Ammonium sulfate is 21-0-0+24S and urea is 46-0-0. So, urea is pretty much always going to be cheaper in terms of $/N. From a homeowner perspective, there's not a huge difference between the two other than the N content and the fact that ammonium sulphate is more acidic (and, of course, has sulphur).

You should water in either type of N as soon as you can. The next morning is no problem at all.

 

Does the max rate for AS differ from max rate of urea?

 

Check out the @thegrassfactor video, all about nitrogen. He answers it really well in the video.

 

If you have high pH soil, you'll want to use ammonium sulfate as your N source.

 

We usually reference the Salt Index of fertilizers when comparing burn potential, since it’s the salt aspect of a mineral that causes this drying effect.
On a weight/weight comparison Urea has a slightly higher salt index, but a #N/#N scale AMS is just about double (not only because you have to apply just about double the AMS to match Urea N value but also the added sulfur that accompanies it). So all things being equal AMS is more of a dessecant than Urea is when looking from the max N application viewpoint.

 

0.2 of N into 2 gallons of water/ksqft is the max I use for foliar on AMS.

If you plan to immediately water it in, then 1lb N/ksqft.

 

I have found that you can safely spray 1# of Urea/M in the evening without much issue and the same goes for AMS but the drier your lawn is the high chance of leaf tip burn. If you want to be safe you can just water it in after application.

 

So 1 pound of Urea or 0.46 pound of N per 1k sqft, letting it dry for a day, probably no big deal? Lawn gets rain or irrigation every 2-3 days.

 

I would spray it in conjunction with an irrigation cycle set for the next morning. Letting it dry on the leaf and sit in the hot sun all day is not recommended. The amount of Nitrogen the grass can uptake from a foliar application is minimal at best and that happens after the first hour.