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Re: Anaerobic conditions in a substrate

Roger S. Miller wrote:

>.....Steve brought up two points that seem to be at odds with one another.  On
>one hand he said that anaerobic conditions in the substrate were necessary
>to keep typically insoluble nutrients like iron and phosphorus in a
>biologically available state.  On the other hand he said that rooted
>plants circulate water in and aerate the substrate.  If the latter were
>completely true, then the former would not happen.
>A point that is often missed when we discuss conditions in the substrate
>is that the substrate doesn't need to be entirely, homogeneously anoxic to
>provide an environment for anaerobic reactions.  Conversely, it doesn't
>need to be completely and homogeneously aerated to provide an environment
>for aerobic reactions.  Each pore space between grains in a substrate is
>potentially a different chemical microenvironment often as small as a few
>microns across.  Local conditions in the pore space are determined by the
>availability of organic compounds, oxygen and other nutrients in the
>immediate vicinity of the pore.
>If we provide a lot of circulation in a substrate, then the circulation
>tends to homogenize everything.  The tendency to homogeneously aerated
>conditions is promoted if the substrate consists of large, regular grains
>of uniform size - like commercial aquarium gravel.  There's little
>opportunity in that kind of substrate to develop closed-off pores with
>chemically unique conditions.
>At the other extreme, if there's little or no circulation, then the
>conditions will tend to be homogeneously anoxic, as the supply of oxygen
>into the substrate will rarely be sufficient to provide for the oxygen
>demand by bacteria feeding on organics compounds in the substrate.  Here's
>where we get smelly and gross conditions.
>In between these two extremes the substrate consists of aerated and
>anoxic zones that support an intimate mixture of anaerobic and aerobic
>environments.  Under these conditions the substrate should be able to
>provide a regular supply of nutrients and support a healthy and diverse
>microbe population that provides for the conversion of nutrients from
>biochemical to (plant-available) mineral forms.
>So moderation appears to be important.  Do the plants alone provide enough
>circulation in the substrate to create those moderate conditions?  The
>reports on this list seem to be mixed.  You could probably say that plants
>alone don't provide enough circulation to create healthy conditions
>in every substrate.  But in some conditions they probably provide plenty
>of circulation.  Certainly, the grain size and the distribution of grains
>sizes in the substrate (with a mixture of sizes being optimal) and the
>amount and reactivity of organics in the substrate (organics in fish
>waste are relatively reactive) are important factors.
>I wonder too, if the amount of circulation provided by the plants will
>depend to a large degree on how much of their nutrient supply is derived
>from the substrate (thus requiring the plants to circulate water to
>transport the nutrients) and how much comes from foliar feeding.  If my
>guess is right, when you provide most of the nutrients in the substrate
>this will promote root feeding by the plants and you will have very good
>substrate conditions.  Conversely, if you provide most of the plant
>nutrients in the water, then you may be promoting homogeneously anaerobic,
>nitrogen-depleted conditions in the substrate.

The two points that Steve brought up:  (1) Aquatic Plants circulate water
and aerate the substrate, and (2) Anaerobic conditions in the substrate are
necessary for making iron and possibly other nutrients, such as phosphorus,
available, are not necessarily mutually exclusive.  Roots of aquatic plants
are able to benefit from the presence of reduced, soluble iron compounds in
the substrate even though they oxygenate ths substrate.  Oxygen in the air
channels of the roots diffuses into the substrate, creating a zone of
higher oxygen content around the root called by microbiologists, the
rhizosphere.  Some reduced iron compounds are oxidized by bacteria in the
rhizosphere and precipitated, but the root still seems to be able to get
what it needs.  The concentration of oxygen decreases further away from the
root, and the concentration of soluble iron increases.  Root hairs can
penetrate into outer regions of the rhizosphere.  In my experience, the
substrate only becomes fully oxygenated and unable to supply iron when it
is densely packed with roots and doesn't have a lot of organic matter in

Aquatic plants also pull water up from the substrate.  See the article in
the "research" section of Tropica's web site.  Provided that there is
organic matter in the substrate and that the substrate isn't complegely
packed with roots, they don't pull water through the substrate so fast that
anaerobic zones can't develop,   The process of moving water from roots to
shoots is thought to be similar to root pressure seen in land plants
resulting in "guttation"---drops of water exuded by leaves.  It is not
linked with nutrient uptake by the roots.

I often use a substrate that I make by mixing ordinary topsoil with water
until it is soupy  and then filtering the mix through window screening.  I
cover about a half inch of this 'soil soup' with an inch or two of gravel
in trays.  I have found that sword plants grown in these trays for 6 months
to a year become iron deficient, and when I pull them from their trays, the
substrate is densely packed with roots and there is a layer of roots
between the bottom of the substrate and the bottom of the tray.  The volume
of the roots is enough to raise the surface of the gravel about a half an

How much organic matter should be in the substrate?  This question is
difficult to answer.  It depends on the plant.  Plants with thick white
roots that rise rapidly to the surface when cut off have large air channels
in their roots and can tolerate high amounts of organic matter.  I recall
an old article in TAG, called "Here's Mud in Your Tank", where the author
mixed 2/3 FRESH manure with soil and reported good growth of a sword plant
in this mix.  Swords, Crypts, Sagittaria have roots with large air
channels.  There are some plants, such as Anubias nana (and probably all
the other A. barteri varieties), whose roots are really designed to cling
to rocks, have very small air channels, and shouldn't be in an anaerobic
substrate at all. Many of the Aponogetons grow poorly when rooted in a peat
soil mix or a manure-soil mix, but do much better in gravel with a little
"soil soup" mixed in the bottom layer.

Paul Krombholz, in steamy, mid-90's, central Mississippi.