Do you mean ammonium sulfate, (NH4)2SO4? If so, ammonium sulfate is
21% nitrogen, but the sulfur in it really acidifies the soil. It can
be a component of a blended fertilizer, but can be purchased separately
if you really want it.
If you have fescue, bluegrass or ryegrass for a lawn I don't think you
want to be applying this stuff every 2-3 months. You could apply 2 to
4 pounds of ammonium sulfate per 1,000 square feet in April or early
May, and then 12 pounds per 1,000 square feet in October or November.
If you have bermudagrass, yes, you can apply about 2 to 4 pounds every
month from April to early August. If you have St. Augustine, don't
apply more than 6 pounds of ammonium sulfate in an entire year or you
will get St. Augustine Decline (that's a disease). If you have zoysia,
forget it because zoysia doens't need much care anyway.