Just found this information on a chart that I've had lying around for about a week:
Kelp is a dynamic accumulator that adds: Sodium, Iodine, Nitrogen, Calcium, Magnesium and Iron to your garden. Kelp and other seeweeds also contain significant amounts of Pot Ash also known as K2O4 or something the like. The short of the long about potash is that it is a Potassium compound that once broken down; adds Potassium to your soil!
Thanks Scott for the info on Potassium!!!
Another great Dynamic Accumulator is coltsfood; adding Boron, Magnesium and Calcium to your soil.
Flaxseed apparently adds Calcium to your soil. I think the best bet would be to grind the seeds up into a flour and spread that on your tomatoe beds but, I use it in my Breads (like the Roti that I made tonight!).
Apparently, Eastern Bracken (obviously after being mulched) adds: Potassium, Phosphorus, Manganese, Copper and Cobalt to the soil.
The list goes on, and on, and on. If anyone is curious about any sort of plant and what it adds to the soil after decomposition just ask. I'll see if it is here.