[Prev][Next][Index]

Re: Fish food as plant fertilizer



> From: Stephen.Pushak at saudan_HAC.COM
> 
> 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 are doing nothing targetted at removing phosphates.  If phosphorus
is captured in algae biomass, the SAEs and others are performing the
chore of removing it but it gets recycled in that case.  Plant
trimming should remove some of it since plant mass has roughly 460 ppm
phosphorus AND plants are able to concentrate it ("Aquarium Heute",
Feb, 1988).

It has been suggested in the past that the iron in laterite, or
laterite itself, is able to capture phosphorus by reaction.  Our tanks
have laterite and substrate circulation via heating coils.  Perhaps
this is where the majority of phosphorus is being bound and explains
the apparent incorrect proportions of N and P in our tanks.

> 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!

Or massive water changes, as we do.  I also feel that because of our
massive water changes, other potentially harmful substances
(alleochemicals, DOCs, who-knows-what-else) are at lower levels than
in "typical" plant tanks. 

> Don't the Dupla substrate tablets contain nitrogen? Are you using
> substrate tablets?

Dupla laterite comes with a dozen or so Duplaplant tablets (which do
NOT contain N or P) and one single, solitary "root starter" tablet
which I assume contains enough N and P to make the substrate "rich"
enough to get things started.  The root starter tablet is crushed,
dissolved and mixed with the laterite in the lower third of the gravel
bed.  This prevents the N and P from getting into the water column and
keeps it in the substrate where roots can get to it.  None of the
Dupla products add N or P after the initial setup is complete. 

> 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!

There is no argument there.  Diana Walstad in TAG has suggested that
fish food is the ONLY fertilizer you need.  I suggest that fish food
(and the natural processing fish provide :-) will supply sufficient
levels of N and P but other elements need to come from other high
quality sources (trusted comercial products, PMDD, etc) and a
carefully constucted substrate.

> > 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. ;-)

"Typical" as in "typical planted tank".  A "typical non-planted tank" 
has nothing to do with this topic. 

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

I have no idea.  Maybe the Dupla laterite has magic phosphate
absorbing stuff added in.  After all, $60 per kg is kind expensive for
just plain dirt.

> >    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.

Hobbyist aquarium books, magazines and certain technical editors have
endlessly repeated the Urban Myth that fish food supplies all you
need.  Since potassium is hard to measure given the dearth of test
kits, how do they know?  Yes, it is simple to control the amount
*added* to a tank, but it is not so simple to measure how much is
actually in the water.  I believe the amount of K in PMDD was arrived
at empirically ("this seems to work good")?  The source referenced
above indicates plant mass is roughly 3600 ppm K and that plants can
concentrate K.  How much actually needs to be in the water?  I don't
know. 

> > 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?

"Soil substrates" are dangerous in the sense that Joe Bob hears how
great potting soil/modelling clay/red Georga clay/manure/etc is for
plants, runs out into his cow pasture to "git some" and tosses a
shovelful of garbage into his aquarium.  His results may vary.  Folks
using commercial laterite know what they are getting (Australia
excepted); folks using "cheap dirt" haven't a clue as to the potential
suitability.  Diana Walstad herself even admits to having a single
source of "good dirt"; other dirt just doesn't work as well.  If you
don't live in Diana's back yard in Research Triangle Park, NC, how can
you take her advice? 

> Do you have any evidence that laterite is "safer" than other
> soils such as loam when used in the _same_ concentration? 

I believe that laterite is used in a much, much lower proportion than
regular dirt.  In the case of Dupla, 1 kg of laterite is used with 80
kg of gravel.  I believe dirt tends to be solid layer at the bottom of
the substrate.  Again, commercial laterite is a carefully controlled
product.  Hand-dug local soil or potting soil from a nursery may or
may not contain harmful by-products. 

> Wouldn't you consider a substrate containing a more appropriate
> texture for aquatic plants "safer" than gravel and clay? 

If you mean "more appropriate" as in "natural", no I would not
consider decomposing Amazon basin muck as something I would call safe.
Nature has appropriate safeguards in place and nature doesn't really
care if a few discus are killed when the substrate is disturbed and
noxious gases are released.  Our tanks have no such safeguards and we
*do* care when a problem occurs.

> Studies have shown a texture between sand and clay seems to be the
> best.

Best for what? Optimum growth rates?  Plants grow fast enough (for me)
in the gravel I use (2-3 mm grain size).  Sand and clay is not
something I would want to deal with in an aquarium where maintenance
is an on-going process.  It may be fine in a situation where plants
are left to run amok (as in nature), but I would prefer not to have
fine clay particles clouding the water for days after a pruning session.

> 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! ;-)

Which is why I use Duplarit-G.  You can depend on the fact that when I
say "laterite" I mean "Dupla laterite". 

> 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.

No argument there.  And if this situation exists in the aquarium, it
is also not a problem UNTIL the substrate is disturbed for whatever
reason and a fish kill occurs.  I would prefer to avoid any potential
problems like that.

> > > 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. 

I agree that efficiency is of interest to scientists studing plant
growth and I'm sure that it makes for a wonderful paper, but it is of
little concern to a practical hobbyist as myself.  I am certainly not
downplaying the role of proper research in plant growth, but I am more
concerned about the limited and fragile ecosystem existing in my
planted tanks.  Fish are as important, or more important, than the
absolute, scientifically proven, optimum plant growth.  I am concerned
about filtration, keeping the tank healthy, preventing fish die-offs,
leaving some margin for error, maintaining multiple plant species
(i.e., not optimized for, say, Madagascar Lace Plants) and presenting
a pleasant appearance.  

George



Reference(s):