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clay, laterite, kitty litter/vermiculite and soil
We should avoid confusing the properties of laterite with the various
clays and other soils. Laterite is quite different, it is chiefly
composed of iron and aluminum oxides/hydroxides. In comparison to
ordinary soil (with a clay & humus component), or any of the various
fine textured clays, laterite has a relatively low CEC (cation exchange
capacity = the ability to adsorb positively charged nutrient ions).
Improving the CEC of your substrate is quite easy; vermiculite or other
expanding layer clays (like kitty litter) are just one option for this.
Fine textured clays or soils with a low organic content are another way.
Humus has a very high CEC and is also be able to adsorb negatively
charged ions (anions). 1 to 5 percent humic material in a substrate is
optimal. Refer to my homepage for a long and technical discussion of
substrate materials (Substrates for aquatic plants under Articles)
Dave Huebert made the following comments in an email discussion last
"In a landmark paper published in Science 207: 987 to 989 (1980)
two Canadians by the name of Carrignan and Kalff clearly demsonstrated
that rooted aquatic plants will absorb the majority of P from the
sediment EVEN UNDER HYPEREUTROPHIC CONDITIONS! This was the first paper
which unambiguously showed the preference of aquatic plants for
sedimentary P. Nitrogen studies are somewhat less clear, mainly because
N is so difficult to work with. Nichols and Keeney, Fresh.Biol.6:145 to
154 were perhaps the first to indicate that rooted aquatic plants
acquire the majority of N from the sediment. After that was an excellent
study by Barko and Smart. They turned the question around somewhat and
asked "which nutrients can be left out of the water". Their results
clearly showed that N was not needed in the water column for optimal
growth. I asked the same type of question in a paper published in 1982
and came up with the same result (Huebert and Gorham (Aquatic Bot. 16:
269 to 283). These experiments, and several others, indicated that with
a fertile substrate the only nutrients required in the water column are
Ca, Mg, K and of course CO2 (I must admit, though, that the evidence for
micronutrients is scant and it is in fact likely that water column
additions of micronutrients is a good strategy ... though my swords and
Sagittaria are completely indifferent to whether or not I add
micronutrients) Little work has been done since the mid eighties since
the dozen or so published studies were fairly clear in their results and
there is not much money for this type of research (its also difficult
and time consuming).
"On the other side of the coin, there have been a plethora of
studies which indicate clearly that rooted aquatic plants will not grow
optimally on a sand or other infertile substrate no matter how richly
you fertilize the water column (perhaps the earliest is by Pond, 1905)"
Laterite certainly provides a certain amount of useable iron in the
substrate. Substrates with very low organic content do not have a low
enough redox potential to provide a continuous supply of substrate iron
and these kinds of aquariums will benefit from carefully controlled
doses of chelated iron in the water. We've also found that the addition
of peat to a soil substrate keeps the redox potential low enough to
provide sufficient iron to maintain moderate growth rates without
resorting to chelated iron dosing. It's probably not wise to combine
really high light levels with significant amounts of chelated iron in
solution as this seems to promote many types of algal growth especially
filamentous algae. NOTE: for these reasons, we do NOT RECOMMEND that you
try a mix and match approach but stick with a single, proven approach.
Quoting Karen Randall:
"I think the strongest merit of laterite is safety. In the hands of
a novice, even _good_ soil can be a mess. Even in the hands of a more
experienced aquatic gardener the _wrong_ soil can be a disaster, and
there is no way to know until you've tried a particular soil how you'll
do with it. TOA approach, and I must say, the one I try to follow in
talking to beginning aquatic gardeners is to help them be successful the
_first_ time. Once the hook is "set"<g> there's plenty of time for
experimentation and improvement. But if they are faced with massive
problems in the first round, they are likely to give up in disgust."
I hope that we can describe some procedures to safely use natural soils
which are also suitable for novices but it should be noted that
following the procedures is more critical.
As Dave Huebert points out above, rooted aquatic plants do best when
they can get an adequate supply of phosphorus from the substrate.
Laterite and other iron compounds in a substrate help to sequester
phosphates. Clays, kitty litter and vermiculite are not able to do this.
Natural, amorphous soils commonly contain significant amounts of iron
(typically 5% according to Diana Walstad). The amount of phosphates in a
soil is extremely variable and for this reason, soils with large amounts
of freshly composted material should be avoided since they readily
release these phosphates into your aquarium water. On the other hand, an
appropriate amount of organic material or phosphates can give your
plants a tremendous boost.
Roger Miller had asked recently about his Cryptocoryne balansae
(reclassified as C crispatula) which only seemed able to support 2 or 3
leaves at a time. Roger, I presume that you are ensuring an adequate
supply of nitrogen in solution and I recall that you had mentioned plans
to experiment with some phosphate fertilization in your substrate. I
wonder if this Crypt melting is really a symptom of a shortage of
phosphorus and a natural reaction of the plant by "recycling" phosphates
from older leaves.
In my 75 gallon tank, I had very good growth of C balansae for several
months without loss of older leaves but more recently the older leaves
have begun to melt and are replaced by new leaves. The substrate in this
tank contains a lower organic material content and much of this is from
peat which is a very poor source of phosphates. I suspect that I am also
beginning to have reduced levels of phosphates. Another plant which has
shown a reduction in growth rates in this tank is Aponogeton crispus.
The fish load in this tank is very low.
Roger, have you tried adding any form of phosphate fertilizer into the
substrate? Probably the best way would be to use something like enriched
clay balls or Jobe's plant sticks which are made from a material
designed to slow the release of the nutrients. I don't think the plastic
coating on Osmocote pellets does an adequate job of this and prefer to
use these inside iron rich clay balls. I'll probably put some of these
into my 75 gal shortly.