Air is 78 per cent nitrogen, and plants need it to grow. Nitrogen gas is chemically very stable because it is made from two atoms that form strong bonds with one another. Breaking these bonds requires lots of energy before they can react.
Lightning can provide this energy, breaking the bonds and leaving the free nitrogen atoms to combine with oxygen in the atmosphere. The resulting compounds are called nitrates, which dissolve in rainwater more readily than nitrogen gas. Most plants cannot use nitrogen gas but they can use nitrates. Because of this, thunderstorm rain is particularly good for fertilising plants.