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Useful forms of nitrogen Re: Cycling

I said:
>Ammonia is called ammonium in water with a PH less than 7.

Karen Randall replied:
>Not quite accurate.  Ammonia and ammonium exist in equilibrium.  There is
>more ammonia at higher pH levels, more ammonium at lower pH levels.
>I'm sure the chemists on the list could elaborate.

Ines Scheurmann states in her book "Aquarium Plants Manual" that: "Water
plants do not absorb nitrogen in its elemental (uncombined) gaseous state,
nor do they take it in as nitrate, as land plants do; rather, they take it
in as ammonium.  Ammonium exists only in acidic water. In the normally
slightly acidic environment of our aquariums, ammonium is unlikely to harm
the fish, but in alkaline water it changes into poisonous ammoia. Ammonium
and ammonia are converted by the filter bacteria first to highly toxic
nitrite and then to relatively harmless nitrate."

> If there are not enough of these fast growers then the end product in the
>water is nitrate which is food for algae as the higher plants
> absorb nitrate through their roots better than from the water directly.
>Some floating type plants such as riccia can take in nitrate directly
>but most don't.

Karen Randall replied:
>This is incorrect.  There is no evidence that I know of to suggest
>that_all_ plants can't benefit from ammonia, ammonium, nitrite and
>nitrate.  It is true
>that studies have shown that plants will preferentially take ammonium first.

Horst and Kipper in "the Optimum Aquarium" said that: "Unfortunately we are
still relatively helpless in the face of one aquaristic problem, and that
is an over supply of nitrate... We know of a number of plants which accept
nitrate as nitrogen and are able to use it, but on the other hand we also
know of sone aquarium plants, such as many types of cryptocorynes, for
which nitrate is useless. In general, nitrogen is needed by plants as
ammonium.  If they receive nitrate, they must reduce it to ammonium."

Susan Romano