RE: [APD] peat in substrate

["Steve Pushak" <teban at powersonic_bc.ca>]

Quintin Holmberg asked:
> the plan for
> the substrate is to use a layer of peat then profile then
> gravel.  ...
> how much of a layer of peat should i put?
> should i mix the peat with some profile or just have a 100%
> peat layer?

Peat has a number of useful characteristics. With regard to layering,
the important consideration is for peat to provide an oxygen sink to
support reduction processes. This means that bacteria slowly decompose
peat & consume oxygen. This allows manganese & iron in the substrate to
reduce thereby dissolving & becoming available to plant roots. Its best
for the peat to be well mixed with a fine mineral component such as soil
which contains the iron minerals. This promotes contact of peat &
mineral soil particles. Therefore its best not to create a layer of
peat, but rather to mix it well with soil. I'm not talking about potting
soil; I mean the ordinary dirt you find in the back yard. Potting soil
may already contain more organic material than you want & probably
contains compost materials that have too high an oxygen demand and decay
to release too much nutrients for practical purposes (at least in my
opinion). In systems without CO2 injection, such as Diana Walstad's
approach, potting soil may work better as a source to provide CO2 by
decomposition. Peat might provide some CO2 but it probably decays too
slowly to provide enough CO2 to meet the requirements advocated by Tom
for a high carbon & nitrogen regime. Remember that Diana also uses
relatively lower lighting levels & this may be one of the reasons her
approach works well.

In order for the soil to provide minerals, it needs to have a fine
texture. A fine texture material maximizes the surface area. Fine sand
is too coarse to have sufficient surface area. You would need to use a
soil with the texture of silt or clay. Silt is moderately fine but is
typically a mixture. Most dirty you find will be a mixture of gravel,
sand, silt & clay.

I have suggested to use no more than about 5% peat to soil by weight or
about 25% by volume. Thomas suggests using a couple of large double hand
scoops. Either way it should provide enough organic material to do the
job.

I haven't read any qualitative descriptions of Profile except what Tom
Barr wrote about it. I believe he described it as a highly porous
material with relatively large grains. Similar porous materials like
vermiculite tend to float and therefore need to be soaked in water for a
week or two before they will sink. Since Profile probably sinks
immediately, I have doubts about its texture & bioavailability of
nutrients. If somebody in the Vancouver area has some Profile, I'd like
to take a look at it & we could even run some simple tests to see how
much iron it solubalizes. The idea would be to mix some with about 5%
peat by weight in a sealed jar of water & then test iron levels after a
week.

There are good reasons not to use fine materials like vermiculite, clay
or dirt. The mess factor is a big one. If you have chelated trace
nutrients, you really don't need to worry too much about iron. On the
other hand, Tom has said that iron availability in the substrate may be
a growth limiting factor when other nutrient needs are taken care of.
You have to ask yourself if you want to deal with pruning or harvesting
a lot of plant material every week. You also have to worry about certain
types of plants colonizing the entire tank.

Steve in Vancouver where the sun is playing hide and go seek with the
clouds but spring is definitely well under way.


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