Our planet’s air itself is, of course, a mixture of gases. Nitrogen forms about 78 percent and oxygen about 21 percent, the rest being minute quantities of argon, carbon dioxide, helium, and so forth.

Lightning shoots through the air and causes nitrogen to form a compound that the raindrops carry down to the earth. There plants use it to grow. The plants, in turn, are eaten by animals or they die and decay. Bacteria acting on decaying plants and animal manure release nitrogen back into the air. The cycle is complete.

NITROGEN CYCLE

Lightning combines nitrogen with oxygen. Rain brings this to earth

Green plants provide food for animals and humans

Bacteria take nitrogen from the air for plants’ use

Bacteria act on decaying plants and animal manure; release nitrogen back into air. Other bacteria produce plant food