Ammonia is used to produce fertilisers by reacting it with either sulfuric or nitric acid to produce ammonium sulfate or nitrate respectively.

The name of a fertiliser can indicate what elements are involved in its production. (See table below)

Preparation of fertiliser in the lab - Reaction of an acid with an alkali

A quantity of alkali is placed in a beaker and a solution of acid is run in until the solution is neutral.
This can be measured with a pH meter or by removing samples for testing with universal indicator.
The neutral solution of salt is evaporated until crystals form.
These are filtered out, washed and dried in a warm oven.

Industrial preparation of fertiliser:

Fertilisers