Nitrate, soil, Calcium and redox

> From: psears at NRCan_gc.ca (Paul Sears)
> > The problem is in the startup period because Fe
> > will not become available until the redox in the Fe layer becomes
> > low enough and the bacterial cultures become established.
> 	I'm not convinced about this.  Terrestrial plants seem to
> get iron from loose soil in small pots.  I suspect they can get
> iron from soils that are not anoxic. 

The mechanisms for terrestrial plants are different. In natural
soils the plants are certainly able to send their roots down to
zones where free iron is available. A common problem which may
occur during flooding is that too much iron can become available
and it interferes with the uptake of other nutrients. In flooded
conditions there is a lack of oxygen in the soil and the resultant
products can kill the plants very quickly. Aquatic plants have to
deal with this environment and they do this by supplying oxygen
through their roots. Some plants do this better than others and
it is highly dependent upon the particular habitat that the plant
is able to grow in. Many, many factors come into play including
the concentrations of sulfer, nitrogen compounds, organic materials,
iron compounds, manganese, as well as pH. In addition the establishment
of the anaerobic bacteria cultures must be important for an aquarium
in which the substrate has been newly placed and the concentration
of oxygen in the substrate will change substantially over a period
of weeks due to biological oxygen demand (BOD).

In reference to house plants and Fe, similar mechanisms occur
but things are a lot more precarious and it's extremely important
that the moisture in the soil is neither too much nor too little.
Often, gardeners help the house plants along with the addition of
chelated Fe in small concentration during watering. 

> > 1) The Ca & Mg ions will displace NO3 which may occupy CEC sites
> > This will liberate nutrients to the water column.
> 	Nitrate is an _anion_.  It will not be held on _cation_
> exchange sites!  Nitrate is difficult to hold anywhere; it is
> _very_ mobile.

Indeed! Ca & Mg will displace other cations but not nitrate.
Nitrate is held in the complex organic molecules and is released
by the bacterial action of decomposition.

> > 2) It will mediate the pH and encourage bacterial activity which
> > will stimulate decay and liberate even more NO3
> 	Could you please explain this?  "mediate?"

Soils are often acidic by virtue of the organic acids. CaCO3 or lime
will mediate the pH by bringing it to a more neutral pH, more favourable
to the bacteria responsible for organic decay. Lime is quite alkaline
and too much could make the pH too high. Lime is used to speed

> 	As someone already pointed out, redox potentials apply to
> equilibrium situations.  They are typically used to see which
> phases are stable in possible corrosion situations (i.e. very
> long-term stuff, with simple component mixtures).  They will
> be of very limited use in a non-equilibrium situation, such as
> we have in a soil substrate.  Also, the reading you get from your
> "redox meter" will depend on exactly which electrode reactions it 
> uses.  I think you are really interested in oxygen concentrations,
> and these may indeed give you some insight into what is happening
> in your substrate, but please don't look at a "redox potential" as
> a magic number that describes everything.

I think it is true that oxygen is one of the dominant agents in 
the chemical and biological processes in the substrate but probably
not the only one. I'm not the one who introduced the use of the
term redox potential in scientific literature on this subject. It
is at best only a gross type of indicator since the concentrations
of so many substances come into play. As has been pointed out by
some others, I need to learn more about the experimental methods
used by aquatic soil researchers before I could use any measuring
devices. Eh (redox potential) is not an independent attribute but
a measure of several factors.

Reference: Drew & Lynch "Soil Anaerobiosis, Microorganisms, and 
Root Function", Annual Review Phytopathology, 1980. 18:37-66

My data on Ca concentration in Vancouver water supplies is from
free  analysis reports supplied by the GRV water utility. Unfortunately,
I can't locate the reports right now. :-)

Steve P