Re: soil works, Dupla doesn't (for me)
Steve Benz, Tuesday, July 23, described plant damage in a tank set up
according to the Dupla formula. He got one sword plant to grow much better
in a pot with some topsoil covered with gravel, while its twin continued to
look sickly in the laterite.
This is the first I've heard of the Dupla method having any problems. The
brown edges that spread inwards do not sound like any kind of nutrient
deficiency that I know of. It seems more likely to me that your plants are
suffering from some kind of toxicity in the soil. If the plant growing in
topsoil is doing fine, then the problem couldn't be a water-borne toxicity.
I noticed that you felt that it was risky going with potting soil. One
comment is that presently you are using topsoil, which is quite different
from potting soil. Topsoil is mostly mineral, and potting soil is almost
completely organic, being mainly composted bark and peat. If your plant is
doing well with topsoil, why not try a few more plants with that? Another
comment is that there are very successful growers of aquatic plants who use
potting soil, others who use peat, topsoil, and, of course, laterite.
Different brands of laterite vary in how much organic matter they contain.
Some laterite vendors have added peat. Laterite by itself is very low in
organic matter. The point is that there are people having success with
soils ranging from almost 100% organic matter to almost no organic matter.
I don't think you have to worry about release of H2S. Even in high organic
soils, such as potting soils, the organic matter has been composted, and
the easily-broken-down organic matter is gone. What remains decomposes
more slowly, and has a lower Biological Oxygen Demand (BOD). Even though
some H2S will be formed within the soil, it will get oxidized to sulfate
before it gets out of the soil. As H2S is diffusing out, O2 will be
diffusing into the soil, and there will be a place, about 1 cm beneath the
surface, where the H2S gets oxidized. Also, you don't have to worry about
H2S bothering the plants. Their roots have air channels that supply
oxygen, and there will be an oxidized zone in the soil surrounding every
root. The level of oxidation around the plant roots is high enough to
precipitate reduced iron as iron oxide, and that is a much higher level of
oxidation (technically called redox potintial) than is necessary to oxidize
Paul Krombholz Tougaloo College, Tougaloo, MS 39174
In steamy Mississippi.