Iris growing formula....any suggestions?

I don't remember where I got this. I've used it before when preparing new beds and it seemed to be fine. I need to prepare new beds and redo some older ones so I thought I'd toss it out there and see if there is anything new or better. Our soil here in my part of Arizona is on the alkaline side and nearly devoid of organic matter. In my yard the soil is a bit sandy, which is ok. Better than clay. And I plan to dig in lots of 'Omni'.
Here's the formula I've used:
gypson-----5lbs. per 100sq. ft.
amonium sulphate-----2lbs. per 100sq. ft..
amonium phosphate----4lbs. per 100sq. ft

Any comments and/or suggestions are welcome. Thanks.