The ground on which we live serves for walking, building houses, but in particular: to grow food in agriculture.
Normally, plants can find all needed substances and minerals out of the ground on which they are growing, but agriculture of farmers is using so many substances, that the ground gets exhausted, has no longer all needed stuff for plants.
So we fertilize the earth with extra substances. We use natural fertilizer (manure), but also artificial manure, like Potassium phosphate, ammonium nitrate, ureum, minerals with Potassium-, Magnesium-, Sodium- and Copper or Iron-ions.
Nitrate is important and contains the element Nitrogen. 80% of the air is Nitrogen, so you would suggest: Nitrogen in abundance.
But a problem is that this is Nitrogen in the air (N2) that not easy transfers to Nitrate.
Some plants are capable to change (N2) into nitrates.
Plants receive most Nitrogen from (artificial) manure, like added ureum and nitrates.
In nature we have a so called Nitrogen cycle:
Artificial fertilizer of course does not only provide nitrates.
The choice of fertilizer depends also on the desired pH in the ground.