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