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Re: slow turnover of water in substrate

> Date: Tue, 4 Nov 1997 13:51:12 +0800 (SGT)
> From: Casey Huang <yhplsing at singnet_com.sg>
> Subject: Slow turnover of water in substrate
> Hi Roger,
> Looks like you are the right person for some explanation.
> I have posted this awhile back but did not get any reply.
> Would appreciate your input.
> Casey Huang
> yhplsing at singnet_com.sg

Well, either Dr. Dave or Paul Krumbolz would be better at this than I am,
but I'll see what I can do.

> - ----------------------------------------------------------------
> Hi All,
> Could someone please offer an explanation concerning
> oxygen requirements for plants root.
> According to TOA there should be slow / small movement
> of water in the substrate through substrate heating so
> as not to have too much oxygen in the substrate inorder
> to get reduction type of processes for the nutrients in
> substrate so that the nutrients can be easily absorbed
> by the plants.

Low-oxygen conditions in a substrate promotes the availability of some
nutrients.  Iron seems to be most commonly cited.  Iron and some other
essential metals form insoluble compounds in their oxidized states, but
form more soluble (and so more readily available) compounds when oxygen is
absent.  Low oxygen supplies also suppress bacterial nitrification, so
ammonium in the substrate remains readily available to plants.

However, plants are inherently aerobic organisms - meaning that they must
have oxygen in their tissues, or they will die.  This includes their
roots, which die without a supply of oxygen.  But aquatic plants are
equipped to supply their roots with the oxygen they need, which allows
them to grow in anoxic substrates.

I've read one account (and just one, I think - this isn't my field of
expertise) of substrate conditions so anoxic that even emergent aquatic
plants couldn't survive - that was in a coastal marsh, not in a pond or
aquarium.  I think that normal garden crops are a different matter - that
most terrestrial crops will die in anoxic soils.  I've also read claims
that aquatic plants will aerate a substrate, but the most well-quantified
study I've seen (it focused on plants in constructed wetlands) showed that
plants didn't maintain aerobic conditions more than a few microns from the
roots. That makes sense to me because it seems like a counter-productive
waste of energy for a plant to aerate more than it has to.  Too bad I
don't have these references on hand.

Some time ago I read a comment by Dr. Dave that indicated that plants need
iron in the substrate but it didn't matter whether the iron was oxidized
or not.  I thought that plants had to have iron in its reduced state.  I
wouldn't mind at all hearing a little more about that.

> According to books about Hydroponics the nutrients are
> mixed by aeration with the air hose in the culture trough
> to have more oxygen in the nutrients to support the
> metabolic processes associated with root formation and
> subsequent growth.
> The same for Aeroponics which is self explanatory from
> Aero in the word Aeroponics.

This is where I break down to speculation, and someone else could probably
give a better answer.  It seems inevitable to me that the plant's task of
providing oxygen to its roots places an energy burden on the plant.  If
the roots are kept under aerobic conditions, then the plant doesn't have
that burden, and the energy can be used for growth or fruiting.  Of
course, if the plant in question is a crop plant (corn, for example) that
can't survive in an anoxic soil, then the problem is worse than any mere
energy burden.

With the plant roots under aerated conditions the gardener has to supply
the plants with all nutrients in a state that the plants can readily use -
so a careful and intensive fertilizing routine is necessary.  I guess the
gardener gets to decide whether saving the plant a little energy is worth
all the time, energy and money that they have to put into the system to
make up the difference.

Most natural soils probably support a fairly wide range of conditions.
One tiny pore space will contain oxygen and support aerobic root growth
and bacterial activity while an adjacent pore will be completely depleted
of oxygen and support anoxic conditions where nutrients are readily
available.  A plant's roots can access both pore spaces and enjoy the
benefits of both aerobic and anaerobic conditions.

I certainly hope that if I screwed things up too much someone will correct
me.  I'd hate to see any misstatements propogated.

Roger Miller

In Albuquerque, where the fall weather is incredibly, boringly perfect.