Ammonium Sulphate is used in most yeast nutrients. as one of the ingredients.
When You Need Yeast Nutrient
My recommendation for using yeast nutrients is to always add them to your yeast starters and when brewing beers with a high starting gravity. I’d say anything over 1.070 warrants a pinch of nutrients.
You also want to use yeast nutrients when you’re brewing a high-adjunct beer. A beer brewed with rice, corn, or other sugar sources won’t have as many “naturally” occurring nutrients because you’re using less malt.
What is in your Yeast Nutrients?
Yeast nutrient provides nitrogen (both organic and nonorganic) and phosphates, as well as minerals like zinc. Yeast need zinc to form alcohol. To put it too simply, yeast nutrient provides all the necessary components for yeast to live, ferment wort, and reproduce.
Nutrient gives yeast the things they need to build their cell walls, produce energy, etc. But an all-malt wort also contains all these things and thus nutrient isn’t always needed. With the exception of zinc.
It’s worth noting that zinc is the only mineral not included in an all-malt wort. If you’re not adding yeast nutrient (assuming it has zinc…because it does), then you should add zinc to every batch.
Having said that, there are plenty of people making good beer that don’t use yeast nutrient…ever.