Re: Substrate vs water column

> From: Miles Morrissey <mmorriss at sophia_smith.edu>
>                 I have recently set up a 20 G long with a vermiculite/soil/
> worm castings substrate covered with a couple of inches of gravel.  I do 
> 20-30 % water changes every other week with ph 7.5 water and have a DIY 
> CO2 infusion setup that keeps the water ph below 7.0.  My question:  Is 
> the substrate enough fertilizer for the plants or do I also need to 
> supplement with fertilizer in the water column?

I'd say it depends upon two things.
- if your tap water contains sufficient Mg & Ca
- if you are adding fish food to supply the major macro-nutrients
which are needed as time goes by.

I don't know if K is present in tap water nor if it is freely available
from decaying material but I suspect the latter may be true.

If you are interested in faster growth rates you may need CO2 injection
in proportion to lighting levels. We tend to forget that CO2 is an
important nutrient source of carbon and oxygen.

The availability of iron from the soil depends upon how much iron
is in the soil and how much oxygen the roots of the plants are putting
into the substrate which will greatly reduce Fe availability as the
substrate becomes root bound. It is likely that you will need to
resort to additions of Fe water in a mature system. Note that some
plants notably Cryptocorynes, may have methods of extracting Fe from 
the substrate even under high oxygen conditions according to observations 
by Paul Krombholz.

Iron is abundant in most soils and available from those with a fine
texture such as those containing clay. Lateritic soils may have as
much as 300mg/g (30% by weight) of iron. Median or typical values for
all soils and sediments are reported to be 40 mg/g. (from Diana
Walstad's article "Iron The Limiting Nutrient for Algae?" in TAG
6:4) Coarse textured materials like sand and gravel will not be able
to provide significant Fe. Granular laterites may provide large
amounts of Fe initially but their texture is not fine enough.

Another point to remember about organic and soil substrates in general
is that they will go through a transition period that lasts several
months upon initial submergence. During this time, depending upon the
amounts and type of organic material, there will be a lot of bacterial
activity and the release of certain compounds which not all plants
are prepared to cope with equally. Aquatic plants are well adapted to
doing this by conducting oxygen to their root system via a air channels.
Does anyone know the scientific name for those air channels?

An old posting by Paul K refers to the aqueous nutrient channel(s) as
the xylem but this might refer to the tissue not the tubes.

I've found that Crypts, Alternanthera spp, and Lobelia (with thick roots 
and leaves) were the fastest early colonizers in a transitional high 
organic soil, then followed closely by Hygrophila spp and Bacopa. 
This may have been more related to the setback by bleaching and the 
thick stemmed and thick leaved plants had a big advantage there. During 
the transitional period I think the plants were also sensitive to having 
too much top leaf material harvested. It is probably also best to use 
healthy plants from another algae free aquarium for the initial planting. 
You really don't want to introduce any types of filamentous algaes and 
established clean, fast growing plants are good for consuming the early, 
high amounts of nutrients which may be released.

 Steve Pushak - spush at hcsd_hac.com - Vancouver, BC, Canada

There are no signposts in the sky to show a man has passed that way
before. There are no channels marked. The flier breaks each second
into new uncharted seas. - Anne Morrow Lindbergh