Where does all that plant mass come from every year?

I was admiring a large 5 m plant that I had cut back to a mere 2 m
last year. The soil appears unafected by all that growth. Why do farmers let
fields lay fallow every other year? To let it rebuild?

It appears that, in an average tree at least, some 90% of the bio mass
isderived from oxygen and carbon in the air. I would guess that if you weighed a
healthy plant, around 90% of that weight would be water, but after removing the
water, the remainder - the actual structural parts - would beprimarily derived
from elements in the air.I had no idea it was that much. That's fascinating! I'm
going to startfermenting my beer next to my plants. See what a little more CO2
does. :-)
Source: mombu.com

As the plant photosythesises it creates glucose which it then turns into starch. As the plant uses the starch for growth it gets bigger and the mass increases!(believe it or not) source: wiki.answers.com

Classroom experiment: http://www.kbs.msu.edu/k12/participants/documents/5th.Plant.Mass.Lesson.pdf