Fish food as plant fertilizer

> From: George Booth <booth at hpmtlgb1_lvld.hp.com>
> Subject: Re: HOLES IN LEAVES
> > From: Stephen.Pushak at saudan_HAC.COM
> > 
> > The plant FAQ says:
> > 
> >   ``Is fish food enough to fertilize my plants?''
> >   
> >    Fish food usually provides enough of the three _macro_nutrients,
> >    nitrogen, phosphate, and potassium (N-P-K), to keep your plants
> >    healthy. [snip comments about trace nutrients]
> > 
> > We should probably revise the wording of this. I don't believe
> > that its really good advice any longer.
> > 
> > It may be true for phosphates but current evidence would indicate
> > that with good lighting and vigorous growth without any other form
> > of supplementation, that a tank will eventually become deficient in 
> > nitrogen and potassium over a period of months even if the substrate 
> > contains soil or an organic fertilizer such as composted manure. 
> We have NEVER had a nitrogen deficiency in our heavily planted,
> heavily "fished", vigorously growing systems.  By actual measurement, 
> nitrates in one of the tanks will increase by roughly 1 mg/l per day. 
> Keeping nitrates in check requires much effort on our part, to the
> tune of 50% water changes biweekly. 

Yes, but how about the phosphates? Certainly you are keeping a much
higher ratio of fish to plants or fish per gallon than many of us.
Perhaps you are removing the excess phosphates by filtration as the
phosphorous is captured in the biomass of green algaes? I submit that
the ratio of phosphorous to nitrogen is much higher in fish food than
is ideal for plants. In strong lighting conditions won't this result
in a green algae bloom? Perhaps you are taking precautions to prevent
this which may be affecting the ratio of phosphates and nitrates
which accumulate in the aquarium water.

We've made no recommendation about the numbers of fish which may
be kept in a plant tank. It would seem that we can't keep too many
fish in a tank unless we resort to heavy filtration strategies
especially if we are feeding generously!

Don't the Dupla substrate tablets contain nitrogen? Are you using
substrate tablets? The nitrogen in a system is not fixed. Unlike
other elements, it can be lost by de-nitrification (conversion to
atmospheric nitrogen which escapes in gaseous form) While all
plant constituent nutrients are removed from the system when we
harvest or trim bio-mass, this may not be sufficient to remove a
super-abundance of one element. Perhaps the plant bio-mass is able
to store those extra phosphates for the future? I suppose you might
be able to contend that not all tanks will become nitrogen deficient
even if the only supplemental fertilizer is via fish food (possibly
at the expense of an excess of green algae ;-) however I think I
can successfully counter that relying upon fish food alone is not
the best strategy for getting luxuriant, healthy plant growth!

> The important context, of course, is what you consider "typical".   

Good point. But perhaps we don't need to consider ourselves as
"typical" aquarists since growing aquatic plants is such an involved
passtime. ;-)

> As far as phosphorus is concerned, we always see very low levels
> whenever we measure it (<0.02 ppm).  I assume this is added by
> feeding. Judging from plant growth, this low level seems sufficient. 
> Given the chemistry of phosphorus, I don't feel qualified to suggest
> that more or less feeding will generate more or less usable
> phosphorus. 

I'm curious how you can get such low levels of phosphates. Doesn't
fish excretement contain fairly high concentrations?

From what I've read so far, very low levels of phosphate are
sufficient for plants. Experiments to limit growth by limiting
phosphates were unsuccessful. It was felt that plants could be
phosphate limited in extremely sterile, sand substrates however
under natural conditions, such a severe shortage of phosphates
does not occur. I don't think it will in the aquarium either.

> Personally, I don't believe fish food will provide sufficient
> potassium (K) for good plant growth.  I'm not sure where this myth
> started and would appreciate some input from our more learned
> subscribers. Since potassium test kits don't seem to be readily
> available, it must be difficult to measure. This leads me to believe  
> that existing advice in publications concerning potassium is
> generally guesswork or hopeful wishing. 

Which publications are you referring to? It is simple enough to
control potassium concentration under lab conditions. The importance
of sufficient potassium has been clearly established in the
scientific literature. But you aren't really contesting that.

> > Sword plants will prosper very well in a very rich substrate
> > so you can follow the Randall method and "pot 'em up"!!
> Sword plants will also prosper in a less dangerous gravel/laterite 
> subtrate, so you need not bother to potting 'em up. 

Dangerous in what sense George? Many folks do use richer substrates
than gravel/laterite in the sense of total organic content and
nutrient content. (but I thought you were not amoung them! ;-) 
Do you have any evidence that laterite is "safer" than other
soils such as loam when used in the _same_ concentration? Wouldn't
you consider a substrate containing a more appropriate texture
for aquatic plants "safer" than gravel and clay? Studies have
shown a texture between sand and clay seems to be the best.

Another point is that we lack a precise definition of the
composition of laterite. Lateritic soils are a general category
which includes a huge variety. (OTOH, Dupla Laterite is a very
specific product! ;-)

I'm not advising the use of a high organic
content substrate for all plant species; to the contrary! I'm looking
for studies which can help us to establish which plants cannot
tolerate high concentrations of various substrate products which
are associated with high organic content and anaerobic substrates.
Aponogetons are sometimes mentioned as such but I'm hesitant to
cite anecdotal evidence. On the other hand, there is lots of
scientific evidence to document the ability of some plants to
thrive in such conditions including substantial H2S concentrations
in the mud. It is theorized that they do this by supplying
considerable oxygen into the substrate via their roots. Elodea
is a champion at this! Transitional plants (which live in seasonal
sloughs) commonly have this characteristic. Stem plants often
have this ability. Purely aquatic plants may not.

I'm also not suggesting we create significant concentrations of
H2S in the substrate. I believe this condition should be simple
to prevent. There may be other products aside from H2S produced
in anaerobic substrates. Phenols (sp?) are mentioned. I'd like
to get a chance to review some material on this subject and have
some references. Perhaps Olga might lend me another favour since
she works at UBC. :-)

There is other evidence which documents a decrease in plant growth
with increasing refractory organic content. I won't go into the
suggested causes for this at this time but there are very interesting
implications to this. However, plant growth is most strongly correlated
to nutrient concentrations in the substrate and secondarily in the
water. Concentration of macro-nutrients is strongly related to the
amount of organic material but not to fully decomposed material (humus).

> > I would also like to point out that most plants absorb nutrients much
> > more readily with their roots than through their leaf surfaces
> > (despite some comments made recently). 
> Perhaps that may be true, but stem plants seem to be able to grow
> extremely well in a "rootless" state.  Trimming stem plants typically
> takes the form of cutting off the top portion, planting that and
> throwing out the existing rooted portion.  If stem plants *depended*
> on the roots for nutrients, this practice would certainly lead to
> disastor.  Which it doesn't. 

Not saying they depend on the roots, just that they absorb nutrients
more _efficiently_ through the roots. See dr dave's comments.