Rrain does not fertilize the vegetation, but the lightning that occurs puts a tiny amount of nitrogen into the soil, The link shows that bacteria is one natural non man made ways that the ground is fertilzed. decaying plant matter also fertilizes the ground.
http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/nitrogen.htm
Microorganisms have a central role in almost all aspects of nitrogen availability and thus for life support on earth:
- some bacteria can convert N 2 into ammonia by the process termed nitrogen fixation; these bacteria are either free-living or form symbiotic associations with plants or other organisms (e.g. termites, protozoa)
- other bacteria bring about transformations of ammonia to nitrate, and of nitrate to N 2 or other nitrogen gases
- many bacteria and fungi degrade organic matter, releasing fixed nitrogen for reuse by other organisms.
All these processes contribute to the nitrogen cycle.
According to the mythos taught by the watchtower, the vegetation was watered by dew.