[helgymatt]

Nice question!

First off, plants will use Ammonium (NH4+), not ammonia(NH3). In the biological process, plants will convert ammonium to nitrite by bacteria call Nitrsomonas. Nitrite is usually very quickly converted to nitrate, by more agressive bacteria, Nitrobacter. Nitrite is known to be toxic to terrestrial plants, even at low levels. The same is true for aquatic plants. So plants can utilize nitrogen either in the form of ammonium or nitrate. Some plants have a preference of one over the other, but most don't.

In a well planted tank, fish waste will will release ammonia, which is toxic to fish. Often, that ammonia quickly gains a hydrogen and forms ammonium, but the process is pH dependent. This article explains it much better than I can.... http://www.extension.iastate.edu/CropNews/2008/0421JohnSawyer.htm

So, ammonia fish toxicity is more of a problem the higher the pH is, but even so, small amounts of ammonia are toxic. In a cycling aquarium, we fertilize with nitriate because the biological process will convert ammonium fertilizer to nitrate anyway. I believe we don't use ammonium fertilizers because they pose algae issues, but I have heard contradictions to this so this may not be true.

 

[helgymatt]

I may be out on a limb here, but I feel pretty confident that the plants can take up ammonium without help from bacterias.

For sure!

And that ammonium to nitrite convertion only happends by bacteria if there are not enough plants to absorb it.

Maybe, but this oxidation will probably always take place at some level no matter how much the plants uptake. Remember that the plants are not absorbing the ammonium all at once.

The third point in this paragraph goes right to the core of the discussion I refered to.
In the link bgzbgz posted, there are a small section about it, and a graph (fig 3)

I'll just paste the section here, since this is what the debate revolved around.

--Snip--
Nitrite Uptake by Plants

Although plants can use nitrite as an N source, the pertinent question for hobbyists is- Do aquatic plants remove the toxic nitrite before the non-toxic nitrate? I could not find enough studies in the scientific literature to state conclusively that they do. However, the chemical reduction of nitrites to ammonium requires less of the plant's energy than the chemical reduction of nitrates to ammonium. (A plant must convert both nitrites and nitrates to ammonium before it can use them to make its proteins.) Thus, it is not surprising that when Spirodela oligorrhiza was grown in media containing both nitrate and nitrite, it preferred nitrite (Fig. 3).
--Snip--

As one can see, this suggests that nitrite may be benificial to aquatic plants, even though it is considdered poisonous to surface plants.
It may be a measuring error (maybee there were nitrobacters in the sample), and it may be that the Spirodela O. does not represent the typical aquatic plant. But there are something there worth to follow, and it would be nice to be more certain about this.

I wouldn't argue that plants cannot take up nitrite, but I will say that at some level it will become toxic, just as very high levels of ammonium and nitrtate can become toxic in plants. I'm sure its the case that it takes much less nitrite to lead to toxicity than nitrate or ammonium. Again, I'm basing most of this on terrestrial plant research and it gets kinda cloudy with submersed plants.

 

[helgymatt]

Diana Walstad's book covers some stuff on nitrite uptake by plants also. Take a look at that if you have it.