Potassium from organic decomposition

To the APD and others:

I've begun the long awaited draft of an article on substrates.

I would like to comment upon the role of organic decomposition
in providing potassium for aquatic plants. Here are some questions:

Are there any low solubility K salts that might precipitate
K in an aquarium? (I don't think so)

Is there significant K released initially from the decay of
plant material, fish food or fish wastes? 

How long does K continue to be released from partially decomposed

What form does K take in living organic material? Are some of
these forms stable for long terms during decomposition?

Is peat, cellose or lignin a source of K?

Are there mineral sources for K in soils?

What happens if there is too much K a) in solution? or b) within
the interstitial water of the substrate? Does it cause osmotic
damage of plant roots (burning?) or have other undesirable
chemical reactions?

Is there any preference for K in the substrate or in solution?