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Re: Chemistry questions - N-P-K ratios

> From: TomWood3 at aol_com
> Subject: Chemistry Questions - N-P-K Ratios (Paul???)
> Paul Sears wrote:
> 	"I don't know why one would expect _cation_ exchange sites to keep
> nitrate, sulphate or phosphate (anions) out of the water column.  They
> would probably not be too efective at hanging on to potassium (single
> charge) either.  In practice, I suspect that the iron oxide-based substrates
> just _might_ hang on to phosphate, and have voiced that suspicion here 
> before.  I would not be surprised if that is the main function
> of laterite.  Nitrate is almost always very mobile, and sulphate usually
> is as well.  Injecting trace elements into a substrate with high CEC
> might result in the Fe, Mn, Zn and Cu being retained there, but the
> chelating agents could prevent that - that is what they are supposed to do."
> Then call me confused (it's okay, I don't mind).  If the _cation_ exchange
> sites aren't much help in holding nutrients, then what is the point of the
> recent substrate wars?

	You may have noticed that I didn't join in.  Cation exchange sites
could and probably would store Fe++ (and Fe+++), Mn++, Cu++ and Zn++,
but given the (usually) huge (by comparison) amounts of Ca++ and Mg++
around, there is the possibility that they would be occupied mainly
by those elements.  I have toyed witht the idea of injecting various
things directly into the substrate, but more with the aim of getting
the plants to grab them than in the hope that the substrate would.  If 
I had a substrate that I aimed to use to store nutrients, my inclination
would be to use a soil-based one with low permeability or a layer of
something not very permeable on top, and inject non-chelated trace
elements into it.
> And if my question was too limiting to _cation_
> exchange sites, is there another mechanism that is at play here?

	Good question.  I would like to see some results from experiments
(preferably analyses of laterite or other substrate that has been around
for a while), rather than endless argument.  I've barely had time in 
the last year or so to do minimal maintenance on my tanks, let alone
do all the experiments I would like to.

> I guess my
> basic question is, can an "ideal" solution similar to PMDD (of whatever
> composition) be devised and applied deep in the substrate (of whatever
> composition) and be expected to stay there and stay in a form usable by
> plants?

	Try it and see.  If you want to contact me directly, please do so.

>  Ya'll got me into this with the PMDD, and now I have huge buckets and
> bags of chemicals

	I think you may have overdone it!  I have a couple of kg or so.

> and the thought of going to the Dupla method where I would
> just follow directions on a box is not particularly attractive.  <g>

	(shudder!)       :)

Paul Sears        Ottawa, Canada