Soil works, Dupla doesn't - for me

Several months ago I finally got a tank that adheres to all aspects of
the Dupla formula - Duplarit in substrate as per instructions, heating
coils, CO2, Dupla Drops & Tabs.

Some plants grow well (ludwigia, ambulia, Valisneria), but others do not -
basically anything with big green leaves has been troublesome.  The typical
symptom was a browning of the leaves at the edges, and slowly creeping toward
the center.  New leaves did not develop this immediately, but soon did.
Over a period of several months, all the sword-type plants I had bought
were just dwindling away, as were sagitaria, anubia and some others.

Obviously, there is some nutrient that the dupla fertilizer either isn't
delivering or perhaps it is delivering but something in my water is
oxidizing or precipitating it out or something. (BTW: my water is naturally
GH 11, KH 4, Ph 7.6 and the Ph is CO2 adjusted down to 6.8 - +/-.5 or so
as I don't have a Ph controller.)

I suppose I could buy a slew of test kits or do PMDD or something to see if
I could figure out exactly what nutrient(s) are deficient, but before I spent
all that time&money, I decided to try out Karen Randall's approach.

I had two amazon sword plants that I had bought at about the same time and
put in either side of my tank.  Both get about the same amount of light,
were about the same size, had been in the tank 2-3 months, and both
were suffering from the symptoms I described above to about an equal degree.
I took one and put it in a plastic pot filled with about a cubic inch (dry)
of topsoil, filled the rest of the pot with gravel, and stuck it back where
it came from.

Within a week the potted planted started showing signs of recovery whereas
the control plant stayed in the same pattern of growth.  Now, six weeks
into the experiment, the control plant is still languishing (largest leaf is
4", none of them are healthy) and the potted plant is flourishing - the
largest leaf is 10" and (at the very least) is triple the biomass of the
control plant.

This doesn't make me want to rip out my substrate and go with potting soil -
I still feel that doing so runs too much of a risk of H2S poisoning due to
rampant anaerobic bacteria growth (or rampant growth of some other harmful
life-form.)  While there are those who are successful with this
approach, I don't think it's a technique to recommend off the bat.

One can still use pots and other methods fairly safely, but lately I've
been giving some thought to another option.  When I was putting together
my aquarium (45 gal), I was surprised that Dupla recommended only about
1 kilogram of laterite for an aquarium of that size.  This small quantity
of laterite made me figure that perhaps a small quantity of topsoil would
give the same results as the small quantity of laterite gives.  I figure
that whatever element that is missing from my fertilizer must be a trace
element, and a kilogram of soil probably contains several hundred miligrams
of most any given mineral, and thus a small amount of topsoil mixed in
with the laterite-layer of gravel may well be as good as a whole layer
of the stuff (assuming that you keep up with your drops and tabs.)

Any thoughts?

                                        - Steve
Steve Benz (steveb at realtime_net)   |   Pragma Systems, Inc.
Author of ZipPiz, a Windows utility for unpacking downloaded archives.
http://www.ccsi.com/~pragma/zippiz.html | ftp://ftp.ccsi.com/pub/pragma