Ratio discussion

Tom Barr

Staff member
Jan 23, 2005
This is a semi old paper that discusses some ratio aspects.


the last line/paragraph is rather telling..............

Here's Paul and

The concentrations of nutrients in aquatic plants
described here( probably reflect their close
association in the basic biochemical processes
of the plants, which are similar across
plant groups (Garten 1976). The weaker relationships
found between N and C and P
and C concentrations (Fig. 4), compared to
the strong, linear relationship between the
concentrations of N and P (Fig. 4; Table l),
probably reflect the important structural role
of C, which uncouples it from metabolicrelated
variation in the concentrations of N
and P. Moreover, N and P are closely associated
in plant biochemistry, particularly
through protein synthesis. Thus, plants with
high N but low P concentrations, or vice
versa, should not be able to produce the
proteins, ATP, ADP, NADP, and nucleic association between the concentrations of
N and P may also explain their close association
in growth regulation, as reflected
in the significant interaction between N and
P detected in most experimental enrichment
experiments of phytoplankton (e.g. Elser
et al. 1990), macroalgae (e.g. Lapointe
and O’Connell 1989), and angiosperms (e.g.
Perez et al. 199 1).
Aquatic ecologists often scale the N and
P contents of aquatic plants to their C concentration
(i.e. C : N and C : P ratios), and
use the N : P atomic ratio to infer the relative
importance of N and P as potential limiting
factors for plant growth. However, since
variability in N and P concentrations is
much greater than that in C concentration
(Figs. l-3), the variability in C : N and C : P
ratios should be dominated by variability
in the N and P concentrations of the plants,
respectively (Duarte 1990). Thus, C : N and
C : P ratios are expected to show inverse relationships
to the concentrations of N and
P, respectively. The existence of strong relationships
between the C : N ratio and N
concentration and between the C : P ratio
and P concentration of aquatic plants was
confirmed by the analyses (Fig. 5; Table 1).
However, the relationships obtained (Table 1).

This reflects what we see when we add CO2 enrichment.
It also shown the strong linkage between Carbon(CO2) and NO3, and PO4 dosing.

If you limit one of them, then it radically influences the others.
Then you have all sorts of issues and confounding factors.

And then we get myths and folks suggesting PO4 limitation is the way, or low NO3, or rich CO2.............if you rule out NO3, PO4 limitations, and CO2 limitations, then you have excellent growth, no algae etc.

Which matches well with the general basics of plant Physiology and biochemistry.
Stable high levels of these allow the plants to run at the best efficacy.
They still grow well at lower levels, but if you have a difference of say 5 ppm NO3, vs 20ppm, then you see a reduction in dry weight by about a factor of 4 times.

So you will need much less CO2 at 5ppm NO3 vs 20ppm NO3.
Similarly, PO4 is the same.
Some folks might want less growth.
But is it best to reduce the rates of growth this way, or with reduced light?
Some argue they are wasting nutrients/water etc..........but the trade off is they are wasting light. Likewise, if you waste nutrients, you also get the most out of CO2/light.

I think the key is reduce the light and reduce the nutrients but keep them a tad higher so that no limitation occurs, so a target about 20ppm is easier to achieve.
This does not mean one has to d water changes or EI, you cna test and dose what you need to keep the nutrients at the higher 20ppm level, and you have more wiggle room, buffer room to do so.

Still, the idea of ratios has never really set well with me for many reasons.
But many claim it to be some savior that it clearly is not. Some suggest that ends justify the means(well it "works") but that does not imply it cannot be improved upon/nor the best trade off, or even that the general hypothesis is even correct.

We can show otherwise via testing and changing these ratios.
Research has show that as well.

Tom Barr


Hi Tom,

thanks for the link. It has been recommended that Ca:Mg should be kept at about 3~4:1, but I do not find that correlation between Ca and Mg in table 1.:confused: