Why do farmers often plants legumes in amongst their other crops?

Answer 1

To replenish the soil nitrogen. Plants need nitrogen for protein (as do we). The roots of legumes are hosts to nitrogen-fixing bacteria, which are able to extract nitrogen (mainly from the air, which is 78% nitrogen) and make it available for the legume. After the legume is harvested, some nitrogen remains in the soil, available for the next crop.

Interestingly, the total amount of nitrogen doesn't vary; whether in the air, the soil, the standing biomass (vegetation) or the tissues of animal bodies, the same atoms are exchanged among forms in what's called the Nitrogen Cycle. An intriguing effect of this is that with every breath someone takes, he's likely taking in an atom or three that was breathed by everyone else on earth, and not just recently; the atoms themselves are indestructable, so in your body now are atoms that used to be part of Lincoln, Caesar, Buddha, your great-grandmother, and everybody else. In fact, everything that participates in the Nitrogen Cycle is represented, so you've also got some lions, some sheep, lots of trees...