a lot of people making the same mistakes (usually because they are acting on the same mistaken presumptions or they are impatient, i.e lowering your pH to at or below neutral while cycling is a common one. And I can understand why. You here somewhere that 7 is a good pH for an AP system, so you latch onto that number without understanding the difference in what goes on in a running system as opposed to a cycling system... Why would you bother since the bacteria don't mind that your well water is 7.8 or whatever. Your actually better off with higher pH during cycling (the bacteria dig it). And since the nitrification process is going to bring it down all by itself anyway, why fight that needless battle? Patience and time is all that was required...Even though there are probably already a hundred posts addressing this very thing, people still seem to do this..?

Anyways...Anything that could possibly come up, in all likelihood already has. Use the "Search" function on this site. You'll see that anything you can think of has probably already been addressed, much of which in great detail by experienced and knowledgeable people. And you'll probably see certain patterns emerge of what is generally "good". It's really not as chaotic as it appears. People get away with different things, with different systems, it is good to strive to recognize the how's and why's...

Amen to that Vald... PATIENCE... especially during cycling... and the first few months...

While I have advocated a certain pH range for optimal performance... and even suggested  means to counter initial problems with systems with high hardness...

I sommetimes rue the day that I did so.... as it often seems to be seized upon within a mindset of continual intervention in peoples systems... without the background knowledge of the processes involved.. and why and/or when any interventions might be desirable...

And yes, please use the "search" function... and research, particularly older forums... the questions have been asked... and answered...

 

[Vlad Jovanovic]

Nope... see Stephens post. I was just 'referring' to his 'referral' whomever that may be...

RupertofOZ said:

Just to clarify again... if I'm the "gentleman" you're referring to Vlad...

I don't deliberatley run my systems at pH 5.5 ... I suggested that was related to recent hydroponics research...

I do however consistantly run between 6.0 - 6.6 ... and may dip below 6.0 at times before I buffer back...

 

The suggestion of alternating your buffer between Potassium and Calcium buffers is certainly wise and advisable... as too much Potassium can inhibit Calcium uptake, and even Nitrates (to a much lesser degree)...

The later, and the need for application of a Calcium buffer... apart from benefit to plant and fish . is a requirement of the nitrification process...

[Stephen Eugene Robinson]

Thanks for the input.  I'll try the search.  Also thanks for the patience.  It is a thing I fight with from time to time.  My system has been going since March, and I have had consistent problems with high nitrates.  Today my problems went away for the very first time.  Needless to say I was happy, and was of the belief that I understood why that was.  The Bernstein book definitely recommends 6.8-7.0 to support all of the living things in the system.  Again, thanks for all the help.  I may just filter thru this form and try to come up with some kind of standard approach anyway just for grins.

 

Steve
 
Vlad Jovanovic said:

The thing is, no two systems (and no two operators) are exactly alike. Period. The water you use, the food you feed your fish, flow rates, the different microbes in these systems (and not just nitrobacter and nitrosomona, come to think of it, even the particular strains N.bacter and N.somona that you happen to have), the amount of oxygen, your height above sea level, the difference in you night time and day time temperatures, stocking densities, amount of algae present, what you want to get out of your system...the list goes on and on...

Yes, to me personally a pH of 7 or 6.8 would make me rather happy. Tomatoes do like a lower pH, and there are a ton of reasons why tomatoes wont blossom or set fruit. Light/dark cycles, not enough difference between night time and day time temps the list goes on, much of which is pH unrelated...  

I am a big believer in not trying to fight  the particulars of a given AP system, let that particular system show you where its most happy at. If you can keep a pH in the mid to upper 6 range, I think you'll be ridin' high (as far as nutrient lock out, plant productivity etc)...Also, it would seem that an AP system needs some time to mature. six months to a year is not that long for a system to sort of stabilize, build up nutrients, build up a web of different organisms, (a lot of which we can't see) etc...

Now I love to micro manage my hydro systems. Boy oh boy don't get me started. Sterilizing everything after each harvest, neurotically checking everything, adding, balancing re-balancing, but I just dont look at an AP system in the same way. Its more of a  natural system that I would like to guide along every now and then, intervening when necessary. Creating a mini eco-system.

There are people on this forum that cannot get their pH below 7.6-7.4 no matter what they do, then you have the gentleman you mentioned who swears he runs his at 5.5. For someone who has super hard water and a buffering media, a pH 5.5 (or even 6.8) would be ridiculous to shoot for. And if you bacteria are ticking away and you system seems fine at 5.5 (though this seems a bit difficult to me) and your fish seem alright, then it would be equally ludicrous to constantly buffer up to 7.6 and try to hold it there, would it not? There are folks who don't add any  supplements (except buffering), then there are folks who add all sorts of stuff, from worm castings to a host of nutrients. Some folks wont even think about heavy feeders like tomatoes being able to do well in the first few months after cycling, while others want and expect round red tomatoes right away...It really depends...hence some of the 'conflicting' opinions. 

It is probably prudent and wise that you intervene when your pH goes down to around 6, (and no, I think tomatoes would do great at an pH of under 6 in an AP system, keeping the rest of the system happy there might be a bit of work) since you like to tinker, you could buffer more often but not buffer so much. It's your system. Or if you are worried about Potassium already, buffer with Potassium Bi-Carbonate half the time and Calcium Hydroxide the other half. At that pH you would hardly have any lock-out problems to worry about. Or heck try to run your system at 5.5 and let us know how you do it. Depending on you and your particular system, it could well work out. Though I could not in good conscience advise you to do so. Still, you shouldn't let me or anyone else for that matter keep you from trying it if that is something you want to do. 

I think if you look at the "Rules of Thumb" section on this site you will find your generally accepted definitions of "good" parameters for all those thing that you mentioned.

[Nate Storey]

you guys are all right- if you're starting and topping off with hard water, forget about any kind of pH control.  from a hydro perspective, lower is better so if your fish don't mind and your nitrification is still cranking, why not? anyone using hard water however can't do this- i talk to so many people who are using acids to lower pH without adressing the carbonates problems in thier systems- or folks who add carbonates and then try to lower pH.  . .

[Vlad Jovanovic]

That's really good to know Nate. I had no idea that such a deficiency could cause such a dramatic difference in nitrate uptake. Is this true with non-fruiting/short term plants like lettuce as well? Are there any set of conditions where this phenomenon becomes more pronounced than others? I have relatively hard water and I'll have 18,000 liters of it, so I probably wont be buffering my system with KHCO3 soon after cycling. I had hoped to use home-brewed worm tea as a foliar spray and thought that that if I use KHCO3 in that way (foliar spary early on) as well, it could be detrimental to the "living web" of beneficial organism that worm tea seems to produce on leaves. Do you feel that this is a safe assumption?

 

My other question if i may is, what are your thoughts on adding chelated (EDTA) micro nutrients (fish safeness)? The product I have access to is:

 Fe 0.76%

Mn 3.48%

Mo 0.485%

Zn 1.02%

Cu 0.76% (this is the one that worries me most, is this a valid concern at this concentration)

I have a separate EDDHA Fe 6% for iron that I was hoping to alternate between with the above product, unless the above product is a no go... 

I would really appreciate your thoughts on all these things.

Nate Storey said:

Vlad, Stephen is right- nitrates can become limited, not in solution but in plant uptake and metabolism if there is another major limiting plant nutrient (K is most common when hydrated lime is being used to raise pH).  You can impact uptake rates by supplementing scarce or absent nutrients- so what Stephen saw in his nitrate drop was probably a real result of nutrient supplementation of an absent or scarce nutrient.  Now the trick Stephen, is to figure out what nutrient your system needs.  What do you raise pH with?  If it's hydrated lime then I guarantee it's most likely a solution K deficiency.  If you switch to KOH for pH moderation, you'll see a bump in nitrate consumption.  Ca and K compete, in solution so most likely your K is precipitating out.  Have you noticed any plant nutrient deficiency symptoms?  Another one might be Mg.  I would encourage the occasional supp. of epsom salts-magnesium sulfate as well.  

I run two 2000 gallon systems consistently between the high fives and about 6.4.  Why? Because I have massive enough BSA that my nitrification rates are still acceptable.  My systems run consistently between 0.5 and 1.0 ppm ammonia as a result, but that's not enough to impact fish health, and definitely maximizes my plant nutrient uptake.  It lets me push my plants on pretty rigorous schedules and turn out top-quality stuff.  Lower pHs will always perform a little better from a plant production perspective, and if your fish are adjusted it usually isn't a problem!

[Nate Storey]

Hi Vlad, you have the right idea.  foliar sprays are really good ways to get around pH limitation by carbonates.  The ideal scenario is RO removal of carbonates- but that isn't always realistic.  Different plants have different nutrient requirements in regards to micro, secondary and primary plant nutrients, so you won't always see a lag in nitrate uptake with some crops while others are experiencing the bottleneck, so it's good to keep track and always be watching for the slightest signs of deficiency.  The whole high pH issue can complicate things because relatively small swings can limit certain nutrients that may already be scarce in solution.  It can be a bit complicated so the trick is to control for as many variables as possible and then treat each deficiency in turn.  Using a foliar spray typically gets around this but locks you into a long-term treatment regime as you aren't really treating the problems in-solution.  

Scenarios where this becomes really pronounced are when there's an increase in algal production, when there is a seasonal temperature drop or increase and when pH runs out of the typical system range.  Basically, when system chemistry is off (not outside of "safe" bounds but outside of "normal" bounds)- nutrient issues can become exaggerated.

As for chelated micro nutrients- they work great but are usually unnecessary.  The biggies in AP (because of low conc.s in feed) are Fe, Mg, Zn, and Ca/K (depending on supp.s).  Most systems have some exposed galvanized steel and Zn is really soluble so Zn is usually not a problem (actually, looking at this product, the amount of Zn might actually be a problem for your fish over time- the plants won't take up that much Zn and it's really really toxic to fish), Mg deficiencies will often show up but don't impact production as much as aesthetic most of the time because they're low level deficiencies, so the Ca/K and Fe deficiencies are the real worries.  Chel. Fe is a required addition to almost all AP systems, Chel. Cu- I'm not so sure.  I've never seen an actual Cu deficiency in an AP system or encountered one that I though was a Cu deficiency- It's actually something that you don't really want making it's way into your fish either.  Mn can be deficient, but I think it's rare, and Mo is almost never deficient in AP systems- there's more than enough in fish feed. . .  I think that staying away from copper is a good idea.  I would stick with the Chel. Fe and treat in very low quantities with that particular product very rarely if ever- perhaps save it in case a weird deficiency shows up.   To be fair, I tend to stay away from most micronutrient supp.s because they're expensive and not (in my experience) necessary.  Most commercial fish feeds have more than enough of these micronutrients to suffice and plants require them in extremely low quantities (i.e. there's enough Mo in a healthy seed for many crops that it can often germinate and go though a substantial part of it's life in a Mo absent environment without exhibiting deficiency- just living off of the Mo in the seed from it's mother plant.)   That's not to say this stuff would hurt your system (except Cu buildup-and it would over time- that amount of Cu is a bit excessive if you ask me. . . and probable the Zn too), that's just to say it's unnecessary.

Sorry about the novel.

Nate

Vlad Jovanovic said:

That's really good to know Nate. I had no idea that such a deficiency could cause such a dramatic difference in nitrate uptake. Is this true with non-fruiting/short term plants like lettuce as well? Are there any set of conditions where this phenomenon becomes more pronounced than others? I have relatively hard water and I'll have 18,000 liters of it, so I probably wont be buffering my system with KHCO3 soon after cycling. I had hoped to use home-brewed worm tea as a foliar spray and thought that that if I use KHCO3 in that way (foliar spary early on) as well, it could be detrimental to the "living web" of beneficial organism that worm tea seems to produce on leaves. Do you feel that this is a safe assumption?

 

My other question if i may is, what are your thoughts on adding chelated (EDTA) micro nutrients (fish safeness)? The product I have access to is:

 Fe 0.76%

Mn 3.48%

Mo 0.485%

Zn 1.02%

Cu 0.76% (this is the one that worries me most, is this a valid concern at this concentration)

I have a separate EDDHA Fe 6% for iron that I was hoping to alternate between with the above product, unless the above product is a no go... 

I would really appreciate your thoughts on all these things.

Nate Storey said:

Vlad, Stephen is right- nitrates can become limited, not in solution but in plant uptake and metabolism if there is another major limiting plant nutrient (K is most common when hydrated lime is being used to raise pH).  You can impact uptake rates by supplementing scarce or absent nutrients- so what Stephen saw in his nitrate drop was probably a real result of nutrient supplementation of an absent or scarce nutrient.  Now the trick Stephen, is to figure out what nutrient your system needs.  What do you raise pH with?  If it's hydrated lime then I guarantee it's most likely a solution K deficiency.  If you switch to KOH for pH moderation, you'll see a bump in nitrate consumption.  Ca and K compete, in solution so most likely your K is precipitating out.  Have you noticed any plant nutrient deficiency symptoms?  Another one might be Mg.  I would encourage the occasional supp. of epsom salts-magnesium sulfate as well.  

I run two 2000 gallon systems consistently between the high fives and about 6.4.  Why? Because I have massive enough BSA that my nitrification rates are still acceptable.  My systems run consistently between 0.5 and 1.0 ppm ammonia as a result, but that's not enough to impact fish health, and definitely maximizes my plant nutrient uptake.  It lets me push my plants on pretty rigorous schedules and turn out top-quality stuff.  Lower pHs will always perform a little better from a plant production perspective, and if your fish are adjusted it usually isn't a problem!

[Vlad Jovanovic]

Thanks Nate. I appreciate the input.

I sorta thought I might be in for some odd deficiency problems as I can't seem to find any fish feed that is formulated for RAS. And none of the distributors/importers (Purina included) here are interested in special importing anything in remotely sane quantities. Hence the micro nutes.

So as far as deficiencies go, it sounds like it's still the "Big 3". Fe/K/Ca. (Provided of course you use a decent feed). 

[TCLynx]

A little seaweed or even sea salt in one form or another should be able to take care of most deficiencies other than the Fe, K and Ca.  And if you have hard water, Ca is likely provided by your water so you will just have to figure out how to provide enough K or keep your Ca and hardness low enough that you are not locking out all the K.  And sounds like you have the Fe product.  I'm trying hard to collect enough rain water that I can top up with it enough to keep my hardness and Ca levels low enough to balance the K using either potassium bicarbonate if the pH is low or just seaweed extract when the pH is not low.

[Terri Mikkola]

A few thoughts on Nitrification

TAN is the measurement of both ammonia (NH3) and ammonium (NH4). Which form is present depends on the pH. High pH = NH3 / Low pH = NH4

Nitrification requires NH3 to be present as a food source for Ammonia Oxidizing Bacteria [AOB] (common genera - nitrobactor & nitropira) At a pH of <= 6.5 AOB begin to starve and conversion to nitrite begins to cease.

Both AOB and Nitrite Oxidizing Bacteria [NOB] (common genera - nitrobactor & nitrospira) require a chemical form of carbon (not organic). CO2 (carbon dioxide) and CaCO3 (calcium carbonate a.ka. alkalinity) are common sources of carbon. Low CO2 and/or alkalinity = low carbon availability = death. In addition, the nitrification process itself is acidic and will consume alkalinity.

Note, there is a complex relationship between CO2, carbonic acid, and alkalinity in water, which I won't go into with this post.

Nitrification is an aerobic process (requires dissolved oxygen). For every lb of NH3-N converted to NO3 it takes 4.3 lbs of oxygen.

These principles are standard for wastewater treatment and aquaculture and are the product of hundreds of studies. So why do we see some AP systems with low pH and alkalinity levels produce adequate levels of nitrogen for the plants?

I have a few ideas - not based on research

The majority of nitrifcation studies were performed with waste streams that had higher ammonia concentrations than what would typically be found in low density aquaponic systems. It could be that Nitrospira or another type is more prevalent than Nitrosomonas in LD AP systems because it can thrive with low levels of NH3 and NO2 as would be seen in an acidic environment. Secondly it could be that the NH4/ NO3 ratio is high due to the acidic environment and the plants are uptaking more ammonium than nitrate.