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[APD] RE: substrate dosing

> I think we can agree that soil rich enough to bear lots of N will rapidly
> give up that N over a few months time so the problem with providing N in
> substrate is how to renew it. 
> I like peat for its stability & virtual lack of N & P nutrients.

Peat can bind N and P in organic complexes which can be remineralized
slowly by bacteria to be made available to plants.

 I also like
> the idea of a natural soil or one containing humus because of the variety
> bacteria that it contains. Natural soils are also more likely to contain
> & Mn than something like earthworm castings.

Fe/Mn will buffer the redox against the organic matter, allowing it go
down, without bottoming out like when you get H2S/Methane production. 
This allows you to chemically buffer the redox(eg adding laterite, flourite
etc) to a relative range.
It's also a nutrient but seldom limiting in  nature(but in our tanks it can

> In terms of substrate enrichment or renewal, I've used clay fertilizer
> but I've never attempted to develop a design to supply N via the substrate
> long term. I'd need to have some idea of how much N a typical, arbitrary
> plant could use up over say a 6 month period. 

Well use KNO3. Add it to agar/gelatin etc. This will diffuse out at a
steady rate, just like electrophoresis gel runs with DNA/proteins etc. 
Even if you have a highly mobile anion like NO3, it will still be released
The agar/gelitin will break down and non problematic.The larger the ball,
the longer the effectiveness within reason.

You can also make a layered sheet of this nutrient holding agar for the
very bottonm of your tank.
Then add balls to supplement later as needed. 

Using that I can calculate the
> number of grams of a specific fertilizer to incorporate into a few clay
> balls for that particular plant & how often to introduce a new ball. 

Well, I don't think that you need this info since much of Fe, NO3 etc will
be used for other processes like bacteria, be removed via water changes,
pruning, N2 gas, organic binding/complexes, inorganic complexes. It does
not all go into plant tissue. 

I don't
> think that denitrification will be a problem because the nitrate and/or
> ammonia can be held within the clay ball where bacteria & the other
> nutrients they require should not be able to penetrate, assuming a
> amount of clay in the ball to prevent a crack letting pore water into the
> interior.

Yes, I don't see large declines in tanks w/o plants and deep substrate.
You can measure this by removing the plants and seeing what the NO3 levels

> This is an hypothetical design; as yet untried.
> How many grams of N (elemental) could a typical plant use?

You need to pick a plant/biomass of plants/light value etc.
Focus the question down to something much more specific.
That might not be what you want, but if you want to accomlish something
like this, it will take awhile to work on each plant in question and some
dry weigh analysis and RGR's:)

You might not see your grandkids:) 
Tom Barr

> Make & state whatever assumptions necessary. I have a pair of 20 gallon
> tanks that need to have the old contents pitched & a new experiment
> designed. I'm open to ideas.
> Steve P

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