This document discusses biological nitrogen fixation. It begins by explaining that nitrogen is an important nutrient for plants but they cannot use atmospheric nitrogen directly. It then describes how certain bacteria are able to fix nitrogen by reducing it to ammonia through a process called nitrogen fixation. There are two main types of biological nitrogen fixation - symbiotic fixation, where bacteria form nodules and fix nitrogen in association with plants like legumes, and non-symbiotic fixation by free-living soil bacteria.