Under ordinary conditions, nitrogen does not react with other elements, so one might not expect to find its compounds in nature, at least not extensively. Yet nitrogen compounds are present in every living cell, and they can be found in all fertile soils, in many foods, in animal products such as wool and feathers, and in many other naturally occurring substances. Somehow, chemically inert nitrogen gas is "fixed," or combined with other elements, from its "free," or elemental, state. During the 1800s, scientists learned that some nitrogen fixation processes occur in nature. Although it was already known that plants absorb nitrogen compounds from the soil through their roots, most people thought that these compounds had to be restored to the soil by the addition of organic or chemical fertilizers.

Then French agricultural chemist Jean Baptiste Boussingault (1802-1887) demonstrated that beans, peas, clover, and other legumes could...