Alfalfa is actually a legume so it fixes nitrogen and essentially does not deplete the soil at all. Micro nutrients only. But it takes a lot of water.
It's going to take a major cultural shift to slow up irrigated farming. Such a high percentage of commercial ag is irrigated.
I believe alfalfa pulls off a lot of potassium, if I recall my HS agronomy class correctly. Also, even though it's a legume, most farmers apply a bit of nitrogen between cutting to push growth for swift recutting (at least around here, though it's not irrigated here).
Sadly the issues with commercial ag won't be resolved until its no longer (financially) advantageous to act so destructively.
Thats some good info. Thanks