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Soil substrates

Recently I read two references supplied to this list by
Dave Huebert concerning optimum substrate composition for
aquatic plant growth. They were

        Smart, R. M. and Barko, J. W. 1985. Laboratory
culture of submersed freshwater macrophytes on natural
sediments. Aquatic Botany 21; 251 to 263.
        Barko, J. W. and Smart, R. M. 1986. Sediment-
related mechanisms of growth limitation in submersed
macrophytes. Ecology 67; 1328 to 1340.

The first of these papers reiterates pretty much what Dave
has been saying all along, that the best substrate is one
of dense, fine-textured lake sediment with less than 20%
organic content by weight. The soil in the researchers
tanks was employed to a depth of four to eight inches.

The authors also talk about the provision of carbon dioxide.
They advocate vigorous aeration and adding baking soda
to increase the alkalinity. For waters of low alkalinity
they indicate that the water be aerated with CO2 enriched
air. Also stated as another reason for additonal CO2 is
that water column and sediment respiration is greater in
the wild than in the laboratory.

Now returning to a previous posting I made about the
problem that I was having getting stem plants to grow in
one inch of soil with only modest circulation. The extra
powerhead that I added to the tank does not appear to
have resulted in any new growth. Lighting is good and
I see no obvious nutrient deficiencies in any plants.
My tapwater is soft. Perhaps a minimum alkalinity is
a necessary condition for this to work.

I noticed a second point about the experiments from
which various conclusions were being drawn. They seemed
to have worked with a few plants specific to North
American waters: hydrilla verticullata, potamogeton
pectinatus, myriophyllum spicatum. I wonder how typical
these are of the tropical ornamental plants we are used
to seeing in the aquarium.

I've since added PMDD to the tank, again with little
noticeable effect. I can only conclude that there is
a CO2 deficit, whether or not the Prandtl boundary is
reduced. I recall that Horst and Kipper tested about
9 ppm CO2 in the river streams of S.E. Asia. That is
less than the probable 1 to 3 ppm of my tank. I had
always thought that 9 ppm CO2 was a minimum level for
plant growth.

Both articles are worth reading if you can find them
in your university library.

Dave Whittaker
ac554 at FreeNet_Carleton.ca