- Why do roots exist?
by Elizabeth Worobel <eworobe-at-cc.UManitoba.CA> (Tue, 18 Jun 1996)
- anaerobic substrates
by eworobe/cc.UManitoba.CA (Sun, 21 Sep 1997)
- Re:Growth problem & anaerobic substrates
by Stephen Pushak <teban/powersonic.bc.ca> (Tue, 23 Sep 1997)
- RE: Nutrient Uptake: Roots v. Leaves
by Charley Bay <Charleyb/Cytomation.com> (Thu, 14 Oct 1999)
by Elizabeth Worobel <eworobe-at-cc.UManitoba.CA>
Date: Tue, 18 Jun 1996
Several different types of experiments have been carried out in attempts
to answer this question. The first type, first tried almost 100 years
ago, asked the question "Do rooted aquatic plants grow better with a
nutrient rich substrate or with a sand substrate and a nutrient rich
water column." The data clearly indicated that rooted aquatic plants,
though they will grow on sand with nutrients supplied in the water
column, grew far better with nutrients supplied through a rich substrate.
These experiments have been repeated many, many times since with many
different types of rooted aquatic plants and the data consistently show
that plants grown on substrates outgrow those grown on sand with
nutrients supplied through the water column.
The second question was "Which nutrients can be supplied exclusively from
the sediment and which must be supplied via the water column." The data
clearly indicate that P and N can be supplied from the sediment and that
S and micronutrients may also be supplied exclusively from the sediment
(the data for N and P is much more extensive). The only nutrients which
are needed in the water column are Mg, K, Ca and of course CO2. These
consistent for several different types of rooted macrophytes on many
different types of sediments.
The third question was "Which nutrients actually are supplied via the
roots from the sediment." This typ of experiment is much more difficult
to carry out but the evidence indicates that N and P are obtained by
rooted aquatic plants from the sediment, even when readily available in
the water column (this includes genera such as Elodea and Myriophyllum
which have small root:shoot ratios).
The fourth question is "Which nutrients can be supplied exclusively from
the water column." As far as I know this remains unanswered as it is
extremely difficult to manipulate the nutrient content of saturated soils.
Date: Sun, 21 Sep 1997
Who said anaerobic substrates are a bad thing? There are several GOOD
THINGS that happen when the substrate is anaerobic;
1. Ferric iron and manganese are reduced to ferrous and manganous forms.
Both are more soluble than the oxidixed species.
2. As an added bonus, when iron is reduced, phosphates are liberated from
the ferric oxyhydroxide colloids that are formed under aerobic conditions.
3. It is true that denitrification occurs but under anaerobic conditions
nitrifying bacteria can quickly fix N2 into organic compounds. The result
is an increase in ammonia. This is beneficial since studies have clearly
and consistently shown that aquatic plants prefer ammonia over nitrate.
4. Aquatic plants in a natural setting ALWAYS have their roots growing in
anaerobic substrates. They have adapted to these conditions and indeed
some species can not produce root hairs UNLESS the substrate is anaerobic.
Additionally, repeated studies with a wide variety of aquatic plants have
shown that plants grown on fertile substrates ALWAYS grow significantly
better than those grown on sand ...even when a full complement of mineral
nutrients is supplied in the water column. Its amusing to me when I see
all this time spent on CO2 injection (especially by beginners) in an
attempt to optimize growth when the fundamentals such as light and substrate
have not been addressed properly.
BAD THINGS that can happen;
Gases such as sulfide, methane, nitrogen or combinations of these can be
formed. The solution is to take the soil you want to use, put it in a
large pail or tub, and submerge it for several weeks. Observe carefully
and if you smell sulfur compounds then try another substrate. Eventually,
like Paul Krombholz, you will find a process that works for you (even
with the unlikeliest of substrate materials :-).
A large nutrient release can occur as the substrate becomes anaerobic.
This may cause algal blooms. Again, to deal with this problem, submerge
your substrate in a large pail and let it sit for some time. Another
solution, of course, is to place 1" to 1 1/2" of coarse sand on top of
the fertile substrate to act as a seal.
by Stephen Pushak <teban/powersonic.bc.ca>
Date: Tue, 23 Sep 1997
Dave Huebert <eworobe-at-cc.UManitoba.CA> wrote:
>Subject: Re: anaerobic substrates
>BAD THINGS that can happen;
>Gases such as sulfide, methane, nitrogen or combinations of
>these can be formed.
I'd like to add one or two (dozen) points to Dave's excellent remarks.
All substrates are anaerobic (or more correctly anoxic, without
free oxygen) below about a half inch of the surface. As you go
deeper down the oxidizing chemicals get used up by certain bacteria
types (aerobic, facultative and anaerobic). This change in chemical
(biochemical) balance is called the redox potential. It is positive
in oxygenated water (600 mV) and decreases below the surface of
the substrate according to depth. It reaches a minimum value (~150 mV)
at about 5-6 centimeters of depth in nature. With unnaturally high
of labile materials (such as from potting soils etc) I think you might
get the -150 mV redox potential at a shallower depth. Note that methane
formation does not occur until below -150 so contrary to what I'd said
in a previous article, sulferous substrate bubbles may be primarily
nitrogen. Certainly if they're not smelly.
The following table helps describe the relative reduction processes
which occur at these redox potentials:
the Sikora & Keeny paper "Further aspects of soil chemistry under
conditions" 1983 in Mires: swamp, bog, fen and moor. Elsevier,
The Netherlands. table 6.1:
Possible systems operating in flooded environments as related to
redox potential (Takai & Kamura 1966 etc...)
System Redox (mV)**2 Micro-organisms involved
O2 disappearance +500 - +350 aerobes
Nitrate disappearance +350 - +100 }
Mn2+ formation below +400 } facultative anaerobes
Fe2+ formation below +400 }
Sulfide formation 0 - -150
Hydrogen, methane form. below -150 obligate anaerobes
Paul K supplied some notes from an older article: MORTIMER, C.H.,
The exchange of dissolved substances between mud and water in lakes. J.
Ecol. 29: 280-329.30: 147-201.
Mortimer made a graph of redox potential versus substrate depth in mud
an eutrophic lake and also in mud from an oligotrophic lake. In the
2 cm. of the eutrophic mud the redox potential went from 600 mv to about
mv. It reached a negative 100 or so mv. at about 5 cm. and then
increased a little with increasing depth to about 0 again. The redox
potential in the oligotrophic mud dropped to about 150 mv. at 5 cm. and
then stayed the same thereafter. He gives ranges for verious reductions
plant nutrients that differe a little from those you cite above:
NO3-----> NO2 0.45 to 0.40 volt
NO2-----> NH4 0.40 to 0.35 volt
Fe+++ ------> Fe++ 0.3 to 0.2 volt
SO4 ------> S 0.1 to 0.06 volt.
Note that the sulfur reduction is to S, not S--.
The presence of labile (decomposible) organic materials below
about 2 inches of depth will probably lead to a redox potential
sufficiently low to produce sulfides. The long and the short of it
is that there isn't much point in putting organic components deeper
than 2 inches except for very small amounts of humus such as you
would get by removing all the organic fibers from a soil as Paul K does
to get his mineral soil. You could mix a small portion of this with
silt. Dupla laterite has about 0.1% humus I think. You need very
little to get the redox low enough to reduce iron and manganese
to their soluble states. A layer of soil 1/2 inch deep is quite
sufficient. The point of having anything deeper, I think, is to
increase the volume so that your can get enough root area for
certain plants to get enough iron. That may be a moot point if
you're going to add chelated iron such as by PMDD, Flourish, Tropica
Master Grow, Dupla-24 etc. Not withstanding, I believe that most
rooted plants grow much better with iron compounds in the substrate
such as laterite, iron rich clay, soils, micronized iron...
These iron compounds are important to the phosphate processes
which Dave aluded to since iron binds with phosphate in such a
way that plant roots can get at it.
When we talk about the bad things in an "anaerobic" substrate,
we should probably use more accurate terminolgy such as low redox
potential coupled with excess labile material. Anaerobic means
without air whereas anoxic means without oxygen. There are other
chemicals which exist in differing concentrations within the
substrate near the surface which also act as oxidizing agents
such as nitrate and at lower redox even sulfate.
>A large nutrient release can occur as the substrate becomes
>anaerobic. This may cause algal blooms. Again, to deal with
>this problem, submerge your substrate in a large pail and
> let it sit for some time. Another solution, of course, is
> to place 1" to 1 1/2" of coarse sand on top of
>the fertile substrate to act as a seal.
I've not been successful in getting coarse sand to act as a
barrier to ammonia and nitrates. I'd recommend people avoid
overly fertile materials or leach them in pails as Dave suggests.
By the way, you can expect a pail of mud-water to go low-redox
and produce mercaptan (sulfer) gases since it has no oxygen
sources such as plants and the soil is probably too deep to
permit oxygenated water to penetrate. I don't know if that's
bad. It should liberate a lot of the nitrogen and phosphorus
compounds so that they can be leached out. I'd like to hear more
about it if somebody tries it.
Paul K has mentioned he doesn't worry about the nutrient release
with most of his soil substrates because he usually grows them
without fish and using daphnia which happily eat the green water
I found ammonia production occured within the first few months
of submergence so this ought to be monitored weekly. Usually
makes stuff grow like mad and not all types of plants were able
to grow well using the earthworm casting substrate. If they had
strong, established root systems, I think most plants would have
grown ok. Crypts had no problems under these conditions and showed
no tendency to Crypt melt. I think they are at a disadvantage
under "typical" conditions and this is why they are thought to be
slow growing. YMMV.
Steve Pushak in Vancouver BC
by Charley Bay <Charleyb/Cytomation.com>
Date: Thu, 14 Oct 1999
> On Wed, 13 Oct 1999, MBCREATIVE wrote:
> > > It is my understanding that aquatic plants DO NOT absorb nutrients
> > > their roots, that the roots are only for anchoring and that the
> vast bulk of
> > > nutrient intake is in the plant above the roots. Anyone care to
> > Wed, 13 Oct 1999 "Roger S. Miller" <email@example.com> replied:
> > That is wrong.
> > Aquatic plants can take in nutrients at either roots or leave. I've
> > that that aquatic plants switch from root feeding to foliar feeding
> > dissolved nutrients are high enough to supply the plant.
> > Taken literally this means that even if the substrate is very rich
> > plant won't use nutrients in the substrate if there are sufficient
> > nutrients in the water column to supply the plants. I'm not sure
> that was
> > the author's intent in writing that passage, but that's how it was
> > written.
> Mon, 11 Oct 1999 Ed Street <firstname.lastname@example.org> also replied:
> > this is simply NOT true for all aquatic plants. While some fall
> into this
> > catagory like java moss which has modified leafs (if you can call it
> > to cling to surfaces you also have others like the sword family
> which DOES
> > have standard roots as we think of them.
We all know that our Anacharis canadensis can't
possibly support it's fifteen feet of growth from
those two or three wimpy roots that it put out.
However, it does have little roots all the way up
its stem in time to assist in uptake to support
the increasingly inefficient growth of aging tissue.
The primary issue in nutrient adsorption is the
gradient between the plant and the environment.
Of course, there is a lot the plant can do to more
efficiently capitalize on the gradient... but in general,
if it's in the water column, it will permeate the
foliage. If it's in the substrate, it will permeate the
roots (assuming the ion is small enough and the
plant doesn't select against it). If the nutrient
is in both, it can be taken up from both foliage
and root system. However, it may be more
"efficient" to uptake a lower concentration in the
water column directly through a more inefficient
mechanism in the foliage than to uptake a relatively
higher concentration in the substrate through a
relatively more efficient mechanism in the
roots, with the requirement that the nutrient is
mobilized systemically for eventual use at the
actual site of growth.
Thus, aquatic plants can and probably usually
do both. Depending on the relative concentrations
of nutrients in the water column and substrate and
target site of nutrient need, as well as the aquatic
plant's morphology (are they "true" roots or not),
the plant will reach some ratio of column/substrate
uptake. The total uptake rate between foliage
and root system will determine plant growth rate
(assuming no limiting nutrient ratios).
Remember that the environments are very different,
though. Terrestrial plants usually have a protective
epidermis that decreases evapotransipiration (water)
loss, provides resistence to abrasion, weathering, or
toxins, may make the foliage unpalatable to
critters, etc. This function is often decreased in
aquatic plants so aquatic plants are often more
successful at uptaking nutrients from the foliage.
Also, it is less typical for terrestrial plants to
have nutrient access at the foliage level other than
CO2, sulfur, and whatever else your acid rain brings
you, (and that's a very inefficient uptake as well,
unless you live next to a smoke stack) so the roots
play an increased role in terrestrial plants. (I have
read that nitrogen fixation in the atmosphere does
occur from lightening storms, and since that's often
the limiting terrestrial nutrient, that's why grass is
so green after the thunderstorm.)
Also remember that nutrient uptake in terrestrial
plants is usually driven by transpiration... water
taken in at the roots, and pulled up the plant
through the osmotic gradient (evaporation at the
leaves leave a lower gradient there, so the water
is pulled up). If you don't transpire your water,
it's pretty hard to get those nutrients up there.
That's the "real" reason plant growth stops on
hot days... the plant can't absorb water faster than
it loses it, the stomata close at the foliage (they
dry out and shut), and the nutrients aren't going
up to where they need to be. Because aquatic
plants typically don't have the osmotic gradient like
terrestrial plants (it's hard to have evaporation
when under water), they are less efficient uptaking
nutrients from the substrate, and need to rely more
heavily on foliage uptake.
Of course, this is a much greater problem
if you are 40-meter oak tree, with terminal
meristems and growth far above the ground.
You've got a long way to go to get those nutrients
to the mitotic (growth) site. Our aquatic stem
plants (Cabomba, Anacharis, Bacopa, etc.)
have terminal meristems like trees do, and grow
from the tip. However, there are far more
rosette type plants like our Echinodorus,
Anubias, and Crypts that have a basal (not terminal)
meristem, like grass: They grow from the
base. Thus, foliage uptake and root uptake
only need to get the nutrients to the "middle"
of the plant, at the base of ground level where
the growth actually occurs (most "growth" of
leaves on our leafy plants that we perceive outside
the plant's "base" is actually not "growth",
but cellular elongation... the cells are all there
when the leaf moves outside the "base" of the
plant [at the basal meristem], but they are very
small; the cells simply stretch and fill out
to make the leaf increase in size. That's why
the most nutricious part of the plant is at the
site of growth... dense DNA, more proteins.)
Sorry this is long... I just get excited about
some topics here.
Isn't this cool stuff?