Increasing the nitrogen content in the soil helps the rate of plant growth. However, plants prefer to grow in an environment that is not overly acidic. Keeping this in mind, would it be wise to add potassium nitrate or ammonium sulphate to encourage plant growth?
You have a choice between potassium nitrate (KNO3) and ammonium sulfate ((NH4)2SO4). Both will add nitrogen to the soil, but how will each of them affect the pH of the soil? Let's look at what each chemical will do in water.
KNO3 is a salt that will readily dissolve in water. It will give the potassium cation and nitrate anion:
KNO3 --> K+ + NO3-
Neither of these ions will affect the pH of the soil. Ammonium sulfate is also a salt that will dissolve in water. It will give the ammonium cation and the sulfate anion:
(NH4)2SO4 --> 2NH4+ + (SO4)2-
The ammonium cation can further break down into ammonia and a proton:
NH4+ --> NH3 + H+
So the ammonium cation will add protons to increase the acidity of the soil and lower the pH. Since you are not looking to increase the acidity of the soil, it would be better to add potassium nitrate instead.