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Re: Bacteria algae and N:P

On Thu, 11 Jan 2001, Tom Barr wrote:

> Also, I found compelling evidence for not letting your N:P ratio fall below
> 16:1 for aquatic plants. Neat stuff. So if you have a NO3 of 5ppm your P
> would/should not fall below 0.3ppm or so.

This number has been around for a while (25 years or so, anyway) as the
value above which aquatic ecosystems tend to be nitrogen limited and below
which they tend to be phosphorus limited.  Be careful, though, because the
ratio is for N and P concentrations, not NO3 and PO4 concentrations.

A NO3:PO4 ratio of 23:1 is equivalent to an N:P ratio of 16:1, as long as
N is available largely as NO3 and P is available largely as PO4.  If
you want nitrogen-limited conditions and have 5 ppm NO3, then you
shouldn't let PO4 exceed about 0.2 ppm.

Of course you may get erratic results when you start with a value that was
derived as an approximate rule-of-thumb for ecosystems and apply it to an

Roger Miller