You can use any form but you will risk having it precipitate. Ammonium Thiosulfate is always in a liquid form, which is a good thing. You can dissolve ammonium sulfate but it will likely fall out of solution and then you will have a mess. Stick with the ATS liquid and you won’t be disappointed.

I wanted to add zinc to our mix so I tried liquid zinc sulfate to get the same Sulfur rate as the ATS but had a chemical reaction in the mix tote where Ammonium Sulfate salted out. I am sure the solubility of the Ammonium Sulfate is less than you would expect. This was not an issue when we put the same mix in a mixing tank and applied it through a fertigation. It is possible, but would say the downside is not worth it.