indicator plants

Now that I've got my pH under control with DIY CO2, I want to
start adjusting my nutrients, ala Sears and Conlin.  Just reading
the discussions on this list has taught me a lot...thanks everyone!

I could use some help observing the needs of my plants.  Using a
very nice table of nutrient deficiency symptoms found in the Krib,
I can spot problems when they're threatening my plants, but I'd 
like to see hints ahead of time so that I can adjust my nutrients
to fix them.  What are some good indicator plants that I can use
to cover the nutrients I'll be adding independently in a PMDD 
approach?  I'd like to not have to buy lots of testkits when it's
possible to just look at my plants and know what they need.  Am
I deluded in thinking this?  Are there articles/postings on this
I haven't found?

I've got duckweed, which I'm using to tell me when I've got too
much phosphate and nitrogen, as well as when I've got too little
iron.  My water sprite stops growing when I don't have enough CO2 
and nutrients in general.  My amazon swords are the first to show
me signs of micronutrient deficiency (but I'd like to catch it
before then). My water hyacinth (sp?) would probably grow in bleach
in a closet, so it's no help except to tell me how rich in nutrients
the water is.  

I've got a few other plants, but I haven't figured out how to 
read them well yet.  Can someone point me to some good indicator
plants or help me hone my skill in observing my plants?  How well
can I expect to be able to discern my nutrient concentrations based
on visual observations alone?  I'll settle for the occasional 
nitrate test, but I'd like to avoid buying expensive kits...I'd 
like to develop a skill, not lab technique.