Re:pH question

[Thomas Barr <tcbiii at earthlink_net>]

Bill pondered:
 
> It was recently posted that the fish and the plants didn't care what the pH
> was.
> 
> OK.

Well, within a range (say 5.8-7.4 or so) folks might be tempted to take this
too far. I sure did on many occasions:)
 
> But in a moderately well-planted tank (about 1.5 growing rooted plants per
> gallon - a new ratio?) with a fish load of about one inch per gallon, 2 watts
> per gallon of light, and no injected CO2, wouldn't the tank work better if
> the pH was on the acid side?  I'm thinking that the ammonia - ammonium
> conversion, which takes place in an acid environment, would make a
> difference.  Right?

Sure. 
BUT non CO2 enriched planted tank with low KH's are often only "acid" at
best for around 12 hours. The other half of the day, the pH is 8-9.
Measure your pH in the morning right before the lights come on, then measure
it right as they go off at night.
I tested two vernal pools that went from 5.9 in the morning to 9.8 in
evening. The KH was 12ppm.
At that pH(9.8) a fair amount of NH4 will be NH3 but generally at the pH's
dealt with using CO2 gas, this is insignificant in both Salt and Freshwater.
Perhaps only 3-6 hours are above 9 generally.  A couple of things can
happen: a plant can shut down and wait till things are better for a few
hours. Many plants/algae dispose of H+ ions that can be used to convert the
NH3 to NH4 and be taken in.

> And in a related question, is the fact that nitrates often have to be
> supplemented in a tank with growing plants due in part to the fact that
> "nitrogen cycle" is bypassed in an acid environment, with the ammonia
> converting to ammonium and being taken up directly by the plants rather than
> being converted to nitrites and nitrates?

No. We have to add the KNO3 etc due to the issue of the supply that the
plants need for growth. There is simply no way to add a bunch of NH4 or NH3
in one single daily dose without killing all the fish etc and also causing
the algae to bloom like mad.

Other wise we could just keep adding fish till we had enough N.
Problem is you get a lot of algae long before NH4/NH3 will kill your fish.
When adding NH4 first you get algae then if you keep adding more you'll get
dead fish.

This is why algae make good bioindicators of aquatic ecosystem health rather
than fish. They tell what and how much of a nutrient is entering a system.
Plants cannot do this or have new species emerge due to nutrient response
except very slowly, but algae are very fast to respond.

NO3 is much more benign and useful for adding for increasing plant growth
without causing algae. Plants need up 2-5ppm of N a day. NO3 can be added
for a few days worth of N, cannot do that with NH4 except perhaps via a
controlled dosing pump if underdosed and fish/critter waste. Anything goes
wrong and the algae will appear.

As far as the nitrogen cycle in the planted tank, ideally all the NH4 will
be removed extremely fast and never have enough to registrar on most test.
This is good since algae loves NH4/NH3. Plants can use either NO3 or NH4,
the difference is subtle in a planted tank, in __SOME__ controlled
experiments with one single species in a non CO2 environment, they seem to
prefer NH4. But is this large enough to see here? I don't think it is that
much. See a counter example below with wheat.

I pulled out DW 's book. I noticed that the Standard error bars overlap with
the NO3 and the NH4 treatments in the figure. I still believe it to be
significant, but I think there is a lot of play with their data on the NH4
treatment. 2ppm of N of either form over two weeks in a non CO2 tank, its
not enough for sustained growth over that time frame. I think with one
single plant in that test is not enough to make a larger statement about all
aquariums plants.

What I do think is a better generalization concerning NH4 and it's role is
that all plants "prefer"(Grow best) a ratio of NH4/NO3. This ratio is
variable depending on  plant species.
 
Plants frequently show a preference to the oxidized form of N (NO3) versus
NH4 in some literature specifically on Nitrogen metabolism such as Haynes
and Goh 1978. Clearly this differs directly from DW's conclusions with NH4
being the _preferred_ form. Wheat is a good example where this is the case.
Although it's not aquatic, many of the plants we keep are not true aquatics
either..........
But one thing seems to be true, at low levels of either NO3 or NH4, the NO3
produces and better growth rate than NH4 does. So if you add NH4, it needs
to be in high concentrations to produce this slightly high production rate
of plant mass.  

It is impractical to add NH4 like we do NO3. The only reasonable source is
fish/critter waste. Some may argue adding it to the gravel etc. I think this
results in too much enrichment(how do you know you have enough or too
much?). Adding mulm and _some_ peat is enough and also natural fish
waste/mulm accumulation is better IMO since that matches the NH4 loading
into the gravel and doesn't exceed the capacity of the tank to process and
use the NH4. With NH4, it is better to error on the lower side than the
higher side due to it's algae inducing nature.

It is very difficult to test how much NH4 the plants are getting relative to
the NO3. While we might think since the NH4 is zero when we measure it and
there fore the plants are being starved this portion of N, the plants may
get it and assimilate the NH4 before any builds up enough to resolve on our
test. It becomes rather difficult to figure the fate of nutrients that are
assimilated so fast and/or re-mineralized quickly.

When the plants are weakened, stunted, this NH4 might make a slight
appearance and you get algae. But the bacteria are quick to turn it into NO3
also. So it does get complicated as to where that nutrient goes. NH4 is the
most difficult nutrient to assess in a planted tank due to the changing of
forms and various sources. Isotope labeling of N in NH4 can help to figure
out what's going on also natural stable Isotopes but it cost some $$ to do
this.

Some NH4 will come from the substrate(these sources are also variable) and
some from the water column. That can make a difference depending on how much
is there in both places and the nutrient status of the plant.

Bottom line: Lots of problems with the "whys" and "Hows" and I think it's
not worth getting into except for adding fish for some NH4 and dosing the
NO3 for the N needs of plants. I think the literature support this
conclusion also along with practical plant tank experiences.

> Would the answer be different in
> an alkaline environment?

Marine systems are slightly alkaline and have this issue but algae uses NH4
namely from a pelagic closed system recycling of nutrients with the trophic
food web. "New production" comes from NO3 upwelling which is an outside
source of N. The only other place N will come from in these systems is
lowing iron rich dust onto the water and causing BGA to grow adding N to the
system.   
  
> Thanks.
> 
> Bill
> 

Regards, 
Tom Barr