" wrote in message
om...
Well, according to Diana Walstad, a soil substrate will help to establish
anaerobic zones, which the plants like


They do? Why do the plants transport O2 to their Rhizosphere then?
They actively transport O2 to their roots and around each root.
This has got help many aerobic bateria in those areas/regions.

and which helps to keep iron
accessible.

Hi Tom,

yes, plant roots do oxygenate the substrate, no doubt. Why do they
do it? According to Walstad, several reasons:

- Oxygenation prevents an excess of soluble iron entering the plant.
(Too much iron is just as bad as too little: too much iron causes
heavy metal poisoning.)

- Roots need oxygen just as the rest of the plant does. Oxygenation
of the roots during the day helps the roots respirate during the night.

- Bacteria use oxygen to oxygenate H2S, which is toxic. The root
oxygenation protects the plant against a substrate that's gone too
anaerobic.

- Root oxygen release can oxydize soluble iron into ironhydroxide
hydronium ions. In turn, this helps the plant to increase it's intake
of zinc and phosphate. (Plants absorb phosphate preferentially
through the roots (depending on the plant, 60% to 99% of
phosphate enters the plant through the roots, even if the water
contains an ample supply of phosphate).)

- Root oxygen provides an aerobic enviornment for symbiotic fungi
that help the plant's nutrient uptake.

- Oxygen acts as an anti-anaerobic bacterial agent and protects
the roots against rot.

But aren't you also adding that to the water column as well?
Anaerobic zones can also produce H2S if there is too much organic
matter in there.

Yes -- if the redox potential drops too low (-215 mV), you get H2S.
Go lower (-244 mV), and you end up with methane (produced by
the reaction: Co2 + 8H+ + 8e- - CH4 + 2H2O). According to
Walstad, anaerobic zones in the substrate (not *too* anaerobic) are
still desirable: They aid in denitification (turning NO3- into NO2-) and
help to keep manganese in sulution as (Mn2+, at +396 mV) and
help to keep iron in solution (Fe2+, at -182 mV). The denitrifying
aspect of anaerobic pockets seems to be the most useful aspect
of a soil substrate (because I can easily add Fe, K, Mn, and so on
with fertilizer).

I've always been puzzled by the fluorite-only approach. Obviously,
many people have good results with that. But I keep thinking that
fluorite-only (especially when combined with substrate heating)
would make it difficult for anaerobic zones to get established

Why do you assume anaerobic substrates are better or worse than
aerobic or sightly aerobic substrate types for plant roots in aquatic
systems?

See above -- Krause also recommends soil and peat substrates,
also quoting anaerobic processes as being desirable.

Flourite w/o cables has pretty close to optimum flow rates. Cable add
too much.
RFUG worked great for me for a decade. That's pretty high flow.
The optimum figure for flow rate to best roots growth was about .49
liters per day of flow per meter squared. Not much.

Right. Very slow flow, so you are not constantly flooding the roots
with oxygen, but you keep the substrate sufficiently aerated to prevent
the bad anaerobic stuff (H2S) from happening. Walstad quotes evidence
of reduced plant growth with substrates that are too oxygenated. The
high-throughput under-gravel filters of the past were great plant killers.
(I never used one myself, but I've had plenty of conversations with
other aquarists who keep telling stories of weekly trips to the LFS to
replenish dying plants...)

Flourite is sandy iron rich clay. Clay has immense internal suface
area and extremely tiny pores and many binding sites.
If the clay is hardened, then kept at certain grain size, the outer
parts of each grain will be fairly aerobic but those tiny internal
pores will become anaerobic so there is a tiny micro habitat gradient
on each grain.
You'll see roots often boring into the grains.
Roots are very active.

Right. I'm sure that's why flourite is considered a good source of iron
because those anaerobic pockets would help to keep the Fe in solution
so the plants can get at it.

But if the nutrients are also supplied in the water column, they will
not take up the nutreints through the roots very much, they will take
it in from leaves and stems mainly if there's enough in the water
column.

Well, it depends on the nutrient: phosphate is taken up preferentially
through the roots, ammonia is taken up preferentially through the leaves.

This has been shown to be the case specifically with a wide variety
oif aquatic plants.
Unless you look at those levels in the water column, it's difficult to
say what amount of interaction the substrate is truly having or not.

One way to test for this is to have a tank with half with and half
with something else for the substrate. Use the same plant etc.
A similar thing can be done for lighting temp colors, different
intensities etc.

I'd probably make it two tanks in a controlled experiment because,
with a single tank, whatever leaches out of the soil substrate might
affect the non-soil part of the tank and ruin the control. It really
would be interesting to try this. But I don't have the room or inclination
at the moment to set up two tanks just to find out why the one I have
already is working so well :-)

(which,
according to Walstad, are one essential ingredient for healthy plants).
The owner of my LFS tells me that eleocharis acicularis will not grow
for him in a fluorite tank. He has two identical tanks that share a single
filter, one with a peat substrate and gravel, the other with fluorite only;
the hair grass won't grow well in the fluorite-only tank and eventually
dies off, whereas it's doing well in the tank with peat in the substrate.

Does great in my tanks. As a matter of fact, I have less trouble with
plant species in onyx/flourite tanks than any substrate I've tried,
I've tried all sorts of mixes.

Interesting. I guess this shows that you can't point at a single factor and
say "x works and y doesn't". I know that lots of people are having
great success with flourite (and I know of people who tore their tank
down to get rid of the soil they had so labouriously put there a few
months earlier...) Most likely, those flourite tanks that work contain
(or lack) one or more other things that, in balance, end up making the
whole thing work. The combination of lots of variables in the right
proportion is probably more important than the level of any single one
of them.

Given that, in nature, plants grow in mud,

All plants do not grow in mud. Many grow in sand, rocky river beds
etc.

Right. Sorry, I should have been more precise.

I know, I visit these plants/places in nature. The nicest looking beds
have rocks or sand. But there are also nice examples of mud based
substrates also.

Most of the crypt pictures I've seen show crypts growing in mud. Yes,
there is often a sand or gravel layer near the top but, as soon as you go
down an inch or so, the substrate is full of accumulated organic material.
(All that leaf litter has to end up somewhere...) Krause has some interesting
illustrations of soil cores taken in various habitats. From memory, pretty
much all of them turn into mud a few inches below the surface at the latest
(and plant roots seem to extend into the mud most of the time).

I'm waiting for my digital underwater camera to try out next week.

I'm envious! :-)

it seems that soil or peat
would come closer to natural conditions than pure gravel or fluorite
substrates, and the good results they give would stand to reason.
(But there is the mess you get every time you uproot a plant --
not pretty.)

I hate the mess too. But Clay is natural and which is what the
flourite/onyx sand is, they don't "make it". It's just natural clay
ground up.

Yes. But I'm still unsure about how I would get anaerobic zones
in fluorite. It seems that it is far too coarse for those to build up.
Maybe the micropores you mentioned are the real secret: you'd
end up with anaerobic areas inside the flourite without having
the mess you get with soil or peat. I honestly don't know...

I agree, I get no discoloration personally, but I'm not adding as
much.
I got use to it years ago, so now I use yellow colored lighting to
mimic the effect:-)

Spoken like a true traditionalist :-)

If you want to keep blackwater fish, which like soft, acidic water,
the peat has the added bonus of helping to keep the pH low. And
the humic acids that leach out of the peat are considered useful too:
they inihibit bacterial blooms and diseases because they act as a
mild antiseptic.

I am really pro peat as well.
That low pH/humic acids will help produce a reductive substrate which
is important in release of those cations like Fe, not just anaerobic
conditions.
Once the plant roots become well established along with some organic
matter, the gravel does very well.

So a little both,
A little element of peat and a little of the flourite/onyx sand does
better than each does alone. Water column dosing and substrate dosing
of nutrients works best IME. Not just one method.

I agree. Dupla seem to agree too: substrate long-term fertilizer (DuplaRoot),
plus water fertilizer (DuplaPlant tablets and DuplaPlant 24 drops). Neglect
either, and the plants won't do so well.

Glad there are still peat folks left. I do add more peat when I know
it's going to be a non CO2 tank. I use ground peat about 1 inch dry
and 3-4 inches of flourite with lots of mulm mixed in. Not much
different than the CO2 enriched set up except for more peat.

Try it and see what you think.

I might do that for my next tank. Despite having harped on about how
great peat is, I still don't like the mess it makes when I pull up a plant.
More fluorite and less peat would obviously make less of a mess.

Cheers,

Michi.

--
Michi Henning Ph: +61 4 1118-2700
Triodia Technologies http://www.triodia.com/staff/michi