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[APD] N budget for a plant

Like Tom, I've used a number of radically differing substrate designs but
have always ended up fertilizing N to some degree, hydroponically; partly
with fish waste & partly with K2NO3 & sometimes with urea or CaNO3. The
highest N organic substrate I used was a mixture of peat & an artificial
soil made with earthworm castings. I think I probably had some vermiculite
in there too but later decided that vermiculite just made too much mess when
you disturbed it. It certainly released ammonia into the water (I measured
it) despite several folks insisting that it shouldn't. Yes, Victoria, there
is a Santa Clays & he does reduce nitrate to ammonia under the substrate.

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 the
substrate is how to renew it. I don't much like the idea of soil-ice cubes;
I think I'd lean away from fertile soils except in pots or large containers.
I like peat for its stability & virtual lack of N & P nutrients. I also like
the idea of a natural soil or one containing humus because of the variety of
bacteria that it contains. Natural soils are also more likely to contain Fe
& Mn than something like earthworm castings.

In terms of substrate enrichment or renewal, I've used clay fertilizer balls
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. 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. 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 generous
amount of clay in the ball to prevent a crack letting pore water into the

This is an hypothetical design; as yet untried.

How many grams of N (elemental) could a typical plant use?

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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