On average, there are 100 lightning strikes happening over the earth per second. That translates to 8,640,000 lightning strikes per day. Apparently, 80% of them are in-cloud flashes and 20% are cloud to ground flashes. And every one of those lightnings converts atmospheric nitrogen (N2) into plant consumable form of nitrogen.
During lightning, air burns at very high temperatures. This tremendous energy causes the (inert) nitrogen and oxygen in the air to combine with rain water to produce nitric and nitrous acids.
The raindrops that you stick out your tongue and catch when it is raining, believe it or not, contains these acids in very small quantities. The acids in the rain water combine with alkaline substances in the soil and form "nitrates." If that sounds like the name of a fertilizer, that's because it is.
These nitrates along with other nitrates from animal manure, bacterial action and fertilizers strengthen the plant.
Just in case you are interested in the chemistry of this, here is how meaningless equations from your chemistry classes from school take on a practical meaning in the context of farming.
Nitric oxide oxidises into nitrogen dioxide in presence of excess oxygen. Nitrogen dioxide may react with rain water to produce nitric and nitrous acids.