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I saw this a while ago but never sent it off:

> Only under the special conditions including substrates relatively high in
> cation exchange capacity and oxygen content can the tremendous surface area
> and nutrient adsorbing power of root hairs be optimized in an aquarium.

Don't roots bring O2 down to the substrate? They are adding the O2 to the
substrate actively. Plus if the water column is saturated or above
saturation with O2, diffusion will occur at a faster rate. If the water
column has high DO and the gravel has a low DO level there will be a
substantial increase in diffusion across the boundary layer. Current of the
water above, grain sizes, root activity.......roots are very active
participants here, some grow as fast or even much faster than the above
ground parts. We don't see this till we remove the offending plant. The
roots are out of sight. Many times we see plants just sit there as others
grow etc, but we don't get the whole picture if all we observe is above
ground growth. 1/2 the plant lies below the ground.
Aquatic plant roots in particular, make their own aerobic rhizosphere. If
there's little nutrient content in the soil, and there's enough in the water
column, they will not spend as much resources on roots. Why would they waste
the energy, resources and effort?

I wouldn't if I were a plant(but I'm not a plant). If I was forced to only
deal with what my environment dealt me, I'd try to do both methods and be
able to switch my resource partitioning by reduced roots growth and instead
put it into increased leaf/stem production.
Similar conclusions have been shown in the literature specifically with a
number of aquatic plants on this topic.

Resource partitioning (where the plant directs its resources) is very
important and holds a great deal of promise in the agricultural field and in
nature if the mechanisms are well understood and/or controllable. Getting 5%
more production is huge news in that field.

> Plants
> in other systems have no cation rich substrate for roots to exploit, so
> don't mass root hairs.

Why would they if they can get it from the water column? Why waste all their
goodies on something they are not getting that much back from?
If you are not getting anything from that old weak leaf, drop it off, it's
wasting energy. 
Not all plant root systems behave the same to resource limitations whether
they be patchy in nature, layered or uniform.

NO3 limitation causes serious reductions in most all photosynthetic
pathways, uptake rates, activity, CO2 uptake etc. And root growth/uptake.

> They are less efficient.

Not true. Why would a plant translocate something from the roots all the way
to the tip of a growing stem if it did not have too? It could get it right
there without having to move anything. The cost of transport is not a
Keeping the water column "lean and clean" doesn't help your plants to grow.
NO3 limitation does some serious uptake and developmental issues on aquatic

The plant is responding to the set of conditions you have given it. Forced
root translocation and limiting nutrients, is far from "optimum"(whatever
the hell that means).
The relative percentages of roots and above ground parts will have a higher
root:stem/leaf ratio. If you fed the water column more, the ratio will be
less. Nothing wrong with a big root system or anything but if the plant has
more resources allocated to it's food making apparatus( healthy leaves), it
will produce more O2, be better able to respond to environmental changes

The down side, and there always is one, is that if you neglect the tank
often, don't dose the water column etc, then the plants will need to rely
more on the roots for nutrients.
Many tanks bounce back and forth with rich to lean water columns turning
on/off these mechanisms. That's likely tough on plants.

I think it gets down to personalities and routines. For folks that can
remember to dose weekly to every 3 days or so, the water column is good. For
folks that cannot do this, the root method is better.
A little of both can also work, but if the tank is getting well fed in the
water column, it will not matter much to the plant's roots whether a lot is
there or not. Iron and NH4 seem to still be taken up even at high water
column levels.     

> Plants in these other
> systems can do quite well taking nutrients up from the water by absorption.
> (Absorption is the drawing in of nutrients as the plant draws in water.)

Via "Bulk or mass flow" I'll assume?
There's big old problem with this notion of getting your nutrients in this
It doesn't take in what the plant needs in the correct ratios. It's
indiscriminate. It can pull in 1 PO4, 1 NO3, 1 NH4, no K, no Mg etc.
This is another problem with many dosing routines, folks will add not enough
of one, while adding too much of another.

Water column levels can vary a fair amount over a week's time.
Substrate levels? Perhaps but not as much I'd say.
Plant's take in the nutrients as the roots moves through the soil, the
bacteria/ fungus make nutrients available, exoenzymes etc. Root cultivated
Microbes alone can dramatically change a substrate (consider a "mature"
substrate vs a new one etc. Why does one grow better than the other?).
Having a plant switch back and forth between starvation in the water column
and trying to get it all by root inception, absorption etc may have a higher
cost. Some nutrients move around more than others so this can be
independent. Over all? I think plants can switch pretty fast between these
allocation methods.

> Higher plants have developed vascular systems which prefer to adsorb through
> roots.

Has this been shown? I've found a little of both mainly iron and perhaps NH4
is the main two that seem to prefer to come from the substrate but water
column dosing does help well for both. Roots take in both NO3/NH4. Leaves
only take in NO3 from what I've read and the references I've come across
thus far. Any plants take the NH4 via the leaves?
Some references show uptake of water column vs the soil to be clearly
opposite of this claim. I would substantiate this further by saying from my
own musings, water column uptake certainly shows remarkable growth for being
so inefficient:-)

But a lean tank will have more rooty plants, a water column a more leafy
plant. Roots are good for getting nutrients, but they also incur a cost to
the plant, they require lots of O2 and lots of metabolic goodies. Which is
better? Well that may be a bigger question. I feel a high water column
uptake, high DO level, less rooty plant makes for a better plant able to
produce more "food"(photosynthate) to allocate in growing leaves, runners
and flowers. On one hand this is evens the playing field with algae but
there are other issues besides the root advantage to a nutrient source.

> application always leads to more questions. Therein lies the
> joy.) Don M.

So do more questions:-)
Thanks, just poking around in the dirt Don:-)
Tom Barr