Nitrates are essential to plant growth, in part because nitrogen is an essential element in protein synthesis.
Photosynthesis only supplies carbohydrates. Farming without an external source of nitrates requires assiduous composting, and the inclusion of those plants that live in symbiosis with 'nitrogen-fixing' bacteria. The crop rotation system of Europe's Middle Ages did this. It is particularly desirable to include the urine of birds and animals in the compost. The traditional growing of maize in company with beans, by the earliest North American natives, was another application of the principle.
But in the ancient Inca civilization, the sun-dried dung of birds was called huano, and was regarded as a divine gift, because it made fields exceptionally productive. It is now known generally as guano.
The stuff was even found in ancient deposits, There came a time when the Europeans learned of this, and the seemingly vast deposits of South American guano were plundered and almost used up.
The Haber-Bosch chemical process came in, and human production of nitrates directly from the air and a suitable source of hydrogen led to today's vast agro-chemical industry.
It was also very helpful in the production of high explosives, taking the world from gunpowder (which uses potassium nitrate) to nitroglycerine, dynamite, and TNT (Tri-Nitro-Toluol)
So much of today's world supply of food is produced with the aid of industrial nitrates, that it is highly questionable whether the present world population can be sustained without them.
Hydrogen is required as a feedstock for ammonia production, which is the first step in making nitrates.
Ammonia is nitrogen trihydride, NH3.
It can be oxidized to produce nitric acid, HNO3, which combines with potassium hydroxide to produce potassium nitrate, or with ammonia itself to make ammonium nitrate, NH4NO3.
Both are fertilizers, and both can be fairly simply used to make explosives.
But there are no hydrogen wells, and in fact the easiest source of hydrogen is carbon tetra-hydride, better known as CH4, methane, which is the chief component of "natural"* gas.
Or if you have a cheap supply of electricity, you can electrolyse dihydrogen monoxide, H2O, better known as water, into its two component gases.
So if the world's "natural"* supply of nitrates is inadequate (which it probably is. **) it is going to cost us a considerable amount of raw energy to make up for it.
This is why the popular notion of biofuels to supplant our dependence upon petroleum has very little chance of success.
But if nuclear power can be harnessed for civilian use to the extent of using significantly more than the three or four parts in a thousand of the uranium that present commercial technology does, the energy problem of continuing to feed 6,000 million people (and more coming) can be solved.
There is a technology for doing this, it is called a breeder reactor.