It depends on the plant in question.
There is no rule because the plants we keep have not all bene tested, only maybe 20 or so.
Wheat for example prefers NO3.
Generally the issue is less defind by increasing growth ratesas it is fish health and algae blooms.
I find it very hypocritical for folks to suggest NH4 dosing, high fish loading etc and then in the same breath tell me that the high NO3 which really are not that high and has a lot more dosing flexibility is better for plants.
NH4 is very toxic to small fish and very useful(Increasingly more as the light intensities are increased) when added can easily induce algae blooms.
Now all aquatic plants do quite well with a trace amount of NH4 from fish etc, and most from KNO3 dosing.
Why folks would like to increase the growth of plants more is really the key question here. At what cost for the method involved?
In terrestrial agriculture, do we have fish and shrimps? No.
Do we have algae? Not anything that bother's crops or farmers.
So it these cases, yes, the addition for some crops is very useful.
Plants use more energy to use NO3, but they can store and have a lot more access to larger amounts by a factor of 100-1000X more than NH4.
I'd rather have a slightly slower growth, than higher growth with a lot more risk to fish health and algae blooms.
If you look at Diana Walstad's book on this topic, you'll see my point.
There is a figure that illustrates that NH4 is preferred in Elodea.
But at what concentration?
Yuo will note that the rate of uptake falls with NH5 as it hits 0.5ppm.
It's zero, the rate of uptake is essentially zero there.
Now look what occurs with NO3 all the way down?
It starts up and is fast when the levels of NH4 are less than about 0.5ppm of NH5.
Try adding 1-2ppm of NH4 to your tank sometime, it'll kill all your fish.
Try adding 10-20ppm of NO3. No effect.
See attached figure
Regards,
Tom Barr
