Nitrogen

"It is the chemist who must come to the rescue of the threatened communities. It is through the laboratory that starvation may ultimately be turned into plenty... The fixation of atmospheric nitrogen is one of the great discoveries, awaiting the genius of chemists." ― William Crookes, Chemical News

I've tried to avoid equations, but this one is too relevant to today's discussion.  Ammonia is necessary to making fertilizer, and it can be easily made with hydrogen and nitrogen, in the Haber-Bosch process: $$ \text{N}_2 + 3 \text{H}_2 \rightarrow 2 \text{NH}_3 $$

Schematic of the Haber-Bosch process.

Nitrogen is essential to life.  Though only 3% of the human body is nitrogen, by weight—9%, if water is ignored—it gives proteins and DNA much of their chemical identity.  We don't simply absorb nitrogen from the atmosphere, like oxygen.  It is supplied through the food we eat, after it has been collected by other plants and animals.  However, modern agriculture has become dependent on artificial fertilizers.  In fact, 80% of the nitrogen in your body comes from the Haber-Bosch process.

This process relies on hydrogen as a reactant, which is abundant on Earth, in the form of water.  Most of our hydrogen does not come from water, though.  95% of it comes from hydrocarbons, with only 5% coming from the electrolysis of water.  The most common process used to manufacture hydrogen is steam reformation of methane.  This process is able to efficiently produce hydrogen at lower cost than electrolysis, though it does produce a substantial amount of carbon dioxide as a by-product.  It also requires about half the energy as electrolysis per kilogram of hydrogen.

In the United States, most hydrogen in used to refine other hydrocarbons—for example, it is used to remove sulfur from petroleum.  Only 1.6 million tonnes of hydrogen could supply our need for ammonia, which would require three times as much hydrogen to be produced by electrolysis. However, other countries use nitrogen more for the production of ammonia, and meeting the world need for cheap fertilizers could be more difficult.