The science that explained this hunger for nitrogen came much later, with the discovery that every building block in a protein or DNA molecule must contain several nitrogen atoms, and that though the air consisted mostly of nitrogen atoms they were bound together in tight pairs, triple covalent bonds between each pair of atoms. Vast energy was needed to break these bonds and make nitrogen useful. In the tropics, frequent lightning strikes provided such energy, keeping the land a little more fertile, while in paddy-rice agriculture, algae and other plants fix nitrogen from the air to replenish
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