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> Paul, you mentioned Hoagland's solution. Can you review for us its ratios
> of N:P:K  and if it is used by researchers to just grow algae or also for
> aquatic plants.

Plants, but they have used it for algae also.
*Elemental ratio's

3.6: 1 : 3.8

Looking at the solution and the compounds that make up the solution I see
that KNO3 is the main source of N but the ratio of NH4 to NO4 is about 1.75
NO3 to 1 NH4. (Source: Epstein 1972)

KNO3:         1M 6.0mls            N  224ppm
Ca(NO3)2*4H2O 1M 4.0mls            K  235ppm
NH4H2PO4      1M 2.0mls            Ca 160ppm
MgSO4*7H2O    1M 1.0mls            P   62ppm
                                   S   32ppm
                                   Mg  24ppm

+ some traces(KCL etc)

> I see that there are lots of other solutions out there, but
> dont know what their various purposes are. It would be interesting to see
> how their ratios relate to Gerloff's critical concentration ratios for
> Elodea I wrote about last week (11-2-8). These and similar non-uniform
> ratios helped me come to the conclusion that our NPK fertilization should
> not be 1:1:1.

I suppose the ratio would work well at 5:1:30. Or 10:1:40 etc. Depends on
your needs. Suppose the tap is loaded with N or P? Your screwed either way
so just coming up with a relative approach using the 3 macro's allows
flexibility for all tap water types. CC ratios work but it depends on
concentration of the 11-2-8 in the medium and frequency of replenishment.

Hoagland solution is pretty concentrated stuff(that way you don't need to
replenish it often). It's levels are set at the highest possible
concentrations without producing toxicity symptoms or salinity stress. Many
researchers dilute the solution and replenish it frequently in order to
minimize fluctuations of nutrient concentrations and in the plant tissues.
> I would also like to hear your opinion (and from others) of the need for
> different relative amounts of N, P and K  vs. sufficient MINIMUM amounts of
> each (in both cases, the max concentrations must not be so high that they
> are harmful).

It would difficult to pin down. I could limit growth with N or P and not
need much traces for example or K. What is defined as minimal? When your
plant dies? Stops growing? Gets holes from a deficiency? Gets covered in
algae? Max growth rate? At the whole plant level this is tricky. You can do
experiments at the micro scale in which these can be determined
biochemically. Different parts of the plant also have different ratios, if
you use all leaves you could get very high MN:Mg ratios but not in the
Just going on observations 10:1:50 is nice but not the minimum. Less than
2ppm of NO3 is my personal cut off point for that species of N. K? about
2-3ppm but some species, fast growers in particular are more sensitive to
variations and lower tolerances IME.
A crypt can/could handle less than 2ppm of NO3(they uptake NH4 namely) and
slow growing vs something like Riccia (fast growing and loves K and NO3)
would react differently.
PO4? I think less than .2ppm certainly but is the .2ppm inorganic P04? You
can run it down to 0.00ppm for a day or two with no ill effects on all 3 but
you'll need to add more after that. Depends on the time frame your talking
about also.  

 Yes, the plant will store excess N-P-K, and I doubt that the
> plant cares if their NPK is 11-2-8 or 15-1-7, BUT it might care if it were
> 10-10-10. 

I 'm sure there'd be too much P at least that the plant would not need
relative to the other nutrients(10-10-10). I really do not know what too
much PO4 is though. 2ppm has proven fine for a number of folks.
So, do we need to be careful about providing relatively TOO MUCH
> of one ingredient (like P) or relatively too little (like N)?  .... In that
> regard, is KNO3 a good source of N and K, because it provides N and K in
> the ratio 14:39 (~1:3). I dont like to overdose K in soft water.

?? What do you consider overdosing? Interference with Ca uptake? Folks in
the bay area have soft water and I deal with tanks there and they have no
problems with 30ppm or so. Certainly nothing that would appear as a Ca issue
that I can tell. I don't go over 50ppm but a 40ppm has been added and Im
sure abuses go on but we hear little horror stories of K
overdosing...........and that's a good thing:)
As a matter of fact I've never heard one, have you?
I've seen some KCL issues but it was the CL, not the K. I use K2SO4 and get
around this issue of CL build up. SO4 is fine and has uptake as S and is a
component in many harder waters.
> Also, do we want to provide the certain ratios to get the best plant
> growth, to produce a certain morphology (color, size, shape),

Well the red color thing we started on a few years ago surely shows some
trends there.
Higher NO3 more green and faster growth in many(most) species.
Lower NO3 redder colors.
PO4 stress also can produce redder colors.
Higher P to lower N ratio helps also in many species.

to get the
> plant to flower/reproduce.....

I think lighting changes are a good method for this.

> OR.... to avoid plant growth problems
> because of the relative or absolute amounts of stored nutrients (luxury
> consumption).... ie what can be the effect of too much?

I think these nutrients would have to be in very high concentrations to kill
the plants or do much harm. But that's from the root to the leaf. Through
the leaf from the water? Take the Hoagland's solution for example, that's
some rich stuff. I still think even at the higher levels of nutrients that
we might keep our plants at are still far away from being toxic.
Max levels for nutrients seldom exceed:
NO3: 40ppm
NH4: .1ppm
PO4: 2ppm
K: 50ppm
FE: 2ppm
CL:? Most likely lower than other nutrients
Cu: again like CL
CO2: Whatever doesn't kill the critters(35ppm?)

I think we be killing off our fish and critters long before it gets toxic
for the plants.
>The outdoor
> gardeners can jump in here to describe the role of fertilizers in
> horticulture. On the other hand, does it have nothing to do with effects on
> the aquatic plant, but instead, is it to AVOID algae growth, perhaps
> because the plant CANNOT soak up and store more than a certain amount of a
> certain nutrient (say P).

But algae has the ***same opportunity*** to get the PO4(or NH4 or NO3 etc)
as the plant if it's in the water column. If there's some there, the algae
can use it. But this doesn't happen _unless_ the plant is doing poorly. I
think something else is going on. It's not resource competition. Both have
had that pressure removed by adding excess of that nutrient and other

It's a simple experiment. Simply remove the limitation and observe. No
algae? Something else must be going on. Look elsewhere for a reasonable

A good possibility:

 Algae photorespire _if_ they are grown in an enriched CO2 (5%)environment
and then transferred to a low CO2 medium, they will show symptoms of
O2(oxygen) inhibition but if they are grown in air( 0.03%) they rapidly
develop the ability to concentrate CO2 and HCO3 internally. Under _this_
condition, the cell no longer photorespire.
This is very likely to play a key role in the process.

>...or because it will be more inclined to leak
> some nutrients out at high storage levels....

These "leakages" may be inhibitors to algae. Plants might produce them when
they are doing well. But..........what about algae vs algae? GW inhibits
other species of algae(not Staghorn, Entromorpha sp and some Cladophora)
Perhaps algae produce extra cellar leakages also that inhibit other algae
from growing.

Perhaps algae can sense when there's someone else at the table. Doesn't
matter if it's a plant or an algae. They wait till there's a chance. Low DO
levels are a good indicator that it's time to try. NH4 levels also.

> and this leaves excess food
> which may contribute to more algae.

The plant leaks "food"(nutrients) and then this is also food for the plants
and the algae. 
This line of thought doesn't address the underlying issue of availability to
the plants and the algae. Algae still have the same access as the plants do
to this "food". Why don't the plants re-sorb this "food"?
Depurination certainly happens at higher and higher internal cell
concentrations. There is a number of studies that address this issue that I
am familiar with. 
But how high does this need to be(to cause algae) and what levels of
leakagae ( and what is leaked) are induced by excessive storage rates?

>This is not a concern for outdoor use.
> There, economics may be a bigger factor.
> Speculation on these topics is certainly allowed.

I like the ideas, but the practical side? Will knowing that in a certain
narrow range of parameters that a plant species will leak PO4 laden residue
into the water column where it may (or may not) be used by algae instead of
the plants help us in the end? I'm skeptical about that issue.
It would be nice to back up things with a good study to explore the theory
behind this. Maybe later.
Tom Barr

> Neil Frank / Aquarian Subjects
> Interesting old books and magazines (but most plant books are already gone)
> http://www.mindspring.com/~aquaristics/aquarian.subjects/