Which plant or plants told you that? :o)
Rainwater contains dissolved nitrogen -- the bestest fertilizer (for everything but legumes).
I ran a pecan orchard in Alabama. We had installed a drip irrigation system to keep the trees watered. We even "fertigated" thru the system, mixing a ration of liquid nitrogen (ammonium nitrate)into the water.
But, after every rainfall, there was a palpable, visible difference in the health of the trees -- noticeable the moment I drove in the gate. They were visibly greener and fuller than they had been the previous day.
The good Lord's trees do love their rainwater...
All the plants are in containers with Miracle Grow Potting Soil MIX which gives them the fertilizer they need for six months. All are in containers large enough for good root systems to form. Plus, the larger containers are made to water from the bottom and they have attached trellis. The other containers are fabric containers so excess water drains from the sides and bottom so they don't get water logged.
I plan to charge admission, probably a hefty five cents, at some point so visitors can view the grow lamp and tiny plants coming up and the lush plants on the deck growing in containers. I'm sure you will want to bring your cameras. Free iced tea for all. Thanks for your attention,
The Marcella Food Plant Farm :o) :o) :o)
I can see a very noticeable difference in our plants of all kinds after a rain. We also run an extensive drip irrigation system to our veggie patch and flower beds and containers for their primary water supply. The drip system was originally planned to be backup, but here in E.Texas it is necessary for survival.