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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?  And if my question was too limiting to _cation_
exchange sites, is there another mechanism that is at play here?  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?  Ya'll got me into this with the PMDD, and now I have huge buckets and
bags of chemicals and the thought of going to the Dupla method where I would
just follow directions on a box is not particularly attractive.  <g>