> From: Stephen.Pushak at saudan_HAC.COM
> Date: Mon, 17 Jun 1996 17:19:49 PDT
> 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. 

As with all generalities, this type of advice needs to be tempered
with the proper context.  In my experience, with our tanks, I feel
just the opposite is true concerning nitrogen.

We maintain an interesting mix of fish in our planted tanks and the
bioload is far heavier than the traditional "Dutch" tank.  A few
assorted tetras just don't ring our chimes.  Perhaps with our tired
old eyes, teeny, weeny fish are just too hard to see. 

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. 

Some have suggested that the wet/dry filters are "nitrate factories",
but recent evidence counters that.  In this same tank, much of the
bioload (8 adult angelfish) was removed in preparation for moving and
food input was reduced accordingly.  Within two weeks, nitrates had
gone from 20 ppm average to 4 ppm.  

From this experience, I can state that feeding can definitely provide
sufficient, and usually excess, nitrogen to a "typical" planted tank.  
The important context, of course, is what you consider "typical".   

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
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. 

> 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. 

> 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. 

George in Sunny and Getting Hotter Colorado