Artificial nitrogen fertilisers are now readily available thanks to the Haber-Bosch process (invented in Germany in the early 20th century). They are made by reacting nitrogen gas (from the atmosphere, most of which is nitrogen) with hydrogen gas to make ammonia compounds which plants need to grow properly. The hydrogen comes from methane which comes from natural gas, a fossil fuel. And the process itself requires high-pressures and temperatures which uses a lot of energy and that, of course, also comes from fossil fuels.
Today these fertilisers are heavily used throughout
the world because they're cheap, convenient and effective
and mean more food can be grown. But there are downsides
and they are big ones: pollution of the atmosphere
and oceans. When farmers scatter it on their fields,
soil microbes convert some of it into nitrous oxide,
a powerful greenhouse gas which adds to global warming.
And because these fertilisers easily dissolve in
water, heavy rains can cause a lot of it to run off
into lakes, rivers and the sea where it creates dead
zones. Find out about these in this video from NASA: