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Re: N:P

On Fri, 12 Jan 2001, Tom Barr wrote:

> >  Be careful, though, because the
> > ratio is for N and P concentrations, not NO3 and PO4 concentrations.
> Oops! Oversight:) Perhaps...


> But consider the the Redfield ratio which I have based this off of
> (C(106)N(16)P(1) and there are other similar ratios for other sources as
> well:
> This balance of constituents implies the following reaction for plant
> photosynthesis:
> 106CO2 + 122H2O + 16 HNO3 + 1 H3PO4 yeilds (106-CH20)(16-NH3)(1-H3PO4) + 138
> O2's. The reverse reaction would be respiration.
>  The ratio is 16 to 1 N:P or N03:PO4. The weights change but not the ratios.
> Did I go wrong somewhere here? If so let me know!

In this case, the 16:1 ratio would be a molar ratio (16 atoms of nitrogen
to 1 atom of phosphorus) and the corresponding weight ratios (i.e., using
concentrations in mg/l) would be 7:1 for N:P ratios and 10:1 for NO3:PO4

For what it's worth, I think the derivation I recalled was different.  It
was from watershed studies that concluded that if the N:P weight ratio in
influent nutrients were above about 16:1 that the system (usually a pond,
lake or wetland) tended to be phosphorus limited.  It's been a long time
since I read that, and I my recollection could be wrong.  I no longer have
ready access to that literature, so if someone else wanted to check it
might clear up some confusion.

Another rule of thumb (easier to remember but more difficult to use) is
that phoshorus levels over about 50 micrograms per liter promotes
eutrophic conditions.  The corresponding phosphate concentration is about
0.15 mg/l.

I'm not sure any of this is especially meaningful.  The key point was made
by both Paul and Tom.  P is an essential nutrient and if we force
conditions where P isn't available then well risk the health of our

Roger Miller