Two things. One you can not tell what a material is by the particle size and shape, or by the color. The physical crystal is orthorhombic ( a cubic structure) when precipitated from a solution, but depending on how it is process it could appear to be a powder, lumps, or spheres.

Two. Personally I would not use a lot of Ammonium Sulfate on my lawn. Remember Ammonium Sulfate is the salt of Sulfuric Acid and Ammonia. While the Ammonia part of the molecule can be used by plants, the Sulfuric acid part will be there nearly forever. To much Ammonium Sulfate and you will start to have a salt build up, probably Calcium, Sodium, Iron, etc. Sulfates.

For this reason you are better off using Ammonium Nitrate, as both parts of the molecule can be used in the plate by different routes.

Ha. Last time I went and asked for Ammonium Nitrate the guy suddenly got a very concerned look on his face. He reluctantly sold me some Ammonium Sulfate instead, he said they stopped selling nitrate.

OTOH, ammonium sulfate will help to bring that soil pH down a bit, which is not at all a bad thing. Ammonium sulfate is a recommended fertilizer for acid loving plants and is a common ingredient in any fert intended for that purpose.

If you can not get Ammonium Nitrate, Urea would be an acceptable substitute. As Ammonium Nitrate there will be no build up as it can be used entirely.

Sulfate (SO4) is an important plant nutrient. Classified as a "secondary" nutrient along with Ca and Mg. It has little if any effect on soil pH. The only "concern" with applying sulfate occurs from accumulations that may create a "black layer" in high maintenance situations where sulfate fertilizers are applied in abundance (e.g. golf courses). The acidifying effect (from AS, Urea etc) is due to the ammonium (NH4) and the release of the hydrogen.

does urea have as much acidifying effect as Ammonium sulfate?

"does urea have as much acidifying effect as Ammonium sulfate"

No. "Ammonium fertilizers: A comparatively minor effect on soil pH when used in appropriate amounts as a nitrogen fertilizer. Diammonium phosphate,
monoammonium phosphate, and ammonium sulfate have more
acidic potential than urea or ammonium nitrate. Calcium nitrate and
potassium nitrate increase soil pH so should be avoided if pH is
already too high."

No. On a pound per pound basis (higher numbers are more acidifying or, in other words, how many pounds of pure lime will one pound of fertilizer product neutralize)) the factors are:

Sulfur coated Urea-1.18

Ammonium Sulfate-1.10

Urea-0.81

Trying to acidify alkaline soils is not very successful. There is just too much CaCo3 in the soil to offset.

Ammonium sulfate will flat wipe out snails and slugs literally overnight. Their eggs will survive, so you might have to repeat in a few weeks.

Sulfur coated Urea-1.18

Sulfur (S) coated Urea is not the same as Ammonium Sulfate.
(NH4)2SO4

What percent of Coating (S) is in the product. I have worked with products where the coating is less that 4% of the product by weight. The Sulfur in Ammonnium Sulfate is about 24%. While I have not read the MSDS sheet I would guess the sulfur in the coated product is elemental sulfur, It reaction in the soil would be completely different that in (NH4)2SO4

The question was:

does urea have as much acidifying effect as Ammonium sulfate?

The answer is:

Ammonium Sulfate-1.10

Urea-0.81

Sulfur coated UREA is 1.18

Sulfate (or the Sulfur atom tied up in the sulfate molecule) has no effect on acidification. On the other hand the biological processing of elemental sulfur (the sulfur coating) does produce an acidifying effect. That is the reason why sulfur coated urea has a greater acidifying effect than uncoated urea.

EDIT: I should clarify: those numbers are a pound per pound basis of product, not on a pound per pound basis of N. For instance 1# of AS will neutralize 1.10 lbs of lime and 1# of urea will neutralize 0.81 lbs of lime. However, to apply 1# of N/k, 5# of AS or 2# of urea needs to be applied. 2# of urea will neutralize 1.62 lbs of lime and 5# of AS will neutralize 5.5 lbs of lime. So per pound of applied N, AS is much more acidifying.

BTW, sulfur coated urea (38-0-0) would be applied at the rate of 2.6# of product to get 1# of N/k. So, although it has the highest pound for pound of product acidifying factor (1.18), when applying at at the 1# N rate, it neutralizes only 3# of lime.

Clear as mud now? :/