KHCO3 in PMDD? & substrate Qs.

	I've  been considering using KHCO3 in a PMDD.  I like the idea of
raising my KH (to say, 3 or 4) while also getting Potassium into my tank.
At present, my KH is'0', and i don't believe the water here in Seattle
ever gets over 1 KH. Will I be able to achieve the (revised) recommended
levels of K without first exceeding my goal of 3-4 KH? Would I be better
off using Potassium Carbonate (K2CO3)?  Which is more soluble? Which will
give me better long-term KH/pH stability? 
	I was also thinking of getting ahold of some Calcium Chloride
(CaCl?) to help with the Calcium levels for my Swords.  Should I consider
getting my Ca from something else? 

	Related to the above, at some point in the future, I hope to set
up a large (100g+) planted tank.  I was thinking of using a substrate
consisting of maybe 1/2 inch of worm castings and peat, topped by 1/2 inch
fine gravel, topped by 1 inch of laterite or an iron-rich clay mixture
(that Steve Pushak found in Calgary? Could you fire that address my way,
Steve?) topped by 3 inches of fine gravel.  I was also thinking of mixing
into that first layer the abovementioned KHCO3 and CaCl in order to
prevent excess acidity in the soil and, of course, to provide K and Ca to
my plants.  What do my fellow APDers make of this plan?