Petro (sic) only explodes in specific air-fuel situations
That seems to be true for ammonia as well, at least according to the Wikipedia page's [1] section on Combustion:
Ammonia does not burn readily or sustain combustion, except under narrow fuel-to-air mixtures of 15–28% ammonia by volume in air.
That doesn't sound too horrible, it feels like gasoline/petrol is easier to combust (although I know it's the fumes that are actually flammable).