" wrote in message
om...

Tom, first of all, let me make it clear that I was quoting Walstad
on these points, so I'm innocent! :-)

- 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.)

So they use O2 as a gate keeper to prevent excess nutrients from
coming into the roots? That sounds awful speculative at best. I have
done work on Spartinia and some other marsh plants that deal with
anaerobic substrates, salt level gradients. I don't buy this agrument.

Well, that's what Walstad says. Here is the precise quote:

"[...] root oxygenation of the rhizosphere counteracts substrate toxins.
For example, excessive soluble iron is potentially toxic to plant roots.
But root oxygen release causes iron to precipitate as iron oxides on the
outside of the root, thus preventing excessive iron from entering the
roots."

The reference she quotes for this passage is:

Armstrong W. 1979. Aereation in higher plants. Adv. Bot. Res. 7: 225-332.

- 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.

Roots need O2 all the time.

Right. But, during the night, the plant can't make oxygen. So, I would think
that the oxygen deposited in the substrate by the roots during the day would
help them get through 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.

(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).)

No they don't, they will perferencially absorb PO4 right through their
leaves and stems. Reference: The journal Freshwater Ecology Feb 2000,
distilled: Aquatic plants(a large cross section of common aquatic
plants, not just one or two) will take in nutrients from the water
column if enough is avaiable.

That's interesting. The reference you quote is in direct conflict with
Walstad. Again, here is the quote:

"Many aquatic plants prefer root uptake of phosphorus. For example,
investigators showed that 3 aquatic plant species, given a choice, took
up more P from the substrate than from the water."

She goes on to describe the experiment; the plants were grown in a two-
chambered tank, with the roots sealed of from the stem and leaves.
Radio-labeled P was used to see where the P in the plant was coming
from. The plants took P preferentially from the roots. Walstad quotes over
90% P from the roots for Myriophyllum brasiliense, 59% for M. spicatum,
74% for Elodea densa, and 99% for Najas flexilis.

The references for this section a

Pedersen O and Sand-Jense K. 1997. Transpiration does not control growth
and nutrient supply in the amphibious plant Mentha aquatica. Pant Cell
Environ. 20: 117-123.

Carignan R and Kalff J. 1980. Phosphorus sources for aquatic weeds: Water
or sediments? Science 207: 987-988.

Moeller RE, Burkholder JM, and Wetzel RG. 1988. Significance of sedimentary
phosphorus to a rooted submersed macrohpyte (Jaja flexilis (Willd.) Rostk. and
Schmidt) and its algal epiphytes. Aquat. Bot. 32: 261-281.

So, I don't know who's right. Maybe it strongly depends on the
species of plant, in which case generalizations such as "P comes
from the roots" or "P comes from the leaves" would be useless.

When there is not enough PO4 or NO3 in the water column, they will
allocate more uptake from the roots if there is a source of nutrients
there but not in the water column.
Basically why transport nutrients, if you can get them right where you
need them?

I have no idea. But, according to Walstad, the P was made available
to both leaves and roots to see which the plant would prefer.

You can also see this in your tank by trying to add a PO4 source to
your gravel versus adding KH2PO4 to your water column.
You'll see immediate(within an hour or less) plant responses from
this.

I didn't know this. What response? Higher respiratory rate? (I would
think that any other response would take more than an hour to detect?)

I think this notion you are making here is valid IF you have an
environment where there is little PO4, NO3 etc in the water column,
but if you test the water column with adequate supply of dissolved
nutrients, the roots don't need to take in the nutrients and transport
them. Plants will take the nutrients where they can get it. But if you
don't test the other side of that coin, one can draw the incorrect
conclusion(s).

Well, I honestly don't know. I was quoting Walstad, who
seems to enjoy an expert reputation. But I believe she is also
considered something of a renegade in the aquatic community,
considering that she advocates putting soil into your tank :-)

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).

NO3=NO2? Plants don't use NO2. They convert NO3 at their cell's
chloroplast membrane to NO2 and then very quickly(this way it cannot
cause damage) into NH4 but NO2 is toxic to plants anywhere else in a
plant.

No, Walstad isn't implying that the NO2 is useful to the plant.
What she is saying is that anaerobic processes are useful because
conversion of NO3 to NO2 is the first stage of denitrification,
and denitrification is useful (albeit for the tank, not the plants, who
need some NO3).

You can also try this out at home in a heavily planted tank.
Try fishless cycling in a fully packed plant tank.

No, I will never try that again, not after the disaster I had last time :-)

The NH4 will decline, the NO2 will appear and the persist for 2-3
weeks, even with a fully planted tank and no outside sources of N
being added.
If plants used it, why aren't they? Simple, they don't use it.

Right. I didn't mean to imply that NO2 was used by plants. Sorry,
I didn't make this clear.

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

I've found examples where this is not true in nature and in my tank.
Plants are very adaptable and dependent not simply on the substrate
and their roots systems but the water column as well, which is much
easier to test for and measure concerning uptake rates and plant's
needs.

Yes. Again, I didn't mean to imply that what goes on in the water
column is not important. It was just that we were talking about why
roots oxygenate the substrate, and how that fits together with people
like Walstad and Krause advocating anaerobic zones in the substrate,
which would seem to be in conflict with what the plants are doing.

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.

But......there's this issues once again, if the plants are supplied
with enough water column nutrients, then the substrate doesn't matter
much if at all except for a mechanical support for attachemnt. You
cannot ignore this issue when addressing aquatic plants. It fouls your
conclusions and data a great deal.

I think this is where you and Walstad differ. At least for P, Walstad
says that the roots are more than just anchors. And, of course, she
is arguing not just about plants, but for a balanced and healthy overall
aquarium environment. For that, according to her, soil substrates are
better than ones without soil. (*Don't* crucify me for this -- this is
Walstad's reasoning, not mine (although I happen to believe her because
I've had good success with soil/peat substrates).)

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...)

I think traditional one did so so.
I use a RFUG grid of tubes like on pg 15 of the Rataj/Horemannii Book.
This worked as well as any plant tank I'd seem until I saw some
Amano's work.
I also wanted a simpler to make substrate and felt there could be a
nice balance between water column and substrate uptake. Why not have
good levels in both places?

I honestly have no personal experience with high-throughput UGFs, so
I can only pass on experiences related to me by other people. Personally,
I don't know many people who told me that their plants were growing like
crazy with an UGF, but I've spoken to lots who said that their plants
were dying all the time.

But it's interesting, isn't it: you report success with such a setup, meaning
that it's definitely possible. Again, I think this goes to show that too much
focus on a single variable isn't all that useful and that the combination of
factors is what is more important.

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.

Yes, I tend to feel this way, although I'm not certain, just
speculating (as usual.

Well, that makes it two of us then :-)

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

See above, I have not found the first part to be the case at all.
But the second part is true.

Like I said, I was quoting Walstad. Another interesting tidbit:
potassium is preferentially taken up through the shoots, rather than
the roots. Walstad reports 5 times the potassium uptake
through the shoots than the roots for Elodea occidentalis.

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 :-)

I have 8 tanks just for such a test. But it'll be a year or two before
I get to anything like that.

Bloody hell: that's what I'd call a dedicated aquarist!

The combination of lots of variables in the right
proportion is probably more important than the level of any single one
of them.

Yep, you got it.
But the relative effects of each vaiable is what is needed to be
considered.
I'm saying you'll get better growth, if you use both a good water
column dosing routine(including macro nutrients) with a decent
substrate. The water column is more important than the substrate in
CO2 enriched tanks with higher growth rates.

Yes, I buy that, definitely. Substrate isn't everything, and comparatively
less important than what's in the water, no doubt. (Otherwise, no-one
would grow anything in plain gravel but, in fact, some people are
growing plants in plain gravel quite nicely, thank you very much ;-)

In non CO2 tanks, this can be true as well but more focus is shifted
to the lower maintenace and slower growth rates which will lend more
need for a good substrate supply of nutrients.

Yes. In particular, iron and CO2 seem to be important factors
coming from the substrate in absence of Fe fertilization and CO2
injection. (Much of the organic matter in the substrate ends up as
CO2 when it decomposes.)

Also, dirt wears out after awhile. No nutrients left. But you can
freeze soupy soil in ice cube trays and push these down under neath
plants etc.
Or clay balls mixed with dirt.
I'm more partial to peat myself.

I agree. Walstad also quotes issues around substrate depletion for
non-fertilized and CO2-injected tanks: eventually, most of the organic
matter has been used up and, suddenly, plant growth is stunted because
one or more essential nutrients have gone missing.

I tested plain old gravel with laterite vs flourite and considering
the make up of each(we had them analyzed by a lab), the physical
stucture was the only real difference I could come up with of why one
did/does better.
The flourite can also be moved around, disturbed etc.

Due to the micro habitat being preserved in each grain, there's not
need to worry about disturbing a larger "macro layer" although some
does form in flourite onyx sand substrates. Just not nearly as a
dependent issue I think.

That makes sense to me. (And, sorry to press the point, it would seem
to support Walstad: if fluorite indeed does support anaerobic processes
in its pores, and plants do better with it, that would seem to indicate
that the anaerobic processes are good for something :-)

Amano's substrates, the Powersands, have peat covered light weight
clay rich high surface area grains.
I use something like Scott's ground peat, no loose moss etc.
Taste great on Chicken:-)

Cultural differences, I'm afraid: as an Australian, I don't know Scott's
peat, so I'm not getting the joke. Is Scott's peat something that's popular
for smoking/barbequeing chicken? If so, I got the joke -- if not, someone
will have to help me out and reduce my ignorance level :-)

Cheers,

Michi.

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